ON THE
ORIGIN OF SPECIES.
"But with regard to the material world, we can at least go so far as
this—we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by
the establishment of general laws."
Whewell: Bridgewater Treatise.
"The only distinct meaning of the word 'natural' is stated,
fixed, or settled; since what is natural as much requires
and presupposes an intelligent agent to render it so, i.e. to
effect it continually or at stated times, as what is supernatural or
miraculous does to effect it for once."
Butler: Analogy of Revealed Religion.
"To conclude, therefore, let no man out of a weak conceit of sobriety,
or an ill-applied moderation, think or maintain, that a man can search
too far or be too well studied in the book of God's word, or in the book
of God's works; divinity or philosophy; but rather let men endeavour an
endless progress or proficience in both."
Bacon: Advancement of Learning.
Down, Bromley, Kent,
October 1st, 1859. (1st Thousand).
ON
THE ORIGIN OF SPECIES
BY MEANS OF NATURAL SELECTION,
OR THE
PRESERVATION OF FAVOURED RACES IN THE STRUGGLE
FOR LIFE.
By CHARLES DARWIN, M.A.,
FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;
AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
ROUND THE WORLD.'
FIFTH THOUSAND.
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1860.
The right of Translation is reserved.
LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
AND CHARING CROSS.
[v]
CONTENTS.
Introduction
Page 1
CHAPTER I.
Variation under Domestication.
Causes of Variability—Effects of Habit—Correlation of
Growth—Inheritance—Character of Domestic
Varieties—Difficulty of distinguishing between Varieties and
Species—Origin of Domestic Varieties from one or more
Species—Domestic Pigeons, their Differences and
Origin—Principle of Selection anciently followed, its
Effects—Methodical and Unconscious Selection—Unknown Origin
of our Domestic Productions—Circumstances favourable to Man's power
of Selection
7-43
CHAPTER II.
Variation under Nature.
Variability—Individual differences—Doubtful
species—Wide ranging, much diffused, and common species vary
most—Species of the larger genera in any country vary more than the
species of the smaller genera—Many of the species of the larger
genera resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges
44-59
[vi]
CHAPTER III.
Struggle for Existence.
Its bearing on natural selection—The term used in a wide
sense—Geometrical powers of increase—Rapid increase of
naturalised animals and plants—Nature of the checks to
increase—Competition universal—Effects of
climate—Protection from the number of individuals—Complex
relations of all animals and plants throughout nature—Struggle for
life most severe between individuals and varieties of the same species;
often severe between species of the same genus—The relation of
organism to organism the most important of all relations
60-79
CHAPTER IV.
Natural Selection.
Natural Selection—its power compared with man's
selection—its power on characters of trifling importance—its
power at all ages and on both sexes—Sexual Selection—On the
generality of intercrosses between individuals of the same
species—Circumstances favourable and unfavourable to Natural
Selection, namely, intercrossing, isolation, number of
individuals—Slow action—Extinction caused by Natural
Selection—Divergence of Character, related to the diversity of
inhabitants of any small area, and to naturalisation—Action of
Natural Selection, through Divergence of Character and Extinction, on the
descendants from a common parent—Explains the Grouping of all
organic beings
80-130
CHAPTER V.
Laws of Variation.
Effects of external conditions—Use and disuse, combined with
natural selection; organs of flight and of
vision—Acclimatisation—Correlation of
growth—Compensation and economy of growth—False
correlations—Multiple, rudimentary, and lowly organised structures
variable—Parts developed in an unusual manner are highly variable:
specific characters more variable than generic: secondary sexual
characters variable—Species of the same genus vary in an analogous
manner—Reversions to long-lost characters—Summary
131-170
[vii]
CHAPTER VI.
Difficulties on Theory.
Difficulties on the theory of descent with
modification—Transitions—Absence or rarity of transitional
varieties—Transitions in habits of life—Diversified habits in
the same species—Species with habits widely different from those of
their allies—Organs of extreme perfection—Means of
transition—Cases of difficulty—Natura non facit
saltum—Organs of small importance—Organs not in all cases
absolutely perfect—The law of Unity of Type and of the Conditions
of Existence embraced by the theory of Natural Selection
171-206
CHAPTER VII.
Instinct.
Instincts comparable with habits, but different in their
origin—Instincts graduated—Aphides and ants—Instincts
variable—Domestic instincts, their origin—Natural instincts
of the cuckoo, ostrich, and parasitic bees—Slave-making
ants—Hive-bee, its cell-making instinct—Difficulties on the
theory of the Natural Selection of instincts—Neuter or sterile
insects—Summary
207-244
CHAPTER VIII.
Hybridism.
Distinction between the sterility of first crosses and of
hybrids—Sterility various in degree, not universal, affected by
close interbreeding, removed by domestication—Laws governing the
sterility of hybrids—Sterility not a special endowment, but
incidental on other differences—Causes of the sterility of first
crosses and of hybrids—Parallelism between the effects of changed
conditions of life and crossing—Fertility of varieties when crossed
and of their mongrel offspring not universal—Hybrids and mongrels
compared independently of their fertility—Summary
245-278
[viii]
CHAPTER IX.
On the Imperfection of the Geological Record.
On the absence of intermediate varieties at the present day—On
the nature of extinct intermediate varieties; on their number—On
the vast lapse of time, as inferred from the rate of deposition and of
denudation—On the poorness of our palæontological
collections—On the intermittence of geological formations—On
the absence of intermediate varieties in any one formation—On the
sudden appearance of groups of species—On their sudden appearance
in the lowest known fossiliferous strata
279-311
CHAPTER X.
On the Geological Succession of Organic Beings.
On the slow and successive appearance of new species—On their
different rates of change—Species once lost do not
reappear—Groups of species follow the same general rules in their
appearance and disappearance as do single species—On
Extinction—On simultaneous changes in the forms of life throughout
the world—On the affinities of extinct species to each other and to
living species—On the state of development of ancient
forms—On the succession of the same types within the same
areas—Summary of preceding and present chapters
312-345
CHAPTER XI.
Geographical Distribution.
Present distribution cannot be accounted for by differences in
physical conditions—Importance of barriers—Affinity of the
productions of the same continent—Centres of creation—Means
of dispersal, by changes of climate and of the level of the land, and by
occasional means—Dispersal during the Glacial period co-extensive
with the world
346-382
CHAPTER XII.
Geographical Distribution—continued.
Distribution of fresh-water productions—On the inhabitants of
oceanic islands—Absence of Batrachians and of terrestrial
Mammals—On the relation of the inhabitants of islands to those of
the nearest mainland—On colonisation from the nearest source with
subsequent modification—Summary of the last and present
chapters
383-410
CHAPTER XIII.
Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.
Classification, groups subordinate to
groups—Natural system—Rules and difficulties in
classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used
in classification—Analogical or adaptive
characters—Affinities, general, complex and
radiating—Extinction separates and defines groups—Morphology, between members of the same class, between
parts of the same individual—Embryology,
laws of, explained by variations not supervening at an early age, and
being inherited at a corresponding age—Rudimentary
organs; their origin explained—Summary
411-458
CHAPTER XIV.
Recapitulation and Conclusion.
Recapitulation of the difficulties on the theory of Natural
Selection—Recapitulation of the general and special circumstances
in its favour—Causes of the general belief in the immutability of
species—How far the theory of natural selection may be
extended—Effects of its adoption on the study of Natural
history—Concluding remarks
459-490
[1]
ON THE ORIGIN OF SPECIES.
INTRODUCTION.
When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants of
that continent. These facts seemed to me to throw some light on the
origin of species—that mystery of mysteries, as it has been called
by one of our greatest philosophers. On my return home, it occurred to
me, in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years' work I allowed myself
to speculate on the subject, and drew up some short notes; these I
enlarged in 1844 into a sketch of the conclusions, which then seemed to
me probable: from that period to the present day I have steadily pursued
the same object. I hope that I may be excused for entering on these
personal details, as I give them to show that I have not been hasty in
coming to a decision.
My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been induced
to do this, as Mr. Wallace, who is now studying the [2]natural history of the
Malay archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. Last year he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published in
the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work—the latter having read my sketch
of 1844—honoured me by thinking it advisable to publish, with Mr.
Wallace's excellent memoir, some brief extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors will have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only the
general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been
grounded; and I hope in a future work to do this. For I am well aware
that scarcely a single point is discussed in this volume on which facts
cannot be adduced, often apparently leading to conclusions directly
opposite to those at which I have arrived. A fair result can be obtained
only by fully stating and balancing the facts and arguments on both sides
of each question; and this cannot possibly be here done.
I much regret that want of space prevents my having the satisfaction
of acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, [3]let this opportunity pass without expressing
my deep obligations to Dr. Hooker, who for the last fifteen years has
aided me in every possible way by his large stores of knowledge and his
excellent judgment.
In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the conclusion
that each species had not been independently created, but had descended,
like varieties, from other species. Nevertheless, such a conclusion, even
if well founded, would be unsatisfactory, until it could be shown how the
innumerable species inhabiting this world have been modified, so as to
acquire that perfection of structure and coadaptation which most justly
excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, &c., as the only possible cause of
variation. In one very limited sense, as we shall hereafter see, this may
be true; but it is preposterous to attribute to mere external conditions,
the structure, for instance, of the woodpecker, with its feet, tail,
beak, and tongue, so admirably adapted to catch insects under the bark of
trees. In the case of the misseltoe, which draws its nourishment from
certain trees, which has seeds that must be transported by certain birds,
and which has flowers with separate sexes absolutely requiring the agency
of certain insects to bring pollen from one flower to the other, it is
equally preposterous to account for the structure of this parasite, with
its relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.
The author of the 'Vestiges of Creation' would, I presume, say that,
after a certain unknown number of [4]generations, some bird had given birth to a
woodpecker, and some plant to the missletoe, and that these had been
produced perfect as we now see them; but this assumption seems to me to
be no explanation, for it leaves the case of the coadaptations of organic
beings to each other and to their physical conditions of life, untouched
and unexplained.
It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best chance
of making out this obscure problem. Nor have I been disappointed; in this
and in all other perplexing cases I have invariably found that our
knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my conviction
of the high value of such studies, although they have been very commonly
neglected by naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a large
amount of hereditary modification is at least possible; and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then
pass on to the variability of species in a state of nature; but I shall,
unfortunately, be compelled to treat this subject far too briefly, as it
can be treated properly only by giving long catalogues of facts. We
shall, however, be enabled to discuss what circumstances are most
favourable to variation. In the next chapter the Struggle for Existence
amongst all organic beings throughout the world, which inevitably follows
from the high geometrical ratio of their [5]increase, will be treated
of. This is the doctrine of Malthus, applied to the whole animal and
vegetable kingdoms. As many more individuals of each species are born
than can possibly survive; and as, consequently, there is a frequently
recurring struggle for existence, it follows that any being, if it vary
however slightly in any manner profitable to itself, under the complex
and sometimes varying conditions of life, will have a better chance of
surviving, and thus be naturally selected. From the strong
principle of inheritance, any selected variety will tend to propagate its
new and modified form.
This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural Selection
almost inevitably causes much Extinction of the less improved forms of
life, and leads to what I have called Divergence of Character. In the
next chapter I shall discuss the complex and little known laws of
variation and of correlation of growth. In the four succeeding chapters,
the most apparent and gravest difficulties on the theory will be given:
namely, first, the difficulties of transitions, or in understanding how a
simple being or a simple organ can be changed and perfected into a highly
developed being or elaborately constructed organ; secondly, the subject
of Instinct, or the mental powers of animals; thirdly, Hybridism, or the
infertility of species and the fertility of varieties when intercrossed;
and fourthly, the imperfection of the Geological Record. In the next
chapter I shall consider the geological succession of organic beings
throughout time; in the eleventh and twelfth, their geographical
distribution throughout space; in the thirteenth, their classification or
mutual affinities, both when mature and in an embryonic condition. In the
last chapter I shall give a [6]brief recapitulation of the whole work, and a
few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due allowance
for our profound ignorance in regard to the mutual relations of all the
beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance, for
they determine the present welfare, and, as I believe, the future success
and modification of every inhabitant of this world. Still less do we know
of the mutual relations of the innumerable inhabitants of the world
during the many past geological epochs in its history. Although much
remains obscure, and will long remain obscure, I can entertain no doubt,
after the most deliberate study and dispassionate judgment of which I am
capable, that the view which most naturalists entertain, and which I
formerly entertained—namely, that each species has been
independently created—is erroneous. I am fully convinced that
species are not immutable; but that those belonging to what are called
the same genera are lineal descendants of some other and generally
extinct species, in the same manner as the acknowledged varieties of any
one species are the descendants of that species. Furthermore, I am
convinced that Natural Selection has been the main but not exclusive
means of modification.
[7]
CHAPTER I.
Variation under Domestication.
Causes of Variability—Effects of Habit—Correlation of
Growth—Inheritance—Character of Domestic
Varieties—Difficulty of distinguishing between Varieties and
Species—Origin of Domestic Varieties from one or more
Species—Domestic Pigeons, their Differences and
Origin—Principle of Selection anciently followed, its
Effects—Methodical and Unconscious Selection—Unknown Origin
of our Domestic Productions—Circumstances favourable to Man's power
of Selection.
When we look to the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. When
we reflect on the vast diversity of the plants and animals which have
been cultivated, and which have varied during all ages under the most
different climates and treatment, I think we are driven to conclude that
this great variability is simply due to our domestic productions having
been raised under conditions of life not so uniform as, and somewhat
different from, those to which the parent-species have been exposed under
nature. There is also, I think, some probability in the view propounded
by Andrew Knight, that this variability may be partly connected with
excess of food. It seems pretty clear that organic beings must be exposed
during several generations to the new conditions of life to cause any
appreciable amount of variation; and that when the organisation has once
begun to vary, it generally continues to vary for many generations. [8]No case is
on record of a variable being ceasing to be variable under cultivation.
Our oldest cultivated plants, such as wheat, still often yield new
varieties: our oldest domesticated animals are still capable of rapid
improvement or modification.
It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by any
clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may be
attributed to the male and female reproductive elements having been
affected prior to the act of conception. Several reasons make me believe
in this; but the chief one is the remarkable effect which confinement or
cultivation has on the function of the reproductive system; this system
appearing to be far more susceptible than any other part of the
organisation, to the action of any change in the conditions of life.
Nothing is more easy than to tame an animal, and few things more
difficult than to get it to breed freely under confinement, even in the
many cases when the male and female unite. How many animals there are
which will not breed, though living long under not very close confinement
in their native country! This is generally attributed to vitiated
instincts; but how many cultivated plants display the utmost vigour, and
yet rarely or never seed! In some few such cases it has been discovered
that very trifling changes, such as a little more or less water at some
particular period of growth, will determine whether or not the plant sets
a seed. I cannot here enter on the copious details which I have collected
on [9]this curious subject; but to show how singular
the laws are which determine the reproduction of animals under
confinement, I may just mention that carnivorous animals, even from the
tropics, breed in this country pretty freely under confinement, with the
exception of the plantigrades or bear family; whereas carnivorous birds,
with the rarest exceptions, hardly ever lay fertile eggs. Many exotic
plants have pollen utterly worthless, in the same exact condition as in
the most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature, perfectly tamed,
long-lived, and healthy (of which I could give numerous instances), yet
having their reproductive system so seriously affected by unperceived
causes as to fail in acting, we need not be surprised at this system,
when it does act under confinement, acting not quite regularly, and
producing offspring not perfectly like their parents.
Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility; and
variability is the source of all the choicest productions of the garden.
I may add, that as some organisms will breed freely under the most
unnatural conditions (for instance, the rabbit and ferret kept in
hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly—perhaps hardly more than in a
state of nature.
A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
[10]Such buds can be propagated by grafting,
&c., and sometimes by seed. These "sports" are extremely rare under
nature, but far from rare under cultivation; and in this case we see that
the treatment of the parent has affected a bud or offset, and not the
ovules or pollen. But it is the opinion of most physiologists that there
is no essential difference between a bud and an ovule in their earliest
stages of formation; so that, in fact, "sports" support my view, that
variability may be largely attributed to the ovules or pollen, or to
both, having been affected by the treatment of the parent prior to the
act of conception. These cases anyhow show that variation is not
necessarily connected, as some authors have supposed, with the act of
generation.
Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young and
the parents, as Müller has remarked, have apparently been exposed to
exactly the same conditions of life; and this shows how unimportant the
direct effects of the conditions of life are in comparison with the laws
of reproduction, of growth, and of inheritance; for had the action of the
conditions been direct, if any of the young had varied, all would
probably have varied in the same manner. To judge how much, in the case
of any variation, we should attribute to the direct action of heat,
moisture, light, food, &c., is most difficult: my impression is, that
with animals such agencies have produced very little direct effect,
though apparently more in the case of plants. Under this point of view,
Mr. Buckman's recent experiments on plants are extremely valuable. When
all or nearly all the individuals exposed to certain conditions are
affected in the same way, the change at first appears to be directly due
to such conditions; but in some cases it can be shown that quite opposite
conditions produce [11]similar changes of structure. Nevertheless
some slight amount of change may, I think, be attributed to the direct
action of the conditions of life—as, in some cases, increased size
from amount of food, colour from particular kinds of food or from light,
and perhaps the thickness of fur from climate.
Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion
to the whole skeleton, than do the same bones in the wild-duck; and I
presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and
inherited development of the udders in cows and goats in countries where
they are habitually milked, in comparison with the state of these organs
in other countries, is another instance of the effect of use. Not a
single domestic animal can be named which has not in some country
drooping ears; and the view suggested by some authors, that the drooping
is due to the disuse of the muscles of the ear, from the animals not
being much alarmed by danger, seems probable.
There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct parts
are very curious; and many instances are given in Isidore Geoffroy St.
Hilaire's great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances of
correlation are quite whimsical: thus [12]cats with blue eyes are
invariably deaf; colour and constitutional peculiarities go together, of
which many remarkable cases could be given amongst animals and plants.
From the facts collected by Heusinger, it appears that white sheep and
pigs are differently affected from coloured individuals by certain
vegetable poisons. Hairless dogs have imperfect teeth: long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks large
feet. Hence, if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly unconsciously modify other parts of
the structure, owing to the mysterious laws of the correlation of
growth.
The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth while
carefully to study the several treatises published on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, &c.;
and it is really surprising to note the endless points in structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic, and
tends to depart in some small degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both those
of slight and those of considerable physiological importance, is endless.
Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and
the best on this subject. No breeder doubts how strong is the tendency to
inheritance: like produces like is his fundamental belief: doubts have
been thrown on this principle by theoretical writers alone. When any
deviation of structure often appears, and we see it in the [13]father and
child, we cannot tell whether it may not be due to the same cause having
acted on both; but when amongst individuals, apparently exposed to the
same conditions, any very rare deviation, due to some extraordinary
combination of circumstances, appears in the parent—say, once
amongst several million individuals—and it reappears in the child,
the mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, &c., appearing in several
members of the same family. If strange and rare deviations of structure
are truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the whole
subject, would be, to look at the inheritance of every character whatever
as the rule, and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why a
peculiarity in different individuals of the same species, or in
individuals of different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characters to its
grandfather or grandmother or other more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes, or to one
sex alone, more commonly but not exclusively to the like sex. It is a
fact of some little importance to us, that peculiarities appearing in the
males of our domestic breeds are often transmitted either exclusively, or
in a much greater degree, to males alone. A much more important rule,
which I think may be trusted, is that, at whatever period of life a
peculiarity first appears, it tends to appear in the offspring at a
corresponding age, though sometimes earlier. In many cases this could not
be otherwise: thus the inherited peculiarities in the horns of cattle
could appear only in [14]the offspring when nearly mature;
peculiarities in the silkworm are known to appear at the corresponding
caterpillar or cocoon stage. But hereditary diseases and some other facts
make me believe that the rule has a wider extension, and that when there
is no apparent reason why a peculiarity should appear at any particular
age, yet that it does tend to appear in the offspring at the same period
at which it first appeared in the parent. I believe this rule to be of
the highest importance in explaining the laws of embryology. These
remarks are of course confined to the first appearance of the
peculiarity, and not to its primary cause, which may have acted on the
ovules or male element; in nearly the same manner as in the crossed
offspring from a short-horned cow by a long-horned bull, the greater
length of horn, though appearing late in life, is clearly due to the male
element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic
varieties, when run wild, gradually but certainly revert in character to
their aboriginal stocks. Hence it has been argued that no deductions can
be drawn from domestic races to species in a state of nature. I have in
vain endeavoured to discover on what decisive facts the above statement
has so often and so boldly been made. There would be great difficulty in
proving its truth: we may safely conclude that very many of the most
strongly-marked domestic varieties could not possibly live in a wild
state. In many cases we do not know what the aboriginal stock was, and so
could not tell whether or not nearly perfect reversion had ensued. It
would be quite necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in its
new home. Nevertheless, as our varieties certainly do occasionally [15]revert
in some of their characters to ancestral forms, it seems to me not
improbable, that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance, of
the cabbage, in very poor soil (in which case, however, some effect would
have to be attributed to the direct action of the poor soil), that they
would to a large extent, or even wholly, revert to the wild aboriginal
stock. Whether or not the experiment would succeed, is not of great
importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion,—that is, to
lose their acquired characters, whilst kept under the same conditions,
and whilst kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations in their structure, in
such case, I grant that we could deduce nothing from domestic varieties
in regard to species. But there is not a shadow of evidence in favour of
this view: to assert that we could not breed our cart and race-horses,
long and short-horned cattle, and poultry of various breeds, and esculent
vegetables, for an almost infinite number of generations, would be
opposed to all experience. I may add, that when under nature the
conditions of life do change, variations and reversions of character
probably do occur; but natural selection, as will hereafter be explained,
will determine how far the new characters thus arising shall be
preserved.
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same
species, also, often have a somewhat monstrous character; by which I
mean, that, although differing [16]from each other, and from other species of
the same genus, in several trifling respects, they often differ in an
extreme degree in some one part, both when compared one with another, and
more especially when compared with all the species in nature to which
they are nearest allied. With these exceptions (and with that of the
perfect fertility of varieties when crossed,—a subject hereafter to
be discussed), domestic races of the same species differ from each other
in the same manner as, only in most cases in a lesser degree than, do
closely-allied species of the same genus in a state of nature. I think
this must be admitted, when we find that there are hardly any domestic
races, either amongst animals or plants, which have not been ranked by
competent judges as mere varieties, and by other competent judges as the
descendants of aboriginally distinct species. If any marked distinction
existed between domestic races and species, this source of doubt could
not so perpetually recur. It has often been stated that domestic races do
not differ from each other in characters of generic value. I think it
could be shown that this statement is hardly correct; but naturalists
differ widely in determining what characters are of generic value; all
such valuations being at present empirical. Moreover, on the view of the
origin of genera which I shall presently give, we have no right to expect
often to meet with generic differences in our domesticated
productions.
When we attempt to estimate the amount of structural difference
between the domestic races of the same species, we are soon involved in
doubt, from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the [17]offspring of any single species, then such
facts would have great weight in making us doubt about the immutability
of the many very closely allied natural species—for instance, of
the many foxes—inhabiting different quarters of the world. I do not
believe, as we shall presently see, that the whole amount of difference
between the several breeds of the dog has been produced under
domestication; I believe that some small part of the difference is due to
their being descended from distinct species. In the case of some other
domesticated species, there is presumptive, or even strong evidence, that
all the breeds have descended from a single wild stock.
It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed an
animal, whether it would vary in succeeding generations, and whether it
would endure other climates? Has the little variability of the ass or
guinea-fowl, or the small power of endurance of warmth by the reindeer,
or of cold by the common camel, prevented their domestication? I cannot
doubt that if other animals and plants, equal in number to our
domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed
for an equal number of generations under domestication, they would vary
on an average as largely as the parent species of our existing
domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants,
I do not think it is possible to come to any definite conclusion, whether
they have descended from one or several wild species. The argument mainly
relied on by those who believe in the multiple origin [18]of our domestic
animals is, that we find in the most ancient records, more especially on
the monuments of Egypt, much diversity in the breeds; and that some of
the breeds closely resemble, perhaps are identical with, those still
existing. Even if this latter fact were found more strictly and generally
true than seems to me to be the case, what does it show, but that some of
our breeds originated there, four or five thousand years ago? But Mr.
Horner's researches have rendered it in some degree probable that man
sufficiently civilized to have manufactured pottery existed in the valley
of the Nile thirteen or fourteen thousand years ago; and who will pretend
to say how long before these ancient periods, savages, like those of
Tierra del Fuego or Australia, who possess a semi-domestic dog, may not
have existed in Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs
have descended from several wild species. Knowing, as we do, that savages
are very fond of taming animals, it seems to me unlikely, in the case of
the dog-genus, which is distributed in a wild state throughout the world,
that since man first appeared one single species alone should have been
domesticated. In regard to sheep and goats I can form no opinion. I
should think, from facts communicated to me by Mr. Blyth, on the habits,
voice, and constitution, &c., of the humped Indian cattle, that these
had descended from a different aboriginal stock from our European cattle;
and several competent judges believe that these latter have had more than
one wild parent. With respect to horses, from reasons which I cannot give
here, I am doubtfully inclined to believe, in opposition to several
authors, that all the races have descended from one [19]wild stock. Mr. Blyth,
whose opinion, from his large and varied stores of knowledge, I should
value more than that of almost any one, thinks that all the breeds of
poultry have proceeded from the common wild Indian fowl (Gallus bankiva).
In regard to ducks and rabbits, the breeds of which differ considerably
from each other in structure, I do not doubt that they have all descended
from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive
characters be ever so slight, has had its wild prototype. At this rate
there must have existed at least a score of species of wild cattle, as
many sheep, and several goats in Europe alone, and several even within
Great Britain. One author believes that there formerly existed in Great
Britain eleven wild species of sheep peculiar to it! When we bear in mind
that Britain has now hardly one peculiar mammal, and France but few
distinct from those of Germany and conversely, and so with Hungary,
Spain, &c., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, &c, we must admit that many
domestic breeds have originated in Europe; for whence could they have
been derived, as these several countries do not possess a number of
peculiar species as distinct parent-stocks? So it is in India. Even in
the case of the domestic dogs of the whole world, which I fully admit
have probably descended from several wild species, I cannot doubt that
there has been an immense amount of inherited variation. Who can believe
that animals closely resembling the Italian greyhound, the bloodhound,
the bull-dog, or Blenheim spaniel, &c.—so unlike all wild
Canidæ—ever existed freely in a state of nature? It has often been
loosely said that all our races of dogs have [20]been produced by the
crossing of a few aboriginal species; but by crossing we can only get
forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must admit the
former existence of the most extreme forms, as the Italian greyhound,
bloodhound, bull-dog, &c., in the wild state. Moreover, the
possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those individual
mongrels, which present any desired character; but that a race could be
obtained nearly intermediate between two extremely different races or
species, I can hardly believe. Sir J. Sebright expressly experimentised
for this object, and failed. The offspring from the first cross between
two pure breeds is tolerably and sometimes (as I have found with pigeons)
extremely uniform, and everything seems simple enough; but when these
mongrels are crossed one with another for several generations, hardly two
of them will be alike, and then the extreme difficulty, or rather utter
hopelessness, of the task becomes apparent. Certainly, a breed
intermediate between two very distinct breeds could not be got
without extreme care and long-continued selection; nor can I find a
single case on record of a permanent race having been thus formed.
On the Breeds of the Domestic Pigeon.—Believing that it
is always best to study some special group, I have, after deliberation,
taken up domestic pigeons. I have kept every breed which I could purchase
or obtain, and have been most kindly favoured with skins from several
quarters of the world, more especially by the Hon. W. Elliot from India,
and by the Hon. C. Murray from Persia. Many treatises in different
languages have been published on pigeons, and some of them are very
important, as being of [21]considerable antiquity. I have associated
with several eminent fanciers, and have been permitted to join two of the
London Pigeon Clubs. The diversity of the breeds is something
astonishing. Compare the English carrier and the short-faced tumbler, and
see the wonderful difference in their beaks, entailing corresponding
differences in their skulls. The carrier, more especially the male bird,
is also remarkable from the wonderful development of the carunculated
skin about the head, and this is accompanied by greatly elongated
eyelids, very large external orifices to the nostrils, and a wide gape of
mouth. The short-faced tumbler has a beak in outline almost like that of
a finch; and the common tumbler has the singular inherited habit of
flying at a great height in a compact flock, and tumbling in the air head
over heels. The runt is a bird of great size, with long, massive beak and
large feet; some of the sub-breeds of runts have very long necks, others
very long wings and tails, others singularly short tails. The barb is
allied to the carrier, but, instead of a very long beak, has a very short
and very broad one. The pouter has a much elongated body, wings, and
legs; and its enormously developed crop, which it glories in inflating,
may well excite astonishment and even laughter. The turbit has a very
short and conical beak, with a line of reversed feathers down the breast;
and it has the habit of continually expanding slightly the upper part of
the œsophagus. The Jacobin has the feathers so much reversed along
the back of the neck that they form a hood, and it has, proportionally to
its size, much elongated wing and tail feathers. The trumpeter and
laugher, as their names express, utter a very different coo from the
other breeds. The fantail has thirty or even forty tail feathers, instead
of twelve or fourteen, the normal number in all members of the great
pigeon family; and these feathers are kept expanded, and are [22]carried so erect
that in good birds the head and tail touch; the oil-gland is quite
aborted. Several other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the bones
of the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The number of the caudal and sacral
vertebræ vary; as does the number of the ribs, together with their
relative breadth and the presence of processes. The size and shape of the
apertures in the sternum are highly variable; so is the degree of
divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and of
the upper part of the œsophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal feathers; the
relative length of wing and tail to each other and to the body; the
relative length of leg and of the feet; the number of scutellæ on the
toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight differs remarkably; as does in some breeds the voice and
disposition. Lastly, in certain breeds, the males and females have come
to differ to a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which if shown
to an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species. Moreover, I
do not believe that any ornithologist would place the [23]English carrier,
the short-faced tumbler, the runt, the barb, pouter, and fantail in the
same genus; more especially as in each of these breeds several
truly-inherited sub-breeds, or species as he might have called them,
could be shown him.
Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely, that
all have descended from the rock-pigeon (Columba livia), including under
this term several geographical races or sub-species, which differ from
each other in the most trifling respects. As several of the reasons which
have led me to this belief are in some degree applicable in other cases,
I will here briefly give them. If the several breeds are not varieties,
and have not proceeded from the rock-pigeon, they must have descended
from at least seven or eight aboriginal stocks; for it is impossible to
make the present domestic breeds by the crossing of any lesser number:
how, for instance, could a pouter be produced by crossing two breeds
unless one of the parent-stocks possessed the characteristic enormous
crop? The supposed aboriginal stocks must all have been rock-pigeons,
that is, not breeding or willingly perching on trees. But besides C.
livia, with its geographical sub-species, only two or three other species
of rock-pigeons are known; and these have not any of the characters of
the domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated, and
yet be unknown to ornithologists; and this, considering their size,
habits, and remarkable characters, seems very improbable; or they must
have become extinct in the wild state. But birds breeding on precipices,
and good fliers, are unlikely to be exterminated; and the common
rock-pigeon, which has the same habits with the domestic breeds, has not
been exterminated [24]even on several of the smaller British
islets, or on the shores of the Mediterranean. Hence the supposed
extermination of so many species having similar habits with the
rock-pigeon seems to me a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of the
world, and, therefore, some of them must have been carried back again
into their native country; but not one has ever become wild or feral,
though the dovecot-pigeon, which is the rock-pigeon in a very slightly
altered state, has become feral in several places. Again, all recent
experience shows that it is most difficult to get any wild animal to
breed freely under domestication; yet on the hypothesis of the multiple
origin of our pigeons, it must be assumed that at least seven or eight
species were so thoroughly domesticated in ancient times by
half-civilized man, as to be quite prolific under confinement.
An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though agreeing
generally in constitution, habits, voice, colouring, and in most parts of
their structure, with the wild rock-pigeon, yet are certainly highly
abnormal in other parts of their structure; we may look in vain
throughout the whole great family of Columbidæ for a beak like that of
the English carrier, or that of the short-faced tumbler, or barb; for
reversed feathers like those of the Jacobin; for a crop like that of the
pouter; for tail-feathers like those of the fantail. Hence it must be
assumed not only that half-civilized man succeeded in thoroughly
domesticating several species, but that he intentionally or by chance
picked out extraordinarily abnormal species; and further, that these very
species have since all become extinct or unknown. So many strange
contingencies seem to me improbable in the highest degree. [25]
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white rump
(the Indian subspecies, C. intermedia of Strickland, having it bluish);
the tail has a terminal dark bar, with the bases of the outer feathers
externally edged with white; the wings have two black bars; some
semi-domestic breeds and some apparently truly wild breeds have, besides
the two black bars, the wings chequered with black. These several marks
do not occur together in any other species of the whole family. Now, in
every one of the domestic breeds, taking thoroughly well-bred birds, all
the above marks, even to the white edging of the outer tail-feathers,
sometimes concur perfectly developed. Moreover, when two birds belonging
to two distinct breeds are crossed, neither of which is blue or has any
of the above-specified marks, the mongrel offspring are very apt suddenly
to acquire these characters; for instance, I crossed some uniformly white
fantails with some uniformly black barbs, and they produced mottled brown
and black birds; these I again crossed together, and one grandchild of
the pure white fantail and pure black barb was of as beautiful a blue
colour, with the white rump, double black wing-bar, and barred and
white-edged tail-feathers, as any wild rock-pigeon! We can understand
these facts, on the well-known principle of reversion to ancestral
characters, if all the domestic breeds have descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, firstly, that all the several
imagined aboriginal stocks were coloured and marked like the rock-pigeon,
although no other existing species is thus coloured and marked, so that
in each separate breed there might be a tendency to revert to the very
same colours and markings. Or, secondly, [26]that each breed, even the
purest, has within a dozen or, at most, within a score of generations,
been crossed by the rock-pigeon: I say within a dozen or twenty
generations, for we know of no fact countenancing the belief that the
child ever reverts to some one ancestor, removed by a greater number of
generations. In a breed which has been crossed only once with some
distinct breed, the tendency to reversion to any character derived from
such cross will naturally become less and less, as in each succeeding
generation there will be less of the foreign blood; but when there has
been no cross with a distinct breed, and there is a tendency in both
parents to revert to a character, which has been lost during some former
generation, this tendency, for all that we can see to the contrary, may
be transmitted undiminished for an indefinite number of generations.
These two distinct cases are often confounded in treatises on
inheritance.
Lastly, the hybrids or mongrels from between all the domestic breeds
of pigeons are perfectly fertile. I can state this from my own
observations, purposely made, on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals clearly distinct being themselves
perfectly fertile. Some authors believe that long-continued domestication
eliminates this strong tendency to sterility: from the history of the dog
I think there is some probability in this hypothesis, if applied to
species closely related together, though it is unsupported by a single
experiment. But to extend the hypothesis so far as to suppose that
species, aboriginally as distinct as carriers, tumblers, pouters, and
fantails now are, should yield offspring perfectly fertile, inter
se, seems to me rash in the extreme.
From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed [27]species of pigeons to
breed freely under domestication; these supposed species being quite
unknown in a wild state, and their becoming nowhere feral; these species
having very abnormal characters in certain respects, as compared with all
other Columbidæ, though so like in most other respects to the
rock-pigeon; the blue colour and various marks occasionally appearing in
all the breeds, both when kept pure and when crossed; the mongrel
offspring being perfectly fertile;—from these several reasons,
taken together, I can feel no doubt that all our domestic breeds have
descended from the Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
varieties, more especially those brought from distant countries, we can
make an almost perfect series between the extremes of structure. Thirdly,
those characters which are mainly distinctive of each breed, for instance
the wattle and length of beak of the carrier, the shortness of that of
the tumbler, and the number of tail-feathers in the fantail, are in each
breed eminently variable; and the explanation of this fact will be
obvious when we come to treat of selection. Fourthly, pigeons have been
watched, and tended with the utmost care, and loved by many people. They
have been domesticated for thousands of years in several quarters of the
world; the earliest known record of pigeons is in the fifth Ægyptian
dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius;
but Mr. Birch informs me that pigeons are given in a bill [28]of fare in the
previous dynasty. In the time of the Romans, as we hear from Pliny,
immense prices were given for pigeons; "nay, they are come to this pass,
that they can reckon up their pedigree and race." Pigeons were much
valued by Akber Khan in India, about the year 1600; never less than
20,000 pigeons were taken with the court. "The monarchs of Iran and Turan
sent him some very rare birds;" and, continues the courtly historian,
"His Majesty by crossing the breeds, which method was never practised
before, has improved them astonishingly." About this same period the
Dutch were as eager about pigeons as were the old Romans. The paramount
importance of these considerations in explaining the immense amount of
variation which pigeons have undergone, will be obvious when we treat of
Selection. We shall then, also, see how it is that the breeds so often
have a somewhat monstrous character. It is also a most favourable
circumstance for the production of distinct breeds, that male and female
pigeons can be easily mated for life; and thus different breeds can be
kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and watched
the several kinds, knowing well how true they bred, I felt fully as much
difficulty in believing that they could have descended from a common
parent, as any naturalist could in coming to a similar conclusion in
regard to the many species of finches, or other large groups of birds, in
nature. One circumstance has struck me much; namely, that all the
breeders of the various domestic animals and the cultivators of plants,
with whom I have ever conversed, or whose treatises I have read, are
firmly convinced that the several breeds to which each has attended, are
descended from so many aboriginally distinct species. [29]Ask, as I have
asked, a celebrated raiser of Hereford cattle, whether his cattle might
not have descended from long-horns, and he will laugh you to scorn. I
have never met a pigeon, or poultry, or duck, or rabbit fancier, who was
not fully convinced that each main breed was descended from a distinct
species. Van Mons, in his treatise on pears and apples, shows how utterly
he disbelieves that the several sorts, for instance a Ribston-pippin or
Codlin-apple, could ever have proceeded from the seeds of the same tree.
Innumerable other examples could be given. The explanation, I think, is
simple: from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that
each race varies slightly, for they win their prizes by selecting such
slight differences, yet they ignore all general arguments, and refuse to
sum up in their minds slight differences accumulated during many
successive generations. May not those naturalists who, knowing far less
of the laws of inheritance than does the breeder, and knowing no more
than he does of the intermediate links in the long lines of descent, yet
admit that many of our domestic races have descended from the same
parents—may they not learn a lesson of caution, when they deride
the idea of species in a state of nature being lineal descendants of
other species?
Selection.—Let us now briefly consider the steps by which
domestic races have been produced, either from one or from several allied
species. Some little effect may, perhaps, be attributed to the direct
action of the external conditions of life, and some little to habit; but
he would be a bold man who would account by such agencies for the
differences of a dray and race horse, a greyhound and bloodhound, a
carrier and tumbler pigeon. One of the most remarkable features in our
domesticated races [30]is that we see in them adaptation, not
indeed to the animal's or plant's own good, but to man's use or fancy.
Some variations useful to him have probably arisen suddenly, or by one
step; many botanists, for instance, believe that the fuller's teazle,
with its hooks, which cannot be rivalled by any mechanical contrivance,
is only a variety of the wild Dipsacus; and this amount of change may
have suddenly arisen in a seedling. So it has probably been with the
turnspit dog; and this is known to have been the case with the ancon
sheep. But when we compare the dray-horse and race-horse, the dromedary
and camel, the various breeds of sheep fitted either for cultivated land
or mountain pasture, with the wool of one breed good for one purpose, and
that of another breed for another purpose; when we compare the many
breeds of dogs, each good for man in very different ways; when we compare
the game-cock, so pertinacious in battle, with other breeds so little
quarrelsome, with "everlasting layers" which never desire to sit, and
with the bantam so small and elegant; when we compare the host of
agricultural, culinary, orchard, and flower-garden races of plants, most
useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly produced
as perfect and as useful as we now see them; indeed, in several cases, we
know that this has not been their history. The key is man's power of
accumulative selection: nature gives successive variations; man adds them
up in certain directions useful to him. In this sense he may be said to
make for himself useful breeds.
The great power of this principle of selection is not hypothetical. It
is certain that several of our eminent breeders have, even within a
single lifetime, modified to [31]a large extent some breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this subject,
and to inspect the animals. Breeders habitually speak of an animal's
organisation as something quite plastic, which they can model almost as
they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably better
acquainted with the works of agriculturists than almost any other
individual, and who was himself a very good judge of an animal, speaks of
the principle of selection as "that which enables the agriculturist, not
only to modify the character of his flock, but to change it altogether.
It is the magician's wand, by means of which he may summon into life
whatever form and mould he pleases." Lord Somerville, speaking of what
breeders have done for sheep, says:—"It would seem as if they had
chalked out upon a wall a form perfect in itself, and then had given it
existence." That most skilful breeder, Sir John Sebright, used to say,
with respect to pigeons, that "he would produce any given feather in
three years, but it would take him six years to obtain head and beak." In
Saxony the importance of the principle of selection in regard to merino
sheep is so fully recognised, that men follow it as a trade: the sheep
are placed on a table and are studied, like a picture by a connoisseur;
this is done three times at intervals of months, and the sheep are each
time marked and classed, so that the very best may ultimately be selected
for breeding.
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; [32]all the best breeders are
strongly opposed to this practice, except sometimes amongst closely
allied sub-breeds. And when a cross has been made, the closest selection
is far more indispensable even than in ordinary cases. If selection
consisted merely in separating some very distinct variety, and breeding
from it, the principle would be so obvious as hardly to be worth notice;
but its importance consists in the great effect produced by the
accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated
eye—differences which I for one have vainly attempted to
appreciate. Not one man in a thousand has accuracy of eye and judgment
sufficient to become an eminent breeder. If gifted with these qualities,
and he studies his subject for years, and devotes his lifetime to it with
indomitable perseverance, he will succeed, and may make great
improvements; if he wants any of these qualities, he will assuredly fail.
Few would readily believe in the natural capacity and years of practice
requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the aboriginal
stock. We have proofs that this is not so in some cases, in which exact
records have been kept; thus, to give a very trifling instance, the
steadily-increasing size of the common gooseberry may be quoted. We see
an astonishing improvement in many florists' flowers, when the flowers of
the present day are compared with drawings made only twenty or thirty
years ago. When a race of plants is once pretty well established, the
seed-raisers do not pick out the best plants, but merely go over their
seed-beds, and pull up the "rogues," as they call the plants that deviate
from the proper standard. With animals this [33]kind of selection is, in
fact, also followed; for hardly any one is so careless as to allow his
worst animals to breed.
In regard to plants, there is another means of observing the
accumulated effects of selection—namely, by comparing the diversity
of flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever part
is valued, in the kitchen-garden, in comparison with the flowers of the
same varieties; and the diversity of fruit of the same species in the
orchard, in comparison with the leaves and flowers of the same set of
varieties. See how different the leaves of the cabbage are, and how
extremely alike the flowers; how unlike the flowers of the heartsease
are, and how alike the leaves; how much the fruit of the different kinds
of gooseberries differ in size, colour, shape, and hairiness, and yet the
flowers present very slight differences. It is not that the varieties
which differ largely in some one point do not differ at all in other
points; this is hardly ever, perhaps never, the case. The laws of
correlation of growth, the importance of which should never be
overlooked, will ensure some differences; but, as a general rule, I
cannot doubt that the continued selection of slight variations, either in
the leaves, the flowers, or the fruit, will produce races differing from
each other chiefly in these characters.
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many treatises
have been published on the subject; and the result has been, in a
corresponding degree, rapid and important. But it is very far from true
that the principle is a modern discovery. I could give several references
to the full acknowledgment of the importance of the principle in works of
high antiquity. In rude and barbarous periods [34]of English history choice
animals were often imported, and laws were passed to prevent their
exportation: the destruction of horses under a certain size was ordered,
and this may be compared to the "roguing" of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese
encyclopædia. Explicit rules are laid down by some of the Roman classical
writers. From passages in Genesis, it is clear that the colour of
domestic animals was at that early period attended to. Savages now
sometimes cross their dogs with wild canine animals, to improve the
breed, and they formerly did so, as is attested by passages in Pliny. The
savages in South Africa match their draught cattle by colour, as do some
of the Esquimaux their teams of dogs. Livingstone shows how much good
domestic breeds are valued by the negroes of the interior of Africa who
have not associated with Europeans. Some of these facts do not show
actual selection, but they show that the breeding of domestic animals was
carefully attended to in ancient times, and is now attended to by the
lowest savages. It would, indeed, have been a strange fact, had attention
not been paid to breeding, for the inheritance of good and bad qualities
is so obvious.
At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything existing in the country. But, for our purpose, a
kind of Selection, which may be called Unconscious, and which results
from every one trying to possess and breed from the best individual
animals, is more important. Thus, a man who intends keeping pointers
naturally tries to get as good dogs as he can, and afterwards breeds from
his own best dogs, but he has no wish or expectation of permanently
altering the breed. Nevertheless I cannot doubt that this process,
continued during centuries, [35]would improve and modify any breed, in the
same way as Bakewell, Collins, &c., by this very same process, only
carried on more methodically, did greatly modify, even during their own
lifetimes, the forms and qualities of their cattle. Slow and insensible
changes of this kind could never be recognised unless actual measurements
or careful drawings of the breeds in question had been made long ago,
which might serve for comparison. In some cases, however, unchanged, or
but little changed individuals of the same breed may be found in less
civilised districts, where the breed has been less improved. There is
reason to believe that King Charles's spaniel has been unconsciously
modified to a large extent since the time of that monarch. Some highly
competent authorities are convinced that the setter is directly derived
from the spaniel, and has probably been slowly altered from it. It is
known that the English pointer has been greatly changed within the last
century, and in this case the change has, it is believed, been chiefly
effected by crosses with the fox-hound; but what concerns us is, that the
change has been effected unconsciously and gradually, and yet so
effectually, that, though the old Spanish pointer certainly came from
Spain, Mr. Borrow has not seen, as I am informed by him, any native dog
in Spain like our pointer.
By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size the
parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer and
others have shown how the cattle of England have increased in weight and
in early maturity, compared with the stock formerly kept in this country.
By comparing the accounts given in old pigeon treatises of carriers and
tumblers with these breeds as now existing in Britain, [36]India, and
Persia, we can, I think, clearly trace the stages through which they have
insensibly passed, and come to differ so greatly from the
rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far
that the breeders could never have expected or even have wished to have
produced the result which ensued—namely, the production of two
distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley
and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from the
original stock of Mr. Bakewell for upwards of fifty years. There is not a
suspicion existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any one instance
from the pure blood of Mr. Bakewell's flock, and yet the difference
between the sheep possessed by these two gentlemen is so great that they
have the appearance of being quite different varieties."
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are so
liable, and such choice animals would thus generally leave more offspring
than the inferior ones; so that in this case there would be a kind of
unconscious selection going on. We see the value set on animals even by
the barbarians of Tierra del Fuego, by their killing and devouring their
old women, in times of dearth, as of less value than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance as distinct
varieties, and whether [37]or not two or more species or races have
become blended together by crossing, may plainly be recognised in the
increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would ever
expect to get a first-rate heartsease or dahlia from the seed of a wild
plant. No one would expect to raise a first-rate melting pear from the
seed of the wild pear, though he might succeed from a poor seedling
growing wild, if it had come from a garden-stock. The pear, though
cultivated in classical times, appears, from Pliny's description, to have
been a fruit of very inferior quality. I have seen great surprise
expressed in horticultural works at the wonderful skill of gardeners, in
having produced such splendid results from such poor materials; but the
art, I cannot doubt, has been simple, and, as far as the final result is
concerned, has been followed almost unconsciously. It has consisted in
always cultivating the best known variety, sowing its seeds, and, when a
slightly better variety has chanced to appear, selecting it, and so
onwards. But the gardeners of the classical period, who cultivated the
best pear they could procure, never thought what splendid fruit we should
eat; though we owe our excellent fruit, in some small degree, to their
having naturally chosen and preserved the best varieties they could
anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact,
that in a vast number of cases we cannot recognise, and therefore do not
know, the wild parent-stocks of the plants which have been longest
cultivated in our flower and kitchen gardens. If it has taken centuries
or thousands of years to improve or modify most of our plants up to their
present [38]standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope, nor
any other region inhabited by quite uncivilised man, has afforded us a
single plant worth culture. It is not that these countries, so rich in
species, do not by a strange chance possess the aboriginal stocks of any
useful plants, but that the native plants have not been improved by
continued selection up to a standard of perfection comparable with that
given to the plants in countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their own
food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed better
in the one country than in the other; and thus by a process of "natural
selection," as will hereafter be more fully explained, two sub-breeds
might be formed. This, perhaps, partly explains what has been remarked by
some authors, namely, that the varieties kept by savages have more of the
character of species than the varieties kept in civilised countries.
On the view here given of the all-important part which selection by
man has played, it becomes at once obvious, how it is that our domestic
races show adaptation in their structure or in their habits to man's
wants or fancies. We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise their differences
being so great in external characters and relatively so slight in
internal parts or organs. Man can hardly select, or only with much
difficulty, any deviation of structure excepting such as is externally
visible; and indeed he rarely cares for what is internal. He can never
act by selection, excepting on variations [39]which are first given to
him in some slight degree by nature. No man would ever try to make a
fantail, till he saw a pigeon with a tail developed in some slight degree
in an unusual manner, or a pouter till he saw a pigeon with a crop of
somewhat unusual size; and the more abnormal or unusual any character was
when it first appeared, the more likely it would be to catch his
attention. But to use such an expression as trying to make a fantail, is,
I have no doubt, in most cases, utterly incorrect. The man who first
selected a pigeon with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical selection. Perhaps the parent bird of
all fantails had only fourteen tail-feathers somewhat expanded, like the
present Java fantail, or like individuals of other and distinct breeds,
in which as many as seventeen tail-feathers have been counted. Perhaps
the first pouter-pigeon did not inflate its crop much more than the
turbit now does the upper part of its œsophagus,—a habit
which is disregarded by all fanciers, as it is not one of the points of
the breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
be set on any slight differences in the individuals of the same species,
be judged of by the value which would now be set on them, after several
breeds have once fairly been established. Many slight differences might,
and indeed do now, arise amongst pigeons, which are rejected as faults or
deviations from the standard of perfection of each breed. The common
goose has not given rise to any marked varieties; hence the Thoulouse and
the common breed, which differ only in colour, that [40]most fleeting of
characters, have lately been exhibited as distinct at our
poultry-shows.
I think these views further explain what has sometimes been
noticed—namely, that we know nothing about the origin or history of
any of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals and
thus improves them, and the improved individuals slowly spread in the
immediate neighbourhood. But as yet they will hardly have a distinct
name, and from being only slightly valued, their history will be
disregarded. When further improved by the same slow and gradual process,
they will spread more widely, and will get recognised as something
distinct and valuable, and will then probably first receive a provincial
name. In semi-civilised countries, with little free communication, the
spreading and knowledge of any new sub-breed will be a slow process. As
soon as the points of value of the new sub-breed are once fully
acknowledged, the principle, as I have called it, of unconscious
selection will always tend,—perhaps more at one period than at
another, as the breed rises or falls in fashion,—perhaps more in
one district than in another, according to the state of civilization of
the inhabitants,—slowly to add to the characteristic features of
the breed, whatever they may be. But the chance will be infinitely small
of any record having been preserved of such slow, varying, and insensible
changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply [41]sufficient, with
extreme care, to allow of the accumulation of a large amount of
modification in almost any desired direction. But as variations
manifestly useful or pleasing to man appear only occasionally, the chance
of their appearance will be much increased by a large number of
individuals being kept; and hence this comes to be of the highest
importance to success. On this principle Marshall has remarked, with
respect to the sheep of parts of Yorkshire, that "as they generally
belong to poor people, and are mostly in small lots, they never
can be improved." On the other hand, nurserymen, from raising large
stocks of the same plants, are generally far more successful than
amateurs in getting new and valuable varieties. The keeping of a large
number of individuals of a species in any country requires that the
species should be placed under favourable conditions of life, so as to
breed freely in that country. When the individuals of any species are
scanty, all the individuals, whatever their quality may be, will
generally be allowed to breed, and this will effectually prevent
selection. But probably the most important point of all, is, that the
animal or plant should be so highly useful to man, or so much valued by
him, that the closest attention should be paid to even the slightest
deviation in the qualities or structure of each individual. Unless such
attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary
just when gardeners began to attend closely to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked out
individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then, there appeared (aided by some [42]crossing with distinct
species) those many admirable varieties of the strawberry which have been
raised during the last thirty or forty years.
In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,—at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is a
great convenience to the fancier, for thus many races may be kept true,
though mingled in the same aviary; and this circumstance must have
largely favoured the improvement and formation of new breeds. Pigeons, I
may add, can be propagated in great numbers and at a very quick rate, and
inferior birds may be freely rejected, as when killed they serve for
food. On the other hand, cats, from their nocturnal rambling habits,
cannot be matched, and, although so much valued by women and children, we
hardly ever see a distinct breed kept up; such breeds as we do sometimes
see are almost always imported from some other country, often from
islands. Although I do not doubt that some domestic animals vary less
than others, yet the rarity or absence of distinct breeds of the cat, the
donkey, peacock, goose, &c., may be attributed in main part to
selection not having been brought into play: in cats, from the difficulty
in pairing them; in donkeys, from only a few being kept by poor people,
and little attention paid to their breeding; in peacocks, from not being
very easily reared and a large stock not kept; in geese, from being
valuable only for two purposes, food and feathers, and more especially
from no pleasure having been felt in the display of distinct breeds.
To sum up on the origin of our Domestic Races of [43]animals and plants. I
believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic beings,
as some authors have thought. The effects of variability are modified by
various degrees of inheritance and of reversion. Variability is governed
by many unknown laws, more especially by that of correlation of growth.
Something may be attributed to the direct action of the conditions of
life. Something must be attributed to use and disuse. The final result is
thus rendered infinitely complex. In some cases, I do not doubt that the
intercrossing of species, aboriginally distinct, has played an important
part in the origin of our domestic productions. When in any country
several domestic breeds have once been established, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided in
the formation of new sub-breeds; but the importance of the crossing of
varieties has, I believe, been greatly exaggerated, both in regard to
animals and to those plants which are propagated by seed. In plants which
are temporarily propagated by cuttings, buds, &c., the importance of
the crossing both of distinct species and of varieties is immense; for
the cultivator here quite disregards the extreme variability both of
hybrids and mongrels, and the frequent sterility of hybrids; but the
cases of plants not propagated by seed are of little importance to us,
for their endurance is only temporary. Over all these causes of Change I
am convinced that the accumulative action of Selection, whether applied
methodically and more quickly, or unconsciously and more slowly, but more
efficiently, is by far the predominant Power.
[44]
CHAPTER II.
Variation under Nature.
Variability—Individual differences—Doubtful
species—Wide ranging, much diffused, and common species vary
most—Species of the larger genera in any country vary more than the
species of the smaller genera—Many of the species of the larger
genera resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges.
Before applying the principles arrived at in the last chapter to
organic beings in a state of nature, we must briefly discuss whether
these latter are subject to any variation. To treat this subject at all
properly, a long catalogue of dry facts should be given; but these I
shall reserve for my future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one definition
has as yet satisfied all naturalists; yet every naturalist knows vaguely
what he means when he speaks of a species. Generally the term includes
the unknown element of a distinct act of creation. The term "variety" is
almost equally difficult to define; but here community of descent is
almost universally implied, though it can rarely be proved. We have also
what are called monstrosities; but they graduate into varieties. By a
monstrosity I presume is meant some considerable deviation of structure
in one part, either injurious to or not useful to the species, and not
generally propagated. Some authors use the term "variation" in a
technical sense, as implying a modification directly due to the physical
conditions of life; and "variations" in this sense are supposed not to be
inherited: but who can say that the dwarfed condition of shells in the
brackish waters of the Baltic, or dwarfed [45]plants on Alpine summits,
or the thicker fur of an animal from far northwards, would not in some
cases be inherited for at least some few generations? and in this case I
presume that the form would be called a variety.
Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring from
the same parents, or which may be presumed to have thus arisen, from
being frequently observed in the individuals of the same species
inhabiting the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same mould. These
individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as man
can accumulate in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the most
experienced naturalist would be surprised at the number of the cases of
variability, even in important parts of structure, which he could collect
on good authority, as I have collected, during a course of years. It
should be remembered that systematists are far from pleased at finding
variability in important characters, and that there are not many men who
will laboriously examine internal and important organs, and compare them
in many specimens of the same species. I should never have expected that
the branching of the main nerves close to the great central ganglion of
an insect would have been variable in the same species; I should have
expected that changes of this nature could have been effected only [46]by slow
degrees: yet quite recently Mr. Lubbock has shown a degree of variability
in these main nerves in Coccus, which may almost be compared to the
irregular branching of the stem of a tree. This philosophical naturalist,
I may add, has also quite recently shown that the muscles in the larvæ of
certain insects are very far from uniform. Authors sometimes argue in a
circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few
naturalists have honestly confessed) which does not vary; and, under this
point of view, no instance of an important part varying will ever be
found: but under any other point of view many instances assuredly can be
given.
There is one point connected with individual differences, which seems
to me extremely perplexing: I refer to those genera which have sometimes
been called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree
which forms to rank as species and which as varieties. We may instance
Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
several genera of Brachiopod shells. In most polymorphic genera some of
the species have fixed and definite characters. Genera which are
polymorphic in one country seem to be, with some few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts seem to be very
perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see in these polymorphic genera variations in points of structure which
are of no service or disservice to the species, and which consequently
have not been seized on and rendered definite by natural selection, as
hereafter will be explained. [47]
Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of these
doubtful and closely-allied forms have permanently retained their
characters in their own country for a long time; for as long, as far as
we know, as have good and true species. Practically, when a naturalist
can unite two forms together by others having intermediate characters, he
treats the one as a variety of the other, ranking the most common, but
sometimes the one first described, as the species, and the other as the
variety. But cases of great difficulty, which I will not here enumerate,
sometimes occur in deciding whether or not to rank one form as a variety
of another, even when they are closely connected by intermediate links;
nor will the commonly-assumed hybrid nature of the intermediate links
always remove the difficulty. In very many cases, however, one form is
ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed;
and here a wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges. [48]
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for me
182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless have
been ranked by some botanists as species, and he has entirely omitted
several highly polymorphic genera. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
gives only 112,—a difference of 139 doubtful forms! Amongst animals
which unite for each birth, and which are highly locomotive, doubtful
forms, ranked by one zoologist as a species and by another as a variety,
can rarely be found within the same country, but are common in separated
areas. How many of those birds and insects in North America and Europe,
which differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as varieties, or,
as they are often called, as geographical races! Many years ago, when
comparing, and seeing others compare, the birds from the separate islands
of the Galapagos Archipelago, both one with another, and with those from
the American mainland, I was much struck how entirely vague and arbitrary
is the distinction between species and varieties. On the islets of the
little Madeira group there are many insects which are characterized as
varieties in Mr. Wollaston's admirable work, but which it cannot [49]be
doubted would be ranked as distinct species by many entomologists. Even
Ireland has a few animals, now generally regarded as varieties, but which
have been ranked as species by some zoologists. Several most experienced
ornithologists consider our British red grouse as only a strongly-marked
race of a Norwegian species, whereas the greater number rank it as an
undoubted species peculiar to Great Britain. A wide distance between the
homes of two doubtful forms leads many naturalists to rank both as
distinct species; but what distance, it has been well asked, will
suffice? if that between America and Europe is ample, will that between
the Continent and the Azores, or Madeira, or the Canaries, or Ireland, be
sufficient? It must be admitted that many forms, considered by
highly-competent judges as varieties, have so perfectly the character of
species that they are ranked by other highly-competent judges as good and
true species. But to discuss whether they are rightly called species or
varieties, before any definition of these terms has been generally
accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism, &c.,
have been brought to bear on the attempt to determine their rank. I will
here give only a single instance,—the well-known one of the
primrose and cowslip, or Primula vulgaris and veris. These plants differ
considerably in appearance; they have a different flavour, and emit a
different odour; they flower at slightly different periods; they grow in
somewhat different stations; they ascend mountains to different heights;
they have different geographical ranges; and lastly, according to very
numerous experiments made during several years by [50]that most careful
observer Gärtner, they can be crossed only with much difficulty. We could
hardly wish for better evidence of the two forms being specifically
distinct. On the other hand, they are united by many intermediate links,
and it is very doubtful whether these links are hybrids; and there is, as
it seems to me, an overwhelming amount of experimental evidence, showing
that they descend from common parents, and consequently must be ranked as
varieties.
Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed that it is
in the best-known countries that we find the greatest number of forms of
doubtful value. I have been struck with the fact, that if any animal or
plant in a state of nature be highly useful to man, or from any cause
closely attract his attention, varieties of it will almost universally be
found recorded. These varieties, moreover, will be often ranked by some
authors as species. Look at the common oak, how closely it has been
studied; yet a German author makes more than a dozen species out of
forms, which are very generally considered as varieties; and in this
country the highest botanical authorities and practical men can be quoted
to show that the sessile and pedunculated oaks are either good and
distinct species or mere varieties.
When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he knows
nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the doubtful
forms. His [51]general tendency will be to make many
species, for he will become impressed, just like the pigeon or poultry
fancier before alluded to, with the amount of difference in the forms
which he is continually studying; and he has little general knowledge of
analogical variation in other groups and in other countries, by which to
correct his first impressions. As he extends the range of his
observations, he will meet with more cases of difficulty; for he will
encounter a greater number of closely-allied forms. But if his
observations be widely extended, he will in the end generally be enabled
to make up his own mind which to call varieties and which species; but he
will succeed in this at the expense of admitting much
variation,—and the truth of this admission will often be disputed
by other naturalists. When, moreover, he comes to study allied forms
brought from countries not now continuous, in which case he can hardly
hope to find the intermediate links between his doubtful forms, he will
have to trust almost entirely to analogy, and his difficulties rise to a
climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species—that is, the forms which in the opinion of
some naturalists come very near to, but do not quite arrive at the rank
of species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other in an insensible series; and a series impresses the
mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any degree
more distinct and permanent, as steps leading to more [52]strongly marked
and more permanent varieties; and at these latter, as leading to
sub-species, and to species. The passage from one stage of difference to
another and higher stage may be, in some cases, due merely to the
long-continued action of different physical conditions in two different
regions; but I have not much faith in this view; and I attribute the
passage of a variety, from a state in which it differs very slightly from
its parent to one in which it differs more, to the action of natural
selection in accumulating (as will hereafter be more fully explained)
differences of structure in certain definite directions. Hence I believe
a well-marked variety may be called an incipient species; but whether
this belief be justifiable must be judged of by the general weight of the
several facts and views given throughout this work.
It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this incipient
state become extinct, or they may endure as varieties for very long
periods, as has been shown to be the case by Mr. Wollaston with the
varieties of certain fossil land-shells in Madeira. If a variety were to
flourish so as to exceed in numbers the parent species, it would then
rank as the species, and the species as the variety; or it might come to
supplant and exterminate the parent species; or both might co-exist, and
both rank as independent species. But we shall hereafter have to return
to this subject.
From these remarks it will be seen that I look at the term species, as
one arbitrarily given for the sake of convenience to a set of individuals
closely resembling each other, and that it does not essentially differ
from the term variety, which is given to less distinct and more
fluctuating forms. The term variety, again, in comparison with mere
individual differences, is also applied arbitrarily, and for mere
convenience' sake. [53]
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance on
this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for my
future work the discussion of these difficulties, and the tables
themselves of the proportional numbers of the varying species. Dr. Hooker
permits me to add, that after having carefully read my manuscript, and
examined the tables, he thinks that the following statements are fairly
well established. The whole subject, however, treated as it necessarily
here is with much brevity, is rather perplexing, and allusions cannot be
avoided to the "struggle for existence," "divergence of character," and
other questions, hereafter to be discussed.
Alph. de Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been
expected, as they become exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is a far
more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which
are most common, that is abound most in individuals, and the species
which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), often give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourishing,
or, as they may be called, the dominant species,—those [54]which range
widely over the world, are the most diffused in their own country, and
are the most numerous in individuals,—which oftenest produce
well-marked varieties, or, as I consider them, incipient species. And
this, perhaps, might have been anticipated; for, as varieties, in order
to become in any degree permanent, necessarily have to struggle with the
other inhabitants of the country, the species which are already dominant
will be the most likely to yield offspring, which, though in some slight
degree modified, still inherit those advantages that enabled their
parents to become dominant over their compatriots.
If the plants inhabiting a country and described in any Flora be
divided into two equal masses, all those in the larger genera being
placed on one side, and all those in the smaller genera on the other
side, a somewhat larger number of the very common and much diffused or
dominant species will be found on the side of the larger genera. This,
again, might have been anticipated; for the mere fact of many species of
the same genus inhabiting any country, shows that there is something in
the organic or inorganic conditions of that country favourable to the
genus; and, consequently, we might have expected to have found in the
larger genera, or those including many species, a large proportional
number of dominant species. But so many causes tend to obscure this
result, that I am surprised that my tables show even a small majority on
the side of the larger genera. I will here allude to only two causes of
obscurity. Fresh-water and salt-loving plants have generally very wide
ranges and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation
to the size of the genera to which the species belong. Again, plants low
in the scale of organisation are [55]generally much more widely diffused than
plants higher in the scale; and here again there is no close relation to
the size of the genera. The cause of lowly-organised plants ranging
widely will be discussed in our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of the
smaller genera; for wherever many closely related species (i.e.
species of the same genus) have been formed, many varieties or incipient
species ought, as a general rule, to be now forming. Where many large
trees grow, we expect to find saplings. Where many species of a genus
have been formed through variation, circumstances have been favourable
for variation; and hence we might expect that the circumstances would
generally be still favourable to variation. On the other hand, if we look
at each species as a special act of creation, there is no apparent reason
why more varieties should occur in a group having many species, than in
one having few.
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into two
nearly equal masses, the species of the larger genera on one side, and
those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the side
of the larger genera present varieties, than on the side of the smaller
genera. Moreover, the species of the large genera which present any
varieties, invariably present a larger average number of varieties than
do the species of the small genera. Both these results follow when
another division is made, and when all the smallest genera, with from
only one to four species, are absolutely excluded from the tables. These
[56]facts are of plain signification on the view
that species are only strongly marked and permanent varieties; for
wherever many species of the same genus have been formed, or where, if we
may use the expression, the manufactory of species has been active, we
ought generally to find the manufactory still in action, more especially
as we have every reason to believe the process of manufacturing new
species to be a slow one. And this certainly is the case, if varieties be
looked at as incipient species; for my tables clearly show as a general
rule that, wherever many species of a genus have been formed, the species
of that genus present a number of varieties, that is of incipient species
beyond the average. It is not that all large genera are now varying much,
and are thus increasing in the number of their species, or that no small
genera are now varying and increasing; for if this had been so, it would
have been fatal to my theory; inasmuch as geology plainly tells us that
small genera have in the lapse of time often increased greatly in size;
and that large genera have often come to their maxima, declined, and
disappeared. All that we want to show is, that where many species of a
genus have been formed, on an average many are still forming; and this
holds good.
There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that there is
no infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been
found between doubtful forms, naturalists are compelled to come to a
determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the
rank of species. Hence the amount of difference is one very important
criterion in settling whether two forms [57]should be ranked as
species or varieties. Now Fries has remarked in regard to plants, and
Westwood in regard to insects, that in large genera the amount of
difference between the species is often exceedingly small. I have
endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted some
sagacious and experienced observers, and, after deliberation, they concur
in this view. In this respect, therefore, the species of the larger
genera resemble varieties, more than do the species of the smaller
genera. Or the case may be put in another way, and it may be said, that
in the larger genera, in which a number of varieties or incipient species
greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for
they differ from each other by a less than usual amount of
difference.
Moreover, the species of the large genera are related to each other,
in the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
certain other species. And what are varieties but groups of forms,
unequally related to each other, and clustered round certain
forms—that is, round their parent-species? Undoubtedly there is one
most important point of difference between varieties and species; namely,
that the amount of difference between varieties, when compared with each
other or with their parent-species, is much less than that between the
species of the same genus. But when we come to discuss the principle, as
I call it, of Divergence of Character, [58]we shall see how this may
be explained, and how the lesser differences between varieties will tend
to increase into the greater differences between species.
There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed scarcely
more than a truism, for if a variety were found to have a wider range
than that of its supposed parent-species, their denominations ought to be
reversed. But there is also reason to believe, that those species which
are very closely allied to other species, and in so far resemble
varieties, often have much restricted ranges. For instance, Mr. H. C.
Watson has marked for me in the well-sifted London Catalogue of plants
(4th edition) 63 plants which are therein ranked as species, but which he
considers as so closely allied to other species as to be of doubtful
value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this
same catalogue, 53 acknowledged varieties are recorded, and these range
over 7.7 provinces; whereas, the species to which these varieties belong
range over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but which
are almost universally ranked by British botanists as good and true
species.
Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species,—except, firstly, by
the discovery of intermediate linking forms, and the occurrence of such
links cannot affect the actual characters of the forms which they
connect; and except, secondly by a certain amount of [59]difference, for
two forms, if differing very little, are generally ranked as varieties,
notwithstanding that intermediate linking forms have not been discovered;
but the amount of difference considered necessary to give to two forms
the rank of species is quite indefinite. In genera having more than the
average number of species in any country, the species of these genera
have more than the average number of varieties. In large genera the
species are apt to be closely, but unequally allied together, forming
little clusters round certain species. Species very closely allied to
other species apparently have restricted ranges. In all these several
respects the species of large genera present a strong analogy with
varieties. And we can clearly understand these analogies, if species have
once existed as varieties, and have thus originated: whereas, these
analogies are utterly inexplicable if each species has been independently
created.
We have, also, seen that it is the most flourishing or dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into new
and distinct species. The larger genera thus tend to become larger; and
throughout nature the forms of life which are now dominant tend to become
still more dominant by leaving many modified and dominant descendants.
But by steps hereafter to be explained, the larger genera also tend to
break up into smaller genera. And thus, the forms of life throughout the
universe become divided into groups subordinate to groups.
[60]
CHAPTER III.
Struggle for Existence.
Bears on natural selection—The term used in a wide
sense—Geometrical powers of increase—Rapid increase of
naturalised animals and plants—Nature of the checks to
increase—Competition universal—Effects of
climate—Protection from the number of individuals—Complex
relations of all animals and plants throughout nature—Struggle for
life most severe between individuals and varieties of the same species;
often severe between species of the same genus—The relation of
organism to organism the most important of all relations.
Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual variability:
indeed I am not aware that this has ever been disputed. It is immaterial
for us whether a multitude of doubtful forms be called species or
sub-species or varieties; what rank, for instance, the two or three
hundred doubtful forms of British plants are entitled to hold, if the
existence of any well-marked varieties be admitted. But the mere
existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us but
little in understanding how species arise in nature. How have all those
exquisite adaptations of one part of the organisation to another part,
and to the conditions of life, and of one distinct organic being to
another being, been perfected? We see these beautiful co-adaptations most
[61]plainly in the woodpecker and missletoe; and
only a little less plainly in the humblest parasite which clings to the
hairs of a quadruped or feathers of a bird; in the structure of the
beetle which dives through the water; in the plumed seed which is wafted
by the gentlest breeze; in short, we see beautiful adaptations everywhere
and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus, arise?
All these results, as we shall more fully see in the next chapter, follow
from the struggle for life. Owing to this struggle for life, any
variation, however slight, and from whatever cause proceeding, if it be
in any degree profitable to an individual of any species, in its
infinitely complex relations to other organic beings and to external
nature, will tend to the preservation of that individual, and will
generally be inherited by its offspring. The offspring, also, will thus
have a better chance of surviving, for, of the many individuals of any
species which are periodically born, but a small number can survive. I
have called this principle, by which each slight variation, if useful, is
preserved, by the term of Natural Selection, in order to mark its
relation to man's power of selection. We have seen that man by selection
can certainly produce great results, and can adapt organic beings to his
own uses, through the accumulation of slight but useful variations, given
to him by the hand of Nature. But Natural Selection, as we shall
hereafter see, is a power incessantly ready for action, and is as [62]immeasurably superior to man's feeble
efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder de Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed to
severe competition. In regard to plants, no one has treated this subject
with more spirit and ability than W. Herbert, Dean of Manchester,
evidently the result of his great horticultural knowledge. Nothing is
easier than to admit in words the truth of the universal struggle for
life, or more difficult—at least I have found it so—than
constantly to bear this conclusion in mind. Yet unless it be thoroughly
engrained in the mind, I am convinced that the whole economy of nature,
with every fact on distribution, rarity, abundance, extinction, and
variation, will be dimly seen or quite misunderstood. We behold the face
of nature bright with gladness, we often see superabundance of food; we
do not see, or we forget that the birds which are idly singing round us
mostly live on insects or seeds, and are thus constantly destroying life;
or we forget how largely these songsters, or their eggs, or their
nestlings, are destroyed by birds and beasts of prey; we do not always
bear in mind, that though food may be now superabundant, it is not so at
all seasons of each recurring year.
I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another, and
including (which is more important) not only the life of the individual,
but success in leaving progeny. Two canine animals in a time of dearth,
may be truly said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle [63]for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a thousand
seeds, of which on an average only one comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds which
already clothe the ground. The missletoe is dependent on the apple and a
few other trees, but can only in a far-fetched sense be said to struggle
with these trees, for if too many of these parasites grow on the same
tree, it will languish and die. But several seedling missletoes, growing
close together on the same branch, may more truly be said to struggle
with each other. As the missletoe is disseminated by birds, its existence
depends on birds; and it may metaphorically be said to struggle with
other fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these several
senses, which pass into each other, I use for convenience' sake the
general term of struggle for existence.
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer destruction
during some period of its life, and during some season or occasional
year, otherwise, on the principle of geometrical increase, its numbers
would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either one
individual with another of the same species, or with the individuals of
distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there [64]can be no artificial
increase of food, and no prudential restraint from marriage. Although
some species may be now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man has
doubled in twenty-five years, and at this rate, in a few thousand years,
there would literally not be standing room for his progeny. Linnæus has
calculated that if an annual plant produced only two seeds—and
there is no plant so unproductive as this—and their seedlings next
year produced two, and so on, then in twenty years there would be a
million plants. The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum
rate of natural increase: it will be under the mark to assume that it
breeds when thirty years old, and goes on breeding till ninety years old,
bringing forth three pair of young in this interval; if this be so, at
the end of the fifth century there would be alive fifteen million
elephants, descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals of
many kinds which have run wild in several parts of the world: if the
statements of the rate of increase of slow-breeding cattle and horses in
South America, and latterly in Australia, had not been well
authenticated, they would have been incredible. So it is with plants:
cases could be given of [65]introduced plants which have become common
throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, now most numerous
over the wide plains of La Plata, clothing square leagues of surface
almost to the exclusion of all other plants, have been introduced from
Europe; and there are plants which now range in India, as I hear from Dr.
Falconer, from Cape Comorin to the Himalaya, which have been imported
from America since its discovery. In such cases, and endless instances
could be given, no one supposes that the fertility of these animals or
plants has been suddenly and temporarily increased in any sensible
degree. The obvious explanation is that the conditions of life have been
very favourable, and that there has consequently been less destruction of
the old and young, and that nearly all the young have been enabled to
breed. In such cases the geometrical ratio of increase, the result of
which never fails to be surprising, simply explains the extraordinarily
rapid increase and wide diffusion of naturalised productions in their new
homes.
In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to increase
at a geometrical ratio, that all would most rapidly stock every station
in which they could any how exist, and that the geometrical tendency to
increase must be checked by destruction at some period of life. Our
familiarity with the larger domestic animals tends, I think, to mislead
us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of
nature an equal number would have somehow to be disposed of.
The only difference between organisms which annually [66]produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the same
country the condor may be the more numerous of the two: the Fulmar petrel
lays but one egg, yet it is believed to be the most numerous bird in the
world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one; but this difference does not determine how many
individuals of the two species can be supported in a district. A large
number of eggs is of some importance to those species which depend on a
rapidly fluctuating amount of food, for it allows them rapidly to
increase in number. But the real importance of a large number of eggs or
seeds is to make up for much destruction at some period of life; and this
period in the great majority of cases is an early one. If an animal can
in any way protect its own eggs or young, a small number may be produced,
and yet the average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced, or the species will become extinct. It
would suffice to keep up the full number of a tree, which lived on an
average for a thousand years, if a single seed were produced once in a
thousand years, supposing that this seed were never destroyed, and could
be ensured to germinate in a fitting place. So that in all cases, the
average number of any animal or plant depends only indirectly on the
number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single
organic being around us may be said to be striving to the utmost to
increase in numbers; that each lives by a struggle at some period of [67]its
life; that heavy destruction inevitably falls either on the young or old,
during each generation or at recurrent intervals. Lighten any check,
mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.
The causes which check the natural tendency of each species to
increase in number are most obscure. Look at the most vigorous species;
by as much as it swarms in numbers, by so much will its tendency to
increase be still further increased. We know not exactly what the checks
are in even one single instance. Nor will this surprise any one who
reflects how ignorant we are on this head, even in regard to mankind, so
incomparably better known than any other animal. This subject has been
ably treated by several authors, and I shall, in my future work, discuss
some of the checks at considerable length, more especially in regard to
the feral animals of South America. Here I will make only a few remarks,
just to recall to the reader's mind some of the chief points. Eggs or
very young animals seem generally to suffer most, but this is not
invariably the case. With plants there is a vast destruction of seeds,
but, from some observations which I have made, I believe that it is the
seedlings which suffer most from germinating in ground already thickly
stocked with other plants. Seedlings, also, are destroyed in vast numbers
by various enemies; for instance, on a piece of ground three feet long
and two wide, dug and cleared, and where there could be no choking from
other plants, I marked all the seedlings of our native weeds as they came
up, and out of the 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants [68]gradually kill the less vigorous, though
fully grown, plants: thus out of twenty species growing on a little plot
of turf (three feet by four) nine species perished from the other species
being allowed to grow up freely.
The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average numbers of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game than
at present, although hundreds of thousands of game animals are now
annually killed. On the other hand, in some cases, as with the elephant
and rhinoceros, none are destroyed by beasts of prey: even the tiger in
India most rarely dares to attack a young elephant protected by its
dam.
Climate plays an important part in determining the average numbers of
a species, and periodical seasons of extreme cold or drought, I believe
to be the most effective of all checks. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my own grounds; and this is
a tremendous destruction, when we remember that ten per cent, is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it
brings on the most severe struggle between the individuals, whether of
the same or of distinct species, which subsist on the same kind of food.
Even when climate, for instance extreme cold, [69]acts directly, it will be
the least vigorous, or those which have got least food through the
advancing winter, which will suffer most. When we travel from south to
north, or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing; and the
change of climate being conspicuous, we are tempted to attribute the
whole effect to its direct action. But this is a false view: we forget
that each species, even where it most abounds, is constantly suffering
enormous destruction at some period of its life, from enemies or from
competitors for the same place and food; and if these enemies or
competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers, and, as each area is already
fully stocked with inhabitants, the other species will decrease. When we
travel southward and see a species decreasing in numbers, we may feel
sure that the cause lies quite as much in other species being favoured,
as in this one being hurt. So it is when we travel northward, but in a
somewhat lesser degree, for the number of species of all kinds, and
therefore of competitors, decreases northwards; hence in going northward,
or in ascending a mountain, we far oftener meet with stunted forms, due
to the directly injurious action of climate, than we do in
proceeding southwards or in descending a mountain. When we reach the
Arctic regions, or snow-capped summits, or absolute deserts, the struggle
for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species,
we may clearly see in the prodigious number of plants in our gardens
which can perfectly well endure our climate, but which never become
naturalised, for they cannot compete with our native plants nor resist
destruction by our native animals. [70]
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue: and
here we have a limiting check independent of the struggle for life. But
even some of these so-called epidemics appear to be due to parasitic
worms, which have from some cause, possibly in part through facility of
diffusion amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite and its
prey.
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of corn
and rape-seed, &c., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can the
birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their numbers
are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such plants in a
garden: I have in this case lost every single seed. This view of the
necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such as that of very
rare plants being sometimes extremely abundant in the few spots where
they do occur; and that of some social plants being social, that is,
abounding in individuals, even on the extreme confines of their range.
For in such cases, we may believe, that a plant could exist only where
the conditions of its life were so favourable that many could exist
together, and thus save the species from utter destruction. I should add
that the good effects of frequent intercrossing, and [71]the ill effects
of close interbreeding, probably come into play in some of these cases;
but on this intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance, which,
though a simple one, has interested me. In Staffordshire, on the estate
of a relation, where I had ample means of investigation, there was a
large and extremely barren heath, which had never been touched by the
hand of man; but several hundred acres of exactly the same nature had
been enclosed twenty-five years previously and planted with Scotch fir.
The change in the native vegetation of the planted part of the heath was
most remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not
counting grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been still
greater, for six insectivorous birds were very common in the plantations,
which were not to be seen on the heath; and the heath was frequented by
two or three distinct insectivorous birds. Here we see how potent has
been the effect of the introduction of a single tree, nothing whatever
else having been done, with the exception that the land had been
enclosed, so that cattle could not enter. But how important an element
enclosure is, I plainly saw near Farnham, in Surrey. Here there are
extensive heaths, with a few clumps of old Scotch firs on the distant
hill-tops: within the last ten years large spaces have been enclosed, and
self-sown firs are now springing up in multitudes, so close together that
all cannot live. [72]When I ascertained that these young trees
had not been sown or planted, I was so much surprised at their numbers
that I went to several points of view, whence I could examine hundreds of
acres of the unenclosed heath, and literally I could not see a single
Scotch fir, except the old planted clumps. But on looking closely between
the stems of the heath, I found a multitude of seedlings and little
trees, which had been perpetually browsed down by the cattle. In one
square yard, at a point some hundred yards distant from one of the old
clumps, I counted thirty-two little trees; and one of them, with
twenty-six rings of growth, had during many years tried to raise its head
above the stems of the heath, and had failed. No wonder that, as soon as
the land was enclosed, it became thickly clothed with vigorously growing
young firs. Yet the heath was so extremely barren and so extensive that
no one would ever have imagined that cattle would have so closely and
effectually searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance of
this; for here neither cattle nor horses nor dogs have ever run wild,
though they swarm southward and northward in a feral state; and Azara and
Rengger have shown that this is caused by the greater number in Paraguay
of a certain fly, which lays its eggs in the navels of these animals when
first born. The increase of these flies, numerous as they are, must be
habitually checked by some means, probably by birds. Hence, if certain
insectivorous birds (whose numbers are probably regulated by hawks or
beasts of prey) were to increase in Paraguay, the flies would
decrease—then cattle and horses would became feral, and this would
certainly greatly [73]alter (as indeed I have observed in parts of
South America) the vegetation: this again would largely affect the
insects; and this, as we just have seen in Staffordshire, the
insectivorous birds, and so onwards in ever-increasing circles of
complexity. We began this series by insectivorous birds, and we have
ended with them, Not that in nature the relations can ever be as simple
as this. Battle within battle must ever be recurring with varying
success; and yet in the long-run the forces are so nicely balanced, that
the face of nature remains uniform for long periods of time, though
assuredly the merest trifle would often give the victory to one organic
being over another. Nevertheless so profound is our ignorance, and so
high our presumption, that we marvel when we hear of the extinction of an
organic being; and as we do not see the cause, we invoke cataclysms to
desolate the world, or invent laws on the duration of the forms of
life!
I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set a
seed. Many of our orchidaceous plants absolutely require the visits of
moths to remove their pollen-masses and thus to fertilise them. I have,
also, reason to believe that humble-bees are indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. From experiments which I have lately tried, I have
found that the visits of bees are necessary for the fertilisation of some
kinds of clover; but humble-bees alone visit the red clover (Trifolium
pratense), as other bees cannot reach the nectar. Hence I have very
little doubt, that if the [74]whole genus of humble-bees became extinct or
very rare in England, the heartsease and red clover would become very
rare, or wholly disappear. The number of humble-bees in any district
depends in a great degree on the number of field-mice, which destroy
their combs and nests; and Mr. H. Newman, who has long attended to the
habits of humble-bees, believes that "more than two-thirds of them are
thus destroyed all over England." Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Mr. Newman
says, "Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the number
of cats that destroy the mice." Hence it is quite credible that the
presence of a feline animal in large numbers in a district might
determine, through the intervention first of mice and then of bees, the
frequency of certain flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally the
most potent, but all concur in determining the average number or even the
existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we are
tempted to attribute their proportional numbers and kinds to what we call
chance. But how false a view is this! Every one has heard that when an
American forest is cut down, a very different vegetation springs up; but
it has been observed that ancient Indian ruins in the Southern United
States, which must formerly have been cleared of trees, now display the
same beautiful diversity and proportion of kinds as in the surrounding
[75]virgin forests. What a struggle between the
several kinds of trees must here have gone on during long centuries, each
annually scattering its seeds by the thousand; what war between insect
and insect—between insects, snails, and other animals with birds
and beasts of prey—all striving to increase, and all feeding on
each other or on the trees or their seeds and seedlings, or on the other
plants which first clothed the ground and thus checked the growth of the
trees! Throw up a handful of feathers, and all must fall to the ground
according to definite laws; but how simple is this problem compared to
the action and reaction of the innumerable plants and animals which have
determined, in the course of centuries, the proportional numbers and
kinds of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on
its prey, lies generally between beings remote in the scale of nature.
This is often the case with those which may strictly be said to struggle
with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invariably will be most
severe between the individuals of the same species, for they frequent the
same districts, require the same food, and are exposed to the same
dangers. In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest soon
decided; for instance, if several varieties of wheat be sown together,
and the mixed seed be resown, some of the varieties which best suit the
soil or climate, or are naturally the most fertile, will beat the others
and so yield more seed, and will consequently in a few years quite
supplant the other varieties. To keep up a mixed stock of even such
extremely close varieties as the variously [76]coloured sweet-peas, they
must be each year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease in numbers
and disappear. So again with the varieties of sheep: it has been asserted
that certain mountain-varieties will starve out other mountain-varieties,
so that they cannot be kept together. The same result has followed from
keeping together different varieties of the medicinal leech. It may even
be doubted whether the varieties of any one of our domestic plants or
animals have so exactly the same strength, habits, and constitution, that
the original proportions of a mixed stock could be kept up for
half-a-dozen generations, if they were allowed to struggle together, like
beings in a state of nature, and if the seed or young were not annually
sorted.
As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species of
the same genus, when they come into competition with each other, than
between species of distinct genera. We see this in the recent extension
over parts of the United States of one species of swallow having caused
the decrease of another species. The recent increase of the missel-thrush
in parts of Scotland has caused the decrease of the song-thrush. How
frequently we hear of one species of rat taking the place of another
species under the most different climates! In Russia the small Asiatic
cockroach has everywhere driven before it its great congener. One species
of charlock will supplant another, and so in other cases. We can dimly
see why the competition should be most severe between allied forms, which
fill nearly the same place in the economy of nature; [77]but probably in
no one case could we precisely say why one species has been victorious
over another in the great battle of life.
A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being is
related, in the most essential yet often hidden manner, to that of all
other organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This is
obvious in the structure of the teeth and talons of the tiger; and in
that of the legs and claws of the parasite which clings to the hair on
the tiger's body. But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle, the relation
seems at first confined to the elements of air and water. Yet the
advantage of plumed seeds no doubt stands in the closest relation to the
land being already thickly clothed by other plants; so that the seeds may
be widely distributed and fall on unoccupied ground. In the water-beetle,
the structure of its legs, so well adapted for diving, allows it to
compete with other aquatic insects, to hunt for its own prey, and to
escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems
at first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and
beans), when sown in the midst of long grass, I suspect that the chief
use of the nutriment in the seed is to favour the growth of the young
seedling, whilst struggling with other plants growing vigorously all
around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know [78]that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wished in imagination to give the plant the
power of increasing in number, we should have to give it some advantage
over its competitors, or over the animals which preyed on it. On the
confines of its geographical range, a change of constitution with respect
to climate would clearly be an advantage to our plant; but we have reason
to believe that only a few plants or animals range so far, that they are
destroyed by the rigour of the climate alone. Not until we reach the
extreme confines of life, in the Arctic regions or on the borders of an
utter desert, will competition cease. The land may be extremely cold or
dry, yet there will be competition between some few species, or between
the individuals of the same species, for the warmest or dampest
spots.
Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the
same as in its former home, yet the conditions of its life will generally
be changed in an essential manner. If we wished to increase its average
numbers in its new home, we should have to modify it in a different way
to what we should have done in its native country; for we should have to
give it some advantage over a different set of competitors or
enemies.
It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on the
mutual relations of all organic beings; a conviction as necessary, as it
seems to be difficult to acquire. All that we can do, is to keep steadily
in mind that each [79]organic being is striving to increase at a
geometrical ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that death is
generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.
[80]
CHAPTER IV.
Natural Selection.
Natural Selection—its power compared with man's
selection—its power on characters of trifling importance—its
power at all ages and on both sexes—Sexual Selection—On the
generality of intercrosses between individuals of the same
species—Circumstances favourable and unfavourable to Natural
Selection, namely, intercrossing, isolation, number of
individuals—Slow action—Extinction caused by Natural
Selection—Divergence of Character, related to the diversity of
inhabitants of any small area, and to naturalisation—Action of
Natural Selection, through Divergence of Character and Extinction, on the
descendants from a common parent—Explains the Grouping of all
organic beings.
How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature? I
think we shall see that it can act most effectually. Let it be borne in
mind in what an endless number of strange peculiarities our domestic
productions, and, in a lesser degree, those under nature, vary; and how
strong the hereditary tendency is. Under domestication, it may be truly
said that the whole organisation becomes in some degree plastic. Let it
be borne in mind how infinitely complex and close-fitting are the mutual
relations of all organic beings to each other and to their physical
conditions of life. Can it, then, be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that other variations
useful in some way to each being in the great and complex battle of life,
should sometimes occur in the course of thousands of generations? If such
do occur, can we doubt [81](remembering that many more individuals are
born than can possibly survive) that individuals having any advantage,
however slight, over others, would have the best chance of surviving and
of procreating their kind? On the other hand, we may feel sure that any
variation in the least degree injurious would be rigidly destroyed. This
preservation of favourable variations and the rejection of injurious
variations, I call Natural Selection. Variations neither useful nor
injurious would not be affected by natural selection, and would be left a
fluctuating element, as perhaps we see in the species called
polymorphic.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants would
almost immediately undergo a change, and some species might become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of some of the
inhabitants, independently of the change of climate itself, would
seriously affect many of the others. If the country were open on its
borders, new forms would certainly immigrate, and this also would
seriously disturb the relations of some of the former inhabitants. Let it
be remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up, if some of the
original inhabitants were in some manner modified; for, had the area been
open to immigration, these same [82]places would have been seized on by
intruders. In such case, every slight modification, which in the course
of ages chanced to arise, and which in any way favoured the individuals
of any of the species, by better adapting them to their altered
conditions, would tend to be preserved; and natural selection would thus
have free scope for the work of improvement.
We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the reproductive
system, causes or increases variability; and in the foregoing case the
conditions of life are supposed to have undergone a change, and this
would manifestly be favourable to natural selection, by giving a better
chance of profitable variations occurring; and unless profitable
variations do occur, natural selection can do nothing. Not that, as I
believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction mere
individual differences, so could Nature, but far more easily, from having
incomparably longer time at her disposal. Nor do I believe that any great
physical change, as of climate, or any unusual degree of isolation to
check immigration, is actually necessary to produce new and unoccupied
places for natural selection to fill up by modifying and improving some
of the varying inhabitants. For as all the inhabitants of each country
are struggling together with nicely balanced forces, extremely slight
modifications in the structure or habits of one inhabitant would often
give it an advantage over others; and still further modifications of the
same kind would often still further increase the advantage. No country
can be named in which all the native inhabitants are now so perfectly
adapted to each other and to the physical conditions under which they
live, that none of [83]them could anyhow be improved; for in all
countries, the natives have been so far conquered by naturalised
productions, that they have allowed foreigners to take firm possession of
the land. And as foreigners have thus everywhere beaten some of the
natives, we may safely conclude that the natives might have been modified
with advantage, so as to have better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not Nature
effect? Man can act only on external and visible characters: Nature cares
nothing for appearances, except in so far as they may be useful to any
being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she tends.
Every selected character is fully exercised by her; and the being is
placed under well-suited conditions of life. Man keeps the natives of
many climates in the same country; he seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a
short beaked pigeon on the same food; he does not exercise a long-backed
or long-legged quadruped in any peculiar manner; he exposes sheep with
long and short wool to the same climate. He does not allow the most
vigorous males to struggle for the females. He does not rigidly destroy
all inferior animals, but protects during each varying season, as far as
lies in his power, all his productions. He often begins his selection by
some half-monstrous form; or at least by some modification prominent
enough to catch his eye, or to be plainly useful to him. Under nature,
the slightest difference of structure or constitution may well turn the
nicely-balanced scale in the struggle for life, and so be [84]preserved. How
fleeting are the wishes and efforts of man! how short his time! and
consequently how poor will his products be, compared with those
accumulated by Nature during whole geological periods. Can we wonder,
then, that Nature's productions should be far "truer" in character than
man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of far
higher workmanship?
It may metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all that
is good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in relation
to its organic and inorganic conditions of life. We see nothing of these
slow changes in progress, until the hand of time has marked the long
lapse of ages, and then so imperfect is our view into long past
geological ages, that we only see that the forms of life are now
different from what they formerly were.
Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to consider
as of very trifling importance, may thus be acted on. When we see
leaf-eating insects green, and bark-feeders mottled-grey; the alpine
ptarmigan white in winter, the red-grouse the colour of heather, and the
black-grouse that of peaty earth, we must believe that these tints are of
service to these birds and insects in preserving them from danger.
Grouse, if not destroyed at some period of their lives, would increase in
countless numbers; they are known to suffer largely from birds of prey;
and hawks are guided by eyesight to their prey—so much so, that on
[85]parts of the Continent persons are warned
not to keep white pigeons, as being the most liable to destruction. Hence
I can see no reason to doubt that natural selection might be most
effective in giving the proper colour to each kind of grouse, and in
keeping that colour, when once acquired, true and constant. Nor ought we
to think that the occasional destruction of an animal of any particular
colour would produce little effect: we should remember how essential it
is in a flock of white sheep to destroy every lamb with the faintest
trace of black. In plants the down on the fruit and the colour of the
flesh are considered by botanists as characters of the most trifling
importance: yet we hear from an excellent horticulturist, Downing, that
in the United States smooth-skinned fruits suffer far more from a beetle,
a curculio, than those with down; that purple plums suffer far more from
a certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh. If,
with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state of
nature, where the trees would have to struggle with other trees and with
a host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or purple fleshed fruit,
should succeed.
In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem quite unimportant, we
must not forget that climate, food, &c., probably produce some slight
and direct effect. It is, however, far more necessary to bear in mind
that there are many unknown laws of correlation of growth, which, when
one part of the organisation is modified through variation, and the
modifications are accumulated by natural selection for [86]the good of the
being, will cause other modifications, often of the most unexpected
nature.
As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at the
same period;—for instance, in the seeds of the many varieties of
our culinary and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm; in the eggs of poultry, and in
the colour of the down of their chickens; in the horns of our sheep and
cattle when nearly adult;—so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any age,
by the accumulation of variations profitable at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its
seeds more and more widely disseminated by the wind, I can see no greater
difficulty in this being effected through natural selection, than in the
cotton-planter increasing and improving by selection the down in the pods
on his cotton-trees. Natural selection may modify and adapt the larva of
an insect to a score of contingencies, wholly different from those which
concern the mature insect. These modifications will no doubt affect,
through the laws of correlation, the structure of the adult; and probably
in the case of those insects which live only for a few hours, and which
never feed, a large part of their structure is merely the correlated
result of successive changes in the structure of their larvæ. So,
conversely, modifications in the adult will probably often affect the
structure of the larva; but in all cases natural selection will ensure
that modifications consequent on other modifications at a different
period of life, shall not be in the least degree injurious: for if they
became so, they would cause the extinction of the species.
Natural selection will modify the structure of the [87]young in
relation to the parent, and of the parent in relation to the young. In
social animals it will adapt the structure of each individual for the
benefit of the community; if each in consequence profits by the selected
change. What natural selection cannot do, is to modify the structure of
one species, without giving it any advantage, for the good of another
species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation. A
structure used only once in an animal's whole life, if of high importance
to it, might be modified to any extent by natural selection; for
instance, the great jaws possessed by certain insects, used exclusively
for opening the cocoon—or the hard tip to the beak of nestling
birds, used for breaking the egg. It has been asserted, that of the best
short-beaked tumbler-pigeons more perish in the egg than are able to get
out of it; so that fanciers assist in the act of hatching. Now, if nature
had to make the beak of a full-grown pigeon very short for the bird's own
advantage, the process of modification would be very slow, and there
would be simultaneously the most rigorous selection of the young birds
within the egg, which had the most powerful and hardest beaks, for all
with weak beaks would inevitably perish: or, more delicate and more
easily broken shells might be selected, the thickness of the shell being
known to vary like every other structure.
Sexual Selection.—Inasmuch as peculiarities often appear
under domestication in one sex and become hereditarily attached to that
sex, the same fact probably occurs under nature, and if so, natural
selection will be able to modify one sex in its functional relations to
the other sex, or in relation to wholly different habits of life in the
two sexes, as is sometimes the case [88]with insects. And this
leads me to say a few words on what I call Sexual Selection. This
depends, not on a struggle for existence, but on a struggle between the
males for possession of the females; the result is not death to the
unsuccessful competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally, the most
vigorous males, those which are best fitted for their places in nature,
will leave most progeny. But in many cases, victory depends not on
general vigour, but on having special weapons, confined to the male sex.
A hornless stag or spurless cock would have a poor chance of leaving
offspring. Sexual selection by always allowing the victor to breed might
surely give indomitable courage, length to the spur, and strength to the
wing to strike in the spurred leg, as well as the brutal cock-fighter,
who knows well that he can improve his breed by careful selection of the
best cocks. How low in the scale of nature the law of battle descends, I
know not; male alligators have been described as fighting, bellowing, and
whirling round, like Indians in a war-dance, for the possession of the
females; male salmons have been seen fighting all day long; male
stag-beetles often bear wounds from the huge mandibles of other males.
The war is, perhaps, severest between the males of polygamous animals,
and these seem oftenest provided with special weapons. The males of
carnivorous animals are already well armed; though to them and to others,
special means of defence may be given through means of sexual selection,
as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw
to the male salmon; for the shield may be as important for victory, as
the sword or spear.
Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, [89]believe that there is the
severest rivalry between the males of many species to attract by singing
the females. The rock-thrush of Guiana, birds of Paradise, and some
others, congregate; and successive males display their gorgeous plumage
and perform strange antics before the females, which, standing by as
spectators, at last choose the most attractive partner. Those who have
closely attended to birds in confinement well know that they often take
individual preferences and dislikes: thus Sir R. Heron has described how
one pied peacock was eminently attractive to all his hen birds. It may
appear childish to attribute any effect to such apparently weak means: I
cannot here enter on the details necessary to support this view; but if
man can in a short time give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no good reason to doubt
that female birds, by selecting, during thousands of generations, the
most melodious or beautiful males, according to their standard of beauty,
might produce a marked effect. I strongly suspect that some well-known
laws, with respect to the plumage of male and female birds, in comparison
with the plumage of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting when the birds
have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by sexual
selection; that is, individual males have had, in successive generations,
some slight advantage over other [90]males, in their weapons, means of defence,
or charms; and have transmitted these advantages to their male offspring.
Yet, I would not wish to attribute all such sexual differences to this
agency: for we see peculiarities arising and becoming attached to the
male sex in our domestic animals (as the wattle in male carriers,
horn-like protuberances in the cocks of certain fowls, &c.), which we
cannot believe to be either useful to the males in battle, or attractive
to the females. We see analogous cases under nature, for instance, the
tuft of hair on the breast of the turkey-cock, which can hardly be either
useful or ornamental to this bird;—indeed, had the tuft appeared
under domestication, it would have been called a monstrosity.
Illustrations of the action of Natural Selection.—In
order to make it clear how, as I believe, natural selection acts, I must
beg permission to give one or two imaginary illustrations. Let us take
the case of a wolf, which preys on various animals, securing some by
craft, some by strength, and some by fleetness; and let us suppose that
the fleetest prey, a deer for instance, had from any change in the
country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest pressed
for food. I can under such circumstances see no reason to doubt that the
swiftest and slimmest wolves would have the best chance of surviving, and
so be preserved or selected,—provided always that they retained
strength to master their prey at this or at some other period of the
year, when they might be compelled to prey on other animals. I can see no
more reason to doubt this, than that man can improve the fleetness of his
greyhounds by careful and methodical selection, or by that unconscious
selection which results from each man trying [91]to keep the best dogs
without any thought of modifying the breed.
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and almost
nightly catching woodcocks or snipes. The tendency to catch rats rather
than mice is known to be inherited. Now, if any slight innate change of
habit or of structure benefited an individual wolf, it would have the
best chance of surviving and of leaving offspring. Some of its young
would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent form of wolf. Or, again, the
wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from the
continued preservation of the individuals best fitted for the two sites,
two varieties might slowly be formed. These varieties would cross and
blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United
States, one with a light greyhound-like form, which pursues deer, and the
other more bulky, with shorter legs, which more frequently attacks the
shepherd's flocks.
Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is [92]effected by glands at the base of the
stipules in some Leguminosæ, and at the back of the leaf of the common
laurel. This juice, though small in quantity, is greedily sought by
insects. Let us now suppose a little sweet juice or nectar to be excreted
by the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species would
thus get crossed; and the act of crossing, we have good reason to believe
(as will hereafter be more fully alluded to), would produce very vigorous
seedlings, which consequently would have the best chance of flourishing
and surviving. Some of these seedlings would probably inherit the
nectar-excreting power. Those individual flowers which had the largest
glands or nectaries, and which excreted most nectar, would be oftenest
visited by insects, and would be oftenest crossed; and so in the long-run
would gain the upper hand. Those flowers, also, which had their stamens
and pistils placed, in relation to the size and habits of the particular
insects which visited them, so as to favour in any degree the transportal
of their pollen from flower to flower, would likewise be favoured or
selected. We might have taken the case of insects visiting flowers for
the sake of collecting pollen instead of nectar; and as pollen is formed
for the sole object of fertilisation, its destruction appears a simple
loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from
flower to flower, and a cross thus effected, although nine-tenths of the
pollen were destroyed, it might still be a great gain to the plant; and
those individuals which produced more and more pollen, and had larger and
larger anthers, would be selected. [93]
When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they can most
effectually do this, I could easily show by many striking instances. I
will give only one—not as a very striking case, but as likewise
illustrating one step in the separation of the sexes of plants, presently
to be alluded to. Some holly-trees bear only male flowers, which have
four stamens producing a rather small quantity of pollen, and a
rudimentary pistil; other holly-trees bear only female flowers; these
have a full-sized pistil, and four stamens with shrivelled anthers, in
which not a grain of pollen can be detected. Having found a female tree
exactly sixty yards from a male tree, I put the stigmas of twenty
flowers, taken from different branches, under the microscope, and on all,
without exception, there were pollen-grains, and on some a profusion of
pollen. As the wind had set for several days from the female to the male
tree, the pollen could not thus have been carried. The weather had been
cold and boisterous, and therefore not favourable to bees, nevertheless
every female flower which I examined had been effectually fertilised by
the bees, accidentally dusted with pollen, having flown from tree to tree
in search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in [94]another flower or on
another plant. In plants under culture and placed under new conditions of
life, sometimes the male organs and sometimes the female organs become
more or less impotent; now if we suppose this to occur in ever so slight
a degree under nature, then as pollen is already carried regularly from
flower to flower, and as a more complete separation of the sexes of our
plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased, would be
continually favoured or selected, until at last a complete separation of
the sexes would be effected.
Let us now turn to the nectar-feeding insects in our imaginary case:
we may suppose the plant of which we have been slowly increasing the
nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give many
facts, showing how anxious bees are to save time; for instance, their
habit of cutting holes and sucking the nectar at the bases of certain
flowers, which they can, with a very little more trouble, enter by the
mouth. Bearing such facts in mind, I can see no reason to doubt that an
accidental deviation in the size and form of the body, or in the
curvature and length of the proboscis, &c., far too slight to be
appreciated by us, might profit a bee or other insect, so that an
individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving descendants.
Its descendants would probably inherit a tendency to a similar slight
deviation of structure. The tubes of the corollas of the common red and
incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty
glance appear to differ in length; yet the hive-bee can easily suck the
nectar out of the incarnate clover, but not out of the common red [95]clover,
which is visited by humble-bees alone; so that whole fields of the red
clover offer in vain an abundant supply of precious nectar to the
hive-bee. Thus it might be a great advantage to the hive-bee to have a
slightly longer or differently constructed proboscis. On the other hand,
I have found by experiment that the fertility of clover depends on bees
visiting and moving parts of the corolla, so as to push the pollen on to
the stigmatic surface. Hence, again, if humble-bees were to become rare
in any country, it might be a great advantage to the red clover to have a
shorter or more deeply divided tube to its corolla, so that the hive-bee
could visit its flowers. Thus I can understand how a flower and a bee
might slowly become, either simultaneously or one after the other,
modified and adapted in the most perfect manner to each other, by the
continued preservation of individuals presenting mutual and slightly
favourable deviations of structure.
I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology;" but we now
seldom hear the action, for instance, of the coast-waves, called a
trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic [96]beings, or of
any great and sudden modification in their structure.
On the Intercrossing of Individuals.—I must here
introduce a short digression. In the case of animals and plants with
separated sexes, it is of course obvious that two individuals must always
(with the exception of the curious and not well-understood cases of
parthenogenesis) unite for each birth; but in the case of hermaphrodites
this is far from obvious. Nevertheless I am strongly inclined to believe
that with all hermaphrodites two individuals, either occasionally or
habitually, concur for the reproduction of their kind. This view was
first suggested by Andrew Knight. We shall presently see its importance;
but I must here treat the subject with extreme brevity, though I have the
materials prepared for an ample discussion. All vertebrate animals, all
insects, and some other large groups of animals, pair for each birth.
Modern research has much diminished the number of supposed
hermaphrodites, and of real hermaphrodites a large number pair; that is,
two individuals regularly unite for reproduction, which is all that
concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are
hermaphrodites. What reason, it may be asked, is there for supposing in
these cases that two individuals ever concur in reproduction? As it is
impossible here to enter on details, I must trust to some general
considerations alone.
In the first place, I have collected so large a body of facts,
showing, in accordance with the almost universal belief of breeders, that
with animals and plants a cross between different varieties, or between
individuals of the same variety but of another strain, gives vigour and
[97]fertility to the offspring; and on the other
hand, that close interbreeding diminishes vigour and fertility;
that these facts alone incline me to believe that it is a general law of
nature (utterly ignorant though we be of the meaning of the law) that no
organic being self-fertilises itself for an eternity of generations; but
that a cross with another individual is occasionally—perhaps at
very long intervals—indispensable.
On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following, which
on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet
what a multitude of flowers have their anthers and stigmas fully exposed
to the weather! but if an occasional cross be indispensable, the fullest
freedom for the entrance of pollen from another individual will explain
this state of exposure, more especially as the plant's own anthers and
pistil generally stand so close together that self-fertilisation seems
almost inevitable. Many flowers, on the other hand, have their organs of
fructification closely enclosed, as in the great papilionaceous or
pea-family; but in several, perhaps in all, such flowers, there is a very
curious adaptation between the structure of the flower and the manner in
which bees suck the nectar; for, in doing this, they either push the
flower's own pollen on the stigma, or bring pollen from another flower.
So necessary are the visits of bees to papilionaceous flowers, that I
have found, by experiments published elsewhere, that their fertility is
greatly diminished if these visits be prevented. Now, it is scarcely
possible that bees should fly from flower to flower, and not carry pollen
from one to the other, to the great good, as I believe, of the plant.
Bees will act like a camel-hair pencil, and it is quite sufficient just
to touch the anthers of [98]one flower and then the stigma of another
with the same brush to ensure fertilisation; but it must not be supposed
that bees would thus produce a multitude of hybrids between distinct
species; for if you bring on the same brush a plant's own pollen and
pollen from another species, the former will have such a prepotent
effect, that it will invariably and completely destroy, as has been shown
by Gärtner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems adapted
solely to ensure self-fertilisation; and no doubt it is useful for this
end: but, the agency of insects is often required to cause the stamens to
spring forward, as Kölreuter has shown to be the case with the barberry;
and in this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if closely-allied forms or
varieties are planted near each other, it is hardly possible to raise
pure seedlings, so largely do they naturally cross. In many other cases,
far from there being any aids for self-fertilisation, there are special
contrivances, as I could show from the writings of C. C. Sprengel and
from my own observations, which effectually prevent the stigma receiving
pollen from its own flower: for instance, in Lobelia fulgens, there is a
really beautiful and elaborate contrivance by which every one of the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited, at least in
my garden, by insects, it never sets a seed, though by placing pollen
from one flower on the stigma of another, I raised plenty of seedlings;
and whilst another species of Lobelia growing close by, which is visited
by bees, seeds freely. In very many other cases, though there [99]be no special
mechanical contrivance to prevent the stigma of a flower receiving its
own pollen, yet, as C. C. Sprengel has shown, and as I can confirm,
either the anthers burst before the stigma is ready for fertilisation, or
the stigma is ready before the pollen of that flower is ready, so that
these plants have in fact separated sexes, and must habitually be
crossed. How strange are these facts! How strange that the pollen and
stigmatic surface of the same flower, though placed so close together, as
if for the very purpose of self-fertilisation, should in so many cases be
mutually useless to each other! How simply are these facts explained on
the view of an occasional cross with a distinct individual being
advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for instance,
I raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind,
and some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens, but by
those of the many other flowers on the same plant. How, then, comes it
that such a vast number of the seedlings are mongrelized? I suspect that
it must arise from the pollen of a distinct variety having a
prepotent effect over a flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct
individuals of the same species. When distinct species are crossed
the case is directly the reverse, for a plant's own pollen is always
prepotent over foreign pollen; but to this subject we shall return in a
future chapter.
In the case of a gigantic tree covered with, [100]innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree,
and at most only from flower to flower on the same tree, and that flowers
on the same tree can be considered as distinct individuals only in a
limited sense. I believe this objection to be valid, but that nature has
largely provided against it by giving to trees a strong tendency to bear
flowers with separated sexes. When the sexes are separated, although the
male and female flowers may be produced on the same tree, we can see that
pollen must be regularly carried from flower to flower; and this will
give a better chance of pollen being occasionally carried from tree to
tree. That trees belonging to all Orders have their sexes more often
separated than other plants, I find to be the case in this country; and
at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa
Gray those of the United States, and the result was as I anticipated. On
the other hand, Dr. Hooker has recently informed me that he finds that
the rule does not hold in Australia; and I have made these few remarks on
the sexes of trees simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which offers
so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in which
terrestrial animals live, and the nature of the fertilising element; for
we know of no means, analogous to the action of insects and of the wind
in the case of plants, by which an occasional cross could be effected
with terrestrial animals without the concurrence of two individuals. Of
aquatic animals, there are many self-fertilising hermaphrodites; but here
[101]currents in the water offer an obvious
means for an occasional cross. And, as in the case of flowers, I have as
yet failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an hermaphrodite
animal with the organs of reproduction so perfectly enclosed within the
body, that access from without and the occasional influence of a distinct
individual can be shown to be physically impossible. Cirripedes long
appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance, elsewhere
to prove that two individuals, though both are self-fertilising
hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even of
the same genus, though agreeing closely with each other in almost their
whole organisation, yet are not rarely, some of them hermaphrodites, and
some of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference between
hermaphrodites and unisexual species, as far as function is concerned,
becomes very small.
From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law of
nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two individuals
is an obvious necessity for each birth; in many others it occurs perhaps
only at long intervals; but in none, as I suspect, can self-fertilisation
go on for perpetuity. [102]
Circumstances favourable to Natural Selection.—This is an
extremely intricate subject. A large amount of inheritable and
diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by
giving a better chance for the appearance within any given period of
profitable variations, will compensate for a lesser amount of variability
in each individual, and is, I believe, an extremely important element of
success. Though nature grants vast periods of time for the work of
natural selection, she does not grant an indefinite period; for as all
organic beings are striving, it may be said, to seize on each place in
the economy of nature, if any one species does not become modified and
improved in a corresponding degree with its competitors, it will soon be
exterminated.
In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when many
men, without intending to alter the breed, have a nearly common standard
of perfection, and all try to get and breed from the best animals, much
improvement and modification surely but slowly follow from this
unconscious process of selection, notwithstanding a large amount of
crossing with inferior animals. Thus it will be in nature; for within a
confined area, with some place in its polity not so perfectly occupied as
might be, natural selection will always tend to preserve all the
individuals varying in the right direction, though in different degrees,
so as better to fill up the unoccupied place. But if the area be large,
its several districts will almost certainly present different conditions
of life; and then if natural selection be modifying and improving a
species in the several districts, there will be intercrossing with the
other individuals of the same species on the confines of each. And in
[103]this case the effects of intercrossing can
hardly be counterbalanced by natural selection always tending to modify
all the individuals in each district in exactly the same manner to the
conditions of each; for in a continuous area, the physical conditions at
least will generally graduate away insensibly from one district to
another. The intercrossing will most affect those animals which unite for
each birth, which wander much, and which do not breed at a very quick
rate. Hence in animals of this nature, for instance in birds, varieties
will generally be confined to separated countries; and this I believe to
be the case. In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but which wander
little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there maintain
itself in a body, so that whatever intercrossing took place would be
chiefly between the individuals of the same new variety. A local variety
when once thus formed might subsequently slowly spread to other
districts. On the above principle, nurserymen always prefer getting seed
from a large body of plants of the same variety, as the chance of
intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to pair
together.
Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and uniform
in character. It will [104]obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have a
better chance of surviving and propagating their kind; and thus, in the
long run, the influence of intercrosses, even at rare intervals, will be
great. If there exist organic beings which never intercross, uniformity
of character can be retained amongst them, as long as their conditions of
life remain the same, only through the principle of inheritance, and
through natural selection destroying any which depart from the proper
type; but if their conditions of life change and they undergo
modification, uniformity of character can be given to their modified
offspring, solely by natural selection preserving the same favourable
variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic
and inorganic conditions of life will generally be in a great degree
uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same manner
in relation to the same conditions. Intercrosses, also, with the
individuals of the same species, which otherwise would have inhabited the
surrounding and differently circumstanced districts, will be prevented.
But isolation probably acts more efficiently in checking the immigration
of better adapted organisms, after any physical change, such as of
climate or elevation of the land, &c.; and thus new places in the
natural economy of the country are left open for the old inhabitants to
struggle for, and become adapted to, through [105]modifications in their
structure and constitution. Lastly, isolation, by checking immigration
and consequently competition, will give time for any new variety to be
slowly improved; and this may sometimes be of importance in the
production of new species. If, however, an isolated area be very small,
either from being surrounded by barriers, or from having very peculiar
physical conditions, the total number of the individuals supported on it
will necessarily be very small; and fewness of individuals will greatly
retard the production of new species through natural selection, by
decreasing the chance of the appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,—that is, have been
produced there, and nowhere else. Hence an oceanic island at first sight
seems to have been highly favourable for the production of new species.
But we may thus greatly deceive ourselves, for to ascertain whether a
small isolated area, or a large open area like a continent, has been most
favourable for the production of new organic forms, we ought to make the
comparison within equal times; and this we are incapable of doing.
Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to believe
that largeness of area is of more importance, more especially in the
production of species, which will prove capable of enduring for a long
period, and of spreading widely. Throughout a great and open area, not
only will there be a better chance of favourable variations arising from
the large number of individuals of the same species [106]there
supported, but the conditions of life are infinitely complex from the
large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved in
a corresponding degree or they will be exterminated. Each new form, also,
as soon as it has been much improved, will be able to spread over the
open and continuous area, and will thus come into competition with many
others. Hence more new places will be formed, and the competition to fill
them will be more severe, on a large than on a small and isolated area.
Moreover, great areas, though now continuous, owing to oscillations of
level, will often have recently existed in a broken condition, so that
the good effects of isolation will generally, to a certain extent, have
concurred. Finally, I conclude that, although small isolated areas
probably have been in some respects highly favourable for the production
of new species, yet that the course of modification will generally have
been more rapid on large areas; and what is more important, that the new
forms produced on large areas, which already have been victorious over
many competitors, will be those that will spread most widely, will give
rise to most new varieties and species, and will thus play an important
part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will be
again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia have
formerly yielded, and apparently are now yielding, before those of the
larger Europæo-Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised on islands. On
a small island, the race for life will have been less severe, and there
will have been less modification and less [107]extermination. Hence,
perhaps, it comes that the flora of Madeira, according to Oswald Heer,
resembles the extinct tertiary flora of Europe. All fresh-water basins,
taken together, make a small area compared with that of the sea or of the
land; and, consequently, the competition between fresh-water productions
will have been less severe than elsewhere; new forms will have been more
slowly formed, and old forms more slowly exterminated. And it is in fresh
water that we find seven genera of Ganoid fishes, remnants of a once
preponderant order: and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders now widely
separated in the natural scale. These anomalous forms may almost be
called living fossils; they have endured to the present day, from having
inhabited a confined area, and from having thus been exposed to less
severe competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a broken condition,
is the most favourable for the production of many new forms of life,
likely to endure long and to spread widely. For the area first existed as
a continent, and the inhabitants, at this period numerous in individuals
and kinds, will have been subjected to very severe competition. When
converted by subsidence into large separate islands, there will still
exist many individuals of the same species on each island: intercrossing
on the confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be [108]prevented, so that new
places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by renewed
elevation, the islands shall be re-converted into a continental area,
there will again be severe competition: the most favoured or improved
varieties will be enabled to spread: there will be much extinction of the
less improved forms, and the relative proportional numbers of the various
inhabitants of the renewed continent will again be changed; and again
there will be a fair field for natural selection to improve still further
the inhabitants, and thus produce new species.
That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of nature,
which can be better occupied by some of the inhabitants of the country
undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and on
the immigration of better adapted forms having been checked. But the
action of natural selection will probably still oftener depend on some of
the inhabitants becoming slowly modified; the mutual relations of many of
the other inhabitants being thus disturbed. Nothing can be effected,
unless favourable variations occur, and variation itself is apparently
always a very slow process. The process will often be greatly retarded by
free intercrossing. Many will exclaim that these several causes are amply
sufficient wholly to stop the action of natural selection. I do not
believe so. On the other hand, I do believe that natural selection always
acts very slowly, often only at long intervals of time, and generally on
only a very few of the inhabitants of the same region at the same time. I
further believe, that this very slow, [109]intermittent action of
natural selection accords perfectly well with what geology tells us of
the rate and manner at which the inhabitants of this world have
changed.
Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the amount
of change, to the beauty and infinite complexity of the coadaptations
between all organic beings, one with another and with their physical
conditions of life, which may be effected in the long course of time by
nature's power of selection.
Extinction.—This subject will be more fully discussed in
our chapter on Geology; but it must be here alluded to from being
intimately connected with natural selection. Natural selection acts
solely through the preservation of variations in some way advantageous,
which consequently endure. But as from the high geometrical ratio of
increase of all organic beings, each area is already fully stocked with
inhabitants, it follows that as each selected and favoured form increases
in number, so will the less favoured forms decrease and become rare.
Rarity, as geology tells us, is the precursor to extinction. We can,
also, see that any form represented by few individuals will, during
fluctuations in the seasons or in the number of its enemies, run a good
chance of utter extinction. But we may go further than this; for as new
forms are continually and slowly being produced, unless we believe that
the number of specific forms goes on perpetually and almost indefinitely
increasing, numbers inevitably must become extinct. That the number of
specific forms has not indefinitely increased, geology shows us plainly;
and indeed we can see reason why they should not have thus increased, for
the number of places in the polity of nature is not indefinitely
great,—not that we [110]have any means of knowing that any one
region has as yet got its maximum of species. Probably no region is as
yet fully stocked, for at the Cape of Good Hope, where more species of
plants are crowded together than in any other quarter of the world, some
foreign plants have become naturalised, without causing, as far as we
know, the extinction of any natives.
Furthermore, the species which are most numerous in individuals will
have the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the greatest
number of recorded varieties, or incipient species. Hence, rare species
will be less quickly modified or improved within any given period, and
they will consequently be beaten in the race for life by the modified
descendants of the commoner species.
From these several considerations I think it inevitably follows, that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct. The
forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,—varieties of the same species, and species of
the same genus or of related genera,—which, from having nearly the
same structure, constitution, and habits, generally come into the
severest competition with each other. Consequently, each new variety or
species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We see the
same process of extermination amongst our domesticated productions,
through the selection of improved forms by man. Many curious [111]instances
could be given showing how quickly new breeds of cattle, sheep, and other
animals, and varieties of flowers, take the place of older and inferior
kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these "were swept away
by the short-horns" (I quote the words of an agricultural writer) "as if
by some murderous pestilence."
Divergence of Character.—The principle, which I have
designated by this term, is of high importance on my theory, and
explains, as I believe, several important facts. In the first place,
varieties, even strongly-marked ones, though having somewhat of the
character of species—as is shown by the hopeless doubts in many
cases how to rank them—yet certainly differ from each other far
less than do good and distinct species. Nevertheless, according to my
view, varieties are species in the process of formation, or are, as I
have called them, incipient species. How, then, does the lesser
difference between varieties become augmented into the greater difference
between species? That this does habitually happen, we must infer from
most of the innumerable species throughout nature presenting well-marked
differences; whereas varieties, the supposed prototypes and parents of
future well-marked species, present slight and ill-defined differences.
Mere chance, as we may call it, might cause one variety to differ in some
character from its parents, and the offspring of this variety again to
differ from its parent in the very same character and in a greater
degree; but this alone would never account for so habitual and large an
amount of difference as that between varieties of the same species and
species of the same genus.
As has always been my practice, let us seek light on [112]this head from
our domestic productions. We shall here find something analogous. A
fancier is struck by a pigeon having a slightly shorter beak; another
fancier is struck by a pigeon having a rather longer beak; and on the
acknowledged principle that "fanciers do not and will not admire a medium
standard, but like extremes," they both go on (as has actually occurred
with tumbler-pigeons) choosing and breeding from birds with longer and
longer beaks, or with shorter and shorter beaks. Again, we may suppose
that at an early period one man preferred swifter horses; another
stronger and more bulky horses. The early differences would be very
slight; in the course of time, from the continued selection of swifter
horses by some breeders, and of stronger ones by others, the differences
would become greater, and would be noted as forming two sub-breeds;
finally, after the lapse of centuries, the sub-breeds would become
converted into two well-established and distinct breeds. As the
differences slowly become greater, the inferior animals with intermediate
characters, being neither very swift nor very strong, will have been
neglected, and will have tended to disappear. Here, then, we see in man's
productions the action of what may be called the principle of divergence,
causing differences, at first barely appreciable, steadily to increase,
and the breeds to diverge in character both from each other and from
their common parent.
But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places in
the polity of nature, and so be enabled to increase in numbers. [113]
We can clearly see this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions) only
by its varying descendants seizing on places at present occupied by other
animals: some of them, for instance, being enabled to feed on new kinds
of prey, either dead or alive; some inhabiting new stations, climbing
trees, frequenting water, and some perhaps becoming less carnivorous. The
more diversified in habits and structure the descendants of our
carnivorous animal became, the more places they would be enabled to
occupy. What applies to one animal will apply throughout all time to all
animals—that is, if they vary—for otherwise natural selection
can do nothing. So it will be with plants. It has been experimentally
proved, that if a plot of ground be sown with one species of grass, and a
similar plot be sown with several distinct genera of grasses, a greater
number of plants and a greater weight of dry herbage can thus be raised.
The same has been found to hold good when first one variety and then
several mixed varieties of wheat have been sown on equal spaces of
ground. Hence, if any one species of grass were to go on varying, and
those varieties were continually selected which differed from each other
in at all the same manner as distinct species and genera of grasses
differ from each other, a greater number of individual plants of this
species of grass, including its modified descendants, would succeed in
living on the same piece of ground. And we well know that each species
and each variety of grass is annually sowing almost countless seeds; and
thus, as it may be said, is striving its utmost to increase its numbers.
[114]Consequently, I cannot doubt that in the
course of many thousands of generations, the most distinct varieties of
any one species of grass would always have the best chance of succeeding
and of increasing in numbers, and thus of supplanting the less distinct
varieties; and varieties, when rendered very distinct from each other,
take the rank of species.
The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any small
piece of ground, could live on it (supposing it not to be in any way
peculiar in its nature), and may be said to be striving to the utmost to
live there; but, it is seen, that where they come into the closest
competition with each other, the advantages of diversification of
structure, with the accompanying differences of habit and constitution,
determine that the inhabitants, which thus jostle each other most
closely, shall, as a general rule, belong to what we call different
genera and orders.
The same principle is seen in the naturalisation of [115]plants through
man's agency in foreign lands. It might have been expected that the
plants which have succeeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different; and
Alph. De Candolle has well remarked in his great and admirable work, that
floras gain by naturalisation, proportionally with the number of the
native genera and species, far more in new genera than in new species. To
give a single instance: in the last edition of Dr. Asa Gray's 'Manual of
the Flora of the Northern United States,' 260 naturalised plants are
enumerated, and these belong to 162 genera. We thus see that these
naturalised plants are of a highly diversified nature. They differ,
moreover, to a large extent from the indigenes, for out of the 162
genera, no less than 100 genera are not there indigenous, and thus a
large proportional addition is made to the genera of these States.
By considering the nature of the plants or animals which have
struggled successfully with the indigenes of any country, and have there
become naturalised, we may gain some crude idea in what manner some of
the natives would have to be modified, in order to gain an advantage over
the other natives; and we may at least safely infer that diversification
of structure, amounting to new generic differences, would be profitable
to them.
The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour in
the organs of the same individual body—a subject so well elucidated
by Milne [116]Edwards. No physiologist doubts that a
stomach adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of any
land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of animals,
with their organisation but little diversified, could hardly compete with
a set more perfectly diversified in structure. It may be doubted, for
instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly representing, as
Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and
rodent mammals, could successfully compete with these well-pronounced
orders. In the Australian mammals, we see the process of diversification
in an early and incomplete stage of development.
After the foregoing discussion, which ought to have been much
amplified, we may, I think, assume that the modified descendants of any
one species will succeed by so much the better as they become more
diversified in structure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how this principle of benefit
being derived from divergence of character, combined with the principles
of natural selection and of extinction, will tend to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large in
its own country; these species are supposed to resemble each other in
unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances.
I have said a large genus, because we have seen in the second chapter,
[117]that on an average more of the species of
large genera vary than of small genera; and the varying species of the
large genera present a greater number of varieties. We have, also, seen
that the species, which are the commonest and the most widely-diffused,
vary more than rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in its
own country. The little fan of diverging dotted lines of unequal lengths
proceeding from (A), may represent its varying offspring. The variations
are supposed to be extremely slight, but of the most diversified nature;
they are not supposed all to appear simultaneously, but often after long
intervals of time; nor are they all supposed to endure for equal periods.
Only those variations which are in some way profitable will be preserved
or naturally selected. And here the importance of the principle of
benefit being derived from divergence of character comes in; for this
will generally lead to the most different or divergent variations
(represented by the outer dotted lines) being preserved and accumulated
by natural selection. When a dotted line reaches one of the horizontal
lines, and is there marked by a small numbered letter, a sufficient
amount of variation is supposed to have been accumulated to have formed a
fairly well-marked variety, such as would be thought worthy of record in
a systematic work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better if
each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1 and
m1. These two varieties will generally continue to be
exposed to the same conditions which made their parents variable, [118]and
the tendency to variability is in itself hereditary, consequently they
will tend to vary, and generally to vary in nearly the same manner as
their parents varied. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their
parent (A) more numerous than most of the other inhabitants of the same
country; they will likewise partake of those more general advantages
which made the genus to which the parent-species belonged, a large genus
in its own country. And these circumstances we know to be favourable to
the production of new varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is
supposed in the diagram to have produced variety a2,
which will, owing to the principle of divergence, differ more from (A)
than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and
s2, differing from each other, and more considerably
from their common parent (A). We may continue the process by similar
steps for any length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go on increasing in
number and diverging in character. In the diagram the process is
represented up to the ten-thousandth generation, and under a condensed
and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in itself
made somewhat irregular. [119]I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium form
may often long endure, and may or may not produce more than one modified
descendant; for natural selection will always act according to the nature
of the places which are either unoccupied or not perfectly occupied by
other beings; and this will depend on infinitely complex relations. But
as a general rule, the more diversified in structure the descendants from
any one species can be rendered, the more places they will be enabled to
seize on, and the more their modified progeny will be increased. In our
diagram the line of succession is broken at regular intervals by small
numbered letters marking the successive forms which have become
sufficiently distinct to be recorded as varieties. But these breaks are
imaginary, and might have been inserted anywhere, after intervals long
enough to have allowed the accumulation of a considerable amount of
divergent variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in character:
this is represented in the diagram by the several divergent branches
proceeding from (A). The modified offspring from the later and more
highly improved branches in the lines of descent, will, it is probable,
often take the place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of the lower
branches not reaching to the upper horizontal lines. In some cases I do
not doubt that the process of modification will be confined to a single
line of descent, and the number of the descendants will not be increased;
although the amount [120]of divergent modification may have been
increased in the successive generations. This case would be represented
in the diagram, if all the lines proceeding from (A) were removed,
excepting that from a1 to a10. In the
same way, for instance, the English race-horse and English pointer have
apparently both gone on slowly diverging in character from their original
stocks, without either having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, a10, f10, and
m10, which, from having diverged in character during
the successive generations, will have come to differ largely, but perhaps
unequally, from each other and from their common parent. If we suppose
the amount of change between each horizontal line in our diagram to be
excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of
sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates the
steps by which the small differences distinguishing varieties are
increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as shown
in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and
m14, all descended from (A). Thus, as I believe,
species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two
species, according to the amount of change supposed to be represented
[121]between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been
produced. In each genus, the species, which are already extremely
different in character, will generally tend to produce the greatest
number of modified descendants; for these will have the best chance of
filling new and widely different places in the polity of nature: hence in
the diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise to
new varieties and species. The other nine species (marked by capital
letters) of our original genus, may for a long period continue to
transmit unaltered descendants; and this is shown in the diagram by the
dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played an
important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate
in each stage of descent their predecessors and their original parent.
For it should be remembered that the competition will generally be most
severe between those forms which are most nearly related to each other in
habits, constitution, and structure. Hence all the intermediate forms
between the earlier and later states, that is between the less and more
improved state of a species, as well as the original parent-species
itself, will generally tend to become extinct. So it probably will be
with many whole collateral lines of descent, which will be conquered by
later and improved lines of descent. If, however, the [122]modified
offspring of a species get into some distinct country, or become quickly
adapted to some quite new station, in which child and parent do not come
into competition, both may continue to exist.
If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a14
to m14); and (I) will have been replaced by six
(n14 to z14) new species.
But we may go further than this. The original species of our genus
were supposed to resemble each other in unequal degrees, as is so
generally the case in nature; species (A) being more nearly related to B,
C, and D, than to the other species; and species (I) more to G, H, K, L,
than to the others. These two species (A) and (I), were also supposed to
be very common and widely diffused species, so that they must originally
have had some advantage over most of the other species of the genus.
Their modified descendants, fourteen in number at the fourteen-thousandth
generation, will probably have inherited some of the same advantages:
they have also been modified and improved in a diversified manner at each
stage of descent, so as to have become adapted to many related places in
the natural economy of their country. It seems, therefore, to me
extremely probable that they will have taken the places of, and thus
exterminated, not only their parents (A) and (I), but likewise some of
the original species which were most nearly related to their parents.
Hence very few of the original species will have transmitted offspring to
the fourteen-thousandth generation. We may suppose that only one (F), of
the two species which were least closely related to the other nine
original species, has transmitted descendants to this late stage of
descent. [123]
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species a14 and z14 will be
much greater than that between the most different of the original eleven
species. The new species, moreover, will be allied to each other in a
widely different manner. Of the eight descendants from (A) the three
marked a14, q14,
p14, will be nearly related from having recently
branched off from a10; b14 and
f14, from having diverged at an earlier period from
a5, will be in some degree distinct from the three
first-named species; and lastly, o14,
e14 and m14, will be nearly related
one to the other, but from having diverged at the first commencement of
the process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six descendants
from (I) will, owing to inheritance alone, differ considerably from the
eight descendants from (A); the two groups, moreover, are supposed to
have gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct,
and have left no descendants. Hence the six new species descended from
(I), and the eight descended from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same genus.
And the two or [124]more parent-species are supposed to have
descended from some one species of an earlier genus. In our diagram, this
is indicated by the broken lines, beneath the capital letters, converging
in sub-branches downwards towards a single point; this point representing
a single species, the supposed single parent of our several new
sub-genera and genera.
It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not
to have diverged much in character, but to have retained the form of (F),
either unaltered or altered only in a slight degree. In this case, its
affinities to the other fourteen new species will be of a curious and
circuitous nature. Having descended from a form which stood between the
two parent-species (A) and (I), now supposed to be extinct and unknown,
it will be in some degree intermediate in character between the two
groups descended from these species. But as these two groups have gone on
diverging in character from the type of their parents, the new species
(F14) will not be directly
intermediate between them, but rather between types of the two groups;
and every naturalist will be able to bring some such case before his
mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when we
come to our chapter on Geology, have to refer again to this subject, and
I think we shall then see that the diagram throws light on the affinities
of extinct beings, which, though generally belonging to the same orders,
or families, or genera, with those now living, yet are often, in some
degree, intermediate in character between existing groups; and we can
understand this fact, for [125]the extinct species lived at very ancient
epochs when the branching lines of descent had diverged less.
I see no reason to limit the process of modification, as now
explained, to the formation of genera alone. If, in our diagram, we
suppose the amount of change represented by each successive group of
diverging dotted lines to be very great, the forms marked
a14 to p14, those marked
b14 and f14, and those marked
o14 to m14, will form three very
distinct genera. We shall also have two very distinct genera descended
from (I); and as these latter two genera, both from continued divergence
of character and from inheritance from a different parent, will differ
widely from the three genera descended from (A), the two little groups of
genera will form two distinct families, or even orders, according to the
amount of divergent modification supposed to be represented in the
diagram. And the two new families, or orders, will have descended from
two species of the original genus; and these two species are supposed to
have descended from one species of a still more ancient and unknown
genus.
We have seen that in each country it is the species of the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts through
one form having some advantage over other forms in the struggle for
existence, it will chiefly act on those which already have some
advantage; and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common. Hence, the
struggle for the production of new and modified descendants, will mainly
lie between the larger groups, which are all trying to increase in
number. One large group will slowly conquer another large group, reduce
its numbers, and thus lessen its chance of further variation and
improvement. Within the same large [126]group, the later and
more highly perfected sub-groups, from branching out and seizing on many
new places in the polity of Nature, will constantly tend to supplant and
destroy the earlier and less improved sub-groups. Small and broken groups
and sub-groups will finally disappear. Looking to the future, we can
predict that the groups of organic beings which are now large and
triumphant, and which are least broken up, that is, which as yet have
suffered least extinction, will for a long period continue to increase.
But which groups will ultimately prevail, no man can predict; for we well
know that many groups, formerly most extensively developed, have now
become extinct. Looking still more remotely to the future, we may predict
that, owing to the continued and steady increase of the larger groups, a
multitude of smaller groups will become utterly extinct, and leave no
modified descendants; and consequently that of the species living at any
one period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on Classification,
but I may add that on this view of extremely few of the more ancient
species having transmitted descendants, and on the view of all the
descendants of the same species making a class, we can understand how it
is that there exist but very few classes in each main division of the
animal and vegetable kingdoms. Although extremely few of the most ancient
species may now have living and modified descendants, yet at the most
remote geological period, the earth may have been as well peopled with
many species of many genera, families, orders, and classes, as at the
present day.
Summary of Chapter.—If during the long course of ages and
under varying conditions of life, organic beings [127]vary at all in the
several parts of their organisation, and I think this cannot be disputed;
if there be, owing to the high geometrical ratio of increase of each
species, a severe struggle for life at some age, season, or year, and
this certainly cannot be disputed; then, considering the infinite
complexity of the relations of all organic beings to each other and to
their conditions of existence, causing an infinite diversity in
structure, constitution, and habits, to be advantageous to them, I think
it would be a most extraordinary fact if no variation ever had occurred
useful to each being's own welfare, in the same manner as so many
variations have occurred useful to man. But if variations useful to any
organic being do occur, assuredly individuals thus characterised will
have the best chance of being preserved in the struggle for life; and
from the strong principle of inheritance they will tend to produce
offspring similarly characterised. This principle of preservation, I have
called, for the sake of brevity, Natural Selection; and it leads to the
improvement of each creature in relation to its organic and inorganic
conditions of life.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as the
adult. Amongst many animals, sexual selection will give its aid to
ordinary selection, by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other
males.
Whether natural selection has really thus acted in nature, in
modifying and adapting the various forms of life to their several
conditions and stations, must be judged of by the general tenour and
balance of evidence given in the following chapters. But we already see
how it entails extinction; and how largely extinction [128]has acted in
the world's history, geology plainly declares. Natural selection, also,
leads to divergence of character; for more living beings can be supported
on the same area the more they diverge in structure, habits, and
constitution, of which we see proof by looking to the inhabitants of any
small spot or to naturalised productions. Therefore during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their chance of
succeeding in the battle for life. Thus the small differences
distinguishing varieties of the same species, steadily tend to increase
till they come to equal the greater differences between species of the
same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary most;
and these tend to transmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection,
as has just been remarked, leads to divergence of character and to much
extinction of the less improved and intermediate forms of life. On these
principles, I believe, the nature of the affinities of all organic beings
may be explained. It is a truly wonderful fact—the wonder of which
we are apt to overlook from familiarity—that all animals and all
plants throughout all time and space should be related to each other in
group subordinate to group, in the manner which we everywhere
behold—namely, varieties of the same species most closely related
together, species of the same genus less closely and unequally related
together, forming sections and sub-genera, species of distinct genera
much less closely related, and genera related in different degrees,
forming [129]sub-families, families, orders,
sub-classes, and classes. The several subordinate groups in any class
cannot be ranked in a single file, but seem rather to be clustered round
points, and these round other points, and so on in almost endless cycles.
On the view that each species has been independently created, I can see
no explanation of this great fact in the classification of all organic
beings; but, to the best of my judgment, it is explained through
inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in
the diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long succession
of extinct species. At each period of growth all the growing twigs have
tried to branch out on all sides, and to overtop and kill the surrounding
twigs and branches, in the same manner as species and groups of species
have tried to overmaster other species in the great battle for life. The
limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was small, budding twigs;
and this connexion of the former and present buds by ramifying branches
may well represent the classification of all extinct and living species
in groups subordinate to groups. Of the many twigs which flourished when
the tree was a mere bush, only two or three, now grown into great
branches, yet survive and bear all the other branches; so with the
species which lived during long-past geological periods, very few now
have living and modified descendants. From the first growth of the tree,
many a limb and branch has decayed and dropped off; and these lost
branches of various [130]sizes may represent those whole orders,
families, and genera which have now no living representatives, and which
are known to us only from having been found in a fossil state. As we here
and there see a thin straggling branch springing from a fork low down in
a tree, and which by some chance has been favoured and is still alive on
its summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise by
growth to fresh buds, and these, if vigorous, branch out and overtop on
all sides many a feebler branch, so by generation I believe it has been
with the great Tree of Life, which fills with its dead and broken
branches the crust of the earth, and covers the surface with its ever
branching and beautiful ramifications.
[131]
CHAPTER V.
Laws of Variation.
Effects of external conditions—Use and disuse, combined with
natural selection; organs of flight and of
vision—Acclimatisation—Correlation of
growth—Compensation and economy of growth—False
correlations—Multiple, rudimentary, and lowly organised structures
variable—Parts developed in an unusual manner are highly variable:
specific characters more variable than generic: secondary sexual
characters variable—Species of the same genus vary in an analogous
manner—Reversions to long-lost characters—Summary.
I have hitherto sometimes spoken as if the variations—so common
and multiform in organic beings under domestication, and in a lesser
degree in those in a state of nature—had been due to chance. This,
of course, is a wholly incorrect expression, but it serves to acknowledge
plainly our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive system
to produce individual differences, or very slight deviations of
structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe that
deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote ancestors
have been exposed during several generations. I have remarked in the
first chapter—but a long catalogue of facts which cannot be here
given would be necessary to show the truth of the remark—that the
reproductive system is eminently susceptible to changes in the conditions
of life; and to [132]this system being functionally disturbed
in the parents, I chiefly attribute the varying or plastic condition of
the offspring. The male and female sexual elements seem to be affected
before that union takes place which is to form a new being. In the case
of "sporting" plants, the bud, which in its earliest condition does not
apparently differ essentially from an ovule, is alone affected. But why,
because the reproductive system is disturbed, this or that part should
vary more or less, we are profoundly ignorant. Nevertheless, we can here
and there dimly catch a faint ray of light, and we may feel sure that
there must be some cause for each deviation of structure, however
slight.
How much direct effect difference of climate, food, &c., produces
on any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that
of plants. We may, at least, safely conclude that such influences cannot
have produced the many striking and complex co-adaptations of structure
between one organic being and another, which we see everywhere throughout
nature. Some little influence may be attributed to climate, food,
&c.: thus, E. Forbes speaks confidently that shells at their southern
limit, and when living in shallow water, are more brightly coloured than
those of the same species further north or from greater depths. Gould
believes that birds of the same species are more brightly coloured under
a clear atmosphere, than when living on islands or near the coast. So
with insects, Wollaston is convinced that residence near the sea affects
their colours. Moquin-Tandon gives a list of plants which when growing
near the sea-shore have their leaves in some degree fleshy, though not
elsewhere fleshy. Several other such cases could be given.
The fact of varieties of one species, when they range [133]into the zone
of habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties. Thus
the species of shells which are confined to tropical and shallow seas are
generally brighter-coloured than those confined to cold and deeper seas.
The birds which are confined to continents are, according to Mr. Gould,
brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants
which live exclusively on the sea-side are very apt to have fleshy
leaves. He who believes in the creation of each species, will have to say
that this shell, for instance, was created with bright colours for a warm
sea; but that this other shell became bright-coloured by variation when
it ranged into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how much of it to
attribute to the accumulative action of natural selection, and how much
to the conditions of life. Thus, it is well known to furriers that
animals of the same species have thicker and better fur the more severe
the climate is under which they have lived; but who can tell how much of
this difference may be due to the warmest-clad individuals having been
favoured and preserved during many generations, and how much to the
direct action of the severe climate? for it would appear that climate has
some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly [134]the conditions
of life act. Again, innumerable instances are known to every naturalist
of species keeping true, or not varying at all, although living under the
most opposite climates. Such considerations as these incline me to lay
very little weight on the direct action of the conditions of life.
Indirectly, as already remarked, they seem to play an important part in
affecting the reproductive system, and in thus inducing variability; and
natural selection will then accumulate all profitable variations, however
slight, until they become plainly developed and appreciable by us.
Effects of Use and Disuse.—From the facts alluded to in
the first chapter, I think there can be little doubt that use in our
domestic animals strengthens and enlarges certain parts, and disuse
diminishes them; and that such modifications are inherited. Under free
nature, we can have no standard of comparison, by which to judge of the
effects of long-continued use or disuse, for we know not the
parent-forms; but many animals have structures which can be explained by
the effects of disuse. As Professor Owen has remarked, there is no
greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck. As the larger ground-feeding
birds seldom take flight except to escape danger, I believe that the
nearly wingless condition of several birds, which now inhabit or have
lately inhabited several oceanic islands, tenanted by no beast of prey,
has been caused by disuse. The ostrich indeed inhabits continents and is
exposed to danger from which it cannot escape by flight, but by kicking
it can defend itself from enemies, as well as any of the smaller [135]quadrupeds. We may imagine that the early
progenitor of the ostrich had habits like those of a bustard, and that as
natural selection increased in successive generations the size and weight
of its body, its legs were used more, and its wings less, until they
became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very often
broken off; he examined seventeen specimens in his own collection, and
not one had even a relic left. In the Onites apelles the tarsi are so
habitually lost, that the insect has been described as not having them.
In some other genera they are present, but in a rudimentary condition. In
the Ateuchus or sacred beetle of the Egyptians, they are totally
deficient. There is not sufficient evidence to induce me to believe that
mutilations are ever inherited; and I should prefer explaining the entire
absence of the anterior tarsi in Ateuchus, and their rudimentary
condition in some other genera, by the long-continued effects of disuse
in their progenitors; for as the tarsi are almost always lost in many
dung-feeding beetles, they must be lost early in life, and therefore
cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of the
550 species inhabiting Madeira, are so far deficient in wings that they
cannot fly; and that of the twenty-nine endemic genera, no less than
twenty-three genera have all their species in this condition! Several
facts, namely, that beetles in many parts of the world are frequently
blown to sea and perish; that the beetles in Madeira, as observed by Mr.
Wollaston, lie much concealed, [136]until the wind lulls and the sun shines;
that the proportion of wingless beetles is larger on the exposed Desertas
than in Madeira itself; and especially the extraordinary fact, so
strongly insisted on by Mr. Wollaston, of the almost entire absence of
certain large groups of beetles, elsewhere excessively numerous, and
which groups have habits of life almost necessitating frequent
flight;—these several considerations have made me believe that the
wingless condition of so many Madeira beetles is mainly due to the action
of natural selection, but combined probably with disuse. For during
thousands of successive generations each individual beetle which flew
least, either from its wings having been ever so little less perfectly
developed or from indolent habit, will have had the best chance of
surviving from not being blown out to sea; and, on the other hand, those
beetles which most readily took to flight would oftenest have been blown
to sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects, their
wings not at all reduced, but even enlarged. This is quite compatible
with the action of natural selection. For when a new insect first arrived
on the island, the tendency of natural selection to enlarge or to reduce
the wings, would depend on whether a greater number of individuals were
saved by successfully battling with the winds, or by giving up the
attempt and rarely or never flying. As with mariners shipwrecked near a
coast, it would have been better for the good swimmers if they had been
able to swim still further, whereas it would have been better for the bad
swimmers if they had not been able to swim at all and had stuck to the
wreck. [137]
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This state
of the eyes is probably due to gradual reduction from disuse, but aided
perhaps by natural selection. In South America, a burrowing rodent, the
tuco-tuco, or Ctenomys, is even more subterranean in its habits than the
mole; and I was assured by a Spaniard, who had often caught them, that
they were frequently blind; one which I kept alive was certainly in this
condition, the cause, as appeared on dissection, having been inflammation
of the nictitating membrane. As frequent inflammation of the eyes must be
injurious to any animal, and as eyes are certainly not indispensable to
animals with subterranean habits, a reduction in their size with the
adhesion of the eyelids and growth of fur over them, might in such case
be an advantage; and if so, natural selection would constantly aid the
effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though
useless, could be in any way injurious to animals living in darkness, I
attribute their loss wholly to disuse. In one of the blind animals,
namely, the cave-rat, the eyes are of immense size; and Professor
Silliman thought that it regained, after living some days in the light,
some slight power of vision. In the same manner as in Madeira the wings
of some of the insects have been enlarged, and the wings of others have
been reduced by natural selection aided by use and disuse, so in the case
of the cave-rat natural selection seems to have struggled with the loss
of light and [138]to have increased the size of the eyes;
whereas with all the other inhabitants of the caves, disuse by itself
seems to have done its work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common
view of the blind animals having been separately created for the American
and European caverns, close similarity in their organisation and
affinities might have been expected; but, as Schiödte and others have
remarked, this is not the case, and the cave-insects of the two
continents are not more closely allied than might have been anticipated
from the general resemblance of the other inhabitants of North America
and Europe. On my view we must suppose that American animals, having
ordinary powers of vision, slowly migrated by successive generations from
the outer world into the deeper and deeper recesses of the Kentucky
caves, as did European animals into the caves of Europe. We have some
evidence of this gradation of habit; for, as Schiödte remarks, "animals
not far remote from ordinary forms, prepare the transition from light to
darkness. Next follow those that are constructed for twilight; and, last
of all, those destined for total darkness." By the time that an animal
had reached, after numberless generations, the deepest recesses, disuse
will on this view have more or less perfectly obliterated its eyes, and
natural selection will often have effected other changes, such as an
increase in the length of the antennæ or palpi, as a compensation for
blindness. Notwithstanding such modifications, we might expect still to
see in the cave-animals of America, affinities to the other inhabitants
of that continent, and in those of Europe, to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from [139]Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding country.
It would be most difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabitants of the two
continents on the ordinary view of their independent creation. That
several of the inhabitants of the caves of the Old and New Worlds should
be closely related, we might expect from the well-known relationship of
most of their other productions. Far from feeling any surprise that some
of the cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved, owing
to the less severe competition to which the inhabitants of these dark
abodes will probably have been exposed.
Acclimatisation.—Habit is hereditary with plants, as in
the period of flowering, in the amount of rain requisite for seeds to
germinate, in the time of sleep, &c., and this leads me to say a few
words on acclimatisation. As it is extremely common for species of the
same genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single
parent, if this view be correct, acclimatisation must be readily effected
during long-continued descent. It is notorious that each species is
adapted to the climate of its own home: species from an arctic or even
from a temperate region cannot endure a tropical climate, or conversely.
So again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they live is
often overrated. [140]We may infer this from our frequent
inability to predict whether or not an imported plant will endure our
climate, and from the number of plants and animals brought from warmer
countries which here enjoy good health. We have reason to believe that
species in a state of nature are limited in their ranges by the
competition of other organic beings quite as much as, or more than, by
adaptation to particular climates. But whether or not the adaptation be
generally very close, we have evidence, in the case of some few plants,
of their becoming, to a certain extent, naturally habituated to different
temperatures, or becoming acclimatised: thus the pines and rhododendrons,
raised from seed collected by Dr. Hooker from trees growing at different
heights on the Himalaya, were found in this country to possess different
constitutional powers of resisting cold. Mr. Thwaites informs me that he
has observed similar facts in Ceylon, and analogous observations have
been made by Mr. H. C. Watson on European species of plants brought from
the Azores to England. In regard to animals, several authentic cases
could be given of species within historical times having largely extended
their range from warmer to cooler latitudes, and conversely; but we do
not positively know that these animals were strictly adapted to their
native climate, but in all ordinary cases we assume such to be the case;
nor do we know that they have subsequently become acclimatised to their
new homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly [141]fertile (a far severer
test) under them, may be used as an argument that a large proportion of
other animals, now in a state of nature, could easily be brought to bear
widely different climates. We must not, however, push the foregoing
argument too far, on account of the probable origin of some of our
domestic animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf or wild dog may perhaps be mingled in our
domestic breeds. The rat and mouse cannot be considered as domestic
animals, but they have been transported by man to many parts of the
world, and now have a far wider range than any other rodent, living free
under the cold climate of Faroe in the north and of the Falklands in the
south, and on many islands in the torrid zones. Hence I am inclined to
look at adaptation to any special climate as a quality readily grafted on
an innate wide flexibility of constitution, which is common to most
animals. On this view, the capacity of enduring the most different
climates by man himself and by his domestic animals, and such facts as
that former species of the elephant and rhinoceros were capable of
enduring a glacial climate, whereas the living species are now all
tropical or sub-tropical in their habits, ought not to be looked at as
anomalies, but merely as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopædias of
China, to be very [142]cautious in transposing animals from one
district to another; for it is not likely that man should have succeeded
in selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts: the result must, I think, be due to
habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which are
born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are said
to withstand certain climates better than others: this is very strikingly
shown in works on fruit trees published in the United States, in which
certain varieties are habitually recommended for the northern, and others
for the southern States; and as most of these varieties are of recent
origin, they cannot owe their constitutional differences to habit. The
case of the Jerusalem artichoke, which is never propagated by seed, and
of which consequently new varieties have not been produced, has even been
advanced—for it is now as tender as ever it was—as proving
that acclimatisation cannot be effected! The case, also, of the
kidney-bean has been often cited for a similar purpose, and with much
greater weight; but until some one will sow, during a score of
generations, his kidney-beans so early that a very large proportion are
destroyed by frost, and then collect seed from the few survivors, with
care to prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.
On the whole, I think we may conclude that habit, [143]use, and
disuse, have, in some cases, played a considerable part in the
modification of the constitution, and of the structure of various organs;
but that the effects of use and disuse have often been largely combined
with, and sometimes overmastered by the natural selection of innate
variations.
Correlation of Growth.—I mean by this expression that the
whole organisation is so tied together during its growth and development,
that when slight variations in any one part occur, and are accumulated
through natural selection, other parts become modified. This is a very
important subject, most imperfectly understood. The most obvious case is,
that modifications accumulated solely for the good of the young or larva,
will, it may safely be concluded, affect the structure of the adult; in
the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several parts
of the body which are homologous, and which, at an early embryonic
period, are alike, seem liable to vary in an allied manner: we see this
in the right and left sides of the body varying in the same manner; in
the front and hind legs, and even in the jaws and limbs, varying
together, for the lower jaw is believed to be homologous with the limbs.
These tendencies, I do not doubt, may be mastered more or less completely
by natural selection: thus a family of stags once existed with an antler
only on one side; and if this had been of any great use to the breed it
might probably have been rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants; and nothing is more
common than the union of homologous parts in normal structures, as the
union of [144]the petals of the corolla into a tube.
Hard parts seem to affect the form of adjoining soft parts; it is
believed by some authors that the diversity in the shape of the pelvis in
birds causes the remarkable diversity in the shape of their kidneys.
Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes,
according to Schlegel, the shape of the body and the manner of swallowing
determine the position of several of the most important viscera.
The nature of the bond of correlation is very frequently quite
obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist, without
our being able to assign any reason. What can be more singular than the
relation between blue eyes and deafness in cats, and the tortoise-shell
colour with the female sex; the feathered feet and skin between the outer
toes in pigeons, and the presence of more or less down on the young birds
when first hatched, with the future colour of their plumage; or, again,
the relation between the hair and teeth in the naked Turkish dog, though
here probably homology comes into play? With respect to this latter case
of correlation, I think it can hardly be accidental, that if we pick out
the two orders of mammalia which are most abnormal in their dermal
covering, viz. Cetacea (whales) and Edentata (armadilloes, scaly
anteaters, &c.), that these are likewise the most abnormal in their
teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility
and, therefore, of natural selection, than that of the difference between
the outer and inner flowers in some Compositous and Umbelliferous plants.
Every one knows the [145]difference in the ray and central florets
of, for instance, the daisy, and this difference is often accompanied
with the abortion of parts of the flower. But, in some Compositous
plants, the seeds also differ in shape and sculpture; and even the ovary
itself, with its accessory parts, differs, as has been described by
Cassini. These differences have been attributed by some authors to
pressure, and the shape of the seeds in the ray-florets in some Compositæ
countenances this idea; but, in the case of the corolla of the
Umbelliferæ, it is by no means, as Dr. Hooker informs me, in species with
the densest heads that the inner and outer flowers most frequently
differ. It might have been thought that the development of the ray-petals
by drawing nourishment from certain other parts of the flower had caused
their abortion; but in some Compositæ there is a difference in the seeds
of the outer and inner florets without any difference in the corolla.
Possibly, these several differences may be connected with some difference
in the flow of nutriment towards the central and external flowers: we
know, at least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an
instance of this, and of a striking case of correlation, that I have
recently observed in some garden pelargoniums, that the central flower of
the truss often loses the patches of darker colour in the two upper
petals; and that when this occurs, the adherent nectary is quite aborted;
when the colour is absent from only one of the two upper petals, the
nectary is only much shortened.
With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C. C.
Sprengel's idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of [146]these two
orders, is so far-fetched, as it may at first appear: and if it be
advantageous, natural selection may have come into play. But in regard to
the differences both in the internal and external structure of the seeds,
which are not always correlated with any differences in the flowers, it
seems impossible that they can be in any way advantageous to the plant:
yet in the Umbelliferæ these differences are of such apparent
importance—the seeds being in some cases, according to Tausch,
orthospermous in the exterior flowers and cœlospermous in the
central flowers,—that the elder De Candolle founded his main
divisions of the order on analogous differences. Hence we see that
modifications of structure, viewed by systematists as of high value, may
be wholly due to unknown laws of correlated growth, and without being, as
far as we can see, of the slightest service to the species.
We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and
these two modifications, having been transmitted to a whole group of
descendants with diverse habits, would naturally be thought to be
correlated in some necessary manner. So, again, I do not doubt that some
apparent correlations, occurring throughout whole orders, are entirely
due to the manner alone in which natural selection can act. For instance,
Alph. De Candolle has remarked that winged seeds are never found in
fruits which do not open: I should explain the rule by the fact that
seeds could not gradually become winged through natural selection, except
in fruits which opened; so that the individual plants producing [147]seeds
which were a little better fitted to be wafted further, might get an
advantage over those producing seed less fitted for dispersal; and this
process could not possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, "in order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain extent
with our domestic productions: if nourishment flows to one part or organ
in excess, it rarely flows, at least in excess, to another part; thus it
is difficult to get a cow to give much milk and to fatten readily. The
same varieties of the cabbage do not yield abundant and nutritious
foliage and a copious supply of oil-bearing seeds. When the seeds in our
fruits become atrophied, the fruit itself gains largely in size and
quality. In our poultry, a large tuft of feathers on the head is
generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying
to economise in every part of the organisation. If under [148]changed
conditions of life a structure before useful becomes less useful, any
diminution, however slight, in its development, will be seized on by
natural selection, for it will profit the individual not to have its
nutriment wasted in building up an useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes,
and of which many other instances could be given: namely, that when a
cirripede is parasitic within another and is thus protected, it loses
more or less completely its own shell or carapace. This is the case with
the male Ibla, and in a truly extraordinary manner with the Proteolepas:
for the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously developed, and
furnished with great nerves and muscles; but in the parasitic and
protected Proteolepas, the whole anterior part of the head is reduced to
the merest rudiment attached to the bases of the prehensile antennæ. Now
the saving of a large and complex structure, when rendered superfluous by
the parasitic habits of the Proteolepas, though effected by slow steps,
would be a decided advantage to each successive individual of the
species; for in the struggle for life to which every animal is exposed,
each individual Proteolepas would have a better chance of supporting
itself, by less nutriment being wasted in developing a structure now
become useless.
Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as it
is rendered superfluous, without by any means causing some other part to
be largely developed in a corresponding degree. And, conversely, that
natural selection may perfectly well succeed in largely developing any
organ, without requiring as a necessary compensation the reduction of
some adjoining part. [149]
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
in varieties and in species, that when any part or organ is repeated many
times in the structure of the same individual (as the vertebræ in snakes,
and the stamens in polyandrous flowers) the number is variable; whereas
the number of the same part or organ, when it occurs in lesser numbers,
is constant. The same author and some botanists have further remarked
that multiple parts are also very liable to variation in structure.
Inasmuch as this "vegetative repetition," to use Prof. Owen's expression,
seems to be a sign of low organisation, the foregoing remark seems
connected with the very general opinion of naturalists, that beings low
in the scale of nature are more variable than those which are higher. I
presume that lowness in this case means that the several parts of the
organisation have been but little specialised for particular functions;
and as long as the same part has to perform diversified work, we can
perhaps see why it should remain variable, that is, why natural selection
should have preserved or rejected each little deviation of form less
carefully than when the part has to serve for one special purpose alone.
In the same way that a knife which has to cut all sorts of things may be
of almost any shape; whilst a tool for some particular object had better
be of some particular shape. Natural selection, it should never be
forgotten, can act on each part of each being, solely through and for its
advantage.
Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to the
general subject of rudimentary and aborted organs; and I will here only
add that their variability seems to be owing to their uselessness, and
therefore to natural selection having no power to check deviations in
their structure. Thus [150]rudimentary parts are left to the free
play of the various laws of growth, to the effects of long-continued
disuse, and to the tendency to reversion.
A part developed in any species in an extraordinary degree or
manner, in comparison with the same part in allied species, tends to be
highly variable.—Several years ago I was much struck with a
remark, nearly to the above effect, published by Mr. Waterhouse. I infer
also from an observation made by Professor Owen, with respect to the
length of the arms of the ourang-outang, that he has come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of this proposition without giving the long array of facts which I
have collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowance for
them. It should be understood that the rule by no means applies to any
part, however unusually developed, unless it be unusually developed in
comparison with the same part in closely allied species. Thus, the bat's
wing is a most abnormal structure in the class mammalia; but the rule
would not here apply, because there is a whole group of bats having
wings; it would apply only if some one species of bat had its wings
developed in some remarkable manner in comparison with the other species
of the same genus. The rule applies very strongly in the case of
secondary sexual characters, when displayed in any unusual manner. The
term, secondary sexual characters, used by Hunter, applies to characters
which are attached to one sex, but are not directly connected with the
act of reproduction. The rule applies to males and females; but as
females more rarely offer remarkable secondary sexual characters, it
applies [151]more rarely to them. The rule being so
plainly applicable in the case of secondary sexual characters, may be due
to the great variability of these characters, whether or not displayed in
any unusual manner—of which fact I think there can be little doubt.
But that our rule is not confined to secondary sexual characters is
clearly shown in the case of hermaphrodite cirripedes; and I may here
add, that I particularly attended to Mr. Waterhouse's remark, whilst
investigating this Order, and I am fully convinced that the rule almost
invariably holds good with cirripedes. I shall, in my future work, give a
list of the more remarkable cases; I will here only briefly give one, as
it illustrates the rule in its largest application. The opercular valves
of sessile cirripedes (rock barnacles) are, in every sense of the word,
very important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma, these
valves present a marvellous amount of diversification: the homologous
valves in the different species being sometimes wholly unlike in shape;
and the amount of variation in the individuals of several of the species
is so great, that it is no exaggeration to state that the varieties
differ more from each other in the characters of these important valves
than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly to
hold good in this class. I cannot make out that it applies to plants, and
this would seriously have shaken my belief in its truth, had not the
great variability in plants made it particularly difficult to compare
their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or
manner in any species, the fair [152]presumption is that it is of high
importance to that species; nevertheless the part in this case is
eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as we now
see them, I can see no explanation. But on the view that groups of
species have descended from other species, and have been modified through
natural selection, I think we can obtain some light. In our domestic
animals, if any part, or the whole animal, be neglected and no selection
be applied, that part (for instance, the comb in the Dorking fowl) or the
whole breed will cease to have a nearly uniform character. The breed will
then be said to have degenerated. In rudimentary organs, and in those
which have been but little specialised for any particular purpose, and
perhaps in polymorphic groups, we see a nearly parallel natural case; for
in such cases natural selection either has not or cannot come into full
play, and thus the organisation is left in a fluctuating condition. But
what here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at the
breeds of the pigeon; see what a prodigious amount of difference there is
in the beak of the different tumblers, in the beak and wattle of the
different carriers, in the carriage and tail of our fantails, &c.,
these being the points now mainly attended to by English fanciers. Even
in the sub-breeds, as in the short-faced tumbler, it is notoriously
difficult to breed them nearly to perfection, and frequently individuals
are born which depart widely from the standard. There may be truly said
to be a constant struggle going on between, on the one hand, the tendency
to reversion to a less modified state, as well as an innate tendency to
further [153]variability of all kinds, and, on the
other hand, the power of steady selection to keep the breed true. In the
long run selection gains the day, and we do not expect to fail so far as
to breed a bird as coarse as a common tumbler from a good short-faced
strain. But as long as selection is rapidly going on, there may always be
expected to be much variability in the structure undergoing modification.
It further deserves notice that these variable characters, produced by
man's selection, sometimes become attached, from causes quite unknown to
us, more to one sex than to the other, generally to the male sex, as with
the wattle of carriers and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the species
branched off from the common progenitor of the genus. This period will
seldom be remote in any extreme degree, as species very rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability,
which has continually been accumulated by natural selection for the
benefit of the species. But as the variability of the
extraordinarily-developed part or organ has been so great and
long-continued within a period not excessively remote, we might, as a
general rule, expect still to find more variability in such parts than in
other parts of the organisation which have remained for a much longer
period nearly constant. And this, I am convinced, is the case. That the
struggle between natural selection on the one hand, and the tendency to
reversion and variability on the other hand, will in the [154]course of time
cease; and that the most abnormally developed organs may be made
constant, I can see no reason to doubt. Hence when an organ, however
abnormal it may be, has been transmitted in approximately the same
condition to many modified descendants, as in the case of the wing of the
bat, it must have existed, according to my theory, for an immense period
in nearly the same state; and thus it comes to be no more variable than
any other structure. It is only in those cases in which the modification
has been comparatively recent and extraordinarily great that we ought to
find the generative variability, as it may be called, still
present in a high degree. For in this case the variability will seldom as
yet have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of
those tending to revert to a former and less modified condition.
The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic. To
explain by a simple example what is meant. If some species in a large
genus of plants had blue flowers and some had red, the colour would be
only a specific character, and no one would be surprised at one of the
blue species varying into red, or conversely; but if all the species had
blue flowers, the colour would become a generic character, and its
variation would be a more unusual circumstance. I have chosen this
example because an explanation is not in this case applicable, which most
naturalists would advance, namely, that specific characters are more
variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera. I
believe this explanation is partly, yet only indirectly, true; I shall,
however, have to [155]return to this subject in our chapter on
Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more
variable than generic; but I have repeatedly noticed in works on natural
history, that when an author has remarked with surprise that some
important organ or part, which is generally very constant
throughout large groups of species, has differed considerably in
closely-allied species, that it has, also, been variable in the
individuals of some of the species. And this fact shows that a character,
which is generally of generic value, when it sinks in value and becomes
only of specific value, often becomes variable, though its physiological
importance may remain the same. Something of the same kind applies to
monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no
doubt, that the more an organ normally differs in the different species
of the same group, the more subject it is to individual anomalies.
On the ordinary view of each species having been independently
created, why should that part of the structure, which differs from the
same part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the view
of species being only strongly marked and fixed varieties, we might
surely expect to find them still often continuing to vary in those parts
of their structure which have varied within a moderately recent period,
and which have thus come to differ. Or to state the case in another
manner:—the points in which all the species of a genus resemble
each other, and in which they differ from the species of some other
genus, are called generic characters; and these characters in common I
attribute to [156]inheritance from a common progenitor, for
it can rarely have happened that natural selection will have modified
several species, fitted to more or less widely-different habits, in
exactly the same manner: and as these so-called generic characters have
been inherited from a remote period, since that period when the species
first branched off from their common progenitor, and subsequently have
not varied or come to differ in any degree, or only in a slight degree,
it is not probable that they should vary at the present day. On the other
hand, the points in which species differ from other species of the same
genus, are called specific characters; and as these specific characters
have varied and come to differ within the period of the branching off of
the species from a common progenitor, it is probable that they should
still often be in some degree variable,—at least more variable than
those parts of the organisation which have for a very long period
remained constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also will
be admitted that species of the same group differ from each other more
widely in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the
males of gallinaceous birds, in which secondary sexual characters are
strongly displayed, with the amount of difference between their females;
and the truth of this proposition will be granted. The cause of the
original variability of secondary sexual characters is not manifest; but
we can see why these characters should not have been rendered as constant
and uniform as other parts of the organisation; for secondary sexual
characters have been accumulated by sexual selection, which [157]is less rigid
in its action than ordinary selection, as it does not entail death, but
only gives fewer offspring to the less favoured males. Whatever the cause
may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus readily have succeeded in giving to the species of the same
group a greater amount of difference in their sexual characters, than in
other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the same
genus differ from each other. Of this fact I will give in illustration
two instances, the first which happen to stand on my list; and as the
differences in these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the tarsi is a
character generally common to very large groups of beetles, but in the
Engidæ, as Westwood has remarked, the number varies greatly; and the
number likewise differs in the two sexes of the same species: again in
fossorial hymenoptera, the manner of neuration of the wings is a
character of the highest importance, because common to large groups; but
in certain genera the neuration differs in the different species, and
likewise in the two sexes of the same species. This relation has a clear
meaning on my view of the subject: I look at all the species of the same
genus as having as certainly descended from the same progenitor, as have
the two sexes of any one of the species. Consequently, whatever part of
the structure of the common progenitor, or of its early descendants,
became variable; variations of this part would, it is highly probable, be
taken advantage of by natural and sexual selection, in order to fit [158]the
several species to their several places in the economy of nature, and
likewise to fit the two sexes of the same species to each other, or to
fit the males and females to different habits of life, or the males to
struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in
common;—that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with the
same part in its congeners; and the slight degree of variability in a
part, however extraordinarily it may be developed, if it be common to a
whole group of species;—that the great variability of secondary
sexual characters, and the great amount of difference in these same
characters between closely allied species;—that secondary sexual
and ordinary specific differences are generally displayed in the same
parts of the organisation,—are all principles closely connected
together. All being mainly due to the species of the same group having
descended from a common progenitor, from whom they have inherited much in
common,—to parts which have recently and largely varied being more
likely still to go on varying than parts which have long been inherited
and have not varied,—to natural selection having more or less
completely, according to the lapse of time, overmastered the tendency to
reversion and to further variability,—to sexual selection being
less rigid than ordinary selection,—and to variations in the same
parts having been accumulated by natural and sexual selection, and having
been thus adapted for secondary sexual, and for ordinary specific
purposes. [159]
Distinct species present analogous variations; and a variety of one
species often assumes some of the characters of an allied species, or
reverts to some of the characters of an early progenitor.—These
propositions will be most readily understood by looking to our domestic
races. The most distinct breeds of pigeons, in countries most widely
apart, present sub-varieties with reversed feathers on the head and
feathers on the feet,—characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter, may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In
the vegetable kingdom we have a case of analogous variation, in the
enlarged stems, or roots as commonly called, of the Swedish turnip and
Ruta baga, plants which several botanists tank as varieties produced by
cultivation from a common parent: if this be not so, the case will then
be one of analogous variation in two so-called distinct species; and to
these a third may be added, namely, the common turnip. According to the
ordinary view of each species having been independently created, we
should have to attribute this similarity in the enlarged stems of these
three plants, not to the vera causa of community of descent, and a
consequent tendency to vary in a like manner, but to three separate yet
closely related acts of creation.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, a white [160]rump, a bar at the end of the tail, with
the outer feathers externally edged near their bases with white. As all
these marks are characteristic of the parent rock-pigeon, I presume that
no one will doubt that this is a case of reversion, and not of a new yet
analogous variation appearing in the several breeds. We may I think
confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring of
two distinct and differently coloured breeds; and in this case there is
nothing in the external conditions of life to cause the reappearance of
the slaty-blue, with the several marks, beyond the influence of the mere
act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations. But
when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations—some say, for a dozen or even a
score of generations. After twelve generations, the proportion of blood,
to use a common expression, of any one ancestor, is only 1 in 2048; and
yet, as we see, it is generally believed that a tendency to reversion is
retained by this very small proportion of foreign blood. In a breed which
has not been crossed, but in which both parents have lost some
character which their progenitor possessed, the tendency, whether strong
or weak, to reproduce the lost character might be, as was formerly
remarked, for all that we can see to the contrary, transmitted for almost
any number of generations. When a character which has been lost in a
breed, reappears after a great number of generations, the most probable
hypothesis is, not that the offspring suddenly takes after an ancestor
some hundred generations [161]distant, but that in each successive
generation there has been a tendency to reproduce the character in
question, which at last, under unknown favourable conditions, gains an
ascendancy. For instance, it is probable that in each generation of the
barb-pigeon, which produces most rarely a blue and black-barred bird,
there has been a tendency in each generation in the plumage to assume
this colour. This view is hypothetical, but could be supported by some
facts; and I can see no more abstract improbability in a tendency to
produce any character being inherited for an endless number of
generations, than in quite useless or rudimentary organs being, as we all
know them to be, thus inherited. Indeed, we may sometimes observe a mere
tendency to produce a rudiment inherited: for instance, in the common
snapdragon (Antirrhinum) a rudiment of a fifth stamen so often appears,
that this plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they would
occasionally vary in an analogous manner; so that a variety of one
species would resemble in some of its characters another species; this
other species being on my view only a well-marked and permanent variety.
But characters thus gained would probably be of an unimportant nature,
for the presence of all important characters will be governed by natural
selection, in accordance with the diverse habits of the species, and will
not be left to the mutual action of the conditions of life and of a
similar inherited constitution. It might further be expected that the
species of the same genus would occasionally exhibit reversions to lost
ancestral characters. As, however, we never know the exact character of
the common ancestor of a group, we could not distinguish these two [162]cases: if, for instance, we did not know
that the rock-pigeon was not feather-footed or turn-crowned, we could not
have told, whether these characters in our domestic breeds were
reversions or only analogous variations; but we might have inferred that
the blueness was a case of reversion, from the number of the markings,
which are correlated with the blue tint, and which it does not appear
probable would all appear together from simple variation. More especially
we might have inferred this, from the blue colour and marks so often
appearing when distinct breeds of diverse colours are crossed. Hence,
though under nature it must generally be left doubtful, what cases are
reversions to an anciently existing character, and what are new but
analogous variations, yet we ought, on my theory, sometimes to find the
varying offspring of a species assuming characters (either from reversion
or from analogous variation) which already occur in some other members of
the same group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable
species in our systematic works, is due to its varieties mocking, as it
were, some of the other species of the same genus. A considerable
catalogue, also, could be given of forms intermediate between two other
forms, which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is afforded by parts or organs of an important and
uniform nature occasionally varying so as to acquire, in some degree, the
character of the same part or organ in an allied species. I have
collected a long list of such cases; but [163]here, as before, I lie
under a great disadvantage in not being able to give them. I can only
repeat that such cases certainly do occur, and seem to me very
remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species
of the same genus, partly under domestication and partly under nature. It
is a case apparently of reversion. The ass not rarely has very distinct
transverse bars on its legs, like those on the legs of the zebra: it has
been asserted that these are plainest in the foal, and from inquiries
which I have made, I believe this to be true. It has also been asserted
that the stripe on each shoulder is sometimes double. The shoulder-stripe
is certainly very variable in length and outline. A white ass, but
not an albino, has been described without either spinal or
shoulder stripe; and these stripes are sometimes very obscure, or
actually quite lost, in dark-coloured asses. The koulan of Pallas is said
to have been seen with a double shoulder-stripe. The hemionus has no
shoulder-stripe; but traces of it, as stated by Mr. Blyth and others,
occasionally appear: and I have been informed by Colonel Poole that the
foals of this species are generally striped on the legs, and faintly on
the shoulder. The quagga, though so plainly barred like a zebra over the
body, is without bars on the legs; but Dr. Gray has figured one specimen
with very distinct zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of all
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut: a faint shoulder-stripe may sometimes
be seen in duns, and I have seen a trace in a [164]bay horse. My son made
a careful examination and sketch for me of a dun Belgian cart-horse with
a double stripe on each shoulder and with leg-stripes; and a man, whom I
can implicitly trust, has examined for me a small dun Welch pony with
three short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined the
breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double and
sometimes treble, is common; the side of the face, moreover, is sometimes
striped. The stripes are plainest in the foal; and sometimes quite
disappear in old horses. Colonel Poole has seen both gray and bay
Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English racehorse the spinal stripe is much commoner in the foal than in
the full-grown animal. Without here entering on further details, I may
state that I have collected cases of leg and shoulder stripes in horses
of very different breeds, in various countries from Britain to Eastern
China; and from Norway in the north to the Malay Archipelago in the
south. In all parts of the world these stripes occur far oftenest in duns
and mouse-duns; by the term dun a large range of colour is included, from
one between brown and black to a close approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species—one of which, the dun, was striped;
and that the above-described appearances are all due to ancient [165]crosses
with the dun stock. But I am not at all satisfied with this theory, and
should be loth to apply it to breeds so distinct as the heavy Belgian
cart-horse, Welch ponies, cobs, the lanky Kattywar race, &c.,
inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and horse
is particularly apt to have bars on its legs: according to Mr. Gosse, in
certain parts of the United States about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any
one would at first have thought that it must have been the product of a
zebra; and Mr. W. C. Martin, in his excellent treatise on the horse, has
given a figure of a similar mule. In four coloured drawings, which I have
seen, of hybrids between the ass and zebra, the legs were much more
plainly barred than the rest of the body; and in one of them there was a
double shoulder-stripe. In Lord Morton's famous hybrid from a chestnut
mare and male quagga, the hybrid, and even the pure offspring
subsequently produced from the mare by a black Arabian sire, were much
more plainly barred across the legs than is even the pure quagga. Lastly,
and this is another most remarkable case, a hybrid has been figured by
Dr. Gray (and he informs me that he knows of a second case) from the ass
and the hemionus; and this hybrid, though the ass seldom has stripes on
his legs and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Welch pony, and even had some
zebra-like stripes on the sides of its face. With respect to this last
fact, I was so convinced that not even a stripe of colour appears from
what would commonly be called an [166]accident, that I was
led solely from the occurrence of the face-stripes on this hybrid from
the ass and hemionus to ask Colonel Poole whether such face-stripes ever
occur in the eminently striped Kattywar breed of horses, and was, as we
have seen, answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears—a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids from
between several of the most distinct species. Now observe the case of the
several breeds of pigeons: they are descended from a pigeon (including
two or three sub-species or geographical races) of a bluish colour, with
certain bars and other marks; and when any breed assumes by simple
variation a bluish tint, these bars and other marks invariably reappear;
but without any other change of form or character. When the oldest and
truest breeds of various colours are crossed, we see a strong tendency
for the blue tint and bars and marks to reappear in the mongrels. I have
stated that the most probable hypothesis to account for the reappearance
of very ancient characters, is—that there is a tendency in
the young of each successive generation to produce the long-lost
character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the
horse-genus the stripes are either plainer or appear more commonly in the
young than in the old. Call the breeds of pigeons, some of which have
bred true for [167]centuries, species; and how exactly
parallel is the case with that of the species of the horse-genus! For
myself, I venture confidently to look back thousands on thousands of
generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our domestic
horse, whether or not it be descended from one or more wild stocks, of
the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to produce
hybrids resembling in their stripes, not their own parents, but other
species of the genus. To admit this view is, as it seems to me, to reject
a real for an unreal, or at least for an unknown, cause. It makes the
works of God a mere mockery and deception; I would almost as soon believe
with the old and ignorant cosmogonists, that fossil shells had never
lived, but had been created in stone so as to mock the shells now living
on the sea-shore.
Summary.—Our ignorance of the laws of variation is
profound. Not in one case out of a hundred can we pretend to assign any
reason why this or that part differs, more or less, from the same part in
the parents. But whenever we have the means of instituting a comparison,
the same laws appear to have acted in producing the lesser differences
between varieties of the same species, and the greater differences
between species of the same genus. The external conditions of life, as
[168]climate and food, &c., seem to have
induced some slight modifications. Habit in producing constitutional
differences, and use in strengthening and disuse in weakening and
diminishing organs, seem to have been more potent in their effects.
Homologous parts tend to vary in the same way, and homologous parts tend
to cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed,
perhaps it tends to draw nourishment from the adjoining parts; and every
part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable
to understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to
any particular function, so that their modifications have not been
closely checked by natural selection. It is probably from this same cause
that organic beings low in the scale of nature are more variable than
those which have their whole organisation more specialised, and are
higher in the scale. Rudimentary organs, from being useless, will be
disregarded by natural selection, and hence probably are variable.
Specific characters—that is, the characters which have come to
differ since the several species of the same genus branched off from a
common parent—are more variable than generic characters, or those
which have long been inherited, and have not differed within this same
period. In these remarks we have referred to special parts or organs
being still variable, because they have recently varied and thus come to
differ; but we have also seen in the second Chapter that the same
principle applies to the whole individual; [169]for in a district where
many species of any genus are found—that is, where there has been
much former variation and differentiation, or where the manufactory of
new specific forms has been actively at work—there, on an average,
we now find most varieties or incipient species. Secondary sexual
characters are highly variable, and such characters differ much in the
species of the same group. Variability in the same parts of the
organisation has generally been taken advantage of in giving secondary
sexual differences to the sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in
comparison with the same part or organ in the allied species, must have
gone through an extraordinary amount of modification since the genus
arose; and thus we can understand why it should often still be variable
in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases
not as yet have had time to overcome the tendency to further variability
and to reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants—which on my view must be a very slow process, requiring
a long lapse of time—in this case, natural selection may readily
have succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly the
same constitution from a common parent and exposed to similar influences
will naturally tend to present analogous variations, and these same
species may occasionally revert to some of the characters of their
ancient progenitors. Although new and important modifications may not
arise from reversion and analogous [170]variation, such
modifications will add to the beautiful and harmonious diversity of
nature.
Whatever the cause may be of each slight difference in the offspring
from their parents—and a cause for each must exist—it is the
steady accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face
of this earth are enabled to struggle with each other, and the best
adapted to survive.
[171]
CHAPTER VI.
Difficulties on Theory.
Difficulties on the theory of descent with
modification—Transitions—Absence or rarity of transitional
varieties—Transitions in habits of life—Diversified habits in
the same species—Species with habits widely different from those of
their allies—Organs of extreme perfection—Means of
transition—Cases of difficulty—Natura non facit
saltum—Organs of small importance—Organs not in all cases
absolutely perfect—The law of Unity of Type and of the Conditions
of Existence embraced by the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so grave
that to this day I can never reflect on them without being staggered;
but, to the best of my judgment, the greater number are only apparent,
and those that are real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following
heads:—Firstly, why, if species have descended from other species
by insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modification
of some animal with wholly different habits? Can we believe that natural
selection could produce, on the one hand, organs of trifling importance,
such as the tail of a giraffe, which serves as a fly-flapper, and, on
the other hand, organs of [172]such wonderful structure, as the eye, of
which we hardly as yet fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct as that which
leads the bee to make cells, which has practically anticipated the
discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?
The two first heads shall be here discussed—Instinct and
Hybridism in separate chapters.
On the absence or rarity of transitional varieties.—As
natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved parent or
other less-favoured forms with which it comes into competition. Thus
extinction and natural selection will, as we have seen, go hand in hand.
Hence, if we look at each species as descended from some other unknown
form, both the parent and all the transitional varieties will generally
have been exterminated by the very process of formation and perfection of
the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record; and
I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed; the
imperfection of the record being chiefly due to organic beings not
inhabiting [173]profound depths of the sea, and to their
remains being embedded and preserved to a future age only in masses of
sediment sufficiently thick and extensive to withstand an enormous amount
of future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur only rarely,
and after enormously long intervals. Whilst the bed of the sea is
stationary or is rising, or when very little sediment is being deposited,
there will be blanks in our geological history. The crust of the earth is
a vast museum; but the natural collections have been made only at
intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the
same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and rarer,
the other becomes more and more frequent, till the one replaces the
other. But if we compare these species where they intermingle, they are
generally as absolutely distinct from each other in every detail of
structure as are specimens taken from the metropolis inhabited by each.
By my theory these allied species have descended from a common parent;
and during the process of modification, each has become adapted to the
conditions of life of its own region, and has supplanted and exterminated
its original parent and all the transitional varieties between its past
and present states. Hence we ought not to expect at the [174]present time
to meet with numerous transitional varieties in each region, though they
must have existed there, and may be embedded there in a fossil condition.
But in the intermediate region, having intermediate conditions of life,
why do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be in
large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during a
long period. Geology would lead us to believe that almost every continent
has been broken up into islands even during the later tertiary periods;
and in such islands distinct species might have been separately formed
without the possibility of intermediate varieties existing in the
intermediate zones. By changes in the form of the land and of climate,
marine areas now continuous must often have existed within recent times
in a far less continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty; for I believe
that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken
condition of areas now continuous has played an important part in the
formation of new species, more especially with freely-crossing and
wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes [175]it is
quite remarkable how abruptly, as Alph. de Candolle has observed, a
common alpine species disappears. The same fact has been noticed by E.
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the all-important
elements of distribution, these facts ought to cause surprise, as climate
and height or depth graduate away insensibly. But when we bear in mind
that almost every species, even in its metropolis, would increase
immensely in numbers, were it not for other competing species; that
nearly all either prey on or serve as prey for others; in short, that
each organic being is either directly or indirectly related in the most
important manner to other organic beings, we must see that the range of
the inhabitants of any country by no means exclusively depends on
insensibly changing physical conditions, but in large part on the
presence of other species, on which it depends, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects (however they may have become so), not
blending one into another by insensible gradations, the range of any one
species, depending as it does on the range of others, will tend to be
sharply defined. Moreover, each species on the confines of its range,
where it exists in lessened numbers, will, during fluctuations in the
number of its enemies or of its prey, or in the seasons, be extremely
liable to utter extermination; and thus its geographical range will come
to be still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each has
a wide range, with a comparatively narrow neutral territory between them,
in which they become rather suddenly rarer and rarer; then, as varieties
do not essentially differ from species, [176]the same rule will
probably apply to both; and if we in imagination adapt a varying species
to a very large area, we shall have to adapt two varieties to two large
areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers from
inhabiting a narrow and lesser area; and practically, as far as I can
make out, this rule holds good with varieties in a state of nature. I
have met with striking instances of the rule in the case of varieties
intermediate between well-marked varieties in the genus Balanus. And it
would appear from information given me by Mr. Watson, Dr. Asa Gray, and
Mr. Wollaston, that generally when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and
therefore conclude that varieties linking two other varieties together
have generally existed in lesser numbers than the forms which they
connect, then, I think, we can understand why intermediate varieties
should not endure for very long periods;—why as a general rule they
should be exterminated and disappear, sooner than the forms which they
originally linked together.
For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on both
sides of it. But a far more important consideration, as I believe, is
that, during the process of further modification, by which two varieties
are supposed on my theory to be converted and perfected into two distinct
species, the two which exist in larger numbers from inhabiting larger
areas, will have a great advantage over the intermediate variety, which
exists [177]in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will always have
a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common forms,
in the race for life, will tend to beat and supplant the less common
forms, for these will be more slowly modified and improved. It is the
same principle which, as I believe, accounts for the common species in
each country, as shown in the second chapter, presenting on an average a
greater number of well-marked varieties than do the rarer species. I may
illustrate what I mean by supposing three varieties of sheep to be kept,
one adapted to an extensive mountainous region; a second to a
comparatively narrow, hilly tract; and a third to wide plains at the
base; and that the inhabitants are all trying with equal steadiness and
skill to improve their stocks by selection; the chances in this case will
be strongly in favour of the great holders on the mountains or on the
plains improving their breeds more quickly than the small holders on the
intermediate narrow, hilly tract; and consequently the improved mountain
or plain breed will soon take the place of the less improved hill breed;
and thus the two breeds, which originally existed in greater numbers,
will come into close contact with each other, without the interposition
of the supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable [178]variations chance to
occur, and until a place in the natural polity of the country can be
better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of climate,
or on the occasional immigration of new inhabitants, and, probably, in a
still more important degree, on some of the old inhabitants becoming
slowly modified, with the new forms thus produced and the old ones acting
and reacting on each other. So that, in any one region and at any one
time, we ought only to see a few species presenting slight modifications
of structure in some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more especially
amongst the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several
representative species and their common parent, must formerly have
existed in each broken portion of the land, but these links will have
been supplanted and exterminated during the process of natural selection,
so that they will no longer exist in a living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative species,
and likewise of acknowledged varieties), exist in the intermediate zones
in lesser numbers than the varieties which they [179]tend to connect. From
this cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted by
the forms which they connect; for these from existing in greater numbers
will, in the aggregate, present more variation, and thus be further
improved through natural selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so often
remarked, to exterminate the parent-forms and the intermediate links.
Consequently evidence of their former existence could be found only
amongst fossil remains, which are preserved, as we shall in a future
chapter attempt to show, in an extremely imperfect and intermittent
record.
On the origin and transitions of organic beings with peculiar
habits and structure.—It has been asked by the opponents of
such views as I hold, how, for instance, a land carnivorous animal could
have been converted into one with aquatic habits; for how could the
animal in its transitional state have subsisted? It would be easy to show
that within the same group carnivorous animals exist having every
intermediate grade between truly aquatic and strictly terrestrial habits;
and as each exists by a struggle for life, it is clear that each is well
adapted in its habits to its place in nature. Look at the Mustela vison
of North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal [180]dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying bat, the
question would have been far more difficult, and I could have given no
answer. Yet I think such difficulties have very little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one or
two instances of transitional habits and structures in closely allied
species of the same genus; and of diversified habits, either constant or
occasional, in the same species. And it seems to me that nothing less
than a long list of such cases is sufficient to lessen the difficulty in
any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from others,
as Sir J. Richardson has remarked, with the posterior part of their
bodies rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which serves
as a parachute and allows them to glide through the air to an astonishing
distance from tree to tree. We cannot doubt that each structure is of use
to each kind of squirrel in its own country, by enabling it to escape
birds or beasts of prey, or to collect food more quickly, or, as there is
reason to believe, by lessening the danger from occasional falls. But it
does not follow from this fact that the structure of each squirrel is the
best that it is possible to conceive under all natural conditions. Let
the climate and vegetation change, let other competing [181]rodents or new
beasts of prey immigrate, or old ones become modified, and all analogy
would lead us to believe that some at least of the squirrels would
decrease in numbers or become exterminated, unless they also became
modified and improved in structure in a corresponding manner. Therefore,
I can see no difficulty, more especially under changing conditions of
life, in the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propagated,
until by the accumulated effects of this process of natural selection, a
perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank-membrane is, also, furnished
with an extensor muscle. Although no graduated links of structure, fitted
for gliding through the air, now connect the Galeopithecus with the other
Lemuridæ, yet I see no difficulty in supposing that such links formerly
existed, and that each had been formed by the same steps as in the case
of the less perfectly gliding squirrels; and that each grade of structure
was useful to its possessor. Nor can I see any insuperable difficulty in
further believing it possible that the membrane-connected fingers and
forearm of the Galeopithecus might be greatly lengthened by natural
selection; and this, as far as the organs of flight are concerned, would
convert it into a bat. In bats which have the wing-membrane extended from
the top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through the
air rather than for flight. [182]
If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have surmised that birds might have existed
which used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed, for
each has to live by a struggle; but it is not necessarily the best
possible under all possible conditions. It must not be inferred from
these remarks that any of the grades of wing-structure here alluded to,
which perhaps may all have resulted from disuse, indicate the natural
steps by which birds have acquired their perfect power of flight; but
they serve, at least, to show what diversified means of transition are
possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land; and seeing that
we have flying birds and mammals, flying insects of the most diversified
types, and formerly had flying reptiles, it is conceivable that
flying-fish, which now glide far through the air, slightly rising and
turning by the aid of their fluttering fins, might have been modified
into perfectly winged animals. If this had been effected, who would have
ever imagined that in an early transitional state they had been
inhabitants of the open ocean, and had used their incipient organs of
flight exclusively, as far as we know, to escape being devoured by other
fish?
When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early [183]transitional grades of the structure will
seldom continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural selection.
Furthermore, we may conclude that transitional grades between structures
fitted for very different habits of life will rarely have been developed
at an early period in great numbers and under many subordinate forms.
Thus, to return to our imaginary illustration of the flying-fish, it does
not seem probable that fishes capable of true flight would have been
developed under many subordinate forms, for taking prey of many kinds in
many ways, on the land and in the water, until their organs of flight had
come to a high stage of perfection, so as to have given them a decided
advantage over other animals in the battle for life. Hence the chance of
discovering species with transitional grades of structure in a fossil
condition will always be less, from their having existed in lesser
numbers, than in the case of species with fully developed structures.
I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case occurs,
it would be easy for natural selection to fit the animal, by some
modification of its structure, for its changed habits, or exclusively for
one of its several different habits. But it is difficult to tell, and
immaterial for us, whether habits generally change first and structure
afterwards; or whether slight modifications of structure lead to changed
habits; both probably often change almost simultaneously. Of cases of
changed habits it will suffice merely to allude to that of the many
British insects which now feed on exotic plants, or exclusively on
artificial substances. Of diversified habits innumerable instances could
be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot [184]and then
proceeding to another, like a kestrel, and at other times standing
stationary on the margin of water, and then dashing like a kingfisher at
a fish. In our own country the larger titmouse (Parus major) may be seen
climbing branches, almost like a creeper; it often, like a shrike, kills
small birds by blows on the head; and I have many times seen and heard it
hammering the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne swimming for
hours with widely open mouth, thus catching, almost like a whale, insects
in the water.
As we sometimes see individuals of a species following habits widely
different from those of their own species and of the other species of the
same genus, we might expect, on my theory, that such individuals would
occasionally have given rise to new species, having anomalous habits, and
with their structure either slightly or considerably modified from that
of their proper type. And such instances do occur in nature. Can a more
striking instance of adaptation be given than that of a woodpecker for
climbing trees and for seizing insects in the chinks of the bark? Yet in
North America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing; and on the
plains of La Plata, where not a tree grows, there is a woodpecker, which
in every essential part of its organisation, even in its colouring, in
the harsh tone of its voice, and undulatory flight, told me plainly of
its close blood-relationship to our common species; yet it is a
woodpecker which never climbs a tree!
Petrels are the most aërial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, its manner of swimming, and
of flying when [185]unwillingly it takes flight, would be
mistaken by any one for an auk or grebe; nevertheless, it is essentially
a petrel, but with many parts of its organisation profoundly modified. On
the other hand, the acutest observer by examining the dead body of the
water-ouzel would never have suspected its sub-aquatic habits; yet this
anomalous member of the strictly terrestrial thrush family wholly
subsists by diving,—grasping the stones with its feet and using its
wings under water.
He who believes that each being has been created as we now see it,
must occasionally have felt surprise when he has met with an animal
having habits and structure not at all in agreement. What can be plainer
than that the webbed feet of ducks and geese are formed for swimming? yet
there are upland geese with webbed feet which rarely or never go near the
water; and no one except Audubon has seen the frigate-bird, which has all
its four toes webbed, alight on the surface of the sea. On the other hand
grebes and coots are eminently aquatic, although their toes are only
bordered by membrane. What seems plainer than that the long toes of
grallatores are formed for walking over swamps and floating plants, yet
the water-hen is nearly as aquatic as the coot; and the landrail nearly
as terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
rudimentary in function, though not in structure. In the frigate-bird,
the deeply-scooped membrane between the toes shows that structure has
begun to change.
He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me only
[186]restating the fact in dignified language.
He who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is
constantly endeavouring to increase in numbers; and that if any one being
vary ever so little, either in habits or structure, and thus gain an
advantage over some other inhabitant of the country, it will seize on the
place of that inhabitant, however different it may be from its own place.
Hence it will cause him no surprise that there should be geese and
frigate-birds with webbed feet, living on the dry land or most rarely
alighting on the water; that there should be long-toed corncrakes living
in meadows instead of in swamps; that there should be woodpeckers where
not a tree grows; that there should be diving thrushes, and petrels with
the habits of auks.
Organs of extreme perfection and complication.—To suppose
that the eye, with all its inimitable contrivances for adjusting the
focus to different distances, for admitting different amounts of light,
and for the correction of spherical and chromatic aberration, could have
been formed by natural selection, seems, I freely confess, absurd in the
highest possible degree. Yet reason tells me, that if numerous gradations
from a perfect and complex eye to one very imperfect and simple, each
grade being useful to its possessor, can be shown to exist; if further,
the eye does vary ever so slightly, and the variations be inherited,
which is certainly the case; and if any variation or modification in the
organ be ever useful to an animal under changing conditions of life, then
the difficulty of believing that a perfect and complex eye could be
formed by natural selection, though insuperable by our imagination, can
hardly be considered real. How a nerve comes to be sensitive to [187]light,
hardly concerns us more than how life itself first originated; but I may
remark that several facts make me suspect that any sensitive nerve may be
rendered sensitive to light, and likewise to those coarser vibrations of
the air which produce sound.
In looking for the gradations by which an organ in any species has
been perfected, we ought to look exclusively to its lineal ancestors; but
this is scarcely ever possible, and we are forced in each case to look to
species of the same group, that is to the collateral descendants from the
same original parent-form, in order to see what gradations are possible,
and for the chance of some gradations having been transmitted from the
earlier stages of descent, in an unaltered or little altered condition.
Amongst existing Vertebrata, we find but a small amount of gradation in
the structure of the eye, and from fossil species we can learn nothing on
this head. In this great class we should probably have to descend far
beneath the lowest known fossiliferous stratum to discover the earlier
stages, by which the eye has been perfected.
In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this low
stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets,
within each of which there is a lens-shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and which
properly act only by excluding lateral pencils of light, are convex at
their upper ends and must act by convergence; and at their lower ends
there seems to be an imperfect vitreous substance. [188]With these facts, here
far too briefly and imperfectly given, which show that there is much
graduated diversity in the eyes of living crustaceans, and bearing in
mind how small the number of living animals is in proportion to those
which have become extinct, I can see no very great difficulty (not more
than in the case of many other structures) in believing that natural
selection has converted the simple apparatus of an optic nerve merely
coated with pigment and invested by transparent membrane, into an optical
instrument as perfect as is possessed by any member of the great
Articulate class.
He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit that
a structure even as perfect as the eye of an eagle might be formed by
natural selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination; though
I have felt the difficulty far too keenly to be surprised at any degree
of hesitation in extending the principle of natural selection to such
startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We
know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that the
eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? If we must compare the
eye to an optical instrument, we ought in imagination to take a thick
layer of transparent tissue, with a nerve sensitive to light beneath, and
then suppose every part of this layer to be continually changing [189]slowly in density, so as to separate into
layers of different densities and thicknesses, placed at different
distances from each other, and with the surfaces of each layer slowly
changing in form. Further we must suppose that there is a power always
intently watching each slight accidental alteration in the transparent
layers; and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; and each to be preserved till a better be
produced, and then the old ones to be destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with unerring
skill each improvement. Let this process go on for millions on millions
of years; and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might thus be
formed as superior to one of glass, as the works of the Creator are to
those of man?
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find out
no such case. No doubt many organs exist of which we do not know the
transitional grades, more especially if we look to much-isolated species,
round which, according to my theory, there has been much extinction. Or
again, if we look to an organ common to all the members of a large class,
for in this latter case the organ must have been first formed at an
extremely remote period, since which all the many members of the class
have been developed; and in order to discover the early transitional
grades through which the organ has [190]passed, we should have
to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous cases
could be given amongst the lower animals of the same organ performing at
the same time wholly distinct functions; thus the alimentary canal
respires, digests, and excretes in the larva of the dragon-fly and in the
fish Cobites. In the Hydra, the animal may be turned inside out, and the
exterior surface will then digest and the stomach respire. In such cases
natural selection might easily specialise, if any advantage were thus
gained, a part or organ, which had performed two functions, for one
function alone, and thus wholly change its nature by insensible steps.
Two distinct organs sometimes perform simultaneously the same function in
the same individual; to give one instance, there are fish with gills or
branchiæ that breathe the air dissolved in the water, at the same time
that they breathe free air in their swimbladders, this latter organ
having a ductus pneumaticus for its supply, and being divided by highly
vascular partitions. In these cases one of the two organs might with ease
be modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then
this other organ might be modified for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a wholly different purpose, namely respiration. The swimbladder has,
also, been worked in as an accessory to the auditory organs of certain
fish, or, for I do not know [191]which view is now generally held, a part
of the auditory apparatus has been worked in as a complement to the
swimbladder. All physiologists admit that the swimbladder is homologous,
or "ideally similar" in position and structure with the lungs of the
higher vertebrate animals: hence there seems to me to be no great
difficulty in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype, of
which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice
of the trachea, with some risk of falling into the lungs, notwithstanding
the beautiful contrivance by which the glottis is closed. In the higher
Vertebrata the branchiæ have wholly disappeared—the slits on the
sides of the neck and the loop-like course of the arteries still marking
in the embryo their former position. But it is conceivable that the now
utterly lost branchiæ might have been gradually worked in by natural
selection for some quite distinct purpose: in the same manner as, on the
view entertained by some naturalists that the branchiæ and dorsal scales
of Annelids are homologous with the wings and wing-covers of insects, it
is probable that organs which at a very ancient period served for
respiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that I
will give one more instance. Pedunculated cirripedes have two minute
folds of skin, [192]called by me the ovigerous frena, which
serve, through the means of a sticky secretion, to retain the eggs until
they are hatched within the sack. These cirripedes have no branchiæ, the
whole surface of the body and sack, including the small frena, serving
for respiration. The Balanidæ or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the sack,
in the well-enclosed shell; but they have large folded branchiæ. Now I
think no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiæ of the other family; indeed, they
graduate into each other. Therefore I do not doubt that little folds of
skin, which originally served as ovigerous frena, but which, likewise,
very slightly aided the act of respiration, have been gradually converted
by natural selection into branchiæ, simply through an increase in their
size and the obliteration of their adhesive glands. If all pedunculated
cirripedes had become extinct, and they have already suffered far more
extinction than have sessile cirripedes, who would ever have imagined
that the branchiæ in this latter family had originally existed as organs
for preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs of
fishes offer another case of special difficulty; it is impossible to
conceive by what steps these wondrous organs have been produced; but, as
Owen and others have remarked, [193]their intimate structure closely resembles
that of common muscle; and as it has lately been shown that Rays have an
organ closely analogous to the electric apparatus, and yet do not, as
Matteucci asserts, discharge any electricity, we must own that we
are far too ignorant to argue that no transition of any kind is
possible.
The electric organs offer another and even more serious difficulty;
for they occur in only about a dozen fishes, of which several are widely
remote in their affinities. Generally when the same organ appears in
several members of the same class, especially if in members having very
different habits of life, we may attribute its presence to inheritance
from a common ancestor; and its absence in some of the members to its
loss through disuse or natural selection. But if the electric organs had
been inherited from one ancient progenitor thus provided, we might have
expected that all electric fishes would have been specially related to
each other. Nor does geology at all lead to the belief that formerly most
fishes had electric organs, which most of their modified descendants have
lost. The presence of luminous organs in a few insects, belonging to
different families and orders, offers a parallel case of difficulty.
Other cases could be given; for instance in plants, the very curious
contrivance of a mass of pollen-grains, borne on a foot-stalk with a
sticky gland at the end, is the same in Orchis and
Asclepias,—genera almost as remote as possible amongst flowering
plants. In all these cases of two very distinct species furnished with
apparently the same anomalous organ, it should be observed that, although
the general appearance and function of the organ may be the same, yet
some fundamental difference can generally be detected. I am inclined to
believe that in nearly the same way as two men have sometimes
independently hit on [194]the very same invention, so natural
selection, working for the good of each being and taking advantage of
analogous variations, has sometimes modified in very nearly the same
manner two parts in two organic beings, which beings owe but little of
their structure in common to inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture by what
transitions organs could have arrived at their present state; yet,
considering that the proportion of living and known forms to the extinct
and unknown is very small, I have been astonished how rarely an organ can
be named, towards which no transitional grade is known to lead. The truth
of this remark is indeed shown by that old but somewhat exaggerated canon
in natural history of "Natura non facit saltum." We meet with this
admission in the writings of almost every experienced naturalist; or, as
Milne Edwards has well expressed it, Nature is prodigal in variety, but
niggard in innovation. Why, on the theory of Creation, should this be so?
Why should all the parts and organs of many independent beings, each
supposed to have been separately created for its proper place in nature,
be so commonly linked together by graduated steps? Why should not Nature
have taken a leap from structure to structure? On the theory of natural
selection, we can clearly understand why she should not; for natural
selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest
and slowest steps.
Organs of little apparent importance.—As natural
selection acts by life and death,—by the preservation of
individuals with any favourable variation, and by the destruction of
those with any unfavourable deviation of structure,—I have
sometimes felt much difficulty in [195]understanding the
origin of simple parts, of which the importance does not seem sufficient
to cause the preservation of successively varying individuals. I have
sometimes felt as much difficulty, though of a very different kind, on
this head, as in the case of an organ as perfect and complex as the
eye.
In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
most trifling characters, such as the down on fruit and the colour of its
flesh, which, from determining the attacks of insects or from being
correlated with constitutional differences, might assuredly be acted on
by natural selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this could
have been adapted for its present purpose by successive slight
modifications, each better and better, for so trifling an object as
driving away flies; yet we should pause before being too positive even in
this case, for we know that the distribution and existence of cattle and
other animals in South America absolutely depends on their power of
resisting the attacks of insects: so that individuals which could by any
means defend themselves from these small enemies, would be able to range
into new pastures and thus gain a great advantage. It is not that the
larger quadrupeds are actually destroyed (except in some rare cases) by
flies, but they are incessantly harassed and their strength reduced, so
that they are more subject to disease, or not so well enabled in a coming
dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a [196]former period, have been transmitted in
nearly the same state, although now become of very slight use; and any
actually injurious deviations in their structure will always have been
checked by natural selection. Seeing how important an organ of locomotion
the tail is in most aquatic animals, its general presence and use for
many purposes in so many land animals, which in their lungs or modified
swimbladders betray their aquatic origin, may perhaps be thus accounted
for. A well-developed tail having been formed in an aquatic animal, it
might subsequently come to be worked in for all sorts of purposes, as a
fly-flapper, an organ of prehension, or as an aid in turning, as with the
dog, though the aid must be slight, for the hare, with hardly any tail,
can double quickly enough.
In the second place, we may sometimes attribute importance to
characters which are really of very little importance, and which have
originated from quite secondary causes, independently of natural
selection. We should remember that climate, food, &c., probably have
some little direct influence on the organisation; that characters
reappear from the law of reversion; that correlation of growth will have
had a most important influence in modifying various structures; and
finally, that sexual selection will often have largely modified the
external characters of animals having a will, to give one male an
advantage in fighting with another or in charming the females. Moreover
when a modification of structure has primarily arisen from the above or
other unknown causes, it may at first have been of no advantage to the
species, but may subsequently have been taken advantage of by the
descendants of the species under new conditions of life and with newly
acquired habits.
To give a few instances to illustrate these latter [197]remarks. If
green woodpeckers alone had existed, and we did not know that there were
many black and pied kinds, I dare say that we should have thought that
the green colour was a beautiful adaptation to hide this tree-frequenting
bird from its enemies; and consequently that it was a character of
importance and might have been acquired through natural selection; as it
is, I have no doubt that the colour is due to some quite distinct cause,
probably to sexual selection. A trailing bamboo in the Malay Archipelago
climbs the loftiest trees by the aid of exquisitely constructed hooks
clustered around the ends of the branches, and this contrivance, no
doubt, is of the highest service to the plant; but as we see nearly
similar hooks on many trees which are not climbers, the hooks on the
bamboo may have arisen from unknown laws of growth, and have been
subsequently taken advantage of by the plant undergoing further
modification and becoming a climber. The naked skin on the head of a
vulture is generally looked at as a direct adaptation for wallowing in
putridity; and so it may be, or it may possibly be due to the direct
action of putrid matter; but we should be very cautious in drawing any
such inference, when we see that the skin on the head of the
clean-feeding male turkey is likewise naked. The sutures in the skulls of
young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable for
this act; but as sutures occur in the skulls of young birds and reptiles,
which have only to escape from a broken egg, we may infer that this
structure has arisen from the laws of growth, and has been taken
advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are [198]immediately made
conscious of this by reflecting on the differences in the breeds of our
domesticated animals in different countries,—more especially in the
less civilised countries where there has been but little artificial
selection. Careful observers are convinced that a damp climate affects
the growth of the hair, and that with the hair the horns are correlated.
Mountain breeds always differ from lowland breeds; and a mountainous
country would probably affect the hind limbs from exercising them more,
and possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and even the head would probably be
affected. The shape, also, of the pelvis might affect by pressure the
shape of the head of the young in the womb. The laborious breathing
necessary in high regions would, we have some reason to believe, increase
the size of the chest; and again correlation would come into play.
Animals kept by savages in different countries often have to struggle for
their own subsistence, and would be exposed to a certain extent to
natural selection, and individuals with slightly different constitutions
would succeed best under different climates; and there is reason to
believe that constitution and colour are correlated. A good observer,
also, states that in cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain
plants; so that colour would be thus subjected to the action of natural
selection. But we are far too ignorant to speculate on the relative
importance of the several known and unknown laws of variation; and I have
here alluded to them only to show that, if we are unable to account for
the characteristic differences of our domestic breeds, which nevertheless
we generally admit to have arisen through ordinary generation, we ought
not to lay too much stress on our ignorance of the precise cause [199]of
the slight analogous differences between species. I might have adduced
for this same purpose the differences between the races of man, which are
so strongly marked; I may add that some little light can apparently be
thrown on the origin of these differences, chiefly through sexual
selection of a particular kind, but without here entering on copious
details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor. They
believe that very many structures have been created for beauty in the
eyes of man, or for mere variety. This doctrine, if true, would be
absolutely fatal to my theory. Yet I fully admit that many structures are
of no direct use to their possessors. Physical conditions probably have
had some little effect on structure, quite independently of any good thus
gained. Correlation of growth has no doubt played a most important part,
and a useful modification of one part will often have entailed on other
parts diversified changes of no direct use. So again characters which
formerly were useful, or which formerly had arisen from correlation of
growth, or from other unknown cause, may reappear from the law of
reversion, though now of no direct use. The effects of sexual selection,
when displayed in beauty to charm the females, can be called useful only
in rather a forced sense. But by far the most important consideration is
that the chief part of the organisation of every being is simply due to
inheritance; and consequently, though each being assuredly is well fitted
for its place in nature, many structures now have no direct relation to
the habits of life of each species. Thus, we can hardly believe that the
webbed feet of the upland [200]goose or of the frigate-bird are of
special use to these birds; we cannot believe that the same bones in the
arm of the monkey, in the fore-leg of the horse, in the wing of the bat,
and in the nipper of the seal, are of special use to these animals. We
may safely attribute these structures to inheritance. But to the
progenitor of the upland goose and of the frigate-bird, webbed feet no
doubt were as useful as they now are to the most aquatic of existing
birds. So we may believe that the progenitor of the seal had not a
nipper, but a foot with five toes fitted for walking or grasping; and we
may further venture to believe that the several bones in the limbs of the
monkey, horse, and bat, which have been inherited from a common
progenitor, were formerly of more special use to that progenitor, or its
progenitors, than they now are to these animals having such widely
diversified habits. Therefore we may infer that these several bones might
have been acquired through natural selection, subjected formerly, as now,
to the several laws of inheritance, reversion, correlation of growth,
&c. Hence every detail of structure in every living creature (making
some little allowance for the direct action of physical conditions) may
be viewed, either as having been of special use to some ancestral form,
or as being now of special use to the descendants of this
form—either directly, or indirectly through the complex laws of
growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang
of the adder, and in the ovipositor of the ichneumon, by which its eggs
are [201]deposited in the living bodies of other insects. If it could be
proved that any part of the structure of any one species had been formed
for the exclusive good of another species, it would annihilate my theory,
for such could not have been produced through natural selection. Although
many statements may be found in works on natural history to this effect,
I cannot find even one which seems to me of any weight. It is admitted
that the rattlesnake has a poison-fang for its own defence and for the
destruction of its prey; but some authors suppose that at the same time
this snake is furnished with a rattle for its own injury, namely, to warn
its prey to escape. I would almost as soon believe that the cat curls the
end of its tail when preparing to spring, in order to warn the doomed
mouse. But I have not space here to enter on this and other such
cases.
Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each. No
organ will be formed, as Paley has remarked, for the purpose of causing
pain or for doing an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will be found on the
whole advantageous. After the lapse of time, under changing conditions of
life, if any part comes to be injurious, it will be modified; or if it be
not so, the being will become extinct, as myriads have become
extinct.
Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than, the other inhabitants of the same country
with which it has to struggle for existence. And we see that this is the
degree of perfection attained under nature. The endemic productions of
New Zealand, for instance, are perfect one compared with another; but
they are now rapidly yielding before the advancing legions of plants [202]and
animals introduced from Europe. Natural selection will not produce
absolute perfection, nor do we always meet, as far as we can judge, with
this high standard under nature. The correction for the aberration of
light is said, on high authority, not to be perfect even in that most
perfect organ, the eye. If our reason leads us to admire with enthusiasm
a multitude of inimitable contrivances in nature, this same reason tells
us, though we may easily err on both sides, that some other contrivances
are less perfect. Can we consider the sting of the wasp or of the bee as
perfect, which, when used against many attacking animals, cannot be
withdrawn, owing to the backward serratures, and so inevitably causes the
death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so
many members of the same great order, and which has been modified but not
perfected for its present purpose, with the poison originally adapted to
cause galls subsequently intensified, we can perhaps understand how it is
that the use of the sting should so often cause the insect's own death:
for if on the whole the power of stinging be useful to the community, it
will fulfil all the requirements of natural selection, though it may
cause the death of some few members. If we admire the truly wonderful
power of scent by which the males of many insects find their females, can
we admire the production for this single purpose of thousands of drones,
which are utterly useless to the community for any other end, and which
are ultimately slaughtered by their industrious and sterile sisters? It
may be difficult, but we ought to admire the savage instinctive hatred of
the queen-bee, which urges her instantly to destroy the [203]young queens
her daughters as soon as born, or to perish herself in the combat; for
undoubtedly this is for the good of the community; and maternal love or
maternal hatred, though the latter fortunately is most rare, is all the
same to the inexorable principle of natural selection. If we admire the
several ingenious contrivances, by which the flowers of the orchis and of
many other plants are fertilised through insect agency, can we consider
as equally perfect the elaboration by our fir-trees of dense clouds of
pollen, in order that a few granules may be wafted by a chance breeze on
to the ovules?
Summary of Chapter.—We have in this chapter discussed
some of the difficulties and objections which may be urged against my
theory. Many of them are very serious; but I think that in the discussion
light has been thrown on several facts, which on the theory of
independent acts of creation are utterly obscure. We have seen that
species at any one period are not indefinitely variable, and are not
linked together by a multitude of intermediate gradations, partly because
the process of natural selection will always be very slow, and will act,
at any one time, only on a very few forms; and partly because the very
process of natural selection almost implies the continual supplanting and
extinction of preceding and intermediate gradations. Closely allied
species, now living on a continuous area, must often have been formed
when the area was not continuous, and when the conditions of life did not
insensibly graduate away from one part to another. When two varieties are
formed in two districts of a continuous area, an intermediate variety
will often be formed, fitted for an intermediate zone; but from reasons
assigned, the intermediate variety will usually exist in lesser numbers
than [204]the two forms which it connects;
consequently the two latter, during the course of further modification,
from existing in greater numbers, will have a great advantage over the
less numerous intermediate variety, and will thus generally succeed in
supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first could only glide through the
air.
We have seen that a species may under new conditions of life change
its habits, or have diversified habits, with some habits very unlike
those of its nearest congeners. Hence we can understand, bearing in mind
that each organic being is trying to live wherever it can live, how it
has arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is more than enough to stagger any one;
yet in the case of any organ, if we know of a long series of gradations
in complexity, each good for its possessor, then, under changing
conditions of life there is no logical impossibility in the acquirement
of any conceivable degree of perfection through natural selection. In the
cases in which we know of no intermediate or transitional states, we
should be very cautious in concluding that none could have existed, for
the homologies of many organs and their intermediate states show that
wonderful metamorphoses in function are at least possible. For instance,
a swim-bladder has apparently been converted into an air-breathing lung.
The same organ having performed [205]simultaneously very different functions,
and then having been specialised for one function; and two very distinct
organs having performed at the same time the same function, the one
having been perfected whilst aided by the other, must often have largely
facilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assert
that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly
accumulated by means of natural selection. But we may confidently believe
that many modifications, wholly due to the laws of growth, and at first
in no way advantageous to a species, have been subsequently taken
advantage of by the still further modified descendants of this species.
We may, also, believe that a part formerly of high importance has often
been retained (as the tail of an aquatic animal by its terrestrial
descendants), though it has become of such small importance that it could
not, in its present state, have been acquired by natural
selection,—a power which acts solely by the preservation of
profitable variations in the struggle for life.
Natural selection will produce nothing in one species for the
exclusive good or injury of another; though it may well produce parts,
organs, and excretions highly useful or even indispensable, or highly
injurious to another species, but in all cases at the same time useful to
the owner. Natural selection in each well-stocked country, must act
chiefly through the competition of the inhabitants one with another, and
consequently will produce perfection, or strength in the battle for life,
only according to the standard of that country. Hence the inhabitants of
one country, generally the smaller one, will often yield, as we see they
do yield, to the inhabitants of another and generally larger country. For
in [206]the larger country there will have existed
more individuals, and more diversified forms, and the competition will
have been severer, and thus the standard of perfection will have been
rendered higher. Natural selection will not necessarily produce absolute
perfection; nor, as far as we can judge by our limited faculties, can
absolute perfection be everywhere found.
On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look only to the present inhabitants of the world, is
not strictly correct, but if we include all those of past times, it must
by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed
on two great laws—Unity of Type, and the Conditions of Existence.
By unity of type is meant that fundamental agreement in structure, which
we see in organic beings of the same class, and which is quite
independent of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions of existence,
so often insisted on by the illustrious Cuvier, is fully embraced by the
principle of natural selection. For natural selection acts by either now
adapting the varying parts of each being to its organic and inorganic
conditions of life; or by having adapted them during long-past periods of
time: the adaptations being aided in some cases by use and disuse, being
slightly affected by the direct action of the external conditions of
life, and being in all cases subjected to the several laws of growth.
Hence, in fact, the law of the Conditions of Existence is the higher law;
as it includes, through the inheritance of former adaptations, that of
Unity of Type.
[207]
CHAPTER VII.
Instinct.
Instincts comparable with habits, but different in their
origin—Instincts graduated—Aphides and ants—Instincts
variable—Domestic instincts, their origin—Natural instincts
of the cuckoo, ostrich, and parasitic
bees—Slave-making-ants—Hive-bee, its cell-making
instinct—Difficulties on the theory of the Natural Selection of
instincts—Neuter or sterile insects—Summary.
The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat
the subject separately, especially as so wonderful an instinct as that of
the hive-bee making its cells will probably have occurred to many
readers, as a difficulty sufficient to overthrow my whole theory. I must
premise, that I have nothing to do with the origin of the primary mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental qualities
of animals within the same class.
I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by this
term; but every one understands what is meant, when it is said that
instinct impels the cuckoo to migrate and to lay her eggs in other birds'
nests. An action, which we ourselves should require experience to enable
us to perform, when performed by an animal, more especially by a very
young one, without any experience, and when performed by many individuals
in the same way, without their knowing for what purpose it is performed,
is usually said to be instinctive. [208]But I could show that
none of these characters of instinct are universal. A little dose, as
Pierre Huber expresses it, of judgment or reason, often comes into play,
even in animals very low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably
accurate notion of the frame of mind under which an instinctive action is
performed, but not of its origin. How unconsciously many habitual actions
are performed, indeed not rarely in direct opposition to our conscious
will! yet they may be modified by the will or reason. Habits easily
become associated with other habits, and with certain periods of time and
states of the body. When once acquired, they often remain constant
throughout life. Several other points of resemblance between instincts
and habits could be pointed out. As in repeating a well-known song, so in
instincts, one action follows another by a sort of rhythm; if a person be
interrupted in a song, or in repeating anything by rote, he is generally
forced to go back to recover the habitual train of thought: so P. Huber
found it was with a caterpillar, which makes a very complicated hammock;
for if he took a caterpillar which had completed its hammock up to, say,
the sixth stage of construction, and put it into a hammock completed up
only to the third stage, the caterpillar simply re-performed the fourth,
fifth, and sixth stages of construction. If, however, a caterpillar were
taken out of a hammock made up, for instance, to the third stage, and
were put into one finished up to the sixth stage, so that much of its
work, was already done for it, far from feeling the benefit of this, it
was much embarrassed, and, in order to complete its hammock, seemed
forced to start from the third stage, where it had left off, and thus
tried to complete the already finished work. [209]
If we suppose any habitual action to become inherited—and I
think it can be shown that this does sometimes happen—then the
resemblance between what originally was a habit and an instinct becomes
so close as not to be distinguished. If Mozart, instead of playing the
pianoforte at three years old with wonderfully little practice, had
played a tune with no practice at all, he might truly be said to have
done so instinctively. But it would be the most serious error to suppose
that the greater number of instincts have been acquired by habit in one
generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been thus acquired.
It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may be
profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what
may be called accidental variations of instincts;—that is of
variations produced by the same unknown causes which produce slight
deviations of bodily structure.
No complex instinct can possibly be produced through [210]natural
selection, except by the slow and gradual accumulation of numerous,
slight, yet profitable, variations. Hence, as in the case of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired—for
these could be found only in the lineal ancestors of each
species—but we ought to find in the collateral lines of descent
some evidence of such gradations; or we ought at least to be able to show
that gradations of some kind are possible; and this we certainly can do.
I have been surprised to find, making allowance for the instincts of
animals having been but little observed except in Europe and North
America, and for no instinct being known amongst extinct species, how
very generally gradations, leading to the most complex instincts, can be
discovered. Changes of instinct may sometimes be facilitated by the same
species having different instincts at different periods of life, or at
different seasons of the year, or when placed under different
circumstances &c.; in which case either one or the other instinct
might be preserved by natural selection. And such instances of diversity
of instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is that
of aphides voluntarily yielding their sweet excretion to ants: that they
do so voluntarily, the following facts show. I removed all the ants from
a group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that
the aphides [211]would want to excrete. I watched them for
some time through a lens, but not one excreted; I then tickled and
stroked them with a hair in the same manner, as well as I could, as the
ants do with their antennæ; but not one excreted. Afterwards I allowed an
ant to visit them, and it immediately seemed, by its eager way of running
about, to be well aware what a rich flock it had discovered; it then
began to play with its antennæ on the abdomen first of one aphis and then
of another; and each aphis, as soon as it felt the antennæ, immediately
lifted up its abdomen and excreted a limpid drop of sweet juice, which
was eagerly devoured by the ant. Even the quite young aphides behaved in
this manner, showing that the action was instinctive, and not the result
of experience. But as the excretion is extremely viscid, it is probably a
convenience to the aphides to have it removed; and therefore probably the
aphides do not instinctively excrete for the sole good of the ants.
Although I do not believe that any animal in the world performs an action
for the exclusive good of another of a distinct species, yet each species
tries to take advantage of the instincts of others, as each takes
advantage of the weaker bodily structure of others. So again, in some few
cases, certain instincts cannot be considered as absolutely perfect; but
as details on this and other such points are not indispensable, they may
be here passed over.
As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to be here given;
but want of space prevents me. I can only assert, that instincts
certainly do vary—for instance, the migratory instinct, both in
extent and direction, and in its total loss. So it is with the nests of
birds, which vary partly [212]in dependence on the situations chosen,
and on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several remarkable
cases of differences in the nests of the same species in the northern and
southern United States. Fear of any particular enemy is certainly an
instinctive quality, as may be seen in nestling birds, though it is
strengthened by experience, and by the sight of fear of the same enemy in
other animals. But fear of man is slowly acquired, as I have elsewhere
shown, by various animals inhabiting desert islands; and we may see an
instance of this, even in England, in the greater wildness of all our
large birds than of our small birds; for the large birds have been most
persecuted by man. We may safely attribute the greater wildness of our
large birds to this cause; for in uninhabited islands large birds are not
more fearful than small; and the magpie, so wary in England, is tame in
Norway, as is the hooded crow in Egypt.
That the general disposition of individuals of the same species, born
in a state of nature, is extremely diversified, can be shown by a
multitude of facts. Several cases also, could be given, of occasional and
strange habits in certain species, which might, if advantageous to the
species, give rise, through natural selection, to quite new instincts.
But I am well aware that these general statements, without facts given in
detail, can produce but a feeble effect on the reader's mind. I can only
repeat my assurance, that I do not speak without good evidence.
The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly considering
a few cases under domestication. We shall thus also be enabled to see the
respective parts which habit and the selection of [213]so-called accidental
variations have played in modifying the mental qualities of our domestic
animals. A number of curious and authentic instances could be given of
the inheritance of all shades of disposition and tastes, and likewise of
the oddest tricks, associated with certain frames of mind or periods of
time. But let us look to the familiar case of the several breeds of dogs:
it cannot be doubted that young pointers (I have myself seen a striking
instance) will sometimes point and even back other dogs the very first
time that they are taken out; retrieving is certainly in some degree
inherited by retrievers; and a tendency to run round, instead of at, a
flock of sheep, by shepherd-dogs. I cannot see that these actions,
performed without experience by the young, and in nearly the same manner
by each individual, performed with eager delight by each breed, and
without the end being known,—for the young pointer can no more know
that he points to aid his master, than the white butterfly knows why she
lays her eggs on the leaf of the cabbage,—I cannot see that these
actions differ essentially from true instincts. If we were to see one
kind of wolf, when young and without any training, as soon as it scented
its prey, stand motionless like a statue, and then slowly crawl forward
with a peculiar gait; and another kind of wolf rushing round, instead of
at, a herd of deer, and driving them to a distant point, we should
assuredly call these actions instinctive. Domestic instincts, as they may
be called, are certainly far less fixed or invariable than natural
instincts; but they have been acted on by far less rigorous selection,
and have been transmitted for an incomparably shorter period, under less
fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are [214]crossed. Thus it is known that a cross
with a bull-dog has affected for many generations the courage and
obstinacy of greyhounds; and a cross with a greyhound has given to a
whole family of shepherd-dogs a tendency to hunt hares. These domestic
instincts, when thus tested by crossing, resemble natural instincts,
which in a like manner become curiously blended together, and for a long
period exhibit traces of the instincts of either parent: for example, Le
Roy describes a dog, whose great-grandfather was a wolf, and this dog
showed a trace of its wild parentage only in one way, by not coming in a
straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit, but
this, I think, is not true. No one would ever have thought of teaching,
or probably could have taught, the tumbler-pigeon to tumble,—an
action which, as I have witnessed, is performed by young birds, that have
never seen a pigeon tumble. We may believe that some one pigeon showed a
slight tendency to this strange habit, and that the long-continued
selection of the best individuals in successive generations made tumblers
what they now are; and near Glasgow there are house-tumblers, as I hear
from Mr. Brent, which cannot fly eighteen inches high without going head
over heels. It may be doubted whether any one would have thought of
training a dog to point, had not some one dog naturally shown a tendency
in this line; and this is known occasionally to happen, as I once saw in
a pure terrier: the act of pointing is probably, as many have thought,
only the exaggerated pause of an animal preparing to spring on its prey.
When the first tendency to point was once displayed, methodical selection
and the inherited effects of compulsory training in each successive
generation would soon complete the [215]work; and unconscious
selection is still at work, as each man tries to procure, without
intending to improve the breed, dogs which will stand and hunt best. On
the other hand, habit alone in some cases has sufficed; no animal is more
difficult to tame than the young of the wild rabbit; scarcely any animal
is tamer than the young of the tame rabbit; but I do not suppose that
domestic rabbits have ever been selected for tameness; and I presume that
we must attribute the whole of the inherited change from extreme wildness
to extreme tameness, simply to habit and long-continued close
confinement.
Natural instincts are lost under domestication: a remarkable instance
of this is seen in those breeds of fowls which very rarely or never
become "broody," that is, never wish to sit on their eggs. Familiarity
alone prevents our seeing how universally and largely the minds of our
domestic animals have been modified by domestication. It is scarcely
possible to doubt that the love of man has become instinctive in the dog.
All wolves, foxes, jackals, and species of the cat genus, when kept tame,
are most eager to attack poultry, sheep, and pigs; and this tendency has
been found incurable in dogs which have been brought home as puppies from
countries, such as Tierra del Fuego and Australia, where the savages do
not keep these domestic animals. How rarely, on the other hand, do our
civilised dogs, even when quite young, require to be taught not to attack
poultry, sheep, and pigs! No doubt they occasionally do make an attack,
and are then beaten; and if not cured, they are destroyed; so that habit,
with some degree of selection, has probably concurred in civilising by
inheritance our dogs. On the other hand, young chickens have lost, wholly
by habit, that fear of the dog and cat which no doubt was originally
instinctive in them, in the same way as it is so plainly instinctive in
[216]young pheasants, though reared under a
hen. It is not that chickens have lost all fear, but fear only of dogs
and cats, for if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves in the
surrounding grass or thickets; and this is evidently done for the
instinctive purpose of allowing, as we see in wild ground-birds, their
mother to fly away. But this instinct retained by our chickens has become
useless under domestication, for the mother-hen has almost lost by disuse
the power of flight.
Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar mental
habits and actions, which at first appeared from what we must in our
ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce such inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection, by considering a few cases. I will
select only three, out of the several which I shall have to discuss in my
future work,—namely, the instinct which leads the cuckoo to lay her
eggs in other birds' nests; the slave-making instinct of certain ants;
and the comb-making power of the hive-bee; these two latter instincts
have generally, and most justly, been ranked by naturalists as the most
wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of
the cuckoo's instinct is, that [217]she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own nest
and sit on her own eggs, those first laid would have to be left for some
time unincubated, or there would be eggs and young birds of different
ages in the same nest. If this were the case, the process of laying and
hatching might be inconveniently long, more especially as she has to
migrate at a very early period; and the first hatched young would
probably have to be fed by the male alone. But the American cuckoo is in
this predicament; for she makes her own nest and has eggs and young
successively hatched, all at the same time. It has been asserted that the
American cuckoo occasionally lays her eggs in other birds' nests; but I
hear on the high authority of Dr. Brewer, that this is a mistake.
Nevertheless, I could give several instances of various birds which have
been known occasionally to lay their eggs in other birds' nests. Now let
us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo; but that occasionally she laid an egg in
another bird's nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage having been taken of
the mistaken maternal instinct of another bird, than by their own
mother's care, encumbered as she can hardly fail to be by having eggs and
young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead me to
believe, that the young thus reared would be apt to follow by inheritance
the occasional and aberrant habit of their mother, and in their turn
would be apt to lay their eggs in other birds' nests, and thus be
successful in rearing their young. By a continued process of this nature,
I believe that the strange instinct of our cuckoo could be, and has been,
[218]generated. I may add that, according to
Dr. Gray and to some other observers, the European cuckoo has not utterly
lost all maternal love and care for her own offspring.
The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with
the Gallinaceæ; and this perhaps explains the origin of a singular
instinct in the allied group of ostriches. For several hen ostriches, at
least in the case of the American species, unite and lay first a few eggs
in one nest and then in another; and these are hatched by the males. This
instinct may probably be accounted for by the fact of the hens laying a
large number of eggs; but, as in the case of the cuckoo, at intervals of
two or three days. This instinct, however, of the American ostrich has
not as yet been perfected; for a surprising number of eggs lie strewed
over the plains, so that in one day's hunting I picked up no less than
twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of
bees of other kinds. This case is more remarkable than that of the
cuckoo; for these bees have not only their instincts but their structure
modified in accordance with their parasitic habits; for they do not
possess the pollen-collecting apparatus which would be necessary if they
had to store food for their own young. Some species, likewise, of
Sphegidæ (wasp-like insects) are parasitic on other species; and M. Fabre
has lately shown good reason for believing that although the Tachytes
nigra generally makes its own burrow and stores it with paralysed prey
for its own larvæ to feed on, yet that when this insect finds a burrow
already made and stored by another sphex, it takes advantage of the
prize, and becomes for the occasion parasitic. In this case, as with the
supposed case of the cuckoo, I can [219]see no difficulty in
natural selection making an occasional habit permanent, if of advantage
to the species, and if the insect whose nest and stored food are thus
feloniously appropriated, be not thus exterminated.
Slave-making instinct.—This remarkable instinct was first
discovered in the Formica (Polyerges) rufescens by Pierre Huber, a better
observer even than his celebrated father. This ant is absolutely
dependent on its slaves; without their aid, the species would certainly
become extinct in a single year. The males and fertile females do no
work. The workers or sterile females, though most energetic and
courageous in capturing slaves, do no other work. They are incapable of
making their own nests, or of feeding their own larvæ. When the old nest
is found inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their jaws.
So utterly helpless are the masters, that when Huber shut up thirty of
them without a slave, but with plenty of the food which they like best,
and with their larvae and pupæ to stimulate them to work, they did
nothing; they could not even feed themselves, and many perished of
hunger. Huber then introduced a single slave (F. fusca), and she
instantly set to work, fed and saved the survivors; made some cells and
tended the larvæ, and put all to rights. What can be more extraordinary
than these well-ascertained facts? If we had not known of any other
slave-making ant, it would have been hopeless to have speculated how so
wonderful an instinct could have been perfected.
Another species, Formica sanguinea, was likewise first discovered by
P. Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith,
of [220]the British Museum, to whom I am much
indebted for information on this and other subjects. Although fully
trusting to the statements of Huber and Mr. Smith, I tried to approach
the subject in a sceptical frame of mind, as any one may well be excused
for doubting the truth of so extraordinary and odious an instinct as that
of making slaves. Hence I will give the observations which I have myself
made, in some little detail. I opened fourteen nests of F. sanguinea, and
found a few slaves in all. Males and fertile females of the slave-species
(F. fusca) are found only in their own proper communities, and have never
been observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in their
appearance is very great. When the nest is slightly disturbed, the slaves
occasionally come out, and like their masters are much agitated and
defend the nest: when the nest is much disturbed and the larvæ and pupæ
are exposed, the slaves work energetically with their masters in carrying
them away to a place of safety. Hence, it is clear, that the slaves feel
quite at home. During the months of June and July, on three successive
years, I have watched for many hours several nests in Surrey and Sussex,
and never saw a slave either leave or enter a nest. As, during these
months, the slaves are very few in number, I thought that they might
behave differently when more numerous; but Mr. Smith informs me that he
has watched the nests at various hours during May, June and August, both
in Surrey and Hampshire, and has never seen the slaves, through present
in large numbers in August, either leave or enter the nest. Hence he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and food
of all kinds. During the present year, however, in the month [221]of July, I
came across a community with an unusually large stock of slaves, and I
observed a few slaves mingled with their masters leaving the nest, and
marching along the same road to a tall Scotch-fir-tree, twenty-five yards
distant, which they ascended together, probably in search of aphides or
cocci. According to Huber, who had ample opportunities for observation,
in Switzerland the slaves habitually work with their masters in making
the nest, and they alone open and close the doors in the morning and
evening; and, as Huber expressly states, their principal office is to
search for aphides. This difference in the usual habits of the masters
and slaves in the two countries, probably depends merely on the slaves
being captured in greater numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the
masters carefully carrying (instead of being carried by, as in the case
of F. rufescens) their slaves in their jaws. Another day my attention was
struck by about a score of the slave-makers haunting the same spot, and
evidently not in search of food; they approached and were vigorously
repulsed by an independent community of the slave-species (F. fusca);
sometimes as many as three of these ants clinging to the legs of the
slave-making F. sanguinea. The latter ruthlessly killed their small
opponents, and carried their dead bodies as food to their nest,
twenty-nine yards distant; but they were prevented from getting any pupæ
to rear as slaves. I then dug up a small parcel of the pupæ of F. fusca
from another nest, and put them down on a bare spot near the place of
combat; they were eagerly seized, and carried off by the tyrants, who
perhaps fancied that, after all, they had been victorious in their late
combat. [222]
At the same time I laid on the same place a small parcel of the pupæ
of another species, F. flava, with a few of these little yellow ants
still clinging to the fragments of the nest. This species is sometimes,
though rarely, made into slaves, as has been described by Mr. Smith.
Although so small a species, it is very courageous, and I have seen it
ferociously attack other ants. In one instance I found to my surprise an
independent community of F. flava under a stone beneath a nest of the
slave-making F. sanguinea; and when I had accidentally disturbed both
nests, the little ants attacked their big neighbours with surprising
courage. Now I was curious to ascertain whether F. sanguinea could
distinguish the pupæ of F. fusca, which they habitually make into slaves,
from those of the little and furious F. flava, which they rarely capture,
and it was evident that they did at once distinguish them: for we have
seen that they eagerly and instantly seized the pupæ of F. fusca, whereas
they were much terrified when they came across the pupæ, or even the
earth from the nest of F. flava, and quickly ran away; but in about a
quarter of an hour, shortly after all the little yellow ants had crawled
away, they took heart and carried off the pupæ.
One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying
the dead bodies of F. fusca (showing that it was not a migration) and
numerous pupæ. I traced a long file of ants burthened with booty, for
about forty yards, to a very thick clump of heath, whence I saw the last
individual of F. sanguinea emerge, carrying a pupa; but I was not able to
find the desolated nest in the thick heath. The nest, however, must have
been close at hand, for two or three individuals of F. fusca were rushing
about in the greatest [223]agitation, and one was perched motionless
with its own pupa in its mouth on the top of a spray of heath, an image
of despair, over its ravaged home.
Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with those
of the continental F. rufescens. The latter does not build its own nest,
does not determine its own migrations, does not collect food for itself
or its young, and cannot even feed itself: it is absolutely dependent on
its numerous slaves. Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely few: the
masters determine when and where a new nest shall be formed, and when
they migrate, the masters carry the slaves. Both in Switzerland and
England the slaves seem to have the exclusive care of the larvæ, and the
masters alone go on slave-making expeditions. In Switzerland the slaves
and masters work together, making and bringing materials for the nest:
both, but chiefly the slaves, tend, and milk as it may be called, their
aphides; and thus both collect food for the community. In England the
masters alone usually leave the nest to collect building materials and
food for themselves, their slaves and larvæ. So that the masters in this
country receive much less service from their slaves than they do in
Switzerland.
By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants, which are not slave-makers, will, as
I have seen, carry off pupæ of other species, if scattered near their
nests, it is possible that such pupæ originally stored as food might
become developed; and the foreign ants thus unintentionally reared would
then follow their proper instincts, and do [224]what work they could.
If their presence proved useful to the species which had seized
them—if it were more advantageous to this species to capture
workers than to procreate them—the habit of collecting pupae
originally for food might by natural selection be strengthened and
rendered permanent for the very different purpose of raising slaves. When
the instinct was once acquired, if carried out to a much less extent even
than in our British F. sanguinea, which, as we have seen, is less aided
by its slaves than the same species in Switzerland, I can see no
difficulty in natural selection increasing and modifying the
instinct—always supposing each modification to be of use to the
species—until an ant was formed as abjectly dependent on its slaves
as is the Formica rufescens.
Cell-making instinct of the Hive-Bee.—I will not here
enter on minute details on this subject, but will merely give an outline
of the conclusions at which I have arrived. He must be a dull man who can
examine the exquisite structure of a comb, so beautifully adapted to its
end, without enthusiastic admiration. We hear from mathematicians that
bees have practically solved a recondite problem, and have made their
cells of the proper shape to hold the greatest possible amount of honey,
with the least possible consumption of precious wax in their
construction. It has been remarked that a skilful workman, with fitting
tools and measures, would find it very difficult to make cells of wax of
the true form, though this is perfectly effected by a crowd of bees
working in a dark hive. Grant whatever instincts you please, and it seems
at first quite inconceivable how they can make all the necessary angles
and planes, or even perceive when they are correctly made. But the
difficulty is not [225]nearly so great as it at first appears:
all this beautiful work can be shown, I think, to follow from a few very
simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered only
as a modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of
work. At one end of a short series we have humble-bees, which use their
old cocoons to hold honey, sometimes adding to them short tubes of wax,
and likewise making separate and very irregular rounded cells of wax. At
the other end of the series we have the cells of the hive-bee, placed in
a double layer: each cell, as is well known, is an hexagonal prism, with
the basal edges of its six sides bevelled so as to fit on to a pyramid,
formed of three rhombs. These rhombs have certain angles, and the three
which form the pyramidal base of a single cell on one side of the comb,
enter into the composition of the bases of three adjoining cells on the
opposite side. In the series between the extreme perfection of the cells
of the hive-bee and the simplicity of those of the humble-bee, we have
the cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in structure
between the hive and humble bee, but more nearly related to the latter:
it forms a nearly regular waxen comb of cylindrical cells, in which the
young are hatched, and, in addition, some large cells of wax for holding
honey. These latter cells are nearly spherical and of nearly equal sizes,
and are aggregated into an irregular mass. But the important point to
notice, is that these cells are always made at that degree of nearness to
each other, that they would have [226]intersected or broken
into each other, if the spheres had been completed; but this is never
permitted, the bees building perfectly flat walls of wax between the
spheres which thus tend to intersect. Hence each cell consists of an
outer spherical portion and of two, three, or more perfectly flat
surfaces, according as the cell adjoins two, three, or more other cells.
When one cell comes into contact with three other cells, which, from the
spheres being nearly of the same size, is very frequently and necessarily
the case, the three flat surfaces are united into a pyramid; and this
pyramid, as Huber has remarked, is manifestly a gross imitation of the
three-sided pyramidal bases of the cell of the hive-bee. As in the cells
of the hive-bee, so here, the three plane surfaces in any one cell
necessarily enter into the construction of three adjoining cells. It is
obvious that the Melipona saves wax by this manner of building; for the
flat walls between the adjoining cells are not double, but are of the
same thickness as the outer spherical portions, and yet each flat portion
forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had made
them of equal sizes and had arranged them symmetrically in a double
layer, the resulting structure would probably have been as perfect as the
comb of the hive-bee. Accordingly I wrote to Professor Miller, of
Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct:—
If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius × √2, or radius × 1.41421 (or at some lesser distance), from
the centres of the six surrounding spheres in the same [227]layer; and at
the same distance from the centres of the adjoining spheres in the other
and parallel layer; then, if planes of intersection between the several
spheres in both layers be formed, there will result a double layer of
hexagonal prisms united together by pyramidal bases formed of three
rhombs; and the rhombs and the sides of the hexagonal prisms will have
every angle identically the same with the best measurements which have
been made of the cells of the hive-bee.
Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as that
of the hive-bee. We must suppose the Melipona to make her cells truly
spherical, and of equal sizes; and this would not be very surprising,
seeing that she already does so to a certain extent, and seeing what
perfectly cylindrical burrows in wood many insects can make, apparently
by turning round on a fixed point. We must suppose the Melipona to
arrange her cells in level layers, as she already does her cylindrical
cells; and we must further suppose, and this is the greatest difficulty,
that she can somehow judge accurately at what distance to stand from her
fellow-labourers when several are making their spheres; but she is
already so far enabled to judge of distance, that she always describes
her spheres so as to intersect largely; and then she unites the points of
intersection by perfectly flat surfaces. We have further to suppose, but
this is no difficulty, that after hexagonal prisms have been formed by
the intersection of adjoining spheres in the same layer, she can prolong
the hexagon to any length requisite to hold the stock of honey; in the
same way as the rude humble-bee adds cylinders of wax to the circular
mouths of her old cocoons. By such [228]modifications of
instincts in themselves not very wonderful,—hardly more wonderful
than those which guide a bird to make its nest,—I believe that the
hive-bee has acquired, through natural selection, her inimitable
architectural powers.
But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, square strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made
them wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere, and of about
the diameter of a cell. It was most interesting to me to observe that
wherever several bees had begun to excavate these basins near together,
they had begun their work at such a distance from each other, that by the
time the basins had acquired the above stated width (i.e. about
the width of an ordinary cell), and were in depth about one sixth of the
diameter of the sphere of which they formed a part, the rims of the
basins intersected or broke into each other. As soon as this occurred,
the bees ceased to excavate, and began to build up flat walls of wax on
the lines of intersection between the basins, so that each hexagonal
prism was built upon the scalloped edge of a smooth basin, instead of on
the straight edges of a three-sided pyramid as in the case of ordinary
cells.
I then put into the hive, instead of a thick, square piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin, that
the bottoms of the basins, if they had been excavated to the same depth
as in the former [229]experiment, would have broken into each
other from the opposite sides. The bees, however, did not suffer this to
happen, and they stopped their excavations in due time; so that the
basins, as soon as they had been a little deepened, came to have flat
bottoms; and these flat bottoms, formed by thin little plates of the
vermilion wax having been left ungnawed, were situated, as far as the eye
could judge, exactly along the planes of imaginary intersection between
the basins on the opposite sides of the ridge of wax. In parts, only
little bits, in other parts, large portions of a rhombic plate had been
left between the opposed basins, but the work, from the unnatural state
of things, had not been neatly performed. The bees must have worked at
very nearly the same rate on the opposite sides of the ridge of vermilion
wax, as they circularly gnawed away and deepened the basins on both
sides, in order to have succeeded in thus leaving flat plates between the
basins, by stopping work along the intermediate planes or planes of
intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of
wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has appeared
to me that the bees do not always succeed in working at exactly the same
rate from the opposite sides; for I have noticed half-completed rhombs at
the base of a just-commenced cell, which were slightly concave on one
side, where I suppose that the bees had excavated too quickly, and convex
on the opposed side, where the bees had worked less quickly. In one
well-marked instance, I put the comb back into the hive, and allowed the
bees to go on working for a short time, and again examined the cell, and
I found that the rhombic [230]plate had been completed, and had become
perfectly flat: it was absolutely impossible, from the extreme
thinness of the little rhombic plate, that they could have effected this
by gnawing away the convex side; and I suspect that the bees in such
cases stand in the opposed cells and push and bend the ductile and warm
wax (which as I have tried is easily done) into its proper intermediate
plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly see
that if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw into this from the opposite
sides, always working circularly as they deepen each cell. They do not
make the whole three-sided pyramidal base of any one cell at the same
time, but only the one rhombic plate which stands on the extreme growing
margin, or the two plates, as the case may be; and they never complete
the upper edges of the rhombic plates, until the hexagonal walls are
commenced. Some of these statements differ from those made by the justly
celebrated elder Huber, but I am convinced of their accuracy; and if I
had space, I could show that they are conformable with my theory.
Huber's statement that the very first cell is excavated out of a
little parallel-sided wall of wax, is not, as far as I have seen,
strictly correct; the first commencement having always been a little hood
of wax; but I will not here enter on these details. We see how important
[231]a part excavation plays in the
construction of the cells; but it would be a great error to suppose that
the bees cannot build up a rough wall of wax in the proper
position—that is, along the plane of intersection between two
adjoining spheres. I have several specimens showing clearly that they can
do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But
the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten to twenty times
thicker than the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a smooth,
very thin wall is left in the middle; the masons always piling up the
cut-away cement, and adding fresh cement, on the summit of the ridge. We
shall thus have a thin wall steadily growing upward; but always crowned
by a gigantic coping. From all the cells, both those just commenced and
those completed, being thus crowned by a strong coping of wax, the bees
can cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth of an inch in
thickness; the plates of the pyramidal basis being about twice as thick.
By this singular manner of building, strength is continually given to the
comb, with the utmost ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee after
working a short time at one cell going to another, so that, as Huber has
stated, [232]a score of individuals work even at the
commencement of the first cell. I was able practically to show this fact,
by covering the edges of the hexagonal walls of a single cell, or the
extreme margin of the circumferential rim of a growing comb, with an
extremely thin layer of melted vermilion wax; and I invariably found that
the colour was most delicately diffused by the bees—as delicately
as a painter could have done with his brush—by atoms of the
coloured wax having been taken from the spot on which it had been placed,
and worked into the growing edges of the cells all round. The work of
construction seems to be a sort of balance struck between many bees, all
instinctively standing at the same relative distance from each other, all
trying to sweep equal spheres, and then building up, or leaving ungnawed,
the planes of intersection between these spheres. It was really curious
to note in cases of difficulty, as when two pieces of comb met at an
angle, how often the bees would pull down and rebuild in different ways
the same cell, sometimes recurring to a shape which they had at first
rejected.
When bees have a place on which they can stand in their proper
positions for working,—for instance, on a slip of wood, placed
directly under the middle of a comb growing downwards so that the comb
has to be built over one face of the slip—in this case the bees can
lay the foundations of one wall of a new hexagon, in its strictly proper
place, projecting beyond the other completed cells. It suffices that the
bees should be enabled to stand at their proper relative distances from
each other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate between
two adjoining spheres; but, as far as I have seen, they never gnaw away
and finish off the angles of a cell till a large part both of that cell
and of [233]the adjoining cells has been built. This
capacity in bees of laying down under certain circumstances a rough wall
in its proper place between two just-commenced cells, is important, as it
bears on a fact, which seems at first quite subversive of the foregoing
theory; namely, that the cells on the extreme margin of wasp-combs are
sometimes strictly hexagonal; but I have not space here to enter on this
subject. Nor does there seem to me any great difficulty in a single
insect (as in the case of a queen-wasp) making hexagonal cells, if she
work alternately on the inside and outside of two or three cells
commenced at the same time, always standing at the proper relative
distance from the parts of the cells just begun, sweeping spheres or
cylinders, and building up intermediate planes. It is even conceivable
that an insect might, by fixing on a point at which to commence a cell,
and then moving outside, first to one point, and then to five other
points, at the proper relative distances from the central point and from
each other, strike the planes of intersection, and so make an isolated
hexagon: but I am not aware that any such case has been observed; nor
would any good be derived from a single hexagon being built, as in its
construction more materials would be required than for a cylinder.
As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the individual
under its conditions of life, it may reasonably be asked, how a long and
graduated succession of modified architectural instincts, all tending
towards the present perfect plan of construction, could have profited the
progenitors of the hive-bee? I think the answer is not difficult: it is
known that bees are often hard pressed to get sufficient nectar; and I am
informed by Mr. Tegetmeier that it has been experimentally found that no
less than from twelve to fifteen pounds of dry sugar [234]are consumed
by a hive of bees for the secretion of each pound of wax; to that a
prodigious quantity of fluid nectar must be collected and consumed by the
bees in a hive for the secretion of the wax necessary for the
construction of their combs. Moreover, many bees have to remain idle for
many days during the process of secretion. A large store of honey is
indispensable to support a large stock of bees during the winter; and the
security of the hive is known mainly to depend on a large number of bees
being supported. Hence the saving of wax by largely saving honey must be
a most important element of success in any family of bees. Of course the
success of any species of bee may be dependent on the number of its
parasites or other enemies, or on quite distinct causes, and so be
altogether independent of the quantity of honey which the bees could
collect. But let us suppose that this latter circumstance determined, as
it probably often does determine, the numbers of a humble-bee which could
exist in a country; and let us further suppose that the community lived
throughout the winter, and consequently required a store of honey: there
can in this case be no doubt that it would be an advantage to our
humble-bee, if a slight modification of her instinct led her to make her
waxen cells near together, so as to intersect a little; for a wall in
common even to two adjoining cells, would save some little wax. Hence it
would continually be more and more advantageous to our humble-bee, if she
were to make her cells more and more regular, nearer together, and
aggregated into a mass, like the cells of the Melipona; for in this case
a large part of the bounding surface of each cell would serve to bound
other cells, and much wax would be saved. Again, from the same cause, it
would be advantageous to the Melipona, if she were to make her cells
closer together, and more regular in every way [235]than at present; for
then, as we have seen, the spherical surfaces would wholly disappear, and
would all be replaced by plane surfaces; and the Melipona would make a
comb as perfect as that of the hive-bee. Beyond this stage of perfection
in architecture, natural selection could not lead; for the comb of the
hive-bee, as far as we can see, is absolutely perfect in economising
wax.
Thus, as I believe, the most wonderful of all known instincts, that of
the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax along
the planes of intersection. The bees, of course, no more knowing that
they swept their spheres at one particular distance from each other, than
they know what are the several angles of the hexagonal prisms and of the
basal rhombic plates. The motive power of the process of natural
selection having been economy of wax; that individual swarm which wasted
least honey in the secretion of wax, having succeeded best, and having
transmitted by inheritance its newly acquired economical instinct to new
swarms, which in their turn will have had the best chance of succeeding
in the struggle for existence.
No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,—cases, in which we cannot see
how an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could [236]hardly have
been acted on by natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that we cannot
account for their similarity by inheritance from a common parent, and
must therefore believe that they have been acquired by independent acts
of natural selection. I will not here enter on these several cases, but
will confine myself to one special difficulty, which at first appeared to
me insuperable, and actually fatal to my whole theory. I allude to the
neuters or sterile females in insect-communities: for these neuters often
differ widely in instinct and in structure from both the males and
fertile females, and yet, from being sterile, they cannot propagate their
kind.
The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can be
shown that some insects and other articulate animals in a state of nature
occasionally become sterile; and if such insects had been social, and it
had been profitable to the community that a number should have been
annually born capable of work, but incapable of procreation, I can see no
very great difficulty in this being effected by natural selection. But I
must pass over this preliminary difficulty. The great difficulty lies in
the working ants differing widely from both the males and the fertile
females in structure, as in the shape of the thorax and in being
destitute of wings and sometimes of eyes, and in instinct. As far as
instinct alone is concerned, the prodigious difference in this respect
between the workers and the perfect females, would have been far better
exemplified by the hive-bee. If a working ant or other neuter insect had
been an animal [237]in the ordinary state, I should have
unhesitatingly assumed that all its characters had been slowly acquired
through natural selection; namely, by an individual having been born with
some slight profitable modification of structure, this being inherited by
its offspring, which again varied and were again selected, and so
onwards. But with the working ant we have an insect differing greatly
from its parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or instinct
to its progeny. It may well be asked how is it possible to reconcile this
case with the theory of natural selection?
First, let it be remembered that we have innumerable instances, both
in our domestic productions and in those in a state of nature, of all
sorts of differences of structure which have become correlated to certain
ages, and to either sex. We have differences correlated not only to one
sex, but to that short period alone when the reproductive system is
active, as in the nuptial plumage of many birds, and in the hooked jaws
of the male salmon. We have even slight differences in the horns of
different breeds of cattle in relation to an artificially imperfect state
of the male sex; for oxen of certain breeds have longer horns than in
other breeds, in comparison with the horns of the bulls or cows of these
same breeds. Hence I can see no real difficulty in any character having
become correlated with the sterile condition of certain members of
insect-communities: the difficulty lies in understanding how such
correlated modifications of structure could have been slowly accumulated
by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied
to the family, as well as to the individual, and may thus gain the [238]desired end. Thus, a well-flavoured
vegetable is cooked, and the individual is destroyed; but the
horticulturist sows seeds of the same stock, and confidently expects to
get nearly the same variety: breeders of cattle wish the flesh and fat to
be well marbled together; the animal has been slaughtered, but the
breeder goes with confidence to the same family. I have such faith in the
powers of selection, that I do not doubt that a breed of cattle, always
yielding oxen with extraordinarily long horns, could be slowly formed by
carefully watching which individual bulls and cows, when matched,
produced oxen with the longest horns; and yet no one ox could ever have
propagated its kind. Thus I believe it has been with social insects: a
slight modification of structure, or instinct, correlated with the
sterile condition of certain members of the community, has been
advantageous to the community: consequently the fertile males and females
of the same community flourished, and transmitted to their fertile
offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, until
that prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many
social insects.
But we have not as yet touched on the climax of the difficulty;
namely, the fact that the neuters of several ants differ, not only from
the fertile females and males, but from each other, sometimes to an
almost incredible degree, and are thus divided into two or even three
castes. The castes, moreover, do not generally graduate into each other,
but are perfectly well defined; being as distinct from each other, as are
any two species of the same genus, or rather as any two genera of the
same family. Thus in Eciton, there are working and soldier neuters, with
jaws and instincts extraordinarily [239]different: in
Cryptocerus, the workers of one caste alone carry a wonderful sort of
shield on their heads, the use of which is quite unknown: in the Mexican
Myrmecocystus, the workers of one caste never leave the nest; they are
fed by the workers of another caste, and they have an enormously
developed abdomen which secretes a sort of honey, supplying the place of
that excreted by the aphides, or the domestic cattle as they may be
called, which our European ants guard or imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,—in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few
alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the
neuters ultimately came to have the desired character. On this view we
ought occasionally to find neuter-insects of the same species, in the
same nest, presenting gradations of structure; and this we do find, even
often, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown how surprisingly the neuters
of several British ants differ from each other in size and sometimes in
colour; and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers [240]are the most numerous;
or that both large and small are numerous, with those of an intermediate
size scanty in numbers. Formica flava has larger and smaller workers,
with some of intermediate size; and, in this species, as Mr. F. Smith has
observed, the larger workers have simple eyes (ocelli), which though
small can be plainly distinguished, whereas the smaller workers have
their ocelli rudimentary. Having carefully dissected several specimens of
these workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their proportionally
lesser size; and I fully believe, though I dare not assert so positively,
that the workers of intermediate size have their ocelli in an exactly
intermediate condition. So that we here have two bodies of sterile
workers in the same nest, differing not only in size, but in their organs
of vision, yet connected by some few members in an intermediate
condition. I may digress by adding, that if the smaller workers had been
the most useful to the community, and those males and females had been
continually selected, which produced more and more of the smaller
workers, until all the workers had come to be in this condition; we
should then have had a species of ant with neuters very nearly in the
same condition with those of Myrmica. For the workers of Myrmica have not
even rudiments of ocelli, though the male and female ants of this genus
have well-developed ocelli.
I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different castes
of neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver ant
(Anomma) of West Africa. The reader will perhaps best appreciate the
amount of difference in these [241]workers, by my giving not the actual
measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house of whom
many were five feet four inches high, and many sixteen feet high; but we
must suppose that the larger workmen had heads four instead of three
times as big as those of the smaller men, and jaws nearly five times as
big. The jaws, moreover, of the working ants of the several sizes
differed wonderfully in shape, and in the form and number of the teeth.
But the important fact for us is, that though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Mr. Lubbock made drawings for me
with the camera lucida of the jaws which I had dissected from the workers
of the several sizes.
With these facts before me, I believe that natural selection, by
acting on the fertile parents, could form a species which should
regularly produce neuters, either all of large size with one form of jaw,
or all of small size with jaws having a widely different structure; or
lastly, and this is our climax of difficulty, one set of workers of one
size and structure, and simultaneously another set of workers of a
different size and structure;—a graduated series having been first
formed, as in the case of the driver ant, and then the extreme forms,
from being the most useful to the community, having been produced in
greater and greater numbers through the natural selection of the parents
which generated them; until none with an intermediate structure were
produced.
Thus, as I believe, the wonderful fact of two distinctly defined
castes of sterile workers existing in the same nest, both widely
different from each other and from [242]their parents, has
originated. We can see how useful their production may have been to a
social community of insects, on the same principle that the division of
labour is useful to civilised man. As ants work by inherited instincts
and by inherited organs or tools, and not by acquired knowledge and
manufactured instruments, a perfect division of labour could be effected
with them only by the workers being sterile; for had they been fertile,
they would have intercrossed, and their instincts and structure would
have become blended. And nature has, as I believe, effected this
admirable division of labour in the communities of ants, by the means of
natural selection. But I am bound to confess, that, with all my faith in
this principle, I should never have anticipated that natural selection
could have been efficient in so high a degree, had not the case of these
neuter insects convinced me of the fact. I have, therefore, discussed
this case, at some little but wholly insufficient length, in order to
show the power of natural selection, and likewise because this is by far
the most serious special difficulty, which my theory has encountered. The
case, also, is very interesting, as it proves that with animals, as with
plants, any amount of modification in structure can be effected by the
accumulation of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without exercise or habit
having come into play. For no amount of exercise, or habit, or volition,
in the utterly sterile members of a community could possibly affect the
structure or instincts of the fertile members, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of Lamarck.
Summary.—I have endeavoured briefly in this chapter [243]to
show that the mental qualities of our domestic animals vary, and that the
variations are inherited. Still more briefly I have attempted to show
that instincts vary slightly in a state of nature. No one will dispute
that instincts are of the highest importance to each animal. Therefore I
can see no difficulty, under changing conditions of life, in natural
selection accumulating slight modifications of instinct to any extent, in
any useful direction. In some cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand,
the fact that instincts are not always absolutely perfect and are liable
to mistakes;—that no instinct has been produced for the exclusive
good of other animals, but that each animal takes advantage of the
instincts of others;—that the canon in natural history, of "Natura
non facit saltum," is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable,—all tend to corroborate the theory of
natural selection.
This theory is, also, strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining
nearly the same instincts. For instance, we can understand on the
principle of inheritance, how it is that the thrush of South America
lines its nest with mud, in the same peculiar manner as does our British
thrush: how it is that the male wrens (Troglodytes) of North America,
build "cock-nests," to roost in, like the males of our distinct
Kitty-wrens,—a habit wholly unlike that of [244]any other known bird.
Finally, it may not be a logical deduction, but to my imagination it is
far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers,—ants making slaves,—the larvae
of ichneumonidæ feeding within the live bodies of caterpillars,—not
as specially endowed or created instincts, but as small consequences of
one general law, leading to the advancement of all organic beings,
namely, multiply, vary, let the strongest live and the weakest die.
[245]
CHAPTER VIII.
Hybridism.
Distinction between the sterility of first crosses and of
hybrids—Sterility various in degree, not universal, affected by
close interbreeding, removed by domestication—Laws governing the
sterility of hybrids—Sterility not a special endowment, but
incidental on other differences—Causes of the sterility of first
crosses and of hybrids—Parallelism between the effects of changed
conditions of life and crossing—Fertility of varieties when crossed
and of their mongrel offspring not universal—Hybrids and mongrels
compared independently of their fertility—Summary.
The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility,
in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country
could hardly have kept distinct had they been capable of crossing freely.
The importance of the fact that hybrids are very generally sterile, has,
I think, been much underrated by some late writers. On the theory of
natural selection the case is especially important, inasmuch as the
sterility of hybrids could not possibly be of any advantage to them, and
therefore could not have been acquired by the continued preservation of
successive profitable degrees of sterility. I hope, however, to be able
to show that sterility is not a specially acquired or endowed quality,
but is incidental on other acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together; namely,
the sterility of two species [246]when first crossed, and the sterility of
the hybrids produced from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the organs themselves are perfect in
structure, as far as the microscope reveals. In the first case the two
sexual elements which go to form the embryo are perfect; in the second
case they are either not at all developed, or are imperfectly developed.
This distinction is important, when the cause of the sterility, which is
common to the two cases, has to be considered. The distinction has
probably been slurred over, owing to the sterility in both cases being
looked on as a special endowment, beyond the province of our reasoning
powers.
The fertility of varieties, that is of the forms known or believed to
have descended from common parents, when intercrossed, and likewise the
fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kölreuter and Gärtner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility. Kölreuter
makes the rule universal; but then he cuts the knot, for in ten cases in
which he found two forms, considered by most authors as distinct species,
quite fertile together, he unhesitatingly ranks [247]them as varieties.
Gärtner, also, makes the rule equally universal; and he disputes the
entire fertility of Kölreuter's ten cases. But in these and in many other
cases, Gärtner is obliged carefully to count the seeds, in order to show
that there is any degree of sterility. He always compares the maximum
number of seeds produced by two species when crossed and by their hybrid
offspring, with the average number produced by both pure parent-species
in a state of nature. But a serious cause of error seems to me to be here
introduced: a plant to be hybridised must be castrated, and, what is
often more important, must be secluded in order to prevent pollen being
brought to it by insects from other plants. Nearly all the plants
experimentised on by Gärtner were potted, and apparently were kept in a
chamber in his house. That these processes are often injurious to the
fertility of a plant cannot be doubted; for Gärtner gives in his table
about a score of cases of plants which he castrated, and artificially
fertilised with their own pollen, and (excluding all cases such as the
Leguminosæ, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gärtner during several years repeatedly
crossed the primrose and cowslip, which we have such good reason to
believe to be varieties, and only once or twice succeeded in getting
fertile seed; as he found the common red and blue pimpernels (Anagallis
arvensis and cœrulea), which the best botanists rank as varieties,
absolutely sterile together; and as he came to the same conclusion in
several other analogous cases; it seems to me that we may well be
permitted to doubt whether many other species are really so sterile, when
intercrossed, as Gärtner believes. [248]
It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so insensibly,
and, on the other hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical purposes it is
most difficult to say where perfect fertility ends and sterility begins.
I think no better evidence of this can be required than that the two most
experienced observers who have ever lived, namely, Kölreuter and Gärtner,
should have arrived at diametrically opposite conclusions in regard to
the very same species. It is also most instructive to compare—but I
have not space here to enter on details—the evidence advanced by
our best botanists on the question whether certain doubtful forms should
be ranked as species or varieties, with the evidence from fertility
adduced by different hybridisers, or by the same author, from experiments
made during different years. It can thus be shown that neither sterility
nor fertility affords any clear distinction between species and
varieties; but that the evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived from other
constitutional and structural differences.
In regard to the sterility of hybrids in successive generations;
though Gärtner was enabled to rear some hybrids, carefully guarding them
from a cross with either pure parent, for six or seven, and in one case
for ten generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in the
first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing [249]that close interbreeding lessens
fertility, and, on the other hand, that an occasional cross with a
distinct individual or variety increases fertility, that I cannot doubt
the correctness of this almost universal belief amongst breeders. Hybrids
are seldom raised by experimentalists in great numbers; and as the
parent-species, or other allied hybrids, generally grow in the same
garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids will generally be fertilised during each
generation by their own individual pollen; and I am convinced that this
would be injurious to their fertility, already lessened by their hybrid
origin. I am strengthened in this conviction by a remarkable statement
repeatedly made by Gärtner, namely, that if even the less fertile hybrids
be artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in artificial
fertilisation pollen is as often taken by chance (as I know from my own
experience) from the anthers of another flower, as from the anthers of
the flower itself which is to be fertilised; so that a cross between two
flowers, though probably on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in progress, so careful an
observer as Gärtner would have castrated his hybrids, and this would have
insured in each generation a cross with a pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of the increase of fertility in the
successive generations of artificially fertilised hybrids may, I
believe, be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third most
experienced hybridiser, namely, the Hon. and [250]Rev. W. Herbert. He is
as emphatic in his conclusion that some hybrids are perfectly
fertile—as fertile as the pure parent-species—as are
Kölreuter and Gärtner that some degree of sterility between distinct
species is a universal law of nature. He experimentised on some of the
very same species as did Gärtner. The difference in their results may, I
think, be in part accounted for by Herbert's great horticultural skill,
and by his having hothouses at his command. Of his many important
statements I will here give only a single one as an example, namely, that
"every ovule in a pod of Crinum capense fertilised by C. revolutum
produced a plant, which (he says) I never saw to occur in a case of its
natural fecundation." So that we here have perfect, or even more than
commonly perfect, fertility in a first cross between two distinct
species.
This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants of certain species of Lobelia
and of some other genera, which can be far more easily fertilised by the
pollen of another and distinct species, than by their own pollen; and all
the individuals of nearly all the species of Hippeastrum seem to be in
this predicament. For these plants have been found to yield seed to the
pollen of a distinct species, though quite sterile with their own pollen,
notwithstanding that their own pollen was found to be perfectly good, for
it fertilised distinct species. So that certain individual plants and all
the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance, a bulb of
Hippeastrum aulicum produced four flowers; three were fertilised by
Herbert with their own pollen, and the fourth was subsequently fertilised
by the pollen of a compound hybrid descended from three other and
distinct [251]species: the result was that "the ovaries
of the three first flowers soon ceased to grow, and after a few days
perished entirely, whereas the pod impregnated by the pollen of the
hybrid made vigorous growth and rapid progress to maturity, and bore good
seed, which vegetated freely." In a letter to me, in 1839, Mr. Herbert
told me that he had then tried the experiment during five years, and he
continued to try it during several subsequent years, and always with the
same result. This result has, also, been confirmed by other observers in
the case of Hippeastrum with its sub-genera, and in the case of some
other genera, as Lobelia, Passiflora and Verbascum. Although the plants
in these experiments appeared perfectly healthy, and although both the
ovules and pollen of the same flower were perfectly good with respect to
other species, yet as they were functionally imperfect in their mutual
self-action, we must infer that the plants were in an unnatural state.
Nevertheless these facts show on what slight and mysterious causes the
lesser or greater fertility of species when crossed, in comparison with
the same species when self-fertilised, sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely dissimilar
in general habit, "reproduced itself as perfectly as if it had been a
natural species from the mountains of Chile." I have taken some pains to
ascertain the degree of fertility of some of the complex crosses of
Rhododendrons, and I am assured that many of them [252]are perfectly fertile.
Mr. C. Noble, for instance, informs me that he raises stocks for grafting
from a hybrid between Rhod. Ponticum and Catawbiense, and that this
hybrid "seeds as freely as it is possible to imagine." Had hybrids, when
fairly treated, gone on decreasing in fertility in each successive
generation, as Gärtner believes to be the case, the fact would have been
notorious to nurserymen. Horticulturists raise large beds of the same
hybrids, and such alone are fairly treated, for by insect agency the
several individuals of the same hybrid variety are allowed to freely
cross with each other, and the injurious influence of close interbreeding
is thus prevented. Any one may readily convince himself of the efficiency
of insect-agency by examining the flowers of the more sterile kinds of
hybrid rhododendrons, which produce no pollen, for he will find on their
stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that is
if the genera of animals are as distinct from each other, as are the
genera of plants, then we may infer that animals more widely separated in
the scale of nature can be more easily crossed than in the case of
plants; but the hybrids themselves are, I think, more sterile. I doubt
whether any case of a perfectly fertile hybrid animal can be considered
as thoroughly well authenticated. It should, however, be borne in mind
that, owing to few animals breeding freely under confinement, few
experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine other finches, but as not one of these nine
species breeds freely in confinement, we have no right to expect that the
first crosses between them and the canary, or that their hybrids, [253]should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of the
same hybrid have been raised at the same time from different parents, so
as to avoid the ill effects of close interbreeding. On the contrary,
brothers and sisters have usually been crossed in each successive
generation, in opposition to the constantly repeated admonition of every
breeder. And in this case, it is not at all surprising that the inherent
sterility in the hybrids should have gone on increasing. If we were to
act thus, and pair brothers and sisters in the case of any pure animal,
which from any cause had the least tendency to sterility, the breed would
assuredly be lost in a very few generations.
Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus and with P. versicolor are perfectly fertile.
There is no doubt that these three pheasants, namely, the common, the
true ring-necked, and the Japan, intercross, and are becoming blended
together in the woods of several parts of England. The hybrids from the
common and Chinese geese (A. cygnoides), species which are so different
that they are generally ranked in distinct genera, have often bred in
this country with either pure parent, and in one single instance they
have bred inter se. This was effected by Mr. Eyton, who raised two
hybrids from the same parents but from different hatches; and from these
two birds he raised no less than eight hybrids (grandchildren of the pure
geese) from one nest. In India, however, these cross-bred geese must be
far more fertile; for I am assured by two eminently capable judges,
namely [254]Mr. Blyth and Capt. Hutton, that whole
flocks of these crossed geese are kept in various parts of the country;
and as they are kept for profit, where neither pure parent-species
exists, they must certainly be highly fertile.
A doctrine which originated with Pallas, has been largely accepted by
modern naturalists; namely, that most of our domestic animals have
descended from two or more wild species, since commingled by
intercrossing. On this view, the aboriginal species must either at first
have produced quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe
in its truth, although it rests on no direct evidence. I believe, for
instance, that our dogs have descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; and analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again there is
reason to believe that our European and the humped Indian cattle are
quite fertile together; but from facts communicated to me by Mr. Blyth, I
think they must be considered as distinct species. On this view of the
origin of many of our domestic animals, we must either give up the belief
of the almost universal sterility of distinct species of animals when
crossed; or we must look at sterility, not as an indelible
characteristic, but as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses [255]and in hybrids, is an extremely general
result; but that it cannot, under our present state of knowledge, be
considered as absolutely universal.
Laws governing the Sterility of first Crosses and of
Hybrids.—We will now consider a little more in detail the
circumstances and rules governing the sterility of first crosses and of
hybrids. Our chief object will be to see whether or not the rules
indicate that species have specially been endowed with this quality, in
order to prevent their crossing and blending together in utter confusion.
The following rules and conclusions are chiefly drawn up from Gärtner's
admirable work on the hybridisation of plants. I have taken much pains to
ascertain how far the rules apply to animals, and considering how scanty
our knowledge is in regard to hybrid animals, I have been surprised to
find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown to
exist; but only the barest outline of the facts can here be given. When
pollen from a plant of one family is placed on the stigma of a plant of a
distinct family, it exerts no more influence than so much inorganic dust.
From this absolute zero of fertility, the pollen of different species of
the same genus applied to the stigma of some one species, yields a
perfect gradation in the number of seeds produced, up to nearly complete
or even quite complete fertility; and, as we have seen, in certain
abnormal cases, even to an excess of fertility, beyond that which the
plant's own pollen will produce. So in hybrids themselves, there are some
which never have produced, and probably never would produce, even [256]with
the pollen of either pure parent, a single fertile seed: but in some of
these cases a first trace of fertility may be detected, by the pollen of
one of the pure parent-species causing the flower of the hybrid to wither
earlier than it otherwise would have done; and the early withering of the
flower is well known to be a sign of incipient fertilisation. From this
extreme degree of sterility we have self-fertilised hybrids producing a
greater and greater number of seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which
are generally confounded together—is by no means strict. There are
many cases, in which two pure species can be united with unusual
facility, and produce numerous hybrid-offspring, yet these hybrids are
remarkably sterile. On the other hand, there are species which can be
crossed very rarely, or with extreme difficulty, but the hybrids, when at
last produced, are very fertile. Even within the limits of the same
genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under the
same circumstances, but depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it is
with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same
capsule and exposed to exactly the same conditions. [257]
By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied species
generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict. A
multitude of cases could be given of very closely allied species which
will not unite, or only with extreme difficulty; and on the other hand of
very distinct species which unite with the utmost facility. In the same
family there may be a genus, as Dianthus, in which very many species can
most readily be crossed; and another genus, as Silene, in which the most
persevering efforts have failed to produce between extremely close
species a single hybrid. Even within the limits of the same genus, we
meet with this same difference; for instance, the many species of
Nicotiana have been more largely crossed than the species of almost any
other genus; but Gärtner found that N. acuminata, which is not a
particularly distinct species, obstinately failed to fertilise, or to be
fertilised by, no less than eight other species of Nicotiana. Very many
analogous facts could be given.
No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked [258]differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass, and
then a male-ass with a mare: these two species may then be said to have
been reciprocally crossed. There is often the widest possible difference
in the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity, or of any
recognisable difference in their whole organisation. On the other hand,
these cases clearly show that the capacity for crossing is connected with
constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kölreuter. To give
an instance: Mirabilis jalapa can easily be fertilised by the pollen of
M. longiflora, and the hybrids thus produced are sufficiently fertile;
but Kölreuter tried more than two hundred times, during eight following
years, to fertilise reciprocally M. longiflora with the pollen of M.
jalapa, and utterly failed. Several other equally striking cases could be
given. Thuret has observed the same fact with certain sea-weeds or Fuci.
Gärtner, moreover, found that this difference of facility in making
reciprocal crosses is extremely common in a lesser degree. He has
observed it even between forms so closely related (as Matthiola annua and
glabra) that many botanists rank them only as varieties. It is also a
remarkable fact, that hybrids raised from reciprocal crosses, though [259]of
course compounded of the very same two species, the one species having
first been used as the father and then as the mother, generally differ in
fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gärtner: for
instance, some species have a remarkable power of crossing with other
species; other species of the same genus have a remarkable power of
impressing their likeness on their hybrid offspring; but these two powers
do not at all necessarily go together. There are certain hybrids which
instead of having, as is usual, an intermediate character between their
two parents, always closely resemble one of them; and such hybrids,
though externally so like one of their pure parent-species, are with rare
exceptions extremely sterile. So again amongst hybrids which are usually
intermediate in structure between their parents, exceptional and abnormal
individuals sometimes are born, which closely resemble one of their pure
parents; and these hybrids are almost always utterly sterile, even when
the other hybrids raised from seed from the same capsule have a
considerable degree of fertility. These facts show how completely
fertility in the hybrid is independent of its external resemblance to
either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced [260]from this cross. That
the fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent. And lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to each
other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from
reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have
been endowed with sterility simply to prevent their becoming confounded
in nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together? Why should the degree of sterility be innately variable in the
individuals of the same species? Why should some species cross with
facility, and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why
should there often be so great a difference in the result of a reciprocal
cross between the same two species? Why, it may even be asked, has the
production of hybrids been permitted? to grant to species the special
power of producing hybrids, and then to stop their further propagation by
different degrees of sterility, not strictly related to the facility of
the first union between their parents, seems to be a strange
arrangement.
The foregoing rules and facts, on the other hand, [261]appear to me
clearly to indicate that the sterility both of first crosses and of
hybrids is simply incidental or dependent on unknown differences, chiefly
in the reproductive systems, of the species which are crossed. The
differences being of so peculiar and limited a nature, that, in
reciprocal crosses between two species the male sexual element of the one
will often freely act on the female sexual element of the other, but not
in a reversed direction. It will be advisable to explain a little more
fully by an example what I mean by sterility being incidental on other
differences, and not a specially endowed quality. As the capacity of one
plant to be grafted or budded on another is so entirely unimportant for
its welfare in a state of nature, I presume that no one will suppose that
this capacity is a specially endowed quality, but will admit that
it is incidental on differences in the laws of growth of the two plants.
We can sometimes see the reason why one tree will not take on another,
from differences in their rate of growth, in the hardness of their wood,
in the period of the flow or nature of their sap, &c.; but in a
multitude of cases we can assign no reason whatever. Great diversity in
the size of two plants, one being woody and the other herbaceous, one
being evergreen and the other deciduous, and adaptation to widely
different climates, does not always prevent the two grafting together. As
in hybridisation, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft trees together belonging to
quite distinct families; and, on the other hand, closely allied species,
and varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no means
absolutely governed by systematic affinity. Although many distinct genera
within the same family have been grafted [262]together, in other
cases species of the same genus will not take on each other. The pear can
be grafted far more readily on the quince, which is ranked as a distinct
genus, than on the apple, which is a member of the same genus. Even
different varieties of the pear take with different degrees of facility
on the quince; so do different varieties of the apricot and peach on
certain varieties of the plum.
As Gärtner found that there was sometimes an innate difference in
different individuals of the same two species in crossing; so
Sagaret believes this to be the case with different individuals of the
same two species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot be
grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different case
from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on their
own roots, and which could be grafted with no great difficulty on another
species, when thus grafted were rendered barren. On the other hand,
certain species of Sorbus, when grafted on other species, yielded twice
as much fruit as when on their own roots. We are reminded by this latter
fact of the extraordinary case of Hippeastrum, Lobelia, &c, which
seeded much more freely when fertilised with the pollen of distinct
species, than when self-fertilised with their own pollen. [263]
We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a rude
degree of parallelism in the results of grafting and of crossing distinct
species. And as we must look at the curious and complex laws governing
the facility with which trees can be grafted on each other as incidental
on unknown differences in their vegetative systems, so I believe that the
still more complex laws governing the facility of first crosses, are
incidental on unknown differences, chiefly in their reproductive systems.
These differences, in both cases, follow to a certain extent, as might
have been expected, systematic affinity, by which every kind of
resemblance and dissimilarity between organic beings is attempted to be
expressed. The facts by no means seem to me to indicate that the greater
or lesser difficulty of either grafting or crossing together various
species has been a special endowment; although in the case of crossing,
the difficulty is as important for the endurance and stability of
specific forms, as in the case of grafting it is unimportant for their
welfare.
Causes of the Sterility of first Crosses and of
Hybrids.—We may now look a little closer at the probable causes
of the sterility of first crosses and of hybrids. These two cases are
fundamentally different, for, as just remarked, in the union of two pure
species the male and female sexual elements are perfect, whereas in
hybrids they are imperfect. Even in first crosses, the greater or lesser
difficulty in effecting a union apparently depends on several distinct
causes. There must sometimes be a physical impossibility in the male
element reaching the ovule, as would be the case with a plant [264]having a
pistil too long for the pollen-tubes to reach the ovarium. It has also
been observed that when pollen of one species is placed on the stigma of
a distantly allied species, though the pollen-tubes protrude, they do not
penetrate the stigmatic surface. Again, the male element may reach the
female element, but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and then
perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated
to me by Mr. Hewitt, who has had great experience in hybridising
gallinaceous birds, that the early death of the embryo is a very frequent
cause of sterility in first crosses. I was at first very unwilling to
believe in this view; as hybrids, when once born, are generally healthy
and long-lived, as we see in the case of the common mule. Hybrids,
however, are differently circumstanced before and after birth: when born
and living in a country where their two parents can live, they are
generally placed under suitable conditions of life. But a hybrid partakes
of only half of the nature and constitution of its mother, and therefore
before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to
conditions in some degree unsuitable, and consequently be liable to
perish at an early period; more especially as all very young beings seem
eminently sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is [265]very different. I have
more than once alluded to a large body of facts, which I have collected,
showing that when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive systems
seriously affected. This, in fact, is the great bar to the domestication
of animals. Between the sterility thus superinduced and that of hybrids,
there are many points of similarity. In both cases the sterility is
independent of general health, and is often accompanied by excess of size
or great luxuriance. In both cases, the sterility occurs in various
degrees; in both, the male element is the most liable to be affected; but
sometimes the female more than the male. In both, the tendency goes to a
certain extent with systematic affinity, for whole groups of animals and
plants are rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other hand, one
species in a group will sometimes resist great changes of conditions with
unimpaired fertility; and certain species in a group will produce
unusually fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement or any exotic plant seed
freely under culture; nor can he tell, till he tries, whether any two
species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary, which
is due, as I believe, to their reproductive systems having been specially
affected, though in a lesser degree than when sterility ensues. So it is
with hybrids, for hybrids in successive generations are eminently liable
to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids [266]are produced by the
unnatural crossing of two species, the reproductive system, independently
of the general state of health, is affected by sterility in a very
similar manner. In the one case, the conditions of life have been
disturbed, though often in so slight a degree as to be inappreciable by
us; in the other case, or that of hybrids, the external conditions have
remained the same, but the organisation has been disturbed by two
different structures and constitutions having been blended into one. For
it is scarcely possible that two organisations should be compounded into
one, without some disturbance occurring in the development, or periodical
action, or mutual relation of the different parts and organs one to
another, or to the conditions of life. When hybrids are able to breed
inter se, they transmit to their offspring from generation to
generation the same compounded organisation, and hence we need not be
surprised that their sterility, though in some degree variable, rarely
diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of hybrids;
for instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions, is
rendered sterile. All that I have attempted to show, is that in two
cases, in some respects allied, sterility is the common result,—in
the one case from the conditions of life having been disturbed, in the
other case from the organisation having been disturbed by two
organisations having been compounded into one.
It may seem fanciful, but I suspect that a similar [267]parallelism
extends to an allied yet very different class of facts. It is an old and
almost universal belief, founded, I think, on a considerable body of
evidence, that slight changes in the conditions of life are beneficial to
all living things. We see this acted on by farmers and gardeners in their
frequent exchanges of seed, tubers, &c., from one soil or climate to
another, and back again. During the convalescence of animals, we plainly
see that great benefit is derived from almost any change in the habits of
life. Again, both with plants and animals, there is abundant evidence,
that a cross between very distinct individuals of the same species, that
is between members of different strains or sub-breeds, gives vigour and
fertility to the offspring. I believe, indeed, from the facts alluded to
in our fourth chapter, that a certain amount of crossing is indispensable
even with hermaphrodites; and that close interbreeding continued during
several generations between the nearest relations, especially if these be
kept under the same conditions of life, always induces weakness and
sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour and
fertility to the offspring. But we have seen that greater changes, or
changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some [268]common but unknown
bond, which is essentially related to the principle of life.
Fertility of Varieties when crossed, and of their Mongrel
offspring.—It may be urged, as a most forcible argument, that
there must be some essential distinction between species and varieties,
and that there must be some error in all the foregoing remarks, inasmuch
as varieties, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But if
we look to varieties produced under nature, we are immediately involved
in hopeless difficulties; for if two hitherto reputed varieties be found
in any degree sterile together, they are at once ranked by most
naturalists as species. For instance, the blue and red pimpernel, the
primrose and cowslip, which are considered by many of our best botanists
as varieties, are said by Gärtner not to be quite fertile when crossed,
and he consequently ranks them as undoubted species. If we thus argue in
a circle, the fertility of all varieties produced under nature will
assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily than
other dogs with foxes, or that certain South American indigenous domestic
dogs do not readily cross with European dogs, the explanation which will
occur to every one, and probably the true one, is that these dogs have
descended from several aboriginally distinct species. Nevertheless the
perfect fertility of so many domestic varieties, differing widely from
each other in appearance, for instance of the pigeon or of the cabbage,
is [269]a remarkable fact; more especially when we
reflect how many species there are, which, though resembling each other
most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties less
remarkable than at first appears. It can, in the first place, be clearly
shown that mere external dissimilarity between two species does not
determine their greater or lesser degree of sterility when crossed; and
we may apply the same rule to domestic varieties. In the second place,
some eminent naturalists believe that a long course of domestication
tends to eliminate sterility in the successive generations of hybrids
which were at first only slightly sterile; and if this be so, we surely
ought not to expect to find sterility both appearing and disappearing
under nearly the same conditions of life. Lastly, and this seems to me by
far the most important consideration, new races of animals and plants are
produced under domestication by man's methodical and unconscious power of
selection, for his own use and pleasure: he neither wishes to select, nor
could select, slight differences in the reproductive system, or other
constitutional differences correlated with the reproductive system. He
supplies his several varieties with the same food; treats them in nearly
the same manner, and does not wish to alter their general habits of life.
Nature acts uniformly and slowly during vast periods of time on the whole
organisation, in any way which may be for each creature's own good; and
thus she may, either directly, or more probably indirectly, through
correlation, modify the reproductive system in the several descendants
from any one species. Seeing this difference in the process of selection,
as carried on by man and nature, we need not be surprised at some
difference in the result.
I have as yet spoken as if the varieties of the same [270]species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility in
the few following cases, which I will briefly abstract. The evidence is
at least as good as that from which we believe in the sterility of a
multitude of species. The evidence is, also, derived from hostile
witnesses, who in all other cases consider fertility and sterility as
safe criterions of specific distinction. Gärtner kept during several
years a dwarf kind of maize with yellow seeds, and a tall variety with
red seeds, growing near each other in his garden; and although these
plants have separated sexes, they never naturally crossed. He then
fertilised thirteen flowers of the one with the pollen of the other; but
only a single head produced any seed, and this one head produced only
five grains. Manipulation in this case could not have been injurious, as
the plants have separated sexes. No one, I believe, has suspected that
these varieties of maize are distinct species; and it is important to
notice that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gärtner did not venture to
consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual fertilisation
is by so much the less easy as their differences are greater. How far
these experiments may be trusted, I know not; but the forms
experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.
The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of experiments
made during many years on nine species of Verbascum, by so good an
observer [271]and so hostile a witness, as Gärtner:
namely, that yellow and white varieties of the same species of Verbascum
when intercrossed produce less seed, than do either coloured varieties
when fertilised with pollen from their own coloured flowers. Moreover, he
asserts that when yellow and white varieties of one species are crossed
with yellow and white varieties of a distinct species, more seed
is produced by the crosses between the similarly coloured flowers, than
between those which are differently coloured. Yet these varieties of
Verbascum present no other difference besides the mere colour of the
flower; and one variety can sometimes be raised from the seed of the
other.
From observations which I have made on certain varieties of hollyhock,
I am inclined to suspect that they present analogous facts.
Kölreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the common
tobacco is more fertile, when crossed with a widely distinct species,
than are the other varieties. He experimentised on five forms, which are
commonly reputed to be varieties, and which he tested by the severest
trial, namely, by reciprocal crosses, and he found their mongrel
offspring perfectly fertile. But one of these five varieties, when used
either as father or mother, and crossed with the Nicotiana glutinosa,
always yielded hybrids not so sterile as those which were produced from
the four other varieties when crossed with N. glutinosa. Hence the
reproductive system of this one variety must have been in some manner and
in some degree modified.
From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety if
infertile in any degree would generally be ranked as species; from man
selecting only [272]external characters in the production of
the most distinct domestic varieties, and from not wishing or being able
to produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think that
the very general fertility of varieties can be proved to be of universal
occurrence, or to form a fundamental distinction between varieties and
species. The general fertility of varieties does not seem to me
sufficient to overthrow the view which I have taken with respect to the
very general, but not invariable, sterility of first crosses and of
hybrids, namely, that it is not a special endowment, but is incidental on
slowly acquired modifications, more especially in the reproductive
systems of the forms which are crossed.
Hybrids and Mongrels compared, independently of their
fertility.—Independently of the question of fertility, the
offspring of species when crossed and of varieties when crossed may be
compared in several other respects. Gärtner, whose strong wish was to
draw a marked line of distinction between species and varieties, could
find very few and, as it seems to me, quite unimportant differences
between the so-called hybrid offspring of species, and the so-called
mongrel offspring of varieties. And, on the other hand, they agree most
closely in very many important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are more
variable than hybrids; but Gärtner admits that hybrids from species which
have long been cultivated are often variable in the first generation; and
I have myself seen striking instances of this fact. Gärtner further
admits that hybrids between very closely allied species are more variable
[273]than those from very distinct species; and
this shows that the difference in the degree of variability graduates
away. When mongrels and the more fertile hybrids are propagated for
several generations an extreme amount of variability in their offspring
is notorious; but some few cases both of hybrids and mongrels long
retaining uniformity of character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.
This greater variability of mongrels than of hybrids does not seem to
me at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies in most cases that there has been
recent variability; and therefore we might expect that such variability
would often continue and be superadded to that arising from the mere act
of crossing. The slight degree of variability in hybrids from the first
cross or in the first generation, in contrast with their extreme
variability in the succeeding generations, is a curious fact and deserves
attention. For it bears on and corroborates the view which I have taken
on the cause of ordinary variability; namely, that it is due to the
reproductive system being eminently sensitive to any change in the
conditions of life, being thus often rendered either impotent or at least
incapable of its proper function of producing offspring identical with
the parent-form. Now hybrids in the first generation are descended from
species (excluding those long cultivated) which have not had their
reproductive systems in any way affected, and they are not variable; but
hybrids themselves have their reproductive systems seriously affected,
and their descendants are highly variable.
But to return to our comparison of mongrels and [274]hybrids: Gärtner states
that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in
degree. Gärtner further insists that when any two species, although most
closely allied to each other, are crossed with a third species, the
hybrids are widely different from each other; whereas if two very
distinct varieties of one species are crossed with another species, the
hybrids do not differ much. But this conclusion, as far as I can make
out, is founded on a single experiment; and seems directly opposed to the
results of several experiments made by Kölreuter.
These alone are the unimportant differences, which Gärtner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents, more
especially in hybrids produced from nearly related species, follows
according to Gärtner the same laws. When two species are crossed, one has
sometimes a prepotent power of impressing its likeness on the hybrid; and
so I believe it to be with varieties of plants. With animals one variety
certainly often has this prepotent power over another variety. Hybrid
plants produced from a reciprocal cross, generally resemble each other
closely; and so it is with mongrels from a reciprocal cross. Both hybrids
and mongrels can be reduced to either pure parent-form, by repeated
crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency in
transmitting likeness running more strongly in one sex than in the other,
both when one species is crossed with another, and when, one variety is
crossed with [275]another variety. For instance, I think
those authors are right, who maintain that the ass has a prepotent power
over the horse, so that both the mule and the hinny more resemble the ass
than the horse; but that the prepotency runs more strongly in the
male-ass than in the female, so that the mule, which is the offspring of
the male-ass and mare, is more like an ass, than is the hinny, which is
the offspring of the female-ass and stallion.
Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but it
can be shown that this does sometimes occur with hybrids; yet I grant
much less frequently with hybrids than with mongrels. Looking to the
cases which I have collected of cross-bred animals closely resembling one
parent, the resemblances seem chiefly confined to characters almost
monstrous in their nature, and which have suddenly appeared—such as
albinism, melanism, deficiency of tail or horns, or additional fingers
and toes; and do not relate to characters which have been slowly acquired
by selection. Consequently, sudden reversions to the perfect character of
either parent would be more likely to occur with mongrels, which are
descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly and
naturally produced. On the whole I entirely agree with Dr. Prosper Lucas,
who, after arranging an enormous body of facts with respect to animals,
comes to the conclusion, that the laws of resemblance of the child to its
parents are the same, whether the two parents differ much or little from
each other, namely in the union of individuals of the same variety, or of
different varieties, or of distinct species.
Laying aside the question of fertility and sterility, [276]in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having been
produced by secondary laws, this similarity would be an astonishing fact.
But it harmonises perfectly with the view that there is no essential
distinction between species and varieties.
Summary of Chapter.—First crosses between forms
sufficiently distinct to be ranked as species, and their hybrids, are
very generally, but not universally, sterile. The sterility is of all
degrees, and is often so slight that the two most careful
experimentalists who have ever lived, have come to diametrically opposite
conclusions in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and is eminently susceptible
of favourable and unfavourable conditions. The degree of sterility does
not strictly follow systematic affinity, but is governed by several
curious and complex laws. It is generally different, and sometimes widely
different, in reciprocal crosses between the same two species. It is not
always equal in degree in a first cross and in the hybrid produced from
this cross.
In the same manner as in grafting trees, the capacity of one species
or variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater or
less facility of one species to unite with another, is incidental on
unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
[277]somewhat analogous degrees of difficulty
in being grafted together in order to prevent them becoming inarched in
our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances;
in some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have
had this system and their whole organisation disturbed by being
compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is supported by a
parallelism of another kind;—namely, that the crossing of forms
only slightly different is favourable to the vigour and fertility of
their offspring; and that slight changes in the conditions of life are
apparently favourable to the vigour and fertility of all organic beings.
It is not surprising that the degree of difficulty in uniting two
species, and the degree of sterility of their hybrid-offspring should
generally correspond, though due to distinct causes; for both depend on
the amount of difference of some kind between the species which are
crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of the hybrids produced from it, and the capacity of
being grafted together—though this latter capacity evidently
depends on widely different circumstances—should all run, to a
certain extent, parallel with the systematic affinity of the forms which
are subjected to experiment; for systematic affinity attempts to express
all kinds of resemblance between all species.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite [278]universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we
are to argue in a circle with respect to varieties in a state of nature;
and when we remember that the greater number of varieties have been
produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In all
other respects, excluding fertility, there is a close general resemblance
between hybrids and mongrels. Finally, then, the facts briefly given in
this chapter do not seem to me opposed to, but even rather to support the
view, that there is no fundamental distinction between species and
varieties.
[279]
CHAPTER IX.
On the Imperfection of the Geological Record.
On the absence of intermediate varieties at the present day—On
the nature of extinct intermediate varieties; on their number—On
the vast lapse of time, as inferred from the rate of deposition and of
denudation—On the poorness of our palæontological
collections—On the intermittence of geological formations—On
the absence of intermediate varieties in any one formation—On the
sudden appearance of groups of species—On their sudden appearance
in the lowest known fossiliferous strata.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific forms,
and their not being blended together by innumerable transitional links,
is a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day, under the circumstances apparently
most favourable for their presence, namely on an extensive and continuous
area with graduated physical conditions. I endeavoured to show, that the
life of each species depends in a more important manner on the presence
of other already defined organic forms, than on climate; and, therefore,
that the really governing conditions of life do not graduate away quite
insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms
which they connect, will generally be beaten out and exterminated during
the course of further modification and improvement. The main cause,
however, of innumerable intermediate links not now occurring everywhere
throughout nature [280]depends on the very process of natural
selection, through which new varieties continually take the places of and
exterminate their parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so must the number of
intermediate varieties, which have formerly existed on the earth, be
truly enormous. Why then is not every geological formation and every
stratum full of such intermediate links? Geology assuredly does not
reveal any such finely graduated organic chain; and this, perhaps, is the
most obvious and gravest objection which can be urged against my theory.
The explanation lies, as I believe, in the extreme imperfection of the
geological record.
In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself, forms directly intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor will
generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons have both descended from the rock-pigeon; if we possessed all the
intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds. These
two breeds, moreover, have become so much modified, that if we had no
historical or indirect evidence regarding their origin, it would not have
been possible to have [281]determined from a mere comparison of their
structure with that of the rock-pigeon, whether they had descended from
this species or from some other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that links
ever existed directly intermediate between them, but between each and an
unknown common parent. The common parent will have had in its whole
organisation much general resemblance to the tapir and to the horse; but
in some points of structure may have differed considerably from both,
even perhaps more than they differ from each other. Hence in all such
cases, we should be unable to recognise the parent-form of any two or
more species, even if we closely compared the structure of the parent
with that of its modified descendants, unless at the same time we had a
nearly perfect chain of the intermediate links.
It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and in
this case direct intermediate links will have existed between
them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare event;
for in all cases the new and improved forms of life tend to supplant the
old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the varieties of the same species at the
present [282]day; and these parent-species, now
generally extinct, have in their turn been similarly connected with more
ancient species; and so on backwards, always converging to the common
ancestor of each great class. So that the number of intermediate and
transitional links, between all living and extinct species, must have
been inconceivably great. But assuredly, if this theory be true, such
have lived upon this earth.
On the lapse of Time.—Independently of our not finding
fossil remains of such infinitely numerous connecting links, it may be
objected, that time will not have sufficed for so great an amount of
organic change, all changes having been effected very slowly through
natural selection. It is hardly possible for me even to recall to the
reader, who may not be a practical geologist, the facts leading the mind
feebly to comprehend the lapse of time. He who can read Sir Charles
Lyell's grand work on the Principles of Geology, which the future
historian will recognise as having produced a revolution in natural
science, yet does not admit how incomprehensively vast have been the past
periods of time, may at once close this volume. Not that it suffices to
study the Principles of Geology, or to read special treatises by
different observers on separate formations, and to mark how each author
attempts to give an inadequate idea of the duration of each formation or
even each stratum. A man must for years examine for himself great piles
of superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything of
the lapse of time, the monuments of which we see around us.
It is good to wander along lines of sea-coast, when formed of
moderately hard rocks, and mark the [283]process of degradation.
The tides in most cases reach the cliffs only for a short time twice a
day, and the waves eat into them only when they are charged with sand or
pebbles; for there is good evidence that pure water can effect little or
nothing in wearing away rock. At last the base of the cliff is
undermined, huge fragments fall down, and these remaining fixed, have to
be worn away, atom by atom, until reduced in size they can be rolled
about by the waves, and then are more quickly ground into pebbles, sand,
or mud. But how often do we see along the bases of retreating cliffs
rounded boulders, all thickly clothed by marine productions, showing how
little they are abraded and how seldom they are rolled about! Moreover,
if we follow for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short length
or round a promontory, that the cliffs are at the present time suffering.
The appearance of the surface and the vegetation show that elsewhere
years have elapsed since the waters washed their base.
He who most closely studies the action of the sea on our shores, will,
I believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller, and
by that excellent observer Mr. Smith of Jordan Hill, are most impressive.
With the mind thus impressed, let any one examine beds of conglomerate
many thousand feet in thickness, which, though probably formed at a
quicker rate than many other deposits, yet, from being formed of worn and
rounded pebbles, each of which bears the stamp of time, are good to show
how slowly the mass has been accumulated. In the Cordillera I estimated
one pile of conglomerate at ten thousand feet in thickness. Let the [284]observer remember Lyell's profound remark
that the thickness and extent of sedimentary formations are the result
and measure of the degradation which the earth's crust has elsewhere
suffered. And what an amount of degradation is implied by the sedimentary
deposits of many countries! Professor Ramsay has given me the maximum
thickness, in most cases from actual measurement, in a few cases from
estimate, of each formation in different parts of Great Britain; and this
is the result:—
Feet.
|
Palæozoic strata (not including igneous beds)
|
57,154
|
Secondary strata
|
13,190
|
Tertiary strata
|
2,240
|
—making altogether 72,584 feet; that is, very nearly thirteen
and three-quarters British miles. Some of the formations, which are
represented in England by thin beds, are thousands of feet in thickness
on the Continent. Moreover, between each successive formation, we have,
in the opinion of most geologists, enormously long blank periods. So that
the lofty pile of sedimentary rocks in Britain, gives but an inadequate
idea of the time which has elapsed during their accumulation; yet what
time this must have consumed! Good observers have estimated that sediment
is deposited by the great Mississippi river at the rate of only 600 feet
in a hundred thousand years. This estimate has no pretension to strict
exactness; yet, considering over what wide spaces very fine sediment is
transported by the currents of the sea, the process of accumulation in
any one area must be extremely slow.
But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember [285]having been much struck with the evidence
of denudation, when viewing volcanic islands, which have been worn by the
waves and pared all round into perpendicular cliffs of one or two
thousand feet in height; for the gentle slope of the lava-streams, due to
their formerly liquid state, showed at a glance how far the hard, rocky
beds had once extended into the open ocean. The same story is still more
plainly told by faults,—those great cracks along which the strata
have been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, the
surface of the land has been so completely planed down by the action of
the sea, that no trace of these vast dislocations is externally
visible.
The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied from
600 to 3000 feet. Prof. Ramsay has published an account of a downthrow in
Anglesea of 2300 feet; and he informs me that he fully believes there is
one in Merionethshire of 12,000 feet; yet in these cases there is nothing
on the surface to show such prodigious movements; the pile of rocks on
the one or other side having been smoothly swept away. The consideration
of these facts impresses my mind almost in the same manner as does the
vain endeavour to grapple with the idea of eternity.
I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the denudation
of the Weald has been a mere trifle, in comparison with that which has
removed masses of our palæozoic strata, in parts ten thousand feet in
thickness, as shown in Prof. Ramsay's masterly memoir on this subject:
yet it is an admirable lesson to stand on the intermediate hilly country
and look on the one hand at the North Downs, and [286]on the other hand at
the South Downs; for, remembering that at no great distance to the west
the northern and southern escarpments meet and close, one can safely
picture to oneself the great dome of rocks which must have covered up the
Weald within so limited a period as since the latter part of the Chalk
formation. The distance from the northern to the southern Downs is about
22 miles, and the thickness of the several formations is on an average
about 1100 feet, as I am informed by Prof. Ramsay. But if, as some
geologists suppose, a range of older rocks underlies the Weald, on the
flanks of which the overlying sedimentary deposits might have accumulated
in thinner masses than elsewhere, the above estimate would be erroneous;
but this source of doubt probably would not greatly affect the estimate
as applied to the western extremity of the district. If, then, we knew
the rate at which the sea commonly wears away a line of cliff of any
given height, we could measure the time requisite to have denuded the
Weald. This, of course cannot be done; but we may, in order to form some
crude notion on the subject, assume that the sea would eat into cliffs
500 feet in height at the rate of one inch in a century. This will at
first appear much too small an allowance; but it is the same as if we
were to assume a cliff one yard in height to be eaten back along a whole
line of coast at the rate of one yard in nearly every twenty-two years. I
doubt whether any rock, even as soft as chalk, would yield at this rate
excepting on the most exposed coasts; though no doubt the degradation of
a lofty cliff would be more rapid from the breakage of the fallen
fragments. On the other hand, I do not believe that any line of coast,
ten or twenty miles in length, ever suffers degradation at the same time
along its whole indented length; and we [287]must remember that
almost all strata contain harder layers or nodules, which from long
resisting attrition form a breakwater at the base. We may at least
confidently believe that no rocky coast 500 feet in height commonly
yields at the rate of a foot per century; for this would be the same in
amount as a cliff one yard in height retreating twelve yards in
twenty-two years; and no one, I think, who has carefully observed the
shape of old fallen fragments at the base of cliffs, will admit any near
approach to such rapid wearing away. Hence, under ordinary circumstances,
I should infer that for a cliff 500 feet in height, a denudation of one
inch per century for the whole length would be a sufficient allowance. At
this rate, on the above data, the denudation of the Weald must have
required 306,662,400 years; or say three hundred million years. But
perhaps it would be safer to allow two or three inches per century, and
this would reduce the number of years to one hundred and fifty or one
hundred million years.
The action of fresh water on the gently inclined Wealden district,
when upraised, could hardly have been great, but it would somewhat reduce
the above estimate. On the other hand, during oscillations of level,
which we know this area has undergone, the surface may have existed for
millions of years as land, and thus have escaped the action of the sea:
when deeply submerged for perhaps equally long periods, it would,
likewise, have escaped the action of the coast-waves. So that it is not
improbable that a longer period than 300 million years has elapsed since
the latter part of the Secondary period.
I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During each
of these years, [288]over the whole world, the land and the
water has been peopled by hosts of living forms. What an infinite number
of generations, which the mind cannot grasp, must have succeeded each
other in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!
On the poorness of our Palæontological collections.—That
our palæontological collections are very imperfect, is admitted by every
one. The remark of that admirable palæontologist, the late Edward Forbes,
should not be forgotten, namely, that numbers of our fossil species are
known and named from single and often broken specimens, or from a few
specimens collected on some one spot. Only a small portion of the surface
of the earth has been geologically explored, and no part with sufficient
care, as the important discoveries made every year in Europe prove. No
organism wholly soft can be preserved. Shells and bones will decay and
disappear when left on the bottom of the sea, where sediment is not
accumulating. I believe we are continually taking a most erroneous view,
when we tacitly admit to ourselves that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed
and preserve fossil remains. Throughout an enormously large proportion of
the ocean, the bright blue tint of the water bespeaks its purity. The
many cases on record of a formation conformably covered, after an
enormous interval of time, by another and later formation, without the
underlying bed having suffered in the interval any wear and tear, seem
explicable only on the view of the bottom of the sea not rarely lying for
ages in an unaltered condition. The remains which do become embedded, if
in sand or gravel, will when the beds are upraised generally be dissolved
[289]by the percolation of rain-water. I
suspect that but few of the very many animals which live on the beach
between high and low watermark are preserved. For instance, the several
species of the Chthamalinæ (a subfamily of sessile cirripedes) coat the
rocks all over the world in infinite numbers: they are all strictly
littoral, with the exception of a single Mediterranean species, which
inhabits deep water and has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary formation: yet it
is now known that the genus Chthamalus existed during the chalk period.
The molluscan genus Chiton offers a partially analogous case.
With respect to the terrestrial productions which lived during the
Secondary and Palæozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with the exception of one species discovered by Sir C. Lyell and
Dr. Dawson in the carboniferous strata of North America, of which shell
several specimens have now been collected. In regard to mammiferous
remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how accidental
and rare is their preservation, far better than pages of detail. Nor is
their rarity surprising, when we remember how large a proportion of the
bones of tertiary mammals have been discovered either in caves or in
lacustrine deposits; and that not a cave or true lacustrine bed is known
belonging to the age of our secondary or palæozoic formations.
But the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely, from
the several formations [290]being separated from each other by wide
intervals of time. When we see the formations tabulated in written works,
or when we follow them in nature, it is difficult to avoid believing that
they are closely consecutive. But we know, for instance, from Sir R.
Murchison's great work on Russia, what wide gaps there are in that
country between the superimposed formations; so it is in North America,
and in many other parts of the world. The most skilful geologist, if his
attention had been exclusively confined to these large territories, would
never have suspected that during the periods which were blank and barren
in his own country, great piles of sediment, charged with new and
peculiar forms of life, had elsewhere been accumulated. And if in each
separate territory, hardly any idea can be formed of the length of time
which has elapsed between the consecutive formations, we may infer that
this could nowhere be ascertained. The frequent and great changes in the
mineralogical composition of consecutive formations, generally implying
great changes in the geography of the surrounding lands, whence the
sediment has been derived, accords with the belief of vast intervals of
time having elapsed between each formation.
But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each other
in close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short
geological period. Along the whole west coast, which is inhabited by a
peculiar marine fauna, tertiary beds are so poorly developed, that no
record of several [291]successive and peculiar marine faunas will
probably be preserved to a distant age. A little reflection will explain
why along the rising coast of the western side of South America, no
extensive formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have been great, from
the enormous degradation of the coast-rocks and from muddy streams
entering the sea. The explanation, no doubt, is, that the littoral and
sub-littoral deposits are continually worn away, as soon as they are
brought up by the slow and gradual rising of the land within the grinding
action of the coast-waves.
We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of the sea, in
which case, judging from the researches of E. Forbes, we may conclude
that the bottom will be inhabited by extremely few animals, and the mass
when upraised will give a most imperfect record of the forms of life
which then existed; or, sediment may be accumulated to any thickness and
extent over a shallow bottom, if it continue slowly to subside. In this
latter case, as long as the rate of subsidence and supply of sediment
nearly balance each other, the sea will remain shallow and favourable for
life, and thus a fossiliferous formation thick enough, when upraised, to
resist any amount of degradation, may be formed.
I am convinced that all our ancient formations, which are rich in
fossils, have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of [292]Geology,
and have been surprised to note how author after author, in treating of
this or that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient tertiary
formation on the west coast of South America, which has been bulky enough
to resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was certainly deposited during a
downward oscillation of level, and thus gained considerable
thickness.
All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but only
where the supply of sediment was sufficient to keep the sea shallow and
to embed and preserve the remains before they had time to decay. On the
other hand, as long as the bed of the sea remained stationary,
thick deposits could not have been accumulated in the shallow
parts, which are the most favourable to life. Still less could this have
happened during the alternate periods of elevation; or, to speak more
accurately, the beds which were then accumulated will have been destroyed
by being upraised and brought within the limits of the coast-action.
Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated by
Sir C. Lyell; and E. Forbes subsequently but independently arrived at a
similar conclusion.
One remark is here worth a passing notice. During periods of elevation
the area of the land and of the [293]adjoining shoal parts of the sea will be
increased, and new stations will often be formed;—all circumstances
most favourable, as previously explained, for the formation of new
varieties and species; but during such periods there will generally be a
blank in the geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (excepting the
productions on the shores of a continent when first broken up into an
archipelago), and consequently during subsidence, though there will be
much extinction, fewer new varieties or species will be formed; and it is
during these very periods of subsidence, that our great deposits rich in
fossils have been accumulated. Nature may almost be said to have guarded
against the frequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if we
confine our attention to any one formation, it becomes more difficult to
understand, why we do not therein find closely graduated varieties
between the allied species which lived at its commencement and at its
close. Some cases are on record of the same species presenting distinct
varieties in the upper and lower parts of the same formation, but, as
they are rare, they may be here passed over. Although each formation has
indisputably required a vast number of years for its deposition, I can
see several reasons why each should not include a graduated series of
links between the species which then lived; but I can by no means pretend
to assign due proportional weight to the following considerations.
Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one species
into another. I am [294]aware that two palæontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or thrice
as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us coming to any just conclusion
on this head. When we see a species first appearing in the middle of any
formation, it would be rash in the extreme to infer that it had not
elsewhere previously existed. So again when we find a species
disappearing before the uppermost layers have been deposited, it would be
equally rash to suppose that it then became wholly extinct. We forget how
small the area of Europe is compared with the rest of the world; nor have
the several stages of the same formation throughout Europe been
correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amount
of migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it only then
first immigrated into that area. It is well known, for instance, that
several species appeared somewhat earlier in the palæozoic beds of North
America than in those of Europe; time having apparently been required for
their migration from the American to the European seas. In examining the
latest deposits of various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or,
conversely, that some are now abundant in the neighbouring sea, but are
rare or absent in this particular deposit. It is an excellent lesson to
reflect on the ascertained amount of migration of the inhabitants of
Europe during the Glacial period, which forms only a part of one whole
geological period; [295]and likewise to reflect on the great
changes of level, on the inordinately great change of climate, on the
prodigious lapse of time, all included within this same glacial period.
Yet it may be doubted whether in any quarter of the world, sedimentary
deposits, including fossil remains, have gone on accumulating
within the same area during the whole of this period. It is not, for
instance, probable that sediment was deposited during the whole of the
glacial period near the mouth of the Mississippi, within that limit of
depth at which marine animals can flourish; for we know what vast
geographical changes occurred in other parts of America during this space
of time. When such beds as were deposited in shallow water near the mouth
of the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to geographical
changes. And in the distant future, a geologist examining these beds,
might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead
of having been really far greater, that is extending from before the
glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will generally
have to be a very thick one; and the species undergoing modification will
have had to live on the same area throughout this whole time. But we have
seen that a thick fossiliferous formation can only be accumulated during
a period of subsidence; and to keep the depth approximately the same,
which is necessary in [296]order to enable the same species to live
on the same space, the supply of sediment must nearly have
counterbalanced the amount of subsidence. But this same movement of
subsidence will often tend to sink the area whence the sediment is
derived, and thus diminish the supply whilst the downward movement
continues. In fact, this nearly exact balancing between the supply of
sediment and the amount of subsidence is probably a rare contingency; for
it has been observed by more than one palæontologist, that very thick
deposits are usually barren of organic remains, except near their upper
or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of different mineralogical composition, we may reasonably suspect
that the process of deposition has been much interrupted, as a change in
the currents of the sea and a supply of sediment of a different nature
will generally have been due to geographical changes requiring much time.
Nor will the closest inspection of a formation give any idea of the time
which its deposition has consumed. Many instances could be given of beds
only a few feet in thickness, representing formations, elsewhere
thousands of feet in thickness, and which must have required an enormous
period for their accumulation; yet no one ignorant of this fact would
have suspected the vast lapse of time represented by the thinner
formation. Many cases could be given of the lower beds of a formation
having been upraised, denuded, submerged, and then re-covered by the
upper beds of the same formation,—facts, showing what wide, yet
easily overlooked, intervals have occurred in its accumulation. In other
cases we have the plainest evidence [297]in great fossilised
trees, still standing upright as they grew, of many long intervals of
time and changes of level during the process of deposition, which would
never even have been suspected, had not the trees chanced to have been
preserved: thus Messrs. Lyell and Dawson found carboniferous beds 1400
feet thick in Nova Scotia, with ancient root-bearing strata, one above
the other, at no less than sixty-eight different levels. Hence, when the
same species occur at the bottom, middle, and top of a formation, the
probability is that they have not lived on the same spot during the whole
period of deposition, but have disappeared and reappeared, perhaps many
times, during the same geological period. So that if such species were to
undergo a considerable amount of modification during any one geological
period, a section would not probably include all the fine intermediate
gradations which must on my theory have existed between them, but abrupt,
though perhaps very slight, changes of form.
It is all-important to remember that naturalists have no golden rule
by which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by close intermediate
gradations. And this from the reasons just assigned we can seldom hope to
effect in any one geological section. Supposing B and C to be two
species, and a third, A, to be found in an underlying bed; even if A were
strictly intermediate between B and C, it would simply be ranked as a
third and distinct species, unless at the same time it could be most
closely connected with either one or both forms by intermediate
varieties. Nor should it be forgotten, as before explained, that A might
be the actual progenitor [298]of B and C, and yet might not at all
necessarily be strictly intermediate between them in all points of
structure. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of a formation, and
unless we obtained numerous transitional gradations, we should not
recognise their relationship, and should consequently be compelled to
rank them all as distinct species.
It is notorious on what excessively slight differences many
palæontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties; and
on this view we do find the kind of evidence of change which on my theory
we ought to find. Moreover, if we look to rather wider intervals, namely,
to distinct but consecutive stages of the same great formation, we find
that the embedded fossils, though almost universally ranked as
specifically different, yet are far more closely allied to each other
than are the species found in more widely separated formations; but to
this subject I shall have to return in the following chapter.
One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally at
first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected
in some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes
are supposed to have been local or [299]confined to some one
spot. Most marine animals have a wide range; and we have seen that with
plants it is those which have the widest range, that oftenest present
varieties; so that with shells and other marine animals, it is probably
those which have had the widest range, far exceeding the limits of the
known geological formations of Europe, which have oftenest given rise,
first to local varieties and ultimately to new species; and this again
would greatly lessen the chance of our being able to trace the stages of
transition in any one geological formation.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until many
specimens have been collected from many places; and in the case of fossil
species this could rarely be effected by palæontologists. We shall,
perhaps, best perceive the improbability of our being enabled to connect
species by numerous, fine, intermediate, fossil links, by asking
ourselves whether, for instance, geologists at some future period will be
able to prove, that our different breeds of cattle, sheep, horses, and
dogs have descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores of
North America, which are ranked by some conchologists as distinct species
from their European representatives, and by other conchologists as only
varieties, are really varieties or are, as it is called, specifically
distinct. This could be effected only by the future geologist discovering
in a fossil state numerous intermediate gradations; and such success
seems to me improbable in the highest degree.
Geological research, though it has added numerous species to existing
and extinct genera, and has made the [300]intervals between some
few groups less wide than they otherwise would have been, yet has done
scarcely anything in breaking down the distinction between species, by
connecting them together by numerous, fine, intermediate varieties; and
this not having been effected, is probably the gravest and most obvious
of all the many objections which may be urged against my views. Hence it
will be worth while to sum up the foregoing remarks, under an imaginary
illustration. The Malay Archipelago is of about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been examined
with any accuracy, excepting those of the United States of America. I
fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide and
shallow seas, probably represents the former state of Europe, whilst most
of our formations were accumulating. The Malay Archipelago is one of the
richest regions of the whole world in organic beings; yet if all the
species were to be collected which have ever lived there, how imperfectly
would they represent the natural history of the world!
But we have every reason to believe that the terrestrial productions
of the archipelago would be preserved in an excessively imperfect manner
in the formations which we suppose to be there accumulating. I suspect
that not many of the strictly littoral animals, or of those which lived
on naked submarine rocks, would be embedded; and those embedded in gravel
or sand, would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could be
preserved.
I believe that fossiliferous formations could be formed [301]in the
archipelago, of thickness sufficient to last to an age as distant in
futurity as the secondary formations lie in the past, only during periods
of subsidence. These periods of subsidence would be separated from each
other by enormous intervals, during which the area would be either
stationary or rising; whilst rising, each fossiliferous formation would
be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America. During the
periods of subsidence there would probably be much extinction of life;
during the periods of elevation, there would be much variation, but the
geological record would then be least perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would exceed the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully
preserved, transitional varieties would merely appear as so many distinct
species. It is, also, probable that each great period of subsidence would
be interrupted by oscillations of level, and that slight climatal changes
would intervene during such lengthy periods; and in these cases the
inhabitants of the archipelago would have to migrate, and no closely
consecutive record of their modifications could be preserved in any one
formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to believe
that it would be chiefly these far-ranging species which would oftenest
produce new varieties; and the varieties would at first generally be
local or confined to one place, but if possessed [302]of any decided
advantage, or when further modified and improved, they would slowly
spread and supplant their parent-forms. When such varieties returned to
their ancient homes, as they would differ from their former state, in a
nearly uniform, though perhaps extremely slight degree, they would,
according to the principles followed by many palæontologists, be ranked
as new and distinct species.
If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite number
of those fine transitional forms, which on my theory assuredly have
connected all the past and present species of the same group into one
long and branching chain of life. We ought only to look for a few links,
some more closely, some more distantly related to each other; and these
links, let them be ever so close, if found in different stages of the
same formation, would, by most palæontologists, be ranked as distinct
species. But I do not pretend that I should ever have suspected how poor
a record of the mutations of life, the best preserved geological section
presented, had not the difficulty of our not discovering innumerable
transitional links between the species which appeared at the commencement
and close of each formation, pressed so hardly on my theory.
On the sudden appearance of whole groups of Allied
Species.—The abrupt manner in which whole groups of species
suddenly appear in certain formations, has been urged by several
palæontologists—for instance, by Agassiz, Pictet, and by none more
forcibly than by Professor Sedgwick—as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging to
the same genera or families, have really [303]started into life all
at once, the fact would be fatal to the theory of descent with slow
modification through natural selection. For the development of a group of
forms, all of which have descended from some one progenitor, must have
been an extremely slow process; and the progenitors must have lived long
ages before their modified descendants. But we continually over-rate the
perfection of the geological record, and falsely infer, because certain
genera or families have not been found beneath a certain stage, that they
did not exist before that stage. We continually forget how large the
world is, compared with the area over which our geological formations
have been carefully examined; we forget that groups of species may
elsewhere have long existed and have slowly multiplied before they
invaded the ancient archipelagoes of Europe and of the United States. We
do not make due allowance for the enormous intervals of time, which have
probably elapsed between our consecutive formations,—longer perhaps
in most cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of
species from some one or some few parent-forms; and in the succeeding
formation such species will appear as if suddenly created.
I may here recall a remark formerly made, namely that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance to fly through the air; but that when this had
been effected, and a few species had thus acquired a great advantage over
other organisms, a comparatively short time would be necessary to produce
many divergent forms, which would be able to spread rapidly and widely
throughout the world.
I will now give a few examples to illustrate these [304]remarks, and
to show how liable we are to error in supposing that whole groups of
species have suddenly been produced. I may recall the well-known fact
that in geological treatises, published not many years ago, the great
class of mammals was always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest known
accumulations of fossil mammals, for its thickness, belongs to the middle
of the secondary series; and one true mammal has been discovered in the
new red sandstone at nearly the commencement of this great series. Cuvier
used to urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America, and in
Europe even as far back as the eocene stage. Had it not been for the rare
accident of the preservation of footsteps in the new red sandstone of the
United States, who would have ventured to suppose that, besides reptiles,
no less than at least thirty kinds of birds, some of gigantic size,
existed during that period? Not a fragment of bone has been discovered in
these beds. Notwithstanding that the number of joints shown in the fossil
impressions correspond with the number in the several toes of living
birds' feet, some authors doubt whether the animals which left the
impressions were really birds. Until quite recently these authors might
have maintained, and some have maintained, that the whole class of birds
came suddenly into existence during an early tertiary period; but now we
know, on the authority of Professor Owen (as may be seen in Lyell's
'Manual'), that a bird certainly lived during the deposition of the upper
greensand.
I may give another instance, which from having passed under my own
eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I have
stated that, from the [305]number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many
species all over the world, from the Arctic regions to the equator,
inhabiting various zones of depths from the upper tidal limits to 50
fathoms; from the perfect manner in which specimens are preserved in the
oldest tertiary beds; from the ease with which even a fragment of a valve
can be recognised; from all these circumstances, I inferred that had
sessile cirripedes existed during the secondary periods, they would
certainly have been preserved and discovered; and as not one species had
then been discovered in beds of this age, I concluded that this great
group had been suddenly developed at the commencement of the tertiary
series. This was a sore trouble to me, adding as I thought one more
instance of the abrupt appearance of a great group of species. But my
work had hardly been published, when a skilful palæontologist, M.
Bosquet, sent me a drawing of a perfect specimen of an unmistakeable
sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this
sessile cirripede was a Chthamalus, a very common, large, and ubiquitous
genus, of which not one specimen has as yet been found even in any
tertiary stratum. Hence we now positively know that sessile cirripedes
existed during the secondary period; and these cirripedes might have been
the progenitors of our many tertiary and existing species.
The case most frequently insisted on by palæontologists of the
apparently sudden appearance of a whole group of species, is that of the
teleostean fishes, low down in the Chalk period. This group includes the
large majority of existing species. Lately, Professor Pictet has carried
their existence one sub-stage further back; and some palæontologists
believe that certain [306]much older fishes, of which the affinities
are as yet imperfectly known, are really teleostean. Assuming, however,
that the whole of them did appear, as Agassiz believes, at the
commencement of the chalk formation, the fact would certainly be highly
remarkable; but I cannot see that it would be an insuperable difficulty
on my theory, unless it could likewise be shown that the species of this
group appeared suddenly and simultaneously throughout the world at this
same period. It is almost superfluous to remark that hardly any
fossil-fish are known from south of the equator; and by running through
Pictet's Palæontology it will be seen that very few species are known
from several formations in Europe. Some few families of fish now have a
confined range; the teleostean fish might formerly have had a similarly
confined range, and after having been largely developed in some one sea,
might have spread widely. Nor have we any right to suppose that the seas
of the world have always been so freely open from south to north as they
are at present. Even at this day, if the Malay Archipelago were converted
into land, the tropical parts of the Indian Ocean would form a large and
perfectly enclosed basin, in which any great group of marine animals
might be multiplied; and here they would remain confined, until some of
the species became adapted to a cooler climate, and were enabled to
double the southern capes of Africa or Australia, and thus reach other
and distant seas.
From these and similar considerations, but chiefly from our ignorance
of the geology of other countries beyond the confines of Europe and the
United States; and from the revolution in our palæontological ideas on
many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic [307]beings throughout the world, as it would
be for a naturalist to land for five minutes on some one barren point in
Australia, and then to discuss the number and range of its
productions.
On the sudden appearance of groups of Allied Species in the lowest
known fossiliferous strata.—There is another and allied
difficulty, which is much graver. I allude to the manner in which numbers
of species of the same group, suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group have descended from one
progenitor, apply with nearly equal force to the earliest known species.
For instance, I cannot doubt that all the Silurian trilobites have
descended from some one crustacean, which must have lived long before the
Silurian age, and which probably differed greatly from any known animal.
Some of the most ancient Silurian animals, as the Nautilus, Lingula,
&c., do not differ much from living species; and it cannot on my
theory be supposed, that these old species were the progenitors of all
the species of the orders to which they belong, for they do not present
characters in any degree intermediate between them. If, moreover, they
had been the progenitors of these orders, they would almost certainly
have been long ago supplanted and exterminated by their numerous and
improved descendants.
Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as,
or probably far longer than, the whole interval from the Silurian age to
the present day; and that during these vast, yet quite unknown, periods
of time, the world swarmed with living creatures. [308]
To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir E. Murchison at their head, are convinced that we
see in the organic remains of the lowest Silurian stratum the dawn of
life on this planet. Other highly competent judges, as Lyell and the late
E. Forbes, dispute this conclusion. We should not forget that only a
small portion of the world is known with accuracy. M. Barrande has lately
added another and lower stage to the Silurian system, abounding with new
and peculiar species. Traces of life have been detected in the Longmynd
beds, beneath Barrande's so-called primordial zone. The presence of
phosphatic nodules and bituminous matter in some of the lowest azoic
rocks, probably indicates the former existence of life at these periods.
But the difficulty of understanding the absence of vast piles of
fossiliferous strata, which on my theory no doubt were somewhere
accumulated before the Silurian epoch, is very great. If these most
ancient beds had been wholly worn away by denudation, or obliterated by
metamorphic action, we ought to find only small remnants of the
formations next succeeding them in age, and these ought to be very
generally in a metamorphosed condition. But the descriptions which we now
possess of the Silurian deposits over immense territories in Russia and
in North America, do not support the view, that the older a formation is,
the more it has always suffered the extremity of denudation and
metamorphism.
The case at present must remain inexplicable; and may be truly urged
as a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains which [309]do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the existing continents of
Europe and North America. But we do not know what was the state of things
in the intervals between the successive formations; whether Europe and
the United States during these intervals existed as dry land, or as a
submarine surface near land, on which sediment was not deposited, or as
the bed of an open and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but not one oceanic island
is as yet known to afford even a remnant of any palæozoic or secondary
formation. Hence we may perhaps infer, that during the palæozoic and
secondary periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed there, palæozoic and
secondary formations would in all probability have been accumulated from
sediment derived from their wear and tear; and would have been at least
partially upheaved by the oscillations of level, which we may fairly
conclude must have intervened during these enormously long periods. If
then we may infer anything from these facts, we may infer that where our
oceans now extend, oceans have extended from the remotest period of which
we have any record; and on the other hand, that where continents now
exist, large tracts of land have existed, subjected no doubt to great
oscillations of level, since the earliest silurian period. The coloured
map appended to my volume on Coral Reefs, led me to conclude that the
great oceans are still mainly areas of [310]subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But have we any right to assume that things have thus
remained from the beginning of this world? Our continents seem to have
been formed by a preponderance, during many oscillations of level, of the
force of elevation; but may not the areas of preponderant movement have
changed in the lapse of ages? At a period immeasurably antecedent to the
silurian epoch, continents may have existed where oceans are now spread
out; and clear and open oceans may have existed where our continents now
stand. Nor should we be justified in assuming that if, for instance, the
bed of the Pacific Ocean were now converted into a continent, we should
there find formations older than the silurian strata, supposing such to
have been formerly deposited; for it might well happen that strata which
had subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might have
undergone far more metamorphic action than strata which have always
remained nearer to the surface. The immense areas in some parts of the
world, for instance in South America, of bare metamorphic rocks, which
must have been heated under great pressure, have always seemed to me to
require some special explanation; and we may perhaps believe that we see
in these large areas, the many formations long anterior to the silurian
epoch in a completely metamorphosed condition.
The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between the
many species which now exist or have existed; the sudden manner [311]in which
whole groups of species appear in our European formations; the almost
entire absence, as at present known, of fossiliferous formations beneath
the Silurian strata, are all undoubtedly of the gravest nature. We see
this in the plainest manner by the fact that all the most eminent
palæontologists, namely Cuvier, Agassiz, Barrande, Falconer, E. Forbes,
&c., and all our greatest geologists, as Lyell, Murchison, Sedgwick,
&c., have unanimously, often vehemently, maintained the immutability
of species. But I have reason to believe that one great authority, Sir
Charles Lyell, from further reflexion entertains grave doubts on this
subject. I feel how rash it is to differ from these authorities, to whom,
with others, we owe all our knowledge. Those who think the natural
geological record in any degree perfect, and who do not attach much
weight to the facts and arguments of other kinds given in this volume,
will undoubtedly at once reject my theory. For my part, following out
Lyell's metaphor, I look at the natural geological record, as a history
of the world imperfectly kept, and written in a changing dialect; of this
history we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has been
preserved; and of each page, only here and there a few lines. Each word
of the slowly-changing language, in which the history is supposed to be
written, being more or less different in the interrupted succession of
chapters, may represent the apparently abruptly changed forms of life,
entombed in our consecutive, but widely separated, formations. On this
view, the difficulties above discussed are greatly diminished, or even
disappear.
[312]
CHAPTER X.
On the Geological Succession of Organic Beings.
On the slow and successive appearance of new species—On their
different rates of change—Species once lost do not
reappear—Groups of species follow the same general rules in their
appearance and disappearance as do single species—On
Extinction—On simultaneous changes in the forms of life throughout
the world—On the affinities of extinct species to each other and to
living species—On the state of development of ancient
forms—On the succession of the same types within the same
areas—Summary of preceding and present chapters.
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and to
make the percentage system of lost and new forms more gradual. In some of
the most recent beds, though undoubtedly of high antiquity if measured by
years, only one or two species are lost forms, and only one or two are
new forms, having here appeared for the first time, either locally, or,
as far as we know, on the face of the earth. If we may trust the
observations of Philippi in Sicily, the successive changes in the marine
inhabitants of that island have been many and most gradual. The secondary
formations are more broken; but, as Bronn has remarked, neither the
appearance [313]nor disappearance of their many now
extinct species has been simultaneous in each separate formation.
Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, in an existing
crocodile associated with many strange and lost mammals and reptiles in
the sub-Himalayan deposits. The Silurian Lingula differs but little from
the living species of this genus; whereas most of the other Silurian
Molluscs and all the Crustaceans have changed greatly. The productions of
the land seem to change at a quicker rate than those of the sea, of which
a striking instance has lately been observed in Switzerland. There is
some reason to believe that organisms, considered high in the scale of
nature, change more quickly than those that are low: though there are
exceptions to this rule. The amount of organic change, as Pictet has
remarked, does not strictly correspond with the succession of our
geological formations; so that between each two consecutive formations,
the forms of life have seldom changed in exactly the same degree. Yet if
we compare any but the most closely related formations, all the species
will be found to have undergone some change. When a species has once
disappeared from the face of the earth, we have reason to believe that
the same identical form never reappears. The strongest apparent exception
to this latter rule, is that of the so-called "colonies" of M. Barrande,
which intrude for a period in the midst of an older formation, and then
allow the pre-existing fauna to reappear; but Lyell's explanation,
namely, that it is a case of temporary migration from a distinct
geographical province, seems to me satisfactory. [314]
These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be
taken advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,—on the variability being of a beneficial
nature, on the power of intercrossing, on the rate of breeding, on the
slowly changing physical conditions of the country, and more especially
on the nature of the other inhabitants with which the varying species
comes into competition. Hence it is by no means surprising that one
species should retain the same identical form much longer than others;
or, if changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand the
apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by the
more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When many
of the inhabitants of a country have become modified and improved, we can
understand, on the principle of competition, and on that of the many
all-important relations of organism to organism, that any form which does
not become in some degree modified and improved, [315]will be liable to be
exterminated. Hence we can see why all the species in the same region do
at last, if we look to wide enough intervals of time, become modified;
for those which do not change will become extinct.
In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is not
equal. Each formation, on this view, does not mark a new and complete act
of creation, but only an occasional scene, taken almost at hazard, in a
slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might be
adapted (and no doubt this has occurred in innumerable instances) to fill
the exact place of another species in the economy of nature, and thus
supplant it; yet the two forms—the old and the new—would not
be identically the same; for both would almost certainly inherit
different characters from their distinct progenitors. For instance, it is
just possible, if our fantail-pigeons were all destroyed, that fanciers,
by striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite [316]incredible
that a fantail, identical with the existing breed, could be raised from
any other species of pigeon, or even from the other well-established
races of the domestic pigeon, for the newly-formed fantail would be
almost sure to inherit from its new progenitor some slight characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single species,
changing more or less quickly, and in a greater or lesser degree. A group
does not reappear after it has once disappeared; or its existence, as
long as it lasts, is continuous. I am aware that there are some apparent
exceptions to this rule, but the exceptions are surprisingly few, so few
that E. Forbes, Pictet, and Woodward (though all strongly opposed to such
views as I maintain) admit its truth; and the rule strictly accords with
my theory. For as all the species of the same group have descended from
some one species, it is clear that as long as any species of the group
have appeared in the long succession of ages, so long must its members
have continuously existed, in order to have generated either new and
modified or the same old and unmodified forms. Species of the genus
Lingula, for instance, must have continuously existed by an unbroken
succession of generations, from the lowest Silurian stratum to the
present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule being a
gradual increase in number, till the group reaches its maximum, and then,
sooner or later, it gradually decreases. If the number of the species of
a genus, or the number of [317]the genera of a family, be represented by
a vertical line of varying thickness, crossing the successive geological
formations in which the species are found, the line will sometimes
falsely appear to begin at its lower end, not in a sharp point, but
abruptly; it then gradually thickens upwards, sometimes keeping for a
space of equal thickness, and ultimately thins out in the upper beds,
marking the decrease and final extinction of the species. This gradual
increase in number of the species of a group is strictly conformable with
my theory; as the species of the same genus, and the genera of the same
family, can increase only slowly and progressively; for the process of
modification and the production of a number of allied forms must be slow
and gradual,—one species giving rise first to two or three
varieties, these being slowly converted into species, which in their turn
produce by equally slow steps other species, and so on, like the
branching of a great tree from a single stem, till the group becomes
large.
On Extinction.—We have as yet spoken only incidentally of
the disappearance of species and of groups of species. On the theory of
natural selection the extinction of old forms and the production of new
and improved forms are intimately connected together. The old notion of
all the inhabitants of the earth having been swept away at successive
periods by catastrophes, is very generally given up, even by those
geologists, as Elie de Beaumont, Murchison, Barrande, &c, whose
general views would naturally lead them to this conclusion. On the
contrary, we have every reason to believe, from the study of the tertiary
formations, that species and groups of species gradually disappear, one
after another, first from one spot, then from another, and finally from
the world. Both single species and whole [318]groups of species last
for very unequal periods; some groups, as we have seen, having endured
from the earliest known dawn of life to the present day; some having
disappeared before the close of the palæozoic period. No fixed law seems
to determine the length of time during which any single species or any
single genus endures. There is reason to believe that the complete
extinction of the species of a group is generally a slower process than
their production: if the appearance and disappearance of a group of
species be represented, as before, by a vertical line of varying
thickness, the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its lower end, which
marks the first appearance and increase in numbers of the species. In
some cases, however, the extermination of whole groups of beings, as of
ammonites towards the close of the secondary period, has been wonderfully
sudden.
The whole subject of the extinction of species has been involved in
the most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a very
late geological period, I was filled with astonishment; for seeing that
the horse, since its introduction by the Spaniards into South America,
has run wild over the whole country and has increased in numbers at an
unparalleled rate, I asked myself what could so recently have
exterminated the former horse under conditions of life apparently so
favourable. But how utterly groundless was my astonishment! [319]Professor Owen
soon perceived that the tooth, though so like that of the existing horse,
belonged to an extinct species. Had this horse been still living, but in
some degree rare, no naturalist would have felt the least surprise at its
rarity; for rarity is the attribute of a vast number of species of all
classes, in all countries. If we ask ourselves why this or that species
is rare, we answer that something is unfavourable in its conditions of
life; but what that something is, we can hardly ever tell. On the
supposition of the fossil horse still existing as a rare species, we
might have felt certain from the analogy of all other mammals, even of
the slow-breeding elephant, and from the history of the naturalisation of
the domestic horse in South America, that under more favourable
conditions it would in a very few years have stocked the whole continent.
But we could not have told what the unfavourable conditions were which
checked its increase, whether some one or several contingencies, and at
what period of the horse's life, and in what degree, they severally
acted. If the conditions had gone on, however slowly, becoming less and
less favourable, we assuredly should not have perceived the fact, yet the
fossil horse would certainly have become rarer and rarer, and finally
extinct;—its place being seized on by some more successful
competitor.
It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. We see in many cases in the more
recent tertiary formations, that rarity precedes extinction; and we know
that this has been the progress of events with those animals which have
been exterminated, either locally or wholly, through [320]man's agency.
I may repeat what I published in 1845, namely, that to admit that species
generally become rare before they become extinct—to feel no
surprise at the rarity of a species, and yet to marvel greatly when it
ceases to exist, is much the same as to admit that sickness in the
individual is the forerunner of death—to feel no surprise at
sickness, but when the sick man dies, to wonder and to suspect that he
died by some unknown deed of violence.
The theory of natural selection is grounded on the belief that each
new variety, and ultimately each new species, is produced and maintained
by having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the less
improved varieties in the same neighbourhood; when much improved it is
transported far and near, like our short-horn cattle, and takes the place
of other breeds in other countries. Thus the appearance of new forms and
the disappearance of old forms, both natural and artificial, are bound
together. In certain flourishing groups, the number of new specific forms
which have been produced within a given time is probably greater than
that of the old specific forms which have been exterminated; but we know
that the number of species has not gone on indefinitely increasing, at
least during the later geological periods, so that looking to later times
we may believe that the production of new forms has caused the extinction
of about the same number of old forms.
The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects. [321]Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one
species, the nearest allies of that species, i.e. the species of
the same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a
new genus, comes to supplant an old genus, belonging to the same family.
But it must often have happened that a new species belonging to some one
group will have seized on the place occupied by a species belonging to a
distinct group, and thus caused its extermination; and if many allied
forms be developed from the successful intruder, many will have to yield
their places; and it will generally be allied forms, which will suffer
from some inherited inferiority in common. But whether it be species
belonging to the same or to a distinct class, which yield their places to
other species which have been modified and improved, a few of the
sufferers may often long be preserved, from being fitted to some peculiar
line of life, or from inhabiting some distant and isolated station, where
they have escaped severe competition. For instance, a single species of
Trigonia, a great genus of shells in the secondary formations, survives
in the Australian seas; and a few members of the great and almost extinct
group of Ganoid fishes still inhabit our fresh waters. Therefore the
utter extinction of a group is generally, as we have seen, a slower
process than its production.
With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palæozoic period and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time [322]between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of a
new area, they will have exterminated in a correspondingly rapid manner
many of the old inhabitants; and the forms which thus yield their places
will commonly be allied, for they will partake of some inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies, on which the existence of each species depends. If
we forget for an instant, that each species tends to increase
inordinately, and that some check is always in action, yet seldom
perceived by us, the whole economy of nature will be utterly obscured.
Whenever we can precisely say why this species is more abundant in
individuals than that; why this species and not another can be
naturalised in a given country; then, and not till then, we may justly
feel surprise why we cannot account for the extinction of this particular
species or group of species.
On the Forms of Life changing almost simultaneously throughout the
World.—Scarcely any palæontological discovery is more striking
than the fact, that the forms of life change almost simultaneously
throughout the world. Thus our European Chalk formation can be recognised
in many distant parts of the world, under the most different climates,
where not a fragment of the mineral chalk itself can be found; namely, in
North [323]America, in equatorial South America, in
Tierra del Fuego, at the Cape of Good Hope, and in the peninsula of
India. For at these distant points, the organic remains in certain beds
present an unmistakeable degree of resemblance to those of the Chalk. It
is not that the same species are met with; for in some cases not one
species is identically the same, but they belong to the same families,
genera, and sections of genera, and sometimes are similarly characterised
in such trifling points as mere superficial sculpture. Moreover other
forms, which are not found in the Chalk of Europe, but which occur in the
formations either above or below, are similarly absent at these distant
points of the world. In the several successive palæozoic formations of
Russia, Western Europe and North America, a similar parallelism in the
forms of life has been observed by several authors: so it is, according
to Lyell, with the several European and North American tertiary deposits.
Even if the few fossil species which are common to the Old and New Worlds
be kept wholly out of view, the general parallelism in the successive
forms of life, in the stages of the widely separated palæozoic and
tertiary periods, would still be manifest, and the several formations
could be easily correlated.
These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge whether
the productions of the land and of fresh water change at distant points
in the same parallel manner. We may doubt whether they have thus changed:
if the Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought
to Europe from La Plata, without any information in regard to their
geological position, no one would have suspected that they had co-existed
with still living sea-shells; but as these anomalous monsters co-existed
with the [324]Mastodon and Horse, it might at least have
been inferred that they had lived during one of the later tertiary
stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year, or
even that it has a very strict geological sense; for if all the marine
animals which live at the present day in Europe, and all those that lived
in Europe during the pleistocene period (an enormously remote period as
measured by years, including the whole glacial epoch), were to be
compared with those now living in South America or in Australia, the most
skilful naturalist would hardly be able to say whether the existing or
the pleistocene inhabitants of Europe resembled most closely those of the
southern hemisphere. So, again, several highly competent observers
believe that the existing productions of the United States are more
closely related to those which lived in Europe during certain later
tertiary stages, than to those which now live here; and if this be so, it
is evident that fossiliferous beds deposited at the present day on the
shores of North America would hereafter be liable to be classed with
somewhat older European beds. Nevertheless, looking to a remotely future
epoch, there can, I think, be little doubt that all the more modern
marine formations, namely, the upper pliocene, the pleistocene and
strictly modern beds, of Europe, North and South America, and Australia,
from containing fossil remains in some degree allied, and from not
including those forms which are only found in the older underlying
deposits, would be correctly ranked as simultaneous in a geological
sense.
The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM. [325]de Verneuil and d'Archiac. After referring
to the parallelism of the palæozoic forms of life in various parts of
Europe, they add, "If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom." M. Barrande has made forcible remarks to precisely the
same effect. It is, indeed, quite futile to look to changes of currents,
climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most
different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation
between the physical conditions of various countries, and the nature of
their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by new varieties arising, which have some
advantage over older forms; and those forms, which are already dominant,
or have some advantage over the other forms in their own country, would
naturally oftenest give rise to new varieties or incipient species; for
these latter must be victorious in a still higher degree in order to be
preserved and to survive. We have distinct evidence on this head, in the
plants which are dominant, that is, which are commonest in their own
homes, and are most widely diffused, having produced the greatest number
of new varieties. It is also natural that the [326]dominant, varying, and
far-spreading species, which already have invaded to a certain extent the
territories of other species, should be those which would have the best
chance of spreading still further, and of giving rise in new countries to
new varieties and species. The process of diffusion may often be very
slow, being dependent on climatal and geographical changes, or on strange
accidents, but in the long run the dominant forms will generally succeed
in spreading. The diffusion would, it is probable, be slower with the
terrestrial inhabitants of distinct continents than with the marine
inhabitants of the continuous sea. We might therefore expect to find, as
we apparently do find, a less strict degree of parallel succession in the
productions of the land than of the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations,
and that severe competition with many already existing forms, would be
highly favourable, as would be the power of spreading into new
territories. A certain amount of isolation, recurring at long intervals
of time, would probably be also favourable, as before explained. One
quarter of the world may have been most favourable for the production of
new and dominant species on the land, and another for those in the waters
of the sea. If two great regions had been for a long period favourably
circumstanced in an equal degree, whenever their inhabitants met, the
battle would be prolonged and severe; and some from one birthplace and
some from the other might be victorious. But in the course of time, the
[327]forms dominant in the highest degree,
wherever produced, would tend everywhere to prevail. As they prevailed,
they would cause the extinction of other and inferior forms; and as these
inferior forms would be allied in groups by inheritance, whole groups
would tend slowly to disappear; though here and there a single member
might long be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant owing to inheritance, and to having
already had some advantage over their parents or over other species;
these again spreading, varying, and producing new species. The forms
which are beaten and which yield their places to the new and victorious
forms, will generally be allied in groups, from inheriting some
inferiority in common; and therefore as new and improved groups spread
throughout the world, old groups will disappear from the world; and the
succession of forms in both ways will everywhere tend to correspond.
There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic remains.
During these long and blank intervals I suppose that the inhabitants of
each region underwent a considerable amount of modification and
extinction, and that there was much migration from [328]other parts of the
world. As we have reason to believe that large areas are affected by the
same movement, it is probable that strictly contemporaneous formations
have often been accumulated over very wide spaces in the same quarter of
the world; but we are far from having any right to conclude that this has
invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly the same period, we should
find in both, from the causes explained in the foregoing paragraphs, the
same general succession in the forms of life; but the species would not
exactly correspond; for there will have been a little more time in the
one region than in the other for modification, extinction, and
immigration.
I suspect that cases of this nature occur in Europe. Mr. Prestwich, in
his admirable Memoirs on the eocene deposits of England and France, is
able to draw a close general parallelism between the successive stages in
the two countries; but when he compares certain stages in England with
those in France, although he finds in both a curious accordance in the
numbers of the species belonging to the same genera, yet the species
themselves differ in a manner very difficult to account for, considering
the proximity of the two areas,—unless, indeed, it be assumed that
an isthmus separated two seas inhabited by distinct, but contemporaneous,
faunas. Lyell has made similar observations on some of the later tertiary
formations. Barrande, also, shows that there is a striking general
parallelism in the successive Silurian deposits of Bohemia and
Scandinavia; nevertheless he finds a surprising amount of difference in
the species. If the several formations in these regions have not been
deposited during the same exact [329]periods,—a formation in one region
often corresponding with a blank interval in the other,—and if in
both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals of
time between them; in this case, the several formations in the two
regions could be arranged in the same order, in accordance with the
general succession of the form of life, and the order would falsely
appear to be strictly parallel; nevertheless the species would not all be
the same in the apparently corresponding stages in the two regions.
On the Affinities of extinct Species to each other, and to living
forms.—Let us now look to the mutual affinities of extinct and
living species. They all fall into one grand natural system; and this
fact is at once explained on the principle of descent. The more ancient
any form is, the more, as a general rule, it differs from living forms.
But, as Buckland long ago remarked, all fossils can be classed either in
still existing groups, or between them. That the extinct forms of life
help to fill up the wide intervals between existing genera, families, and
orders, cannot be disputed. For if we confine our attention either to the
living or to the extinct alone, the series is far less perfect than if we
combine both into one general system. With respect to the Vertebrata,
whole pages could be filled with striking illustrations from our great
palaeontologist, Owen, showing how extinct animals fall in between
existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two
most distinct orders of mammals; but Owen has discovered so many fossil
links, that he has had to alter the whole classification of these two
orders; and has placed certain pachyderms in the same sub-order with
ruminants: for example, he dissolves by fine gradations the apparently
[330]wide difference between the pig and the
camel. In regard to the Invertebrata, Barrande, and a higher authority
could not be named, asserts that he is every day taught that Palaeozoic
animals, though belonging to the same orders, families, or genera with
those living at the present day, were not at this early epoch limited in
such distinct groups as they now are.
Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If by
this term it is meant that an extinct form is directly intermediate in
all its characters between two living forms, the objection is probably
valid. But I apprehend that in a perfectly natural classification many
fossil species would have to stand between living species, and some
extinct genera between living genera, even between genera belonging to
distinct families. The most common case, especially with respect to very
distinct groups, such as fish and reptiles, seems to be, that supposing
them to be distinguished at the present day from each other by a dozen
characters, the ancient members of the same two groups would be
distinguished by a somewhat lesser number of characters, so that the two
groups, though formerly quite distinct, at that period made some small
approach to each other.
It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted to
those groups which have undergone much change in the course of geological
ages; and it would be difficult to prove the truth of the proposition,
for every now and then even a living animal, as the Lepidosiren, is
discovered having affinities directed towards very distinct groups. Yet
if we compare the older Reptiles and [331]Batrachians, the older
Fish, the older Cephalopods, and the eocene Mammals, with the more recent
members of the same classes, we must admit that there is some truth in
the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex,
I must request the reader to turn to the diagram in the fourth chapter.
We may suppose that the numbered letters represent genera, and the dotted
lines diverging from them the species in each genus. The diagram is much
too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may
be considered as extinct. The three existing genera,
a14, q14, p14, will
form a small family; b14 and f14 a
closely allied family or sub-family; and o14,
e14, m14, a third family. These three
families, together with the many extinct genera on the several lines of
descent diverging from the parent-form (A), will form an order; for all
will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence of
character, which was formerly illustrated by this diagram, the more
recent any form is, the more it will generally differ from its ancient
progenitor. Hence we can understand the rule that the most ancient
fossils differ most from existing forms. We must not, however, assume
that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on
many and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its slightly
altered conditions of [332]life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.
All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct
forms, supposed to be embedded in the successive formations, were
discovered at several points low down in the series, the three existing
families on the uppermost line would be rendered less distinct from each
other. If, for instance, the genera a1,
a5, a10, f8,
m3, m6, m9, were
disinterred, these three families would be so closely linked together
that they probably would have to be united into one great family, in
nearly the same manner as has occurred with ruminants and pachyderms. Yet
he who objected to call the extinct genera, which thus linked the living
genera of three families together, intermediate in character, would be
justified, as they are intermediate, not directly, but only by a long and
circuitous course through many widely different forms. If many extinct
forms were to be discovered above one of the middle horizontal lines or
geological formations —for instance, above No. VI.—but none
from beneath this line, then only the two families on the left hand
(namely, a14, &c, and
b14,&c.) would have to be united into one family;
and the two other families (namely, a14 to
f14 now including five genera, and
o14 to m14) would yet remain
distinct. These two families, however, would be less distinct from each
other than they were before the discovery of the fossils. If, for
instance, we suppose the existing genera of the two families to differ
from each [333]other by a dozen characters, in this case
the genera, at the early period marked VI., would differ by a lesser
number of characters; for at this early stage of descent they have not
diverged in character from the common progenitor of the order, nearly so
much as they subsequently diverged. Thus it comes that ancient and
extinct genera are often in some slight degree intermediate in character
between their modified descendants, or between their collateral
relations.
In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will have
endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no right
to expect, except in very rare cases, to fill up wide intervals in the
natural system, and thus unite distinct families or orders. All that we
have a right to expect, is that those groups, which have within known
geological periods undergone much modification, should in the older
formations make some slight approach to each other; so that the older
members should differ less from each other in some of their characters
than do the existing members of the same groups; and this by the
concurrent evidence of our best palæontologists seems frequently to be
the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory manner.
And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period
in the earth's history will be intermediate in general character between
that which preceded and that which succeeded it. Thus, the species which
lived at the sixth great stage of descent in the [334]diagram are the
modified offspring of those which lived at the fifth stage, and are the
parents of those which became still more modified at the seventh stage;
hence they could hardly fail to be nearly intermediate in character
between the forms of life above and below. We must, however, allow for
the entire extinction of some preceding forms, and in any one region for
the immigration of new forms from other regions, and for a large amount
of modification, during the long and blank intervals between the
successive formations. Subject to these allowances, the fauna of each
geological period undoubtedly is intermediate in character, between the
preceding and succeeding faunas. I need give only one instance, namely,
the manner in which the fossils of the Devonian system, when this system
was first discovered, were at once recognised by palæontologists as
intermediate in character between those of the overlying carboniferous,
and underlying Silurian system. But each fauna is not necessarily exactly
intermediate, as unequal intervals of time have elapsed between
consecutive formations.
It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, mastodons and elephants, when arranged by Dr.
Falconer in two series, first according to their mutual affinities and
then according to their periods of existence, do not accord in
arrangement. The species extreme in character are not the oldest, or the
most recent; nor are those which are intermediate in character,
intermediate in age. But supposing for an instant, in this and other such
cases, that the record of the first appearance and disappearance of the
species was perfect, we have no reason to believe that forms successively
produced necessarily endure for [335]corresponding lengths of time: a very
ancient form might occasionally last much longer than a form elsewhere
subsequently produced, especially in the case of terrestrial productions
inhabiting separated districts. To compare small things with great: if
the principal living and extinct races of the domestic pigeon were
arranged as well as they could be in serial affinity, this arrangement
would not closely accord with the order in time of their production, and
still less with the order of their disappearance; for the parent
rock-pigeon now lives; and many varieties between the rock-pigeon and the
carrier have become extinct; and carriers which are extreme in the
important character of length of beak originated earlier than
short-beaked tumblers, which are at the opposite end of the series in
this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palæontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the Chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his firm belief in the immutability of species. He
who is acquainted with the distribution of existing species over the
globe, will not attempt to account for the close resemblance of the
distinct species in closely consecutive formations, by the physical
conditions of the ancient areas having remained nearly the same. Let it
be remembered that the forms of life, at least those inhabiting the sea,
have changed almost simultaneously throughout the world, and therefore
under the most different climates and conditions. Consider the [336]prodigious
vicissitudes of climate during the pleistocene period, which includes the
whole glacial period, and note how little the specific forms of the
inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals have
intervened between successive formations, we ought not to expect to find,
as I attempted to show in the last chapter, in any one or two formations
all the intermediate varieties between the species which appeared at the
commencement and close of these periods; but we ought to find after
intervals, very long as measured by years, but only moderately long as
measured geologically, closely allied forms, or, as they have been called
by some authors, representative species; and these we assuredly do find.
We find, in short, such evidence of the slow and scarcely sensible
mutation of specific forms, as we have a just right to expect to
find.
On the state of Development of Ancient Forms.—There has
been much discussion whether recent forms are more highly developed than
ancient. I will not here enter on this subject, for naturalists have not
as yet defined to each other's satisfaction what is meant by high and low
forms. The best definition probably is, that the higher forms have their
organs more distinctly specialised for different functions; and as such
division of physiological labour seems to be an advantage to each being,
natural selection will constantly tend in so far to make the later and
more modified forms higher than their early progenitors, or than the
slightly modified descendants of such progenitors. In a more general
sense the [337]more recent forms must, on my theory, be
higher than the more ancient; for each new species is formed by having
had some advantage in the struggle for life over other and preceding
forms. If under a nearly similar climate, the eocene inhabitants of one
quarter of the world were put into competition with the existing
inhabitants of the same or some other quarter, the eocene fauna or flora
would certainly be beaten and exterminated; as would a secondary fauna by
an eocene, and a palæozoic fauna by a secondary fauna. I do not doubt
that this process of improvement has affected in a marked and sensible
manner the organisation of the more recent and victorious forms of life,
in comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the highest
in their own class, may have beaten the highest molluscs. From the
extraordinary manner in which European productions have recently spread
over New Zealand, and have seized on places which must have been
previously occupied, we may believe, if all the animals and plants of
Great Britain were set free in New Zealand, that in the course of time a
multitude of British forms would become thoroughly naturalized there, and
would exterminate many of the natives. On the other hand, from what we
see now occurring in New Zealand, and from hardly a single inhabitant of
the southern hemisphere having become wild in any part of Europe, we may
doubt, if all the productions of New Zealand were set free in Great
Britain, whether any considerable number would be enabled to seize on
places now occupied by our native plants and animals. Under this point of
view, the productions of Great Britain may be said to be higher than
those of New Zealand. Yet the most skilful naturalist from an examination
of the [338]species of the two countries could not
have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz accords
well with the theory of natural selection. In a future chapter I shall
attempt to show that the adult differs from its embryo, owing to
variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fish strictly
belong to their own proper classes, though some of these old forms are in
a slight degree less distinct from each other than are the typical
members of the same groups at the present day, it would be vain to look
for animals having the common embryological character of the Vertebrata,
until beds far beneath the lowest Silurian strata are discovered—a
discovery of which the chance is very small.
On the Succession of the same Types within the same [339]areas, during
the later tertiary periods.—Mr. Clift many years ago showed
that the fossil mammals from the Australian caves were closely allied to
the living marsupials of that continent. In South America, a similar
relationship is manifest, even to an uneducated eye, in the gigantic
pieces of armour like those of the armadillo, found in several parts of
La Plata; and Professor Owen has shown in the most striking manner that
most of the fossil mammals, buried there in such numbers, are related to
South American types. This relationship is even more clearly seen in the
wonderful collection of fossil bones made by MM. Lund and Clausen in the
caves of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this "law of the succession of
types,"—on "this wonderful relationship in the same continent
between the dead and the living." Professor Owen has subsequently
extended the same generalisation to the mammals of the Old World. We see
the same law in this author's restorations of the extinct and gigantic
birds of New Zealand. We see it also in the birds of the caves of Brazil.
Mr. Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between
the extinct and living land-shells of Madeira; and between the extinct
and living brackish-water shells of the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under the
same latitude, would attempt to account, on the one hand, by dissimilar
physical conditions for the dissimilarity of the inhabitants of these two
continents, [340]and, on the other hand, by similarity of
conditions, for the uniformity of the same types in each during the later
tertiary periods. Nor can it be pretended that it is an immutable law
that marsupials should have been chiefly or solely produced in Australia;
or that Edentata and other American types should have been solely
produced in South America. For we know that Europe in ancient times was
peopled by numerous marsupials; and I have shown in the publications
above alluded to, that in America the law of distribution of terrestrial
mammals was formerly different from what it now is. North America
formerly partook strongly of the present character of the southern half
of the continent; and the southern half was formerly more closely allied,
than it is at present, to the northern half. In a similar manner we know
from Falconer and Cautley's discoveries, that northern India was formerly
more closely related in its mammals to Africa than it is at the present
time. Analogous facts could be given in relation to the distribution of
marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed
greatly from those of another continent, so will their modified
descendants still differ in nearly the same manner and degree. But after
very long intervals of time and after great geographical changes,
permitting much inter-migration, the feebler will yield to the more
dominant forms, and there will be nothing immutable in the laws of past
and present distribution. [341]
It may be asked in ridicule, whether I suppose that the megatherium
and other allied huge monsters have left behind them in South America,
the sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become wholly
extinct, and have left no progeny. But in the caves of Brazil, there are
many extinct species which are closely allied in size and in other
characters to the species still living in South America; and some of
these fossils may be the actual progenitors of living species. It must
not be forgotten that, on my theory, all the species of the same genus
have descended from some one species; so that if six genera, each having
eight species, be found in one geological formation, and in the next
succeeding formation there be six other allied or representative genera
with the same number of species, then we may conclude that only one
species of each of the six older genera has left modified descendants,
constituting the six new genera. The other seven species of the old
genera have all died out and have left no progeny. Or, which would
probably be a far commoner case, two or three species of two or three
alone of the six older genera will have been the parents of the six new
genera; the other old species and the other whole old genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of South
America, still fewer genera and species will have left modified
blood-descendants.
Summary of the preceding and present Chapters.—I have
attempted to show that the geological record is extremely imperfect; that
only a small portion of the globe has been geologically explored with
care; that [342]only certain classes of organic beings
have been largely preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, is absolutely as
nothing compared with the incalculable number of generations which must
have passed away even during a single formation; that, owing to
subsidence being necessary for the accumulation of fossiliferous deposits
thick enough to resist future degradation, enormous intervals of time
have elapsed between the successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will
have been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is, perhaps,
short compared with the average duration of specific forms; that
migration has played an important part in the first appearance of new
forms in any one area and formation; that widely ranging species are
those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological record
extremely imperfect, and will to a large extent explain why we do not
find interminable varieties, connecting together all the extinct and
existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record,
will rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages of
the same great formation. He may disbelieve in the enormous intervals of
time which have elapsed between our consecutive formations; he [343]may
overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent, sudden
coming in of whole groups of species. He may ask where are the remains of
those infinitely numerous organisms which must have existed long before
the first bed of the Silurian system was deposited: I can answer this
latter question only hypothetically, by saying that as far as we can see,
where our oceans now extend they have for an enormous period extended,
and where our oscillating continents now stand they have stood ever since
the Silurian epoch; but that long before that period, the world may have
presented a wholly different aspect; and that the older continents,
formed of formations older than any known to us, may now all be in a
metamorphosed condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great leading facts in
palæontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of
different classes do not necessarily change together, or at the same
rate, or in the same degree; yet in the long run that all undergo
modification to some extent. The extinction of old forms is the almost
inevitable consequence of the production of new forms. We can understand
why when a species has once disappeared it never reappears. Groups of
species increase in numbers slowly, and endure for unequal periods of
time; for the process of modification is necessarily slow, and depends on
many complex contingencies. The dominant species of the larger dominant
groups tend to leave many modified [344]descendants, and thus
new sub-groups and groups are formed. As these are formed, the species of
the less vigorous groups, from their inferiority inherited from a common
progenitor, tend to become extinct together, and to leave no modified
offspring on the face of the earth. But the utter extinction of a whole
group of species may often be a very slow process, from the survival of a
few descendants, lingering in protected and isolated situations. When a
group has once wholly disappeared, it does not reappear; for the link of
generation has been broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to people
the world with allied, but modified, descendants; and these will
generally succeed in taking the places of those groups of species which
are their inferiors in the struggle for existence. Hence, after long
intervals of time, the productions of the world will appear to have
changed simultaneously.
We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to divergence
of character, why the more ancient a form is, the more it generally
differs from those now living. Why ancient and extinct forms often tend
to fill up gaps between existing forms, sometimes blending two groups
previously classed as distinct into one; but more commonly only bringing
them a little closer together. The more ancient a form is, the more
often, apparently, it displays characters in some degree intermediate
between groups now distinct; for the more ancient a form is, the more
nearly it will be related to, and consequently resemble, the common
progenitor of groups, since [345]become widely divergent. Extinct forms are
seldom directly intermediate between existing forms; but are intermediate
only by a long and circuitous course through many extinct and very
different forms. We can clearly see why the organic remains of closely
consecutive formations are more closely allied to each other, than are
those of remote formations; for the forms are more closely linked
together by generation: we can clearly see why the remains of an
intermediate formation are intermediate in character.
The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palæontologists, that organisation on
the whole has progressed. If it should hereafter be proved that ancient
animals resemble to a certain extent the embryos of more recent animals
of the same class, the fact will be intelligible. The succession of the
same types of structure within the same areas during the later geological
periods ceases to be mysterious, and is simply explained by
inheritance.
If then the geological record be as imperfect as I believe it to be,
and it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural selection
are greatly diminished or disappear. On the other hand, all the chief
laws of palæontology plainly proclaim, as it seems to me, that species
have been produced by ordinary generation: old forms having been
supplanted by new and improved forms of life, produced by the laws of
variation still acting round us, and preserved by Natural Selection.
[346]
CHAPTER XI.
Geographical Distribution.
Present distribution cannot be accounted for by differences in
physical conditions—Importance of barriers—Affinity of the
productions of the same continent—Centres of creation—Means
of dispersal, by changes of climate and of the level of the land, and by
occasional means—Dispersal during the Glacial period co-extensive
with the world.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions. Of
late, almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth: for if we exclude the northern parts where the circumpolar land is
almost continuous, all authors agree that one of the most fundamental
divisions in geographical distribution is that between the New and Old
Worlds; yet if we travel over the vast American continent, from the
central parts of the United States to its extreme southern point, we meet
with the most diversified conditions; the most humid districts, arid
deserts, lofty mountains, grassy plains, forests, marshes, lakes, and
great rivers, under almost every temperature. There is hardly a climate
or condition in the Old World which cannot be paralleled in the
New—at least as closely as the same species generally require; for
it is a most rare case to find a group of organisms confined to any small
spot, having conditions peculiar in only a slight [347]degree; for instance,
small areas in the Old World could be pointed out hotter than any in the
New World, yet these are not inhabited by a peculiar fauna or flora.
Notwithstanding this parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25°
and 35°, we shall find parts extremely similar in all their conditions,
yet it would not be possible to point out three faunas and floras more
utterly dissimilar. Or again we may compare the productions of South
America south of lat. 35° with those north of 25°, which consequently
inhabit a considerably different climate, and they will be found
incomparably more closely related to each other, than they are to the
productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the
sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference of nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude: for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
[348]lofty and continuous mountain-ranges, and
of great deserts, and sometimes even of large rivers, we find different
productions; though as mountain-chains, deserts, &c., are not as
impassable, or likely to have endured so long as the oceans separating
continents, the differences are very inferior in degree to those
characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are
more distinct, with hardly a fish, shell, or crab in common, than those
of the eastern and western shores of South and Central America; yet these
great faunas are separated only by the narrow, but impassable, isthmus of
Panama. Westward of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emigrants; here we
have a barrier of another kind, and as soon as this is passed we meet in
the eastern islands of the Pacific, with another and totally distinct
fauna. So that here three marine faunas range far northward and
southward, in parallel lines not far from each other, under corresponding
climates; but from being separated from each other by impassable
barriers, either of land or open sea, they are wholly distinct. On the
other hand, proceeding still further westward from the eastern islands of
the tropical parts of the Pacific, we encounter no impassable barriers,
and we have innumerable islands as halting-places, or continuous coasts,
until after travelling over a hemisphere we come to the shores of Africa;
and over this vast space we meet with no well-defined and distinct marine
faunas. Although hardly one shell, crab or fish is common to the
above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fish range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of
the Pacific [349]and the eastern shores of Africa, on
almost exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statements, is
the affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is a
law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance, from
north to south never fails to be struck by the manner in which successive
groups of beings, specifically distinct, yet clearly related, replace
each other. He hears from closely allied, yet distinct kinds of birds,
notes nearly similar, and sees their nests similarly constructed, but not
quite alike, with eggs coloured in nearly the same manner. The plains
near the Straits of Magellan are inhabited by one species of Rhea
(American ostrich), and northward the plains of La Plata by another
species of the same genus; and not by a true ostrich or emu, like those
found in Africa and Australia under the same latitude. On these same
plains of La Plata, we see the agouti and bizcacha, animals having nearly
the same habits as our hares and rabbits and belonging to the same order
of Rodents, but they plainly display an American type of structure. We
ascend the lofty peaks of the Cordillera and we find an alpine species of
bizcacha; we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the American type.
Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ in geological structure,
the inhabitants, though they may be all peculiar species, are essentially
American. We may look back to past ages, as shown in the last chapter,
and we find American types then prevalent on [350]the American continent
and in the American seas. We see in these facts some deep organic bond,
prevailing throughout space and time, over the same areas of land and
water, and independent of their physical conditions. The naturalist must
feel little curiosity, who is not led to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which
alone, as far as we positively know, produces organisms quite like, or,
as we see in the case of varieties, nearly like each other. The
dissimilarity of the inhabitants of different regions may be attributed
to modification through natural selection, and in a quite subordinate
degree to the direct influence of different physical conditions. The
degree of dissimilarity will depend on the migration of the more dominant
forms of life from one region into another having been effected with more
or less ease, at periods more or less remote;—on the nature and
number of the former immigrants;—and on their action and reaction,
in their mutual struggles for life;—the relation of organism to
organism being, as I have already often remarked, the most important of
all relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in their
own widely-extended homes will have the best chance of seizing on new
places, when they spread into new countries. In their new homes they will
be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification, we can understand how it is
that sections of genera, whole genera, [351]and even families are
confined to the same areas, as is so commonly and notoriously the
case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so
far as it profits the individual in its complex struggle for life, so the
degree of modification in different species will be no uniform quantity.
If, for instance, a number of species, which stand in direct competition
with each other, migrate in a body into a new and afterwards isolated
country, they will be little liable to modification; for neither
migration nor isolation in themselves can do anything. These principles
come into play only by bringing organisms into new relations with each
other, and in a lesser degree with the surrounding physical conditions.
As we have seen in the last chapter that some forms have retained nearly
the same character from an enormously remote geological period, so
certain species have migrated over vast spaces, and have not become
greatly modified.
On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification, there
is not much difficulty in believing that they may have migrated from the
same region; for during the vast geographical and climatal changes which
will have supervened since ancient times, almost any amount of migration
is possible. But in many other cases, in which we have reason to believe
that the species of a genus have been produced within comparatively
recent times, there is great difficulty on this head. It [352]is also
obvious that the individuals of the same species, though now inhabiting
distant and isolated regions, must have proceeded from one spot, where
their parents were first produced: for, as explained in the last chapter,
it is incredible that individuals identically the same should ever have
been produced through natural selection from parents specifically
distinct.
We are thus brought to the question which has been largely discussed
by naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it, rejects the vera causa of ordinary
generation with subsequent migration, and calls in the agency of a
miracle. It is universally admitted, that in most cases the area
inhabited by a species is continuous; and when a plant or animal inhabits
two points so distant from each other, or with an interval of such a
nature, that the space could not be easily passed over by migration, the
fact is given as something remarkable and exceptional. The capacity of
migrating across the sea is more distinctly limited in terrestrial
mammals, than perhaps in any other organic beings; and, accordingly, we
find no inexplicable cases of the same mammal inhabiting distant points
of the world. No geologist will feel any difficulty in such cases as
Great Britain having been formerly united to Europe, and consequently
possessing the same quadrupeds. But if the same species can be produced
at two separate points, why do we not find a single mammal common to
Europe and [353]Australia or South America? The conditions
of life are nearly the same, so that a multitude of European animals and
plants have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every kind
have had on distribution, is intelligible only on the view that the great
majority of species have been produced on one side alone, and have not
been able to migrate to the other side. Some few families, many
sub-families, very many genera, and a still greater number of sections of
genera are confined to a single region; and it has been observed by
several naturalists, that the most natural genera, or those genera in
which the species are most closely related to each other, are generally
local, or confined to one area. What a strange anomaly it would be, if,
when coming one step lower in the series, to the individuals of the same
species, a directly opposite rule prevailed; and species were not local,
but had been produced in two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how
the same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the [354]formerly continuous range of many species.
So that we are reduced to consider whether the exceptions to continuity
of range are so numerous and of so grave a nature, that we ought to give
up the belief, rendered probable by general considerations, that each
species has been produced within one area, and has migrated thence as far
as it could. It would be hopelessly tedious to discuss all the
exceptional cases of the same species, now living at distant and
separated points; nor do I for a moment pretend that any explanation
could be offered of many such cases. But after some preliminary remarks,
I will discuss a few of the most striking classes of facts; namely, the
existence of the same species on the summits of distant mountain-ranges,
and at distant points in the arctic and antarctic regions; and secondly
(in the following chapter), the wide distribution of freshwater
productions; and thirdly, the occurrence of the same terrestrial species
on islands and on the mainland, though separated by hundreds of miles of
open sea. If the existence of the same species at distant and isolated
points of the earth's surface, can in many instances be explained on the
view of each species having migrated from a single birthplace; then,
considering our ignorance with respect to former climatal and
geographical changes and various occasional means of transport, the
belief that this has been the universal law, seems to me incomparably the
safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during some
part of their migration) from the area inhabited by their progenitor. If
it can be shown to be almost invariably the case, that a region, of which
[355]most of its inhabitants are closely
related to, or belong to the same genera with the species of a second
region, has probably received at some former period immigrants from this
other region, my theory will be strengthened; for we can clearly
understand, on the principle of modification, why the inhabitants of a
region should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by
inheritance to the inhabitants of the continent. Cases of this nature are
common, and are, as we shall hereafter more fully see, inexplicable on
the theory of independent creation. This view of the relation of species
in one region to those in another, does not differ much (by substituting
the word variety for species) from that lately advanced in an ingenious
paper by Mr. Wallace, in which he concludes, that "every species has come
into existence coincident both in space and time with a pre-existing
closely allied species." And I now know from correspondence, that this
coincidence he attributes to generation with modification.
The previous remarks on "single and multiple centres of creation" do
not directly bear on another allied question,—namely whether all
the individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have
descended from a succession of improved varieties, which will never have
blended with other individuals or varieties, but will have supplanted
each other; so that, at each [356]successive stage of modification and
improvement, all the individuals of each variety will have descended from
a single parent. But in the majority of cases, namely, with all organisms
which habitually unite for each birth, or which often intercross, I
believe that during the slow process of modification the individuals of
the species will have been kept nearly uniform by intercrossing; so that
many individuals will have gone on simultaneously changing, and the whole
amount of modification will not have been due, at each stage, to descent
from a single parent. To illustrate what I mean: our English racehorses
differ slightly from the horses of every other breed; but they do not owe
their difference and superiority to descent from any single pair, but to
continued care in selecting and training many individuals during many
generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of
dispersal.
Means of Dispersal.—Sir C. Lyell and other authors have
ably treated this subject. I can give here only the briefest abstract of
the more important facts. Change of climate must have had a powerful
influence on migration: a region when its climate was different may have
been a high road for migration, but now be impassable; I shall, however,
presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a
narrow isthmus now separates two marine faunas; submerge it, or let it
formerly have been submerged, and the two faunas will now blend or may
formerly have blended: where the sea now extends, land may at a former
period have connected islands or [357]possibly even
continents together, and thus have allowed terrestrial productions to
pass from one to the other. No geologist will dispute that great
mutations of level have occurred within the period of existing organisms.
Edward Forbes insisted that all the islands in the Atlantic must recently
have been connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothetically bridged over every ocean,
and have united almost every island to some mainland. If indeed the
arguments used by Forbes are to be trusted, it must be admitted that
scarcely a single island exists which has not recently been united to
some continent. This view cuts the Gordian knot of the dispersal of the
same species to the most distant points, and removes many a difficulty:
but to the best of my judgment we are not authorized in admitting such
enormous geographical changes within the period of existing species. It
seems to me that we have abundant evidence of great oscillations of level
in our continents; but not of such vast changes in their position and
extension, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked, as I believe, by rings of coral or atolls standing over them.
Whenever it is fully admitted, as I believe it will some day be, that
each species has proceeded from a single birthplace, and when in the
course of time we know something definite about the means of
distribution, we shall be enabled to speculate with security on the
former extension of the land. But I do not believe that it will ever be
proved that within the [358]recent period continents which are now
quite separate, have been continuously, or almost continuously, united
with each other, and with the many existing oceanic islands. Several
facts in distribution,—such as the great difference in the marine
faunas on the opposite sides of almost every continent,—the close
relation of the tertiary inhabitants of several lands and even seas to
their present inhabitants,—a certain degree of relation (as we
shall hereafter see) between the distribution of mammals and the depth of
the sea,—these and other such facts seem to me opposed to the
admission of such prodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by Forbes and admitted
by his many followers. The nature and relative proportions of the
inhabitants of oceanic islands likewise seem to me opposed to the belief
of their former continuity with continents. Nor does their almost
universally volcanic composition favour the admission that they are the
wrecks of sunken continents;—if they had originally existed as
mountain-ranges on the land, some at least of the islands would have been
formed, like other mountain-summits, of granite, metamorphic schists, old
fossiliferous or other such rocks, instead of consisting of mere piles of
volcanic matter.
I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution. I
shall here confine myself to plants. In botanical works, this or that
plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a few
experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that [359]out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived an
immersion of 137 days. For convenience' sake I chiefly tried small seeds,
without the capsule or fruit; and as all of these sank in a few days,
they could not be floated across wide spaces of the sea, whether or not
they were injured by the salt-water. Afterwards I tried some larger
fruits, capsules, &c., and some of these floated for a long time. It
is well known what a difference there is in the buoyancy of green and
seasoned timber; and it occurred to me that floods might wash down plants
or branches, and that these might be dried on the banks, and then by a
fresh rise in the stream be washed into the sea. Hence I was led to dry
stems and branches of 94 plants with ripe fruit, and to place them on
sea-water. The majority sank quickly, but some which whilst green floated
for a very short time, when dried floated much longer; for instance, ripe
hazel-nuts sank immediately, but when dried they floated for 90 days, and
afterwards when planted they germinated; an asparagus plant with ripe
berries floated for 23 days, when dried it floated for 85 days, and the
seeds afterwards germinated; the ripe seeds of Helosciadium sank in two
days, when dried they floated for above 90 days, and afterwards
germinated. Altogether out of the 94 dried plants, 18 floated for above
28 days, and some of the 18 floated for a very much longer period. So
that as 64/87 seeds germinated after an immersion of 28 days; and as
18/94 plants with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above 28 days, as
far as we may infer anything from these scanty facts, we may conclude
that the seeds of 14/100 plants of any country might be floated by
sea-currents during 28 days, and would retain their power of germination.
In Johnston's Physical Atlas, the average [360]rate of the several
Atlantic currents is 33 miles per diem (some currents running at the rate
of 60 miles per diem); on this average, the seeds of 14/100 plants
belonging to one country might be floated across 924 miles of sea to
another country; and when stranded, if blown to a favourable spot by an
inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in
a much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried 98 seeds, mostly different from mine; but he
chose many large fruits and likewise seeds from plants which live near
the sea; and this would have favoured the average length of their
flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or
branches with the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was that 18/98 of
his seeds floated for 42 days, and were then capable of germination. But
I do not doubt that plants exposed to the waves would float for a less
time than those protected from violent movement as in our experiments.
Therefore it would perhaps be safer to assume that the seeds of about
10/100 plants of a flora, after having been dried, could be floated
across a space of sea 900 miles in width, and would then germinate. The
fact of the larger fruits often floating longer than the small, is
interesting; as plants with large seeds or fruit could hardly be
transported by any other means; and Alph. de Candolle has shown that such
plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, [361]even on those in the
midst of the widest oceans; and the natives of the coral-islands in the
Pacific, procure stones for their tools, solely from the roots of drifted
trees, these stones being a valuable royal tax. I find on examination,
that when irregularly shaped stones are embedded in the roots of trees,
small parcels of earth are very frequently enclosed in their interstices
and behind them,—so perfectly that not a particle could be washed
away in the longest transport: out of one small portion of earth thus
completely enclosed by wood in an oak about 50 years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcasses of birds, when floating
on the sea, sometimes escape being immediately devoured; and seeds of
many kinds in the crops of floating birds long retain their vitality:
peas and vetches, for instance, are killed by even a few days' immersion
in sea-water; but some taken out of the crop of a pigeon, which had
floated on artificial salt-water for 30 days, to my surprise nearly all
germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may I think safely assume that under such circumstances their
rate of flight would often be 35 miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of nutritious
seeds passing through the intestines of a bird; but hard seeds of fruit
pass uninjured through even the digestive organs of a turkey. In the
course of two months, I picked up in my garden 12 kinds of seeds, out of
the excrement of small birds, and these seemed perfect, and some of them,
which I tried, germinated. [362]But the following fact is more important:
the crops of birds do not secrete gastric juice, and do not in the least
injure, as I know by trial, the germination of seeds; now after a bird
has found and devoured a large supply of food, it is positively asserted
that all the grains do not pass into the gizzard for 12 or even 18 hours.
A bird in this interval might easily be blown to the distance of 500
miles, and hawks are known to look out for tired birds, and the contents
of their torn crops might thus readily get scattered. Mr. Brent informs
me that a friend of his had to give up flying carrier-pigeons from France
to England, as the hawks on the English coast destroyed so many on their
arrival. Some hawks and owls bolt their prey whole, and after an interval
of from twelve to twenty hours, disgorge pellets, which, as I know from
experiments made in the Zoological Gardens, include seeds capable of
germination. Some seeds of the oat, wheat, millet, canary, hemp, clover,
and beet germinated after having been from twelve to twenty-one hours in
the stomachs of different birds of prey; and two seeds of beet grew after
having been thus retained for two days and fourteen hours. Freshwater
fish, I find, eat seeds of many land and water plants: fish are
frequently devoured by birds, and thus the seeds might be transported
from place to place. I forced many kinds of seeds into the stomachs of
dead fish, and then gave their bodies to fishing-eagles, storks, and
pelicans; these birds after an interval of many hours, either rejected
the seeds in pellets or passed them in their excrement; and several of
these seeds retained their power of germination. Certain seeds, however,
were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I can
show that earth sometimes adheres to them: in one instance I removed
twenty-two grains [363]of dry argillaceous earth from one foot of
a partridge, and in this earth there was a pebble quite as large as the
seed of a vetch. Thus seeds might occasionally be transported to great
distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions of
quails which annually cross the Mediterranean; and can we doubt that the
earth adhering to their feet would sometimes include a few minute seeds?
But I shall presently have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones,
and have even carried brushwood, bones, and the nest of a land-bird, I
can hardly doubt that they must occasionally have transported seeds from
one part to another of the arctic and antarctic regions, as suggested by
Lyell; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of the species
of plants common to Europe, in comparison with the plants of other
oceanic islands nearer to the mainland, and (as remarked by Mr. H. C.
Watson) from the somewhat northern character of the flora in comparison
with the latitude, I suspected that these islands had been partly stocked
by ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell
wrote to M. Hartung to inquire whether he had observed erratic boulders
on these islands, and he answered that he had found large fragments of
granite and other rocks, which do not occur in the archipelago. Hence we
may safely infer that icebergs formerly landed their rocky burthens on
the shores of these mid-ocean islands, and it is at least possible that
they may have brought thither the seeds of northern plants.
Considering that the several above means of transport, and that
several other means, which without [364]doubt remain to be
discovered, have been in action year after year, for centuries and tens
of thousands of years, it would I think be a marvellous fact if many
plants had not thus become widely transported. These means of transport
are sometimes called accidental, but this is not strictly correct: the
currents of the sea are not accidental, nor is the direction of prevalent
gales of wind. It should be observed that scarcely any means of transport
would carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
sea-water; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would not
by such means become mingled in any great degree; but would remain as
distinct as we now see them to be. The currents, from their course, would
never bring seeds from North America to Britain, though they might and do
bring seeds from the West Indies to our western shores, where, if not
killed by so long an immersion in salt-water, they could not endure our
climate. Almost every year, one or two land-birds are blown across the
whole Atlantic Ocean, from North America to the western shores of Ireland
and England; but seeds could be transported by these wanderers only by
one means, namely, in dirt sticking to their feet, which is in itself a
rare accident. Even in this case, how small would the chance be of a seed
falling on favourable soil, and coming to maturity! But it would be a
great error to argue that because a well-stocked island, like Great
Britain, has not, as far as is known [365](and it would be very
difficult to prove this), received within the last few centuries, through
occasional means of transport, immigrants from Europe or any other
continent, that a poorly-stocked island, though standing more remote from
the mainland, would not receive colonists by similar means. I do not
doubt that out of twenty seeds or animals transported to an island, even
if far less well-stocked than Britain, scarcely more than one would be so
well fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological time,
whilst an island was being upheaved and formed, and before it had become
fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed, which
chanced to arrive, if fitted for the climate, would be sure to germinate
and survive.
Dispersal during the Glacial period.—The identity of many
plants and animals, on mountain-summits, separated from each other by
hundreds of miles of lowlands, where the Alpine species could not
possibly exist, is one of the most striking cases known of the same
species living at distant points, without the apparent possibility of
their having migrated from one to the other. It is indeed a remarkable
fact to see so many of the same plants living on the snowy regions of the
Alps or Pyrenees, and in the extreme northern parts of Europe; but it is
far more remarkable, that the plants on the White Mountains, in the
United States of America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with those on the loftiest
mountains of Europe. Even as long ago as 1747, such facts led Gmelin to
conclude that the [366]same species must have been independently
created at several distinct points; and we might have remained in this
same belief, had not Agassiz and others called vivid attention to the
Glacial period, which, as we shall immediately see, affords a simple
explanation of these facts. We have evidence of almost every conceivable
kind, organic and inorganic, that within a very recent geological period,
central Europe and North America suffered under an Arctic climate. The
ruins of a house burnt by fire do not tell their tale more plainly, than
do the mountains of Scotland and Wales, with their scored flanks,
polished surfaces, and perched boulders, of the icy streams with which
their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old glaciers,
are now clothed by the vine and maize. Throughout a large part of the
United States, erratic boulders, and rocks scored by drifted icebergs and
coast-ice, plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then
pass away, as formerly occurred. As the cold came on, and as each more
southern zone became fitted for arctic beings and ill-fitted for their
former more temperate inhabitants, the latter would be supplanted and
arctic productions would take their places. The inhabitants of the more
temperate regions would at the same time travel southward, unless they
were stopped by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had
reached [367]its maximum, we should have a uniform
arctic fauna and flora, covering the central parts of Europe, as far
south as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by
arctic plants and animals, and these would be nearly the same with those
of Europe; for the present circumpolar inhabitants, which we suppose to
have everywhere travelled southward, are remarkably uniform round the
world. We may suppose that the Glacial period came on a little earlier or
later in North America than in Europe, so will the southern migration
there have been a little earlier or later; but this will make no
difference in the final result.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed ground,
always ascending higher and higher, as the warmth increased, whilst their
brethren were pursuing their northern journey. Hence, when the warmth had
fully returned, the same arctic species, which had lately lived in a body
together on the lowlands of the Old and New Worlds, would be left
isolated on distant mountain-summits (having been exterminated on all
lesser heights) and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of Europe.
We can thus also understand the fact that the Alpine plants of each
mountain-range are more especially related to the arctic forms living due
north or nearly due north of them: for the migration as the cold came on,
and the re-migration on the returning warmth, will generally [368]have been
due south and north. The Alpine plants, for example, of Scotland, as
remarked by Mr. H. C. Watson, and those of the Pyrenees, as remarked by
Ramond, are more especially allied to the plants of northern Scandinavia;
those of the United States to Labrador; those of the mountains of Siberia
to the arctic regions of that country. These views, grounded as they are
on the perfectly well-ascertained occurrence of a former Glacial period,
seem to me to explain in so satisfactory a manner the present
distribution of the Alpine and Arctic productions of Europe and America,
that when in other regions we find the same species on distant
mountain-summits, we may almost conclude without other evidence, that a
colder climate permitted their former migration across the low
intervening tracts, since become too warm for their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe
to have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very late
period have marched a little further north, and subsequently have
retreated to their present homes; but I have met with no satisfactory
evidence with respect to this intercalated slightly warmer period, since
the Glacial period.
The arctic forms, during their long southern migration and
re-migration northward, will have been exposed to nearly the same
climate, and, as is especially to be noticed, they will have kept in a
body together; consequently their mutual relations will not have been
much disturbed, and, in accordance with the principles inculcated in this
volume, they will not have been liable to much modification. But with our
Alpine productions, left isolated from the moment of the returning
warmth, [369]first at the bases and ultimately on the
summits of the mountains, the case will have been somewhat different; for
it is not likely that all the same arctic species will have been left on
mountain ranges distant from each other, and have survived there ever
since; they will, also, in all probability have become mingled with
ancient Alpine species, which must have existed on the mountains before
the commencement of the Glacial epoch, and which during its coldest
period will have been temporarily driven down to the plains; they will,
also, have been exposed to somewhat different climatal influences. Their
mutual relations will thus have been in some degree disturbed;
consequently they will have been liable to modification; and this we find
has been the case; for if we compare the present Alpine plants and
animals of the several great European mountain-ranges, though very many
of the species are identically the same, some present varieties, some are
ranked as doubtful forms, and some few are distinct yet closely allied or
representative species.
In illustrating what, as I believe, actually took place during the
Glacial period, I assumed that at its commencement the arctic productions
were as uniform round the polar regions as they are at the present day.
But the foregoing remarks on distribution apply not only to strictly
arctic forms, but also to many sub-arctic and to some few northern
temperate forms, for some of these are the same on the lower mountains
and on the plains of North America and Europe; and it may be reasonably
asked how I account for the necessary degree of uniformity of the
sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the [370]Atlantic Ocean and by
the extreme northern part of the Pacific. During the Glacial period, when
the inhabitants of the Old and New Worlds lived further southwards than
at present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We
have good reason to believe that during the newer Pliocene period, before
the Glacial epoch, and whilst the majority of the inhabitants of the
world were specifically the same as now, the climate was warmer than at
the present day. Hence we may suppose that the organisms now living under
the climate of latitude 60°, during the Pliocene period lived further
north under the Polar Circle, in latitude 66°-67°; and that the strictly
arctic productions then lived on the broken land still nearer to the
pole. Now if we look at a globe, we shall see that under the Polar Circle
there is almost continuous land from western Europe, through Siberia, to
eastern America. And to this continuity of the circumpolar land, and to
the consequent freedom for intermigration under a more favourable
climate, I attribute the necessary amount of uniformity in the sub-arctic
and northern temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected to
large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of the
same plants and animals inhabited the almost continuous circumpolar land;
and that these plants and animals, both in the Old and [371]New Worlds,
began slowly to migrate southwards as the climate became less warm, long
before the commencement of the Glacial period. We now see, as I believe,
their descendants, mostly in a modified condition, in the central parts
of Europe and the United States. On this view we can understand the
relationship, with very little identity, between the productions of North
America and Europe,—a relationship which is most remarkable,
considering the distance of the two areas, and their separation by the
Atlantic Ocean. We can further understand the singular fact remarked on
by several observers, that the productions of Europe and America during
the later tertiary stages were more closely related to each other than
they are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they must have been completely cut off from
each other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and have had to compete with them;
and in the other great region, with those of the Old World. Consequently
we have here everything favourable for much modification,—for far
more modification than with the Alpine productions, left isolated, within
a much more recent period, on the several mountain-ranges and on the
arctic lands of the two Worlds. Hence it has come, that when we compare
[372]the now living productions of the
temperate regions of the New and Old Worlds, we find very few identical
species (though Asa Gray has lately shown that more plants are identical
than was formerly supposed), but we find in every great class many forms,
which some naturalists rank as geographical races, and others as distinct
species; and a host of closely allied or representative forms which are
ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary representative
forms on the eastern and western shores of temperate North America; and
the still more striking case of many closely allied crustaceans (as
described in Dana's admirable work), of some fish and other marine
animals, in the Mediterranean and in the seas of Japan,—areas now
separated by a continent and by nearly a hemisphere of equatorial
ocean.
These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants of
the temperate lands of North America and Europe, are inexplicable on the
theory of creation. We cannot say that they have been created alike, in
correspondence with the nearly similar physical conditions of the areas;
for if we compare, for instance, certain parts of South America with the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their
inhabitants utterly dissimilar. [373]
But we must return to our more immediate subject, the Glacial period.
I am convinced that Forbes's view may be largely extended. In Europe we
have the plainest evidence of the cold period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. Along the Himalaya, at points 900 miles
apart, glaciers have left the marks of their former low descent; and in
Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. South
of the equator, we have some direct evidence of former glacial action in
New Zealand; and the same plants, found on widely separated mountains in
that island, tell the same story. If one account which has been published
can be trusted, we have direct evidence of glacial action in the
south-eastern corner of Australia.
Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36°-37°, and
on the shores of the Pacific, where the climate is now so different, as
far south as lat. 46°; erratic boulders have, also, been noticed on the
Rocky Mountains. In the Cordillera of Equatorial South America, glaciers
once extended far below their present level. In central Chili I was
astonished at the structure of a vast mound of detritus, about 800 feet
in height, crossing a valley of the Andes; and this I now feel convinced
was a gigantic moraine, left far below any existing glacier. Further
south on both sides of the continent, from lat. 41° to the southernmost
extremity, we have the clearest evidence of former glacial action, in
huge boulders transported far from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at
these several far distant points on [374]opposite sides of the
world. But we have good evidence in almost every case, that the epoch was
included within the latest geological period. We have, also, excellent
evidence, that it endured for an enormous time, as measured by years, at
each point. The cold may have come on, or have ceased, earlier at one
point of the globe than at another, but seeing that it endured for long
at each, and that it was contemporaneous in a geological sense, it seems
to me probable that it was, during a part at least of the period,
actually simultaneous throughout the world. Without some distinct
evidence to the contrary, we may at least admit as probable that the
glacial action was simultaneous on the eastern and western sides of North
America, in the Cordillera under the equator and under the warmer
temperate zones, and on both sides of the southern extremity of the
continent. If this be admitted, it is difficult to avoid believing that
the temperature of the whole world was at this period simultaneously
cooler. But it would suffice for my purpose, if the temperature was at
the same time lower along certain broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty of
the flowering plants of Tierra del Fuego, forming no inconsiderable part
of its scanty flora, are common to Europe, enormously remote as these two
points are; and there are many closely allied species. On the lofty
mountains of equatorial America a host of peculiar species belonging to
European genera occur. On the highest mountains of Brazil, some few
European genera were found by Gardner, which do not exist in the wide
[375]intervening hot countries. So on the Silla
of Caraccas the illustrious Humboldt long ago found species belonging to
genera characteristic of the Cordillera. On the mountains of Abyssinia,
several European forms and some few representatives of the peculiar flora
of the Cape of Good Hope occur. At the Cape of Good Hope a very few
European species, believed not to have been introduced by man, and on the
mountains, some few representative European forms are found, which have
not been discovered in the intertropical parts of Africa. On the
Himalaya, and on the isolated mountain-ranges of the peninsula of India,
on the heights of Ceylon, and on the volcanic cones of Java, many plants
occur, either identically the same or representing each other, and at the
same time representing plants of Europe, not found in the intervening hot
lowlands. A list of the genera collected on the loftier peaks of Java
raises a picture of a collection made on a hill in Europe! Still more
striking is the fact that southern Australian forms are clearly
represented by plants growing on the summits of the mountains of Borneo.
Some of these Australian forms, as I hear from Dr. Hooker, extend along
the heights of the peninsula of Malacca, and are thinly scattered, on the
one hand over India and on the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Müller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the Flora
of New Zealand,' by Dr. Hooker, analogous and striking facts are given in
regard to the plants of that large island. Hence we see that throughout
the world, the plants growing on the [376]more lofty mountains,
and on the temperate lowlands of the northern and southern hemispheres,
are sometimes identically the same; but they are much oftener
specifically distinct, though related to each other in a most remarkable
manner.
This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Prof. Dana, that "it is certainly a
wonderful fact that New Zealand should have a closer resemblance in its
Crustacea to Great Britain, its antipode, than to any other part of the
world." Sir J. Richardson, also, speaks of the reappearance on the shores
of New Zealand, Tasmania, &c., of northern forms of fish. Dr. Hooker
informs me that twenty-five species of Algæ are common to New Zealand and
to Europe, but have not been found in the intermediate tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of
the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic." Many of the forms living on the
mountains of the warmer regions of the earth and in the southern
hemisphere are of doubtful value, being ranked by some naturalists as
specifically distinct, by others as varieties; but some are certainly
identical, and many, though closely related to northern forms, must be
ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large [377]body of geological
evidence, that the whole world, or a large part of it, was during the
Glacial period simultaneously much colder than at present. The Glacial
period, as measured by years, must have been very long; and when we
remember over what vast spaces some naturalised plants and animals have
spread within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical plants
and other productions will have retreated from both sides towards the
equator, followed in the rear by the temperate productions, and these by
the arctic; but with the latter we are not now concerned. The tropical
plants probably suffered much extinction; how much no one can say;
perhaps formerly the tropics supported as many species as we see at the
present day crowded together at the Cape of Good Hope, and in parts of
temperate Australia. As we know that many tropical plants and animals can
withstand a considerable amount of cold, many might have escaped
extermination during a moderate fall of temperature, more especially by
escaping into the lowest, most protected, and warmest districts. But the
great fact to bear in mind is, that all tropical productions will have
suffered to a certain extent. On the other hand, the temperate
productions, after migrating nearer to the equator, though they will have
been placed under somewhat new conditions, will have suffered less. And
it is certain that many temperate plants, if protected from the inroads
of competitors, can withstand a much warmer climate than their own.
Hence, it seems to me possible, bearing in mind that the tropical
productions were in a suffering state and could not have presented a firm
front against intruders, that a certain number of the more vigorous and
dominant temperate forms might have penetrated the native ranks and have
reached or [378]even crossed the equator. The invasion
would, of course, have been greatly favoured by high land, and perhaps by
a dry climate; for Dr. Falconer informs me that it is the damp with the
heat of the tropics which is so destructive to perennial plants from a
temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropical natives. The
mountain-ranges north-west of the Himalaya, and the long line of the
Cordillera, seem to have afforded two great lines of invasion: and it is
a striking fact, lately communicated to me by Dr. Hooker, that all the
flowering plants, about forty-six in number, common to Tierra del Fuego
and to Europe still exist in North America, which must have lain on the
line of march. But I do not doubt that some temperate productions entered
and crossed even the lowlands of the tropics at the period when
the cold was most intense,—when arctic forms had migrated some
twenty-five degrees of latitude from their native country and covered the
land at the foot of the Pyrenees. At this period of extreme cold, I
believe that the climate under the equator at the level of the sea was
about the same with that now felt there at the height of six or seven
thousand feet. During this the coldest period, I suppose that large
spaces of the tropical lowlands were clothed with a mingled tropical and
temperate vegetation, like that now growing with strange luxuriance at
the base of the Himalaya, as graphically described by Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the [379]lowlands; those which had not reached the
equator would re-migrate northward or southward towards their former
homes; but the forms, chiefly northern, which had crossed the equator,
would travel still further from their homes into the more temperate
latitudes of the opposite hemisphere. Although we have reason to believe
from geological evidence that the whole body of arctic shells underwent
scarcely any modification during their long southern migration and
re-migration northward, the case may have been wholly different with
those intruding forms which settled themselves on the intertropical
mountains, and in the southern hemisphere. These being surrounded by
strangers will have had to compete with many new forms of life; and it is
probable that selected modifications in their structure, habits, and
constitutions will have profited them. Thus many of these wanderers,
though still plainly related by inheritance to their brethren of the
northern or southern hemispheres, now exist in their new homes as
well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical plants and allied forms have apparently migrated from the north
to the south, than in a reversed direction. We see, however, a few
southern vegetable forms on the mountains of Borneo and Abyssinia. I
suspect that this preponderant migration from north to south is due to
the greater extent of land in the north, and to the northern forms having
existed in their own homes in greater numbers, and having consequently
been advanced through natural selection and competition to a higher stage
of perfection or dominating power, than the southern forms. And thus,
when they became commingled during the Glacial period, the northern forms
[380]were enabled to beat the less powerful
southern forms. Just in the same manner as we see at the present day,
that very many European productions cover the ground in La Plata, and in
a lesser degree in Australia, and have to a certain extent beaten the
natives; whereas extremely few southern forms have become naturalised in
any part of Europe, though hides, wool, and other objects likely to carry
seeds have been largely imported into Europe during the last two or three
centuries from La Plata, and during the last thirty or forty years from
Australia. Something of the same kind must have occurred on the
intertropical mountains: no doubt before the Glacial period they were
stocked with endemic Alpine forms; but these have almost everywhere
largely yielded to the more dominant forms, generated in the larger areas
and more efficient workshops of the north. In many islands the native
productions are nearly equalled or even outnumbered by the naturalised;
and if the natives have not been actually exterminated, their numbers
have been greatly reduced, and this is the first stage towards
extinction. A mountain is an island on the land; and the intertropical
mountains before the Glacial period must have been completely isolated;
and I believe that the productions of these islands on the land yielded
to those produced within the larger areas of the north, just in the same
way as the productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.
I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain to
be solved. I do not pretend to [381]indicate the exact lines and means of
migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise to
new groups of forms, and others have remained unaltered. We cannot hope
to explain such facts, until we can say why one species and not another
becomes naturalised by man's agency in a foreign land; why one ranges
twice or thrice as far, and is twice or thrice as common, as another
species within their own homes.
I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed.
I will only say that as far as regards the occurrence of identical
species at points so enormously remote as Kerguelen Land, New Zealand,
and Fuegia, I believe that towards the close of the Glacial period,
icebergs, as suggested by Lyell, have been largely concerned in their
dispersal. But the existence of several quite distinct species, belonging
to genera exclusively confined to the south, at these and other distant
points of the southern hemisphere, is, on my theory of descent with
modification, a far more remarkable case of difficulty. For some of these
species are so distinct, that we cannot suppose that there has been time
since the commencement of the Glacial period for their migration, and for
their subsequent modification to the necessary degree. The facts seem to
me to indicate that peculiar and very distinct species have migrated in
radiating lines from some common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer period,
before the commencement of the Glacial period, when the antarctic lands,
now covered with ice, supported a highly peculiar [382]and isolated flora. I
suspect that before this flora was exterminated by the Glacial epoch, a
few forms were widely dispersed to various points of the southern
hemisphere by occasional means of transport, and by the aid, as
halting-places, of existing and now sunken islands: By these means, as I
believe, the southern shores of America, Australia, New Zealand, have
become slightly tinted by the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one
of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the
present distribution both of the same and of allied forms of life can be
explained. The living waters may be said to have flowed during one short
period from the north and from the south, and to have crossed at the
equator; but to have flowed with greater force from the north so as to
have freely inundated the south. As the tide leaves its drift in
horizontal lines, though rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands to a
great height under the equator. The various beings thus left stranded may
be compared with savage races of man, driven up and surviving in the
mountain-fastnesses of almost every land, which serve as a record, full
of interest to us, of the former inhabitants of the surrounding
lowlands.
[383]
CHAPTER XII.
Geographical Distribution—continued.
Distribution of fresh-water productions—On the inhabitants of
oceanic islands—Absence of Batrachians and of terrestrial
Mammals—On the relation of the inhabitants of islands to those of
the nearest mainland—On colonisation from the nearest source with
subsequent modification—Summary of the last and present
chapters.
As lakes and river-systems are separated from each other by barriers
of land, it might have been thought that fresh-water productions would
not have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to quite different classes,
an enormous range, but allied species prevail in a remarkable manner
throughout the world. I well remember, when first collecting in the fresh
waters of Brazil, feeling much surprise at the similarity of the
fresh-water insects, shells, &c., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to
wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to [384]fish,
I believe that the same species never occur in the fresh waters of
distant continents. But on the same continent the species often range
widely and almost capriciously; for two river-systems will have some fish
in common and some different. A few facts seem to favour the possibility
of their occasional transport by accidental means; like that of the live
fish not rarely dropped by whirlwinds in India, and the vitality of their
ova when removed from the water. But I am inclined to attribute the
dispersal of fresh-water fish mainly to slight changes within the recent
period in the level of the land, having caused rivers to flow into each
other. Instances, also, could be given of this having occurred during
floods, without any change of level. We have evidence in the loess of the
Rhine of considerable changes of level in the land within a very recent
geological period, and when the surface was peopled by existing land and
fresh-water shells. The wide difference of the fish on opposite sides of
continuous mountain-ranges, which from an early period must have parted
river-systems and completely prevented their inosculation, seems to lead
to this same conclusion. With respect to allied fresh-water fish
occurring at very distant points of the world, no doubt there are many
cases which cannot at present be explained: but some fresh-water fish
belong to very ancient forms, and in such cases there will have been
ample time for great geographical changes, and consequently time and
means for much migration. In the second place, salt-water fish can with
care be slowly accustomed to live in fresh water; and, according to
Valenciennes, there is hardly a single group of fishes confined
exclusively to fresh water, so that we may imagine that a marine member
of a fresh-water group might travel far along the shores of the sea, and
[385]subsequently become modified and adapted
to the fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to
be transported by birds, and they are immediately killed by sea-water, as
are the adults. I could not even understand how some naturalised species
have rapidly spread throughout the same country. But two facts, which I
have observed—and no doubt many others remain to be
observed—throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duckweed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a duck's
feet, which might represent those of a bird sleeping in a natural pond,
in an aquarium, where many ova of fresh-water shells were hatching; and I
found that numbers of the extremely minute and just-hatched shells
crawled on the feet, and clung to them so firmly that when taken out of
the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched
molluscs, though aquatic in their nature, survived on the duck's feet, in
damp air, from twelve to twenty hours; and in this length of time a duck
or heron might fly at least six or seven hundred miles, and would be sure
to alight on a pool or rivulet, if blown across sea to an oceanic island
or to any other distant point. Sir Charles Lyell also [386]informs me
that a Dyticus has been caught with an Ancylus (a fresh-water shell like
a limpet) firmly adhering to it; and a water-beetle of the same family, a
Colymbetes, once flew on board the 'Beagle,' when forty-five miles
distant from the nearest land: how much farther it might have flown with
a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and to
the most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have only
a very few aquatic members; for these latter seem immediately to acquire,
as if in consequence, a very wide range. I think favourable means of
dispersal explain this fact. I have before mentioned that earth
occasionally, though rarely, adheres in some quantity to the feet and
beaks of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds of
this order I can show are the greatest wanderers, and are occasionally
found on the most remote and barren islands in the open ocean; they would
not be likely to alight on the surface of the sea, so that the dirt would
not be washed off their feet; when making land, they would be sure to fly
to their natural fresh-water haunts. I do not believe that botanists are
aware how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I
took in February three table-spoonfuls of mud from three different
points, beneath water, on the edge of a little pond; this mud when dry
weighed only 6Ÿ ounces; I kept it covered up in my study for six months,
pulling up and counting each plant as it grew; the plants were [387]of many
kinds, and were altogether 537 in number; and yet the viscid mud was all
contained in a breakfast cup! Considering these facts, I think it would
be an inexplicable circumstance if water-birds did not transport the
seeds of fresh-water plants to vast distances, and if consequently the
range of these plants was not very great. The same agency may have come
into play with the eggs of some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they reject
many other kinds after having swallowed them; even small fish swallow
seeds of moderate size, as of the yellow water-lily and Potamogeton.
Herons and other birds, century after century, have gone on daily
devouring fish; they then take flight and go to other waters, or are
blown across the sea; and we have seen that seeds retain their power of
germination, when rejected in pellets or in excrement, many hours
afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks on
this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and getting
a hearty meal of fish, would probably reject from its stomach a pellet
containing the seeds of the Nelumbium undigested; or the seeds might be
dropped by the bird whilst feeding its young, in the same way as fish are
known sometimes to be dropped.
In considering these several means of distribution, [388]it should be
remembered that when a pond or stream is first formed, for instance, on a
rising islet, it will be unoccupied; and a single seed or egg will have a
good chance of succeeding. Although there will always be a struggle for
life between the individuals of the species, however few, already
occupying any pond, yet as the number of kinds is small, compared with
those on the land, the competition will probably be less severe between
aquatic than between terrestrial species; consequently an intruder from
the waters of a foreign country, would have a better chance of seizing on
a place, than in the case of terrestrial colonists. We should, also,
remember that some, perhaps many, freshwater productions are low in the
scale of nature, and that we have reason to believe that such low beings
change or become modified less quickly than the high; and this will give
longer time than the average for the migration of the same aquatic
species. We should not forget the probability of many species having
formerly ranged as continuously as fresh-water productions ever can
range, over immense areas, and having subsequently become extinct in
intermediate regions. But the wide distribution of fresh-water plants and
of the lower animals, whether retaining the same identical form or in
some degree modified, I believe mainly depends on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful
gardener, thus takes her seeds from a bed of a particular nature, and
drops them in another equally well fitted for them.
On the Inhabitants of Oceanic Islands.—We now come to the
last of the three classes of facts, which I [389]have selected as
presenting the greatest amount of difficulty, on the view that all the
individuals both of the same and of allied species have descended from a
single parent; and therefore have all proceeded from a common birthplace,
notwithstanding that in the course of time they have come to inhabit
distant points of the globe. I have already stated that I cannot honestly
admit Forbes's view on continental extensions, which, if legitimately
followed out, would lead to the belief that within the recent period all
existing islands have been nearly or quite joined to some continent. This
view would remove many difficulties, but it would not, I think, explain
all the facts in regard to insular productions. In the following remarks
I shall not confine myself to the mere question of dispersal; but shall
consider some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de Candolle
admits this for plants, and Wollaston for insects. If we look to the
large size and varied stations of New Zealand, extending over 780 miles
of latitude, and compare its flowering plants, only 750 in number, with
those on an equal area at the Cape of Good Hope or in Australia, we must,
I think, admit that something quite independently of any difference in
physical conditions has caused so great a difference in number. Even the
uniform county of Cambridge has 847 plants, and the little island of
Anglesea 764, but a few ferns and a few introduced plants are included in
these numbers, and the comparison in some other respects is not quite
fair. We have evidence that the barren island of Ascension aboriginally
possessed under half-a-dozen flowering plants; [390]yet many have become
naturalised on it, as they have on New Zealand and on every other oceanic
island which can be named. In St. Helena there is reason to believe that
the naturalised plants and animals have nearly or quite exterminated many
native productions. He who admits the doctrine of the creation of each
separate species, will have to admit, that a sufficient number of the
best adapted plants and animals have not been created on oceanic islands;
for man has unintentionally stocked them from various sources far more
fully and perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (i.e. those found
nowhere else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on any
continent, and then compare the area of the islands with that of the
continent, we shall see that this is true. This fact might have been
expected on my theory, for, as already explained, species occasionally
arriving after long intervals in a new and isolated district, and having
to compete with new associates, will be eminently liable to modification,
and will often produce groups of modified descendants. But it by no means
follows, that, because in an island nearly all the species of one class
are peculiar, those of another class, or of another section of the same
class, are peculiar; and this difference seems to depend partly on the
species which do not become modified having immigrated with facility and
in a body, so that their mutual relations have not been much disturbed;
and partly on the frequent arrival of unmodified immigrants from the
mother-country, and the consequent intercrossing with them. With respect
to the effects of this intercrossing, [391]it should be remembered
that the offspring of such crosses would almost certainly gain in vigour;
so that even an occasional cross would produce more effect than might at
first have been anticipated. To give a few examples: in the Galapagos
Islands nearly every land-bird, but only two out of the eleven marine
birds, are peculiar; and it is obvious that marine birds could arrive at
these islands more easily than land-birds. Bermuda, on the other hand,
which lies at about the same distance from North America as the Galapagos
Islands do from South America, and which has a very peculiar soil, does
not possess one endemic land-bird; and we know from Mr. J. M. Jones's
admirable account of Bermuda, that very many North American birds, during
their great annual migrations, visit either periodically or occasionally
this island. Madeira does not possess one peculiar bird, and many
European and African birds are almost every year blown there, as I am
informed by Mr. E. V. Harcourt. So that these two islands of Bermuda and
Madeira have been stocked by birds, which for long ages have struggled
together in their former homes, and have become mutually adapted to each
other; and when settled in their new homes, each kind will have been kept
by the others to their proper places and habits, and will consequently
have been little liable to modification. Any tendency to modification
will, also, have been checked by intercrossing with the unmodified
immigrants from the mother-country. Madeira, again, is inhabited by a
wonderful number of peculiar land-shells, whereas not one species of
sea-shell is confined to its shores: now, though we do not know how
sea-shells are dispersed, yet we can see that their eggs or larvae,
perhaps attached to seaweed or floating timber, or to the feet of
wading-birds, might be transported far more easily than [392]land-shells,
across three or four hundred miles of open sea. The different orders of
insects in Madeira apparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take the
place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
shown that the proportional numbers of the different orders are very
different from what they are elsewhere. Such cases are generally
accounted for by the physical conditions of the islands; but this
explanation seems to me not a little doubtful. Facility of immigration, I
believe, has been at least as important as the nature of the
conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than the adaptation of hooked
seeds for transportal by the wool and fur of quadrupeds. This case
presents no difficulty on my view, for a hooked seed might be transported
to an island by some other means; and the plant then becoming slightly
modified, but still retaining its hooked seeds, would form an endemic
species, having as useless an appendage as any rudimentary
organ,—for instance, as the shrivelled wings under the soldered
elytra of many insular beetles. Again, islands often possess trees or
bushes belonging to orders which elsewhere include only herbaceous
species; now trees, as Alph. de Candolle has shown, generally have,
whatever the cause may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands; and an herbaceous plant, though
it would have no chance of [393]successfully competing in stature with a
fully developed tree, when established on an island and having to compete
with herbaceous plants alone, might readily gain an advantage by growing
taller and taller and overtopping the other plants. If so, natural
selection would often tend to add to the stature of herbaceous plants
when growing on an oceanic island, to whatever order they belonged, and
thus convert them first into bushes and ultimately into trees.
With respect to the absence of whole orders on oceanic islands, Bory
St. Vincent long ago remarked that Batrachians (frogs, toads, newts) have
never been found on any of the many islands with which the great oceans
are studded. I have taken pains to verify this assertion, and I have
found it strictly true. I have, however, been assured that a frog exists
on the mountains of the great island of New Zealand; but I suspect that
this exception (if the information be correct) may be explained through
glacial agency. This general absence of frogs, toads, and newts on so
many oceanic islands cannot be accounted for by their physical
conditions; indeed it seems that islands are peculiarly well fitted for
these animals; for frogs have been introduced into Madeira, the Azores,
and Mauritius, and have multiplied so as to become a nuisance. But as
these animals and their spawn are known to be immediately killed by
sea-water, on my view we can see that there would be great difficulty in
their transportal across the sea, and therefore why they do not exist on
any oceanic island. But why, on the theory of creation, they should not
have been created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found
a single [394]instance, free from doubt, of a
terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not support
small mammals, for they occur in many parts of the world on very small
islands, if close to a continent; and hardly an island can be named on
which our smaller quadrupeds have not become naturalised and greatly
multiplied. It cannot be said, on the ordinary view of creation, that
there has not been time for the creation of mammals; many volcanic
islands are sufficiently ancient, as shown by the stupendous degradation
which they have suffered and by their tertiary strata: there has also
been time for the production of endemic species belonging to other
classes; and on continents it is thought that mammals appear and
disappear at a quicker rate than other and lower animals. Though
terrestrial mammals do not occur on oceanic islands, aërial mammals do
occur on almost every island. New Zealand possesses two bats found
nowhere else in the world: Norfolk Island, the Viti Archipelago, the
Bonin Islands, the Caroline and Marianne Archipelagoes, and Mauritius,
all possess their peculiar bats. Why, it may be asked, has the supposed
creative force produced bats and no other mammals on remote islands? On
my view this question can easily be answered; for no [395]terrestrial
mammal can be transported across a wide space of sea, but bats can fly
across. Bats have been seen wandering by day far over the Atlantic Ocean;
and two North American species either regularly or occasionally visit
Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
Tomes, who has specially studied this family, that many of the same
species have enormous ranges, and are found on continents and on far
distant islands. Hence we have only to suppose that such wandering
species have been modified through natural selection in their new homes
in relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence in
both of the same mammiferous species or of allied species in a more or
less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago, which
is traversed near Celebes by a space of deep ocean; and this space
separates two widely distinct mammalian faunas. On either side the
islands are situated on moderately deep submarine banks, and they are
inhabited by closely allied or identical quadrupeds. No doubt some few
anomalies occur in this great archipelago, and there is much difficulty
in forming a judgment in some cases owing to the probable naturalisation
of certain mammals through man's agency; but we shall soon have much
light thrown on the natural history of this archipelago by the admirable
zeal and researches of Mr. Wallace. I have not as yet had time to [396]follow up this subject in all other
quarters of the world; but as far as I have gone, the relation generally
holds good. We see Britain separated by a shallow channel from Europe,
and the mammals are the same on both sides; we meet with analogous facts
on many islands separated by similar channels from Australia. The West
Indian Islands stand on a deeply submerged bank, nearly 1000 fathoms in
depth, and here we find American forms, but the species and even the
genera are distinct. As the amount of modification in all cases depends
to a certain degree on the lapse of time, and as during changes of level
it is obvious that islands separated by shallow channels are more likely
to have been continuously united within a recent period to the mainland
than islands separated by deeper channels, we can understand the frequent
relation between the depth of the sea and the degree of affinity of the
mammalian inhabitants of islands with those of a neighbouring
continent,—an inexplicable relation on the view of independent acts
of creation.
All the foregoing remarks on the inhabitants of oceanic
islands,—namely, the scarcity of kinds—the richness in
endemic forms in particular classes or sections of classes,—the
absence of whole groups, as of batrachians, and of terrestrial mammals
notwithstanding the presence of aërial bats,—the singular
proportions of certain orders of plants,—herbaceous forms having
been developed into trees, &c.,—seem to me to accord better
with the view of occasional means of transport having been largely
efficient in the long course of time, than with the view of all our
oceanic islands having been formerly connected by continuous land with
the nearest continent; for on this latter view the migration would
probably have been more complete; and if modification be admitted, all
the forms of life would have been more [397]equally modified, in
accordance with the paramount importance of the relation of organism to
organism.
I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote islands,
whether still retaining the same specific form or modified since their
arrival, could have reached their present homes. But the probability of
many islands having existed as halting-places, of which not a wreck now
remains, must not be overlooked. I will here give a single instance of
one of the cases of difficulty. Almost all oceanic islands, even the most
isolated and smallest, are inhabited by land-shells, generally by endemic
species, but sometimes by species found elsewhere. Dr. Aug. A. Gould has
given several interesting cases in regard to the land-shells of the
islands of the Pacific. Now it is notorious that land-shells are very
easily killed by salt; their eggs, at least such as I have tried, sink in
sea-water and are killed by it. Yet there must be, on my view, some
unknown, but highly efficient means for their transportal. Would the
just-hatched young occasionally crawl on and adhere to the feet of birds
roosting on the ground, and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over the
mouth of the shell, might be floated in chinks of drifted timber across
moderately wide arms of the sea. And I found that several species did in
this state withstand uninjured an immersion in sea-water during seven
days: one of these shells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days, and it perfectly
recovered. As this species has a thick calcareous operculum, I removed
it, and when it had formed a new membranous one, I immersed it for
fourteen days in sea-water, and it recovered and crawled away: but more
experiments are wanted on this head. [398]
The most striking and important fact for us in regard to the
inhabitants of islands, is their affinity to those of the nearest
mainland, without being actually the same species. Numerous instances
could be given of this fact. I will give only one, that of the Galapagos
Archipelago, situated under the equator, between 500 and 600 miles from
the shores of South America. Here almost every product of the land and
water bears the unmistakeable stamp of the American continent. There are
twenty-six land-birds, and twenty-five of these are ranked by Mr. Gould
as distinct species, supposed to have been created here; yet the close
affinity of most of these birds to American species in every character,
in their habits, gestures, and tones of voice, was manifest. So it is
with the other animals, and with nearly all the plants, as shown by Dr.
Hooker in his admirable memoir on the Flora of this archipelago. The
naturalist, looking at the inhabitants of these volcanic islands in the
Pacific, distant several hundred miles from the continent, yet feels that
he is standing on American land. Why should this be so? why should the
species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact
there is a considerable dissimilarity in all these respects. On the other
hand, there is a considerable degree of resemblance in the volcanic nature of
the soil, in climate, height, and size of the islands, between the
Galapagos and Cape de Verde Archipelagos: but what an entire and absolute
difference in their inhabitants! The inhabitants of the Cape de Verde
Islands are related to [399]those of Africa, like those of the
Galapagos to America. I believe this grand fact can receive no sort of
explanation on the ordinary view of independent creation; whereas on the
view here maintained, it is obvious that the Galapagos Islands would be
likely to receive colonists, whether by occasional means of transport or
by formerly continuous land, from America; and the Cape de Verde Islands
from Africa; and that such colonists would be liable to
modification;—the principle of inheritance still betraying their
original birthplace.
Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of the
nearest continent, or of other near islands. The exceptions are few, and
most of them can be explained. Thus the plants of Kerguelen Land, though
standing nearer to Africa than to America, are related, and that very
closely, as we know from Dr. Hooker's account, to those of America: but
on the view that this island has been mainly stocked by seeds brought
with earth and stones on icebergs, drifted by the prevailing currents,
this anomaly disappears. New Zealand in its endemic plants is much more
closely related to Australia, the nearest mainland, than to any other
region: and this is what might have been expected; but it is also plainly
related to South America, which, although the next nearest continent, is
so enormously remote, that the fact becomes an anomaly. But this
difficulty almost disappears on the view that both New Zealand, South
America, and other southern lands were long ago partially stocked from a
nearly intermediate though distant point, namely from the antarctic
islands, when they were clothed with vegetation, before the commencement
of the Glacial period. The affinity, which, though feeble, I am assured
by Dr. Hooker is real, between the flora of the south-western corner of
Australia and of the Cape of Good [400]Hope, is a far more
remarkable case, and is at present inexplicable: but this affinity is
confined to the plants, and will, I do not doubt, be some day
explained.
The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus the
several islands of the Galapagos Archipelago are tenanted, as I have
elsewhere shown, in a quite marvellous manner, by very closely related
species; so that the inhabitants of each separate island, though mostly
distinct, are related in an incomparably closer degree to each other than
to the inhabitants of any other part of the world. And this is just what
might have been expected on my view, for the islands are situated so near
each other that they would almost certainly receive immigrants from the
same original source, or from each other. But this dissimilarity between
the endemic inhabitants of the islands may be used as an argument against
my views; for it may be asked, how has it happened in the several islands
situated within sight of each other, having the same geological nature,
the same height, climate, &c., that many of the immigrants should
have been differently modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises in chief part from the
deeply-seated error of considering the physical conditions of a country
as the most important for its inhabitants; whereas it cannot, I think, be
disputed that the nature of the other inhabitants, with which each has to
compete, is as least as important, and generally a far more important
element of success. Now if we look to those inhabitants of the Galapagos
Archipelago which are found in other parts of the world (laying on one
side for the moment the [401]endemic species, which cannot be here
fairly included, as we are considering how they have come to be modified
since their arrival), we find a considerable amount of difference in the
several islands. This difference might indeed have been expected on the
view of the islands having been stocked by occasional means of
transport—a seed, for instance, of one plant having been brought to
one island, and that of another plant to another island. Hence when in
former times an immigrant settled on any one or more of the islands, or
when it subsequently spread from one island to another, it would
undoubtedly be exposed to different conditions of life in the different
islands, for it would have to compete with different sets of organisms: a
plant for instance, would find the best-fitted ground more perfectly
occupied by distinct plants in one island than in another, and it would
be exposed to the attacks of somewhat different enemies. If then it
varied, natural selection would probably favour different varieties in
the different islands. Some species, however, might spread and yet retain
the same character throughout the group, just as we see on continents
some species spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the British
Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and
sweep across the archipelago, and gales of wind are extraordinarily rare;
so that the islands are far more effectually separated from each other
than they appear to be on a map. Nevertheless a good many [402]species, both
those found in other parts of the world and those confined to the
archipelago, are common to the several islands, and we may infer from
certain facts that these have probably spread from some one island to the
others. But we often take, I think, an erroneous view of the probability
of closely-allied species invading each other's territory, when put into
free intercommunication. Undoubtedly if one species has any advantage
whatever over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places in
nature, both probably will hold their own places and keep separate for
almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species would
thus spread; but we should remember that the forms which become
naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct species, belonging in a
large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, are distinct on each; thus
there are three closely-allied species of mocking-thrush, each confined
to its own island. Now let us suppose the mocking-thrush of Chatham
Island to be blown to Charles Island, which has its own mocking-thrush:
why should it succeed in establishing itself there? We may safely infer
that Charles Island is well stocked with its own species, for annually
more eggs are laid there than can possibly be reared; and we may infer
that the mocking-thrush peculiar to Charles Island is at least as well
fitted for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact bearing
on this [403]subject; namely, that Madeira and the
adjoining islet of Porto Santo possess many distinct but representative
land-shells, some of which live in crevices of stone; and although large
quantities of stone are annually transported from Porto Santo to Madeira, yet this
latter island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European
land-shells, which no doubt had some advantage over the indigenous
species. From these considerations I think we need not greatly marvel at
the endemic and representative species, which inhabit the several islands
of the Galapagos Archipelago, not having universally spread from island
to island. In many other instances, as in the several districts of the
same continent, pre-occupation has probably played an important part in
checking the commingling of species under the same conditions of life.
Thus, the south-east and south-west corners of Australia have nearly the
same physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of that
region whence colonists could most readily have been derived,—the
colonists having been subsequently modified and better fitted to their
new homes,—is of the widest application throughout nature. We see
this on every mountain, in every lake and marsh. For Alpine species,
excepting in so far as the same forms, chiefly of plants, have spread
widely throughout the world during the recent Glacial epoch, are related
to those of the surrounding lowlands;—thus we have in South
America, Alpine humming-birds, Alpine rodents, Alpine plants, [404]&c.,
all of strictly American forms, and it is obvious that a mountain, as it
became slowly upheaved, would naturally be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting
in so far as great facility of transport has given the same general forms
to the whole world. We see this same principle in the blind animals
inhabiting the caves of America and of Europe. Other analogous facts
could be given. And it will, I believe, be universally found to be true,
that wherever in two regions, let them be ever so distant, many
closely-allied or representative species occur, there will likewise be
found some identical species, showing, in accordance with the foregoing
view, that at some former period there has been intercommunication or
migration between the two regions. And wherever many closely-allied
species occur, there will be found many forms which some naturalists rank
as distinct species, and some as varieties; these doubtful forms showing
us the steps in the process of modification.
This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way. Mr.
Gould remarked to me long ago, that in those genera of birds which range
over the world, many of the species have very wide ranges. I can hardly
doubt that this rule is generally true, though it would be difficult to
prove it. Amongst mammals, we see it strikingly displayed in Bats, and in
a lesser degree in the Felidæ and Canidæ. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many
individual species have [405]enormous ranges. It is not meant that in
world-ranging genera all the species have a wide range, or even that they
have on an average a wide range; but only that some of the species
range very widely; for the facility with which widely-ranging species
vary and give rise to new forms will largely determine their average
range. For instance, two varieties of the same species inhabit America
and Europe, and the species thus has an immense range; but, if the
variation had been a little greater, the two varieties would have been
ranked as distinct species, and the common range would have been greatly
reduced. Still less is it meant, that a species which apparently has the
capacity of crossing barriers and ranging widely, as in the case of
certain powerfully-winged birds, will necessarily range widely; for we
should never forget that to range widely implies not only the power of
crossing barriers, but the more important power of being victorious in
distant lands in the struggle for life with foreign associates. But on
the view of all the species of a genus having descended from a single
parent, though now distributed to the most remote points of the world, we
ought to find, and I believe as a general rule we do find, that some at
least of the species range very widely; for it is necessary that the
unmodified parent should range widely, undergoing modification during its
diffusion, and should place itself under diverse conditions favourable
for the conversion of its offspring, firstly into new varieties and
ultimately into new species.
In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off from
a common parent at a remote epoch; so that in such cases there will have
been ample time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration of some of the
species into all [406]quarters of the world, where they may have
become slightly modified in relation to their new conditions. There is,
also, some reason to believe from geological evidence that organisms low
in the scale within each great class, generally change at a slower rate
than the higher forms; and consequently the lower forms will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with the seeds and eggs of many low forms
being very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms is, the more widely it is
apt to range.
The relations just discussed,—namely, low and slowly-changing
organisms ranging more widely than the high,—some of the species of
widely-ranging genera themselves ranging widely,—such facts, as
alpine, lacustrine, and marsh productions being related (with the
exceptions before specified) to those on the surrounding low lands and
dry lands, though these stations are so different,—the very close
relation of the distinct species which inhabit the islets of the same
archipelago,—and especially the striking relation of the
inhabitants of each whole archipelago or island to those of the nearest
mainland,—are, I think, utterly inexplicable on the ordinary view
of the independent creation of each species, but are explicable on the
view of colonisation from the nearest or readiest source, together with
the subsequent modification and better adaptation of the colonists to
their new homes.
Summary of last and present Chapters.—In these chapters I
have endeavoured to show, that if we make due allowance for our ignorance
of the full effects of all [407]the changes of climate and of the level of
the land, which have certainly occurred within the recent period, and of
other similar changes which may have occurred within the same period; if
we remember how profoundly ignorant we are with respect to the many and
curious means of occasional transport,—a subject which has hardly
ever been properly experimentised on; if we bear in mind how often a
species may have ranged continuously over a wide area, and then have
become extinct in the intermediate tracts, I think the difficulties in
believing that all the individuals of the same species, wherever located,
have descended from the same parents, are not insuperable. And we are led
to this conclusion, which has been arrived at by many naturalists under
the designation of single centres of creation, by some general
considerations, more especially from the importance of barriers and from
the analogical distribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life change most slowly, enormous periods of time being thus granted for
their migration, I do not think that the difficulties are insuperable;
though they often are in this case, and in that of the individuals of the
same species, extremely great.
As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at
some little length the means of dispersal of fresh-water productions.
[408]
If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all the
grand leading facts of geographical distribution are explicable on the
theory of migration (generally of the more dominant forms of life),
together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether of
land or water, which separate our several zoological and botanical
provinces. We can thus understand the localisation of sub-genera, genera,
and families; and how it is that under different latitudes, for instance
in South America, the inhabitants of the plains and mountains, of the
forests, marshes, and deserts, are in so mysterious a manner linked
together by affinity, and are likewise linked to the extinct beings which
formerly inhabited the same continent. Bearing in mind that the mutual
relation of organism to organism is of the highest importance, we can see
why two areas having nearly the same physical conditions should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since new inhabitants entered one region;
according to the nature of the communication which allowed certain forms
and not others to enter, either in greater or lesser numbers; according
or not, as those which entered happened to come in more or less direct
competition with each other and with the aborigines; and according as the
immigrants were capable of varying more or less rapidly, there would
ensue in different regions, independently of their physical conditions,
infinitely diversified conditions of life,—there would be an almost
endless amount of organic action and reaction,—and we should find,
as we do find, some groups of beings greatly, and some only slightly
modified,—some [409]developed in great force, some existing in
scanty numbers—in the different great geographical provinces of the
world.
On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all
its species common to other quarters of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands possess
their own peculiar species of aërial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a more
or less modified condition, and the depth of the sea between an island
and the mainland. We can clearly see why all the inhabitants of an
archipelago, though specifically distinct on the several islets, should
be closely related to each other, and likewise be related, but less
closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however
distant from each other, there should be a correlation, in the presence
of identical species, of varieties, of doubtful species, and of distinct
but representative species.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and group
of species is continuous in time; for the exceptions to the rule are so
few, that they may [410]fairly be attributed to our not having as
yet discovered in an intermediate deposit the forms which are therein
absent, but which occur above and below: so in space, it certainly is the
general rule that the area inhabited by a single species, or by a group
of species, is continuous; and the exceptions, which are not rare, may,
as I have attempted to show, be accounted for by migration at some former
period under different conditions or by occasional means of transport,
and by the species having become extinct in the intermediate tracts. Both
in time and space, species and groups of species have their points of
maximum development. Groups of species, belonging either to a certain
period of time, or to a certain area, are often characterised by trifling
characters in common, as of sculpture or colour. In looking to the long
succession of ages, as in now looking to distant provinces throughout the
world, we find that some organisms differ little, whilst others belonging
to a different class, or to a different order, or even only to a
different family of the same order, differ greatly. In both time and
space the lower members of each class generally change less than the
higher; but there are in both cases marked exceptions to the rule. On my
theory these several relations throughout time and space are
intelligible; for whether we look to the forms of life which have changed
during successive ages within the same quarter of the world, or to those
which have changed after having migrated into distant quarters, in both
cases the forms within each class have been connected by the same bond of
ordinary generation; and the more nearly any two forms are related in
blood, the nearer they will generally stand to each other in time and
space; in both cases the laws of variation have been the same, and
modifications have been accumulated by the same power of natural
selection.
[411]
CHAPTER XIII.
Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.
Classification, groups subordinate to
groups—Natural system—Rules and difficulties in
classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used
in classification—Analogical or adaptive
characters—Affinities, general, complex and
radiating—Extinction separates and defines groups—Morphology, between members of the same class, between
parts of the same individual—Embryology,
laws of, explained by variations not supervening at an early age, and
being inherited at a corresponding age—Rudimentary
organs; their origin explained—Summary.
From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in groups
under groups. This classification is evidently not arbitrary like the
grouping of the stars in constellations. The existence of groups would
have been of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different
in nature; for it is notorious how commonly members of even the same
sub-group have different habits. In our second and fourth chapters, on
Variation and on Natural Selection, I have attempted to show that it is
the widely ranging, the much diffused and common, that is the dominant
species belonging to the larger genera, which vary most. The varieties,
or incipient species, thus produced ultimately become converted, as I
believe, into new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant [412]species.
Consequently the groups which are now large, and which generally include
many dominant species, tend to go on increasing indefinitely in size. I
further attempted to show that from the varying descendants of each
species trying to occupy as many and as different places as possible in
the economy of nature, there is a constant tendency in their characters
to diverge. This conclusion was supported by looking at the great
diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in
naturalisation.
I attempted also to show that there is a constant tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the less divergent, the less improved, and
preceding forms. I request the reader to turn to the diagram illustrating
the action, as formerly explained, of these several principles; and he
will see that the inevitable result is that the modified descendants
proceeding from one progenitor become broken up into groups subordinate
to groups. In the diagram each letter on the uppermost line may represent
a genus including several species; and all the genera on this line form
together one class, for all have descended from one ancient but unseen
parent, and, consequently, have inherited something in common. But the
three genera on the left hand have, on this same principle, much in
common, and form a sub-family, distinct from that including the next two
genera on the right hand, which diverged from a common parent at the
fifth stage of descent. These five genera have also much, though less, in
common; and they form a family distinct from that including the three
genera still further to the right hand, which diverged at a still earlier
period. And all these genera, descended from (A), form an order distinct
from the [413]genera descended from (I). So that we here
have many species descended from a single progenitor grouped into genera;
and the genera are included in, or subordinate to, sub-families,
families, and orders, all united into one class. Thus, the grand fact in
natural history of the subordination of group under group, which, from
its familiarity, does not always sufficiently strike us, is in my
judgment explained.
Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging together
those living objects which are most alike, and for separating those which
are most unlike; or as an artificial means for enunciating, as briefly as
possible, general propositions,—that is, by one sentence to give
the characters common, for instance, to all mammals, by another those
common to all carnivora, by another those common to the dog-genus, and
then by adding a single sentence, a full description is given of each
kind of dog. The ingenuity and utility of this system are indisputable.
But many naturalists think that something more is meant by the Natural
System; they believe that it reveals the plan of the Creator; but unless
it be specified whether order in time or space, or what else is meant by
the plan of the Creator, it seems to me that nothing is thus added to our
knowledge. Such expressions as that famous one of Linnæus, and which we
often meet with in a more or less concealed form, that the characters do
not make the genus, but that the genus gives the characters, seem to
imply that something more is included in our classification, than mere
resemblance. I believe that something more is included; and that
propinquity of descent,—the only known cause of the similarity of
organic beings,—is the bond, hidden as it is by various degrees of
[414]modification, which is partially revealed
to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits of
life, and the general place of each being in the economy of nature, would
be of very high importance in classification. Nothing can be more false.
No one regards the external similarity of a mouse to a shrew, of a dugong
to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely "adaptive or analogical characters;" but to
the consideration of these resemblances we shall have to recur. It may
even be given as a general rule, that the less any part of the
organisation is concerned with special habits, the more important it
becomes for classification. As an instance: Owen, in speaking of the
dugong, says, "The generative organs being those which are most remotely
related to the habits and food of an animal, I have always regarded as
affording very clear indications of its true affinities. We are least
likely in the modifications of these organs to mistake a merely adaptive
for an essential character." So with plants, how remarkable it is that
the organs of vegetation, on which their whole life depends, are of
little signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!
We must not, therefore, in classifying, trust to resemblances in parts
of the organisation, however important [415]they may be for the
welfare of the being in relation to the outer world. Perhaps from this
cause it has partly arisen, that almost all naturalists lay the greatest
stress on resemblances in organs of high vital or physiological
importance. No doubt this view of the classificatory importance of organs
which are important is generally, but by no means always, true. But their
importance for classification, I believe, depends on their greater
constancy throughout large groups of species; and this constancy depends
on such organs having generally been subjected to less change in the
adaptation of the species to their conditions of life. That the mere
physiological importance of an organ does not determine its
classificatory value, is almost shown by the one fact, that in allied
groups, in which the same organ, as we have every reason to suppose, has
nearly the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being
struck with this fact; and it has been fully acknowledged in the writings
of almost every author. It will suffice to quote the highest authority,
Robert Brown, who in speaking of certain organs in the Proteaceæ, says
their generic importance, "like that of all their parts, not only in this
but, as I apprehend, in every natural family, is very unequal, and in
some cases seems to be entirely lost." Again in another work he says, the
genera of the Connaraceæ "differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or valvular æstivation.
Any one of these characters singly is frequently of more than generic
importance, though here even when all taken together they appear
insufficient to separate Cnestis from Connarus." To give an example
amongst insects, in one great division of the Hymenoptera, the antennæ,
as Westwood has remarked, are most constant in structure; [416]in another
division they differ much, and the differences are of quite subordinate
value in classification; yet no one probably will say that the antennae
in these two divisions of the same order are of unequal physiological
importance. Any number of instances could be given of the varying
importance for classification of the same important organ within the same
group of beings.
Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in
exhibiting the close affinity between Ruminants and Pachyderms. Robert
Brown has strongly insisted on the fact that the rudimentary florets are
of the highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an open passage from
the nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles—the inflection of the
angle of the jaws in Marsupials—the manner in which the wings of
insects are folded—mere colour in certain Algæ—mere
pubescence on parts of the flower in grasses—the nature of the
dermal covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of hair, this
external and trifling character would, I think, have been considered by
naturalists as important an aid in determining the degree of affinity of
this strange creature to [417]birds and reptiles, as an approach in
structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other characters of more
or less importance. The value indeed of an aggregate of characters is
very evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has been
found, that a classification founded on any single character, however
important that may be, has always failed; for no part of the organisation
is universally constant. The importance of an aggregate of characters,
even when none are important, alone explains, I think, that saying of
Linnæus, that the characters do not give the genus, but the genus gives
the characters; for this saying seems founded on an appreciation of many
trifling points of resemblance, too slight to be defined. Certain plants,
belonging to the Malpighiaceæ, bear perfect and degraded flowers; in the
latter, as A. de Jussieu has remarked, "the greater number of the
characters proper to the species, to the genus, to the family, to the
class, disappear, and thus laugh at our classification." But when
Aspicarpa produced in France, during several years, only degraded
flowers, departing so wonderfully in a number of the most important
points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be
retained amongst the Malpighiaceæ. This case seems to me well to
illustrate the spirit with which our classifications are sometimes
necessarily founded.
Practically when naturalists are at work, they do [418]not trouble
themselves about the physiological value of the characters which they use
in defining a group, or in allocating any particular species. If they
find a character nearly uniform, and common to a great number of forms,
and not common to others, they use it as one of high value; if common to
some lesser number, they use it as of subordinate value. This principle
has been broadly confessed by some naturalists to be the true one; and by
none more clearly than by that excellent botanist, Aug. St. Hilaire. If
certain characters are always found correlated with others, though no
apparent bond of connexion can be discovered between them, especial value
is set on them. As in most groups of animals, important organs, such as
those for propelling the blood, or for aërating it, or those for
propagating the race, are found nearly uniform, they are considered as
highly serviceable in classification; but in some groups of animals all
these, the most important vital organs, are found to offer characters of
quite subordinate value.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications of
course include all ages of each species. But it is by no means obvious,
on the ordinary view, why the structure of the embryo should be more
important for this purpose than that of the adult, which alone plays its
full part in the economy of nature. Yet it has been strongly urged by
those great naturalists, Milne Edwards and Agassiz, that embryonic
characters are the most important of any in the classification of
animals; and this doctrine has very generally been admitted as true. The
same fact holds good with flowering plants, of which the two main
divisions have been founded on characters derived from the
embryo,—on the number and position of the [419]embryonic leaves or
cotyledons, and on the mode of development of the plumule and radicle. In
our discussion on embryology, we shall see why such characters are so
valuable, on the view of classification tacitly including the idea of
descent.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but in the case of crustaceans, such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large groups
of closely allied forms. Temminck insists on the utility or even
necessity of this practice in certain groups of birds; and it has been
followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary. Several
of the best botanists, such as Mr. Bentham and others, have strongly
insisted on their arbitrary value. Instances could be given amongst
plants and insects, of a group of forms, first ranked by practised
naturalists as only a genus, and then raised to the rank of a sub-family
or family; and this has been done, not because further research has
detected important structural differences, at first overlooked, but
because numerous allied species, with slightly different grades of
difference, have been subsequently discovered. [420]
All the foregoing rules and aids and difficulties in classification
are explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination
and relation to the other groups, must be strictly genealogical in order
to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different degrees
of modification which they have undergone; and this is expressed by the
forms being ranked under different genera, families, sections, or orders.
The reader will best understand what is meant, if he will take the
trouble of referring to the diagram in the fourth chapter. We will
suppose the letters A to L to represent allied genera, which lived during
the Silurian epoch, and these have descended from a species which existed
at an unknown anterior period. Species of three of these genera (A, F,
and I) have transmitted modified descendants to the present day,
represented by the fifteen genera (a14 to
z14) on the uppermost horizontal line. Now all these
modified descendants from a single species, are represented as related in
blood or descent to the same [421]degree; they may metaphorically be called
cousins to the same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended from A, now broken
up into two or three families, constitute a distinct order from those
descended from I, also broken up into two families. Nor can the existing
species, descended from A, be ranked in the same genus with the parent A;
or those from I, with the parent I. But the existing genus F14 may be supposed to have been but
slightly modified; and it will then rank with the parent-genus F; just as
some few still living organic beings belong to Silurian genera. So that
the amount or value of the differences between organic beings all related
to each other in the same degree in blood, has come to be widely
different. Nevertheless their genealogical arrangement remains
strictly true, not only at the present time, but at each successive
period of descent. All the modified descendants from A will have
inherited something in common from their common parent, as will all the
descendants from I; so will it be with each subordinate branch of
descendants, at each successive period. If, however, we choose to suppose
that any of the descendants of A or of I have been so much modified as to
have more or less completely lost traces of their parentage, in this
case, their places in a natural classification will have been more or
less completely lost,—as sometimes seems to have occurred with
existing organisms. All the descendants of the genus F, along its whole
line of descent, are supposed to have been but little modified, and they
yet form a single genus. But this genus, though much isolated, will still
occupy its proper intermediate position; for F originally was
intermediate in character between A and I, and the several genera
descended from these two genera will [422]have inherited to a
certain extent their characters. This natural arrangement is shown, as
far as is possible on paper, in the diagram, but in much too simple a
manner. If a branching diagram had not been used, and only the names of
the groups had been written in a linear series, it would have been still
less possible to have given a natural arrangement; and it is notoriously
not possible to represent in a series, on a flat surface, the affinities
which we discover in nature amongst the beings of the same group. Thus,
on the view which I hold, the natural system is genealogical in its
arrangement, like a pedigree; but the degrees of modification which the
different groups have undergone, have to be expressed by ranking them
under different so-called genera, sub-families, families, sections,
orders, and classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford the
best classification of the various languages now spoken throughout the
world; and if all extinct languages, and all intermediate and slowly
changing dialects, had to be included, such an arrangement would, I
think, be the only possible one. Yet it might be that some very ancient
language had altered little, and had given rise to few new languages,
whilst others (owing to the spreading and subsequent isolation and states
of civilisation of the several races, descended from a common race) had
altered much, and had given rise to many new languages and dialects. The
various degrees of difference in the languages from the same stock, would
have to be expressed by groups subordinate to groups; but the proper or
even only possible arrangement would still be genealogical; and this
would be strictly natural, as [423]it would connect together all languages,
extinct and modern, by the closest affinities, and would give the
filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of the
existence of groups subordinate to groups, is the same with varieties as
with species, namely, closeness of descent with various degrees of
modification. Nearly the same rules are followed in classifying
varieties, as with species. Authors have insisted on the necessity of
classing varieties on a natural instead of an artificial system; we are
cautioned, for instance, not to class two varieties of the pine-apple
together, merely because their fruit, though the most important part,
happens to be nearly identical; no one puts the swedish and common
turnips together, though the esculent and thickened stems are so similar.
Whatever part is found to be most constant, is used in classing
varieties: thus the great agriculturist Marshall says the horns are very
useful for this purpose with cattle, because they are less variable than
the shape or colour of the body, &c.; whereas with sheep the horns
are much less serviceable, because less constant. In classing varieties,
I apprehend if we had a real pedigree, a genealogical classification
would be universally preferred; and it has been attempted by some
authors. For we might feel sure, whether there had been more or less
modification, the principle of inheritance would keep the forms together
which were allied in the greatest number of points. In tumbler pigeons,
though some sub-varieties differ from the others [424]in the important
character of having a longer beak, yet all are kept together from having
the common habit of tumbling; but the short-faced breed has nearly or
quite lost this habit; nevertheless, without any reasoning or thinking on
the subject, these tumblers are kept in the same group, because allied in
blood and alike in some other respects. If it could be proved that the
Hottentot had descended from the Negro, I think he would be classed under
the Negro group, however much he might differ in colour and other
important characters from negroes.
With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, or that of a species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult, and yet no
one dreams of separating them. The naturalist includes as one species the
several larval stages of the same individual, however much they may
differ from each other and from the adult; as he likewise includes the
so-called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes
monsters; he includes varieties, not solely because they closely resemble
the parent-form, but because they are descended from it. He who believes
that the cowslip is descended from the primrose, or conversely, ranks
them together as a single species, and gives a single definition. As soon
as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), which
had previously been ranked as three distinct genera, were known to be
sometimes produced on the same spike, they were immediately included as a
single species. [425]
As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvæ
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, though in these cases the modification has been greater in
degree, and has taken a longer time to complete? I believe it has thus
been unconsciously used; and only thus can I understand the several rules
and guides which have been followed by our best systematists. We have no
written pedigrees; we have to make out community of descent by
resemblances of any kind. Therefore we choose those characters which, as
far as we can judge, are the least likely to have been modified in
relation to the conditions of life to which each species has been
recently exposed. Rudimentary structures on this view are as good as, or
even sometimes better than, other parts of the organisation. We care not
how trifling a character may be—let it be the mere inflection of
the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers—if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms with such different habits, only by its
inheritance from a common parent. We may err in this respect in regard to
single points of structure, but when several characters, let them be ever
so trifling, occur together throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent, that
these characters have been inherited from a common ancestor. [426]And we
know that such correlated or aggregated characters have especial value in
classification.
We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and yet
be safely classed with them. This may be safely done, and is often done,
as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet if these extreme forms
are connected together by a chain of intermediate groups, we may at once
infer their community of descent, and we put them all into the same
class. As we find organs of high physiological importance—those
which serve to preserve life under the most diverse conditions of
existence—are generally the most constant, we attach especial value
to them; but if these same organs, in another group or section of a
group, are found to differ much, we at once value them less in our
classification. We shall hereafter, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play in
classing large and widely-distributed genera, because all the species of
the same genus, inhabiting any distinct and isolated region, have in all
probability descended from the same parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this distinction, and he has been ably followed
by Macleay and others. The resemblance, in the shape of the body and in
the fin-like anterior limbs, between the dugong, which is a
pachydermatous animal, and the whale, and between both these mammals and
fishes, is analogical. Amongst insects there are innumerable [427]instances:
thus Linnæus, misled by external appearances, actually classed an
homopterous insect as a moth. We see something of the same kind even in
our domestic varieties, as in the thickened stems of the common and
swedish turnip. The resemblance of the greyhound and racehorse is hardly
more fanciful than the analogies which have been drawn by some authors
between very distinct animals. On my view of characters being of real
importance for classification, only in so far as they reveal descent, we
can clearly understand why analogical or adaptive character, although of
the utmost importance to the welfare of the being, are almost valueless
to the systematist. For animals, belonging to two most distinct lines of
descent, may readily become adapted to similar conditions, and thus
assume a close external resemblance; but such resemblances will not
reveal—will rather tend to conceal their blood-relationship to
their proper lines of descent. We can also understand the apparent
paradox, that the very same characters are analogical when one class or
order is compared with another, but give true affinities when the members
of the same class or order are compared one with another: thus the shape
of the body and fin-like limbs are only analogical when whales are
compared with fishes, being adaptations in both classes for swimming
through the water; but the shape of the body and fin-like limbs serve as
characters exhibiting true affinity between the several members of the
whale family; for these cetaceans agree in so many characters, great and
small, that we cannot doubt that they have inherited their general shape
of body and structure of limbs from a common ancestor. So it is with
fishes.
As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,—to
inhabit for instance [428]the three elements of land, air, and
water,—we can perhaps understand how it is that a numerical
parallelism has sometimes been observed between the sub-groups in
distinct classes. A naturalist, struck by a parallelism of this nature in
any one class, by arbitrarily raising or sinking the value of the groups
in other classes (and all our experience shows that this valuation has
hitherto been arbitrary), could easily extend the parallelism over a wide
range; and thus the septenary, quinary, quaternary, and ternary
classifications have probably arisen.
As the modified descendants of dominant species, belonging to the
larger genera, tend to inherit the advantages, which made the groups to
which they belong large and their parents dominant, they are almost sure
to spread widely, and to seize on more and more places in the economy of
nature. The larger and more dominant groups thus tend to go on increasing
in size; and they consequently supplant many smaller and feebler groups.
Thus we can account for the fact that all organisms, recent and extinct,
are included under a few great orders, under still fewer classes, and all
in one great natural system. As showing how few the higher groups are in
number, and how widely spread they are throughout the world, the fact is
striking, that the discovery of Australia has not added a single insect
belonging to a new class; and that in the vegetable kingdom, as I learn
from Dr. Hooker, it has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight degree
intermediate between existing groups. A few [429]old and intermediate
parent-forms having occasionally transmitted to the present day
descendants but little modified, will give to us our so-called osculant
or aberrant groups. The more aberrant any form is, the greater must be
the number of connecting forms which on my theory have been exterminated
and utterly lost. And we have some evidence of aberrant forms having
suffered severely from extinction, for they are generally represented by
extremely few species; and such species as do occur are generally very
distinct from each other, which again implies extinction. The genera
Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species instead of by a
single one; but such richness in species, as I find after some
investigation, does not commonly fall to the lot of aberrant genera. We
can, I think, account for this fact only by looking at aberrant forms as
failing groups conquered by more successful competitors, with a few
members preserved by some unusual coincidence of favourable
circumstances.
Mr. Waterhouse has remarked that, when a member belonging to one group
of animals exhibits an affinity to a quite distinct group, this affinity
in most cases is general and not special: thus, according to Mr.
Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, and not to any one marsupial species more than to
another. As the points of affinity of the bizcacha to Marsupials are
believed to be real and not merely adaptive, they are due on my theory to
inheritance in common. Therefore we must suppose either that all Rodents,
including the bizcacha, branched off from some very ancient Marsupial,
which will have had a character in some degree intermediate with respect
to all existing Marsupials; or [430]that both Rodents and Marsupials branched
off from a common progenitor, and that both groups have since undergone
much modification in divergent directions. On either view we may suppose
that the bizcacha has retained, by inheritance, more of the character of
its ancient progenitor than have other Rodents; and therefore it will not
be specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character of
their common progenitor, or of an early member of the group. On the other
hand, of all Marsupials, as Mr. Waterhouse has remarked, the phascolomys
resembles most nearly, not any one species, but the general order of
Rodents. In this case, however, it may be strongly suspected that the
resemblance is only analogical, owing to the phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of
distinct orders of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from a common parent, together with
their retention by inheritance of some characters in common, we can
understand the excessively complex and radiating affinities by which all
the members of the same family or higher group are connected together.
For the common parent of a whole family of species, now broken up by
extinction into distinct groups and sub-groups, will have transmitted
some of its characters, modified in various ways and degrees, to all; and
the several species will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the
diagram so often referred to), mounting up through many predecessors. As
it is difficult to show the blood-relationship between the numerous
kindred [431]of any ancient and noble family, even by
the aid of a genealogical tree, and almost impossible to do this without
this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram,
the various affinities which they perceive between the many living and
extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the several
groups in each class. We may thus account even for the distinctness of
whole classes from each other—for instance, of birds from all other
vertebrate animals—by the belief that many ancient forms of life
have been utterly lost, through which the early progenitors of birds were
formerly connected with the early progenitors of the other vertebrate
classes. There has been less entire extinction of the forms of life which
once connected fishes with batrachians. There has been still less in some
other classes, as in that of the Crustacea, for here the most wonderfully
diverse forms are still tied together by a long, but broken, chain of
affinities. Extinction has only separated groups: it has by no means made
them; for if every form which has ever lived on this earth were suddenly
to reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all would
blend together by steps as fine as those between the finest existing
varieties, nevertheless a natural classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the
diagram: the letters, A to L, may represent eleven Silurian genera, some
of which have produced large groups of modified descendants. Every
intermediate link between these eleven genera and their primordial
parent, and every [432]intermediate link in each branch and
sub-branch of their descendants, may be supposed to be still alive; and
the links to be as fine as those between the finest varieties. In this
case it would be quite impossible to give any definition by which the
several members of the several groups could be distinguished from their
more immediate parents; or these parents from their ancient and unknown
progenitor. Yet the natural arrangement in the diagram would still hold
good; and, on the principle of inheritance, all the forms descended from
A, or from I, would have something in common. In a tree we can specify
this or that branch, though at the actual fork the two unite and blend
together. We could not, as I have said, define the several groups; but we
could pick out types, or forms, representing most of the characters of
each group, whether large or small, and thus give a general idea of the
value of the differences between them. This is what we should be driven
to, if we were ever to succeed in collecting all the forms in any class
which have lived throughout all time and space. We shall certainly never
succeed in making so perfect a collection: nevertheless, in certain
classes, we are tending in this direction; and Milne Edwards has lately
insisted, in an able paper, on the high importance of looking to types,
whether or not we can separate and define the groups to which such types
belong.
Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals of
both sexes and of all ages, although having few characters in common,
[433]under one species; we use descent in
classing acknowledged varieties, however different they may be from their
parent; and I believe this element of descent is the hidden bond of
connexion which naturalists have sought under the term of the Natural
System. On this idea of the natural system being, in so far as it has
been perfected, genealogical in its arrangement, with the grades of
difference between the descendants from a common parent, expressed by the
terms genera, families, orders, &c., we can understand the rules
which we are compelled to follow in our classification. We can understand
why we value certain resemblances far more than others; why we are
permitted to use rudimentary and useless organs, or others of trifling
physiological importance; why, in comparing one group with a distinct
group, we summarily reject analogical or adaptive characters, and yet use
these same characters within the limits of the same group. We can clearly
see how it is that all living and extinct forms can be grouped together
in one great system; and how the several members of each class are
connected together by the most complex and radiating lines of affinities.
We shall never, probably, disentangle the inextricable web of affinities
between the members of any one class; but when we have a distinct object
in view, and do not look to some unknown plan of creation, we may hope to
make sure but slow progress.
Morphology.—We have seen that the members of the same
class, independently of their habits of life, resemble each other in the
general plan of their organisation. This resemblance is often expressed
by the term "unity of type;" or by saying that the several parts and
organs in the different species of the class are homologous. The whole
subject is included under [434]the general name of Morphology. This is
the most interesting department of natural history, and may be said to be
its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include similar bones, in the
same relative positions? Geoffroy St. Hilaire has insisted strongly on
the high importance of relative connexion in homologous organs: the parts
may change to almost any extent in form and size, and yet they always
remain connected together in the same order. We never find, for instance,
the bones of the arm and forearm, or of the thigh and leg, transposed.
Hence the same names can be given to the homologous bones in widely
different animals. We see the same great law in the construction of the
mouths of insects: what can be more different than the immensely long
spiral proboscis of a sphinx-moth, the curious folded one of a bee or
bug, and the great jaws of a beetle?—yet all these organs, serving
for such different purposes, are formed by infinitely numerous
modifications of an upper lip, mandibles, and two pairs of maxillæ.
Analogous laws govern the construction of the mouths and limbs of
crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by the
doctrine of final causes. The hopelessness of the attempt has been
expressly admitted by Owen in his most interesting work on the 'Nature of
Limbs.' On the ordinary view of the independent creation of each being,
we can only say that so it is;—that it has so pleased the Creator
to construct each animal and plant.
The explanation is manifest on the theory of the [435]natural
selection of successive slight modifications,—each modification
being profitable in some way to the modified form, but often affecting by
correlation of growth other parts of the organisation. In changes of this
nature, there will be little or no tendency to modify the original
pattern, or to transpose parts. The bones of a limb might be shortened
and widened to any extent, and become gradually enveloped in thick
membrane, so as to serve as a fin; or a webbed foot might have all its
bones, or certain bones, lengthened to any extent, and the membrane
connecting them increased to any extent, so as to serve as a wing: yet in
all this great amount of modification there will be no tendency to alter
the framework of bones or the relative connexion of the several parts. If
we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, had its limbs constructed on the existing general
pattern, for whatever purpose they served, we can at once perceive the
plain signification of the homologous construction of the limbs
throughout the whole class. So with the mouths of insects, we have only
to suppose that their common progenitor had an upper lip, mandibles, and
two pair of maxillæ, these parts being perhaps very simple in form; and
then natural selection, acting on some originally created form, will
account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally lost,
by the atrophy and ultimately by the complete abortion of certain parts,
by the soldering together of other parts, and by the doubling or
multiplication of others,—variations which we know to be within the
limits of possibility. In the paddles of the extinct gigantic
sea-lizards, and in the mouths of certain suctorial crustaceans, the [436]general pattern seems to have been thus to
a certain extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual. Most
physiologists believe that the bones of the skull are homologous
with—that is correspond in number and in relative connexion
with—the elemental parts of a certain number of vertebræ. The
anterior and posterior limbs in each member of the vertebrate and
articulate classes are plainly homologous. We see the same law in
comparing the wonderfully complex jaws and legs in crustaceans. It is
familiar to almost every one, that in a flower the relative position of
the sepals, petals, stamens, and pistils, as well as their intimate
structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed
into another; and we can actually see in embryonic crustaceans and in
many other animals, and in flowers, that organs, which when mature become
extremely different, are at an early stage of growth exactly alike.
How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinary shaped pieces of bone? As Owen has remarked, the benefit
derived from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same construction in
the skulls of birds. Why should similar bones have been created in the
formation of the wing and leg of a bat, used as they are for such totally
different purposes? Why should one crustacean, which has an extremely
complex [437]mouth formed of many parts, consequently
always have fewer legs; or conversely, those with many legs have simpler
mouths? Why should the sepals, petals, stamens, and pistils in any
individual flower, though fitted for such widely different purposes, be
all constructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebræ
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages; and
in flowering plants, we see a series of successive spiral whorls of
leaves. An indefinite repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or little-modified
forms; therefore we may readily believe that the unknown progenitor of
the vertebrata possessed many vertebræ; the unknown progenitor of the
articulata, many segments; and the unknown progenitor of flowering
plants, many spiral whorls of leaves. We have formerly seen that parts
many times repeated are eminently liable to vary in number and structure;
consequently it is quite probable that natural selection, during a
long-continued course of modification, should have seized on a certain
number of the primordially similar elements, many times repeated, and
have adapted them to the most diverse purposes. And as the whole amount
of modification will have been effected by slight successive steps, we
need not wonder at discovering in such parts or organs, a certain degree
of fundamental resemblance, retained by the strong principle of
inheritance.
In the great class of molluscs, though we can homologise the parts of
one species with those of other and distinct species, we can indicate but
few serial homologies; that is, we are seldom enabled to say that one
[438]part or organ is homologous with another
in the same individual. And we can understand this fact; for in molluscs,
even in the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part, as we find in the other great
classes of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebræ, both jaws and legs, &c.,—as having
been metamorphosed, not one from the other, but from some common element.
Naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent,
primordial organs of any kind—vertebræ in the one case and legs in
the other—have actually been modified into skulls or jaws. Yet so
strong is the appearance of a modification of this nature having
occurred, that naturalists can hardly avoid employing language having
this plain signification. On my view these terms may be used literally;
and the wonderful fact of the jaws, for instance, of a crab retaining
numerous characters, which they would probably have retained through
inheritance, if they had really been metamorphosed during a long course
of descent from true legs, or from some simple appendage, is
explained.
Embryology.—It has already been casually remarked that
certain organs in the individual, which when mature become widely
different and serve for different purposes, are in the embryo exactly
alike. The embryos, also, of distinct animals within the same class are
often strikingly similar: a better proof of this cannot be given, than a
[439]circumstance mentioned by Agassiz, namely,
that having forgotten to ticket the embryo of some vertebrate animal, he
cannot now tell whether it be that of a mammal, bird, or reptile. The
vermiform larvæ of moths, flies, beetles, &c., resemble each other
much more closely than do the mature insects; but in the case of larvæ,
the embryos are active, and have been adapted for special lines of life.
A trace of the law of embryonic resemblance, sometimes lasts till a
rather late age: thus birds of the same genus, and of closely allied
genera, often resemble each other in their first and second plumage; as
we see in the spotted feathers in the thrush group. In the cat tribe,
most of the species are striped or spotted in lines; and stripes can be
plainly distinguished in the whelp of the lion. We occasionally though
rarely see something of this kind in plants: thus the embryonic leaves of
the ulex or furze, and the first leaves of the phyllodineous acaceas, are
pinnate or divided like the ordinary leaves of the leguminosæ.
The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions,—in the young mammal which is nourished in the womb of
its mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in such a
relation, than we have to believe that the same bones in the hand of a
man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one will suppose that the stripes on the whelp of
a lion, or the spots on the young blackbird, [440]are of any use to these
animals, or are related to the conditions to which they are exposed.
The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. From such special adaptations,
the similarity of the larvæ or active embryos of allied animals is
sometimes much obscured; and cases could be given of the larvæ of two
species, or of two groups of species, differing quite as much, or even
more, from each other than do their adult parents. In most cases,
however, the larvæ, though active, still obey, more or less closely, the
law of common embryonic resemblance. Cirripedes afford a good instance of
this: even the illustrious Cuvier did not perceive that a barnacle was,
as it certainly is, a crustacean; but a glance at the larva shows this to
be the case in an unmistakeable manner. So again the two main divisions
of cirripedes, the pedunculated and sessile, which differ widely in
external appearance, have larvæ in all their stages barely
distinguishable.
The embryo in the course of development generally rises in
organisation: I use this expression, though I am aware that it is hardly
possible to define clearly what is meant by the organisation being higher
or lower. But no one probably will dispute that the butterfly is higher
than the caterpillar. In some cases, however, the mature animal is
generally considered as lower in the scale than the larva, as with
certain parasitic crustaceans. To refer once again to cirripedes: the
larvæ in the first stage have three pairs of legs, a very simple single
eye, and a probosciformed mouth, with which they feed largely, for they
increase much in [441]size. In the second stage, answering to
the chrysalis stage of butterflies, they have six pairs of beautifully
constructed natatory legs, a pair of magnificent compound eyes, and
extremely complex antennæ; but they have a closed and imperfect mouth,
and cannot feed: their function at this stage is, to search by their
well-developed organs of sense, and to reach by their active powers of
swimming, a proper place on which to become attached and to undergo their
final metamorphosis. When this is completed they are fixed for life:
their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennæ, and their two eyes are
now reconverted into a minute, single, and very simple eye-spot. In this
last and complete state, cirripedes may be considered as either more
highly or more lowly organised than they were in the larval condition.
But in some genera the larvæ become developed either into hermaphrodites
having the ordinary structure, or into what I have called complemental
males: and in the latter, the development has assuredly been retrograde;
for the male is a mere sack, which lives for a short time, and is
destitute of mouth, stomach, or other organ of importance, excepting for
reproduction.
We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or
the fin of a porpoise, should not have been sketched out with all the
parts in proper proportion, as soon as any structure became visible in
the embryo. And in some whole groups of animals and in certain members of
other groups, the embryo does not at any period differ widely from the
[442]adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed;" and again
in spiders, "there is nothing worthy to be called a metamorphosis." The
larvæ of insects, whether adapted to the most diverse and active habits,
or quite inactive, being fed by their parents or placed in the midst of
proper nutriment, yet nearly all pass through a similar worm-like stage
of development; but in some few cases, as in that of Aphis, if we look to
the admirable drawings by Professor Huxley of the development of this
insect, we see no trace of the vermiform stage.
How, then, can we explain these several facts in
embryology,—namely the very general, but not universal difference
in structure between the embryo and the adult;—of parts in the same
individual embryo, which ultimately become very unlike and serve
for diverse purposes, being at this early period of growth
alike;—of embryos of different species within the same class,
generally, but not universally, resembling each other;—of the
structure of the embryo not being closely related to its conditions of
existence, except when the embryo becomes at any period of life active
and has to provide for itself;—of the embryo apparently having
sometimes a higher organisation than the mature animal, into which it is
developed? I believe that all these facts can be explained, as follows,
on the view of descent with modification.
It is commonly assumed, perhaps from monstrosities often affecting the
embryos at a very early period, that slight variations necessarily appear
at an equally early period. But we have little evidence on this
head—indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various [443]fancy animals,
cannot positively tell, until some time after the animal has been born,
what its merits or form will ultimately turn out. We see this plainly in
our own children; we cannot always tell whether the child will be tall or
short, or what its precise features will be. The question is not, at what
period of life any variation has been caused, but at what period it is
fully displayed. The cause may have acted, and I believe generally has
acted, even before the embryo is formed; and the variation may be due to
the male and female sexual elements having been affected by the
conditions to which either parent, or their ancestors, have been exposed.
Nevertheless an effect thus caused at a very early period, even before
the formation of the embryo, may appear late in life; as when an
hereditary disease, which appears in old age alone, has been communicated
to the offspring from the reproductive element of one parent. Or again,
as when the horns of cross-bred cattle have been affected by the shape of
the horns of either parent. For the welfare of a very young animal, as
long as it remains in its mother's womb, or in the egg, or as long as it
is nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or
later in life. It would not signify, for instance, to a bird which
obtained its food best by having a long beak, whether or not it assumed a
beak of this particular length, as long as it was fed by its parents.
Hence, I conclude, that it is quite possible, that each of the many
successive modifications, by which each species has acquired its present
structure, may have supervened at a not very early period of life; and
some direct evidence from our domestic animals supports this view. But in
other cases it is quite possible that each successive modification, or
[444]most of them, may have appeared at an
extremely early period.
I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears in
the parent, it tends to reappear at a corresponding age in the offspring.
Certain variations can only appear at corresponding ages, for instance,
peculiarities in the caterpillar, cocoon, or imago states of the
silk-moth; or, again, in the horns of almost full-grown cattle. But
further than this, variations which, for all that we can see, might have
appeared earlier or later in life, tend to appear at a corresponding age
in the offspring and parent. I am far from meaning that this is
invariably the case; and I could give a good many cases of variations
(taking the word in the largest sense) which have supervened at an
earlier age in the child than in the parent.
These two principles, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some authors
who have written on Dogs, maintain that the greyhound and bulldog, though
appearing so different, are really varieties most closely allied, and
have probably descended from the same wild stock; hence I was curious to
see how far their puppies differed from each other: I was told by
breeders that they differed just as much as their parents, and this,
judging by the eye, seemed almost to be the case; but on actually
measuring the old dogs and their six-days old puppies, I found that the
puppies had not nearly acquired their full amount of proportional
difference. So, again, I was told that the foals of cart and race-horses
differed as much as the full-grown animals; and this surprised me
greatly, as I think it probable that the difference between these two
breeds has been wholly [445]caused by selection under domestication;
but having had careful measurements made of the dam and of a three-days
old colt of a race and heavy cart-horse, I find that the colts have by no
means acquired their full amount of proportional difference.
As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared young
pigeons of various breeds, within twelve hours after being hatched; I
carefully measured the proportions (but will not here give details) of
the beak, width of mouth, length of nostril and of eyelid, size of feet
and length of leg, in the wild stock, in pouters, fantails, runts, barbs,
dragons, carriers, and tumblers. Now some of these birds, when mature,
differ so extraordinarily in length and form of beak, that they would, I
cannot doubt, be ranked in distinct genera, had they been natural
productions. But when the nestling birds of these several breeds were
placed in a row, though most of them could be distinguished from each
other, yet their proportional differences in the above specified several
points were incomparably less than in the full-grown birds. Some
characteristic points of difference—for instance, that of the width
of mouth—could hardly be detected in the young. But there was one
remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon and of the other
breeds, in all its proportions, almost exactly as much as in the adult
state.
The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties. Fanciers
select their horses, dogs, and pigeons, for breeding, when they are
nearly grown up: they are indifferent whether the desired qualities and
structures have been acquired earlier or [446]later in life, if the
full-grown animal possesses them. And the cases just given, more
especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at an
early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced tumbler,
which when twelve hours old had acquired its proper proportions, proves
that this is not the universal rule; for here the characteristic
differences must either have appeared at an earlier period than usual,
or, if not so, the differences must have been inherited, not at the
corresponding, but at an earlier age.
Now let us apply these facts and the above two principles—which
latter, though not proved true, can be shown to be in some degree
probable—to species in a state of nature. Let us take a genus of
birds, descended on my theory from some one parent-species, and of which
the several new species have become modified through natural selection in
accordance with their diverse habits. Then, from the many slight
successive steps of variation having supervened at a rather late age, and
having been inherited at a corresponding age, the young of the new
species of our supposed genus will manifestly tend to resemble each other
much more closely than do the adults, just as we have seen in the case of
pigeons. We may extend this view to whole families or even classes. The
fore-limbs, for instance, which served as legs in the parent-species, may
have become, by a long course of modification, adapted in one descendant
to act as hands, in another as paddles, in another as wings; and on the
above two principles—namely of each successive modification
supervening at a rather late age, and being inherited at a [447]corresponding
late age—the fore-limbs in the embryos of the several descendants
of the parent-species will still resemble each other closely, for they
will not have been modified. But in each of our new species, the
embryonic fore-limbs will differ greatly from the fore-limbs in the
mature animal; the limbs in the latter having undergone much modification
at a rather late period of life, and having thus been converted into
hands, or paddles, or wings. Whatever influence long-continued exercise
or use on the one hand, and disuse on the other, may have in modifying an
organ, such influence will mainly affect the mature animal, which has
come to its full powers of activity and has to gain its own living; and
the effects thus produced will be inherited at a corresponding mature
age. Whereas the young will remain unmodified, or be modified in a lesser
degree, by the effects of use and disuse.
In certain cases the successive steps of variation might supervene,
from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that at
which it first appeared. In either case (as with the short-faced tumbler)
the young or embryo would closely resemble the mature parent-form. We
have seen that this is the rule of development in certain whole groups of
animals, as with cuttle-fish and spiders, and with a few members of the
great class of insects, as with Aphis. With respect to the final cause of
the young in these cases not undergoing any metamorphosis, or closely
resembling their parents from their earliest age, we can see that this
would result from the two following contingencies: firstly, from the
young, during a course of modification carried on for many generations,
having to provide for their own wants at a very early stage [448]of
development, and secondly, from their following exactly the same habits
of life with their parents; for in this case, it would be indispensable
for the existence of the species, that the child should be modified at a
very early age in the same manner with its parents, in accordance with
their similar habits. Some further explanation, however, of the embryo
not undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be constructed
in a slightly different manner, then, on the principle of inheritance at
corresponding ages, the active young or larvæ might easily be rendered by
natural selection different to any conceivable extent from their parents.
Such differences might, also, become correlated with successive stages of
development; so that the larvæ, in the first stage, might differ greatly
from the larvæ in the second stage, as we have seen to be the case with
cirripedes. The adult might become fitted for sites or habits, in which
organs of locomotion or of the senses, &c., would be useless; and in
this case the final metamorphosis would be said to be retrograde.
As all the organic beings, extinct and recent, which have ever lived
on this earth have to be classed together, and as all have been connected
by the finest gradations, the best, or indeed, if our collections were
nearly perfect, the only possible arrangement, would be genealogical.
Descent being on my view the hidden bond of connexion which naturalists
have been seeking under the term of the natural system. On this view we
can understand how it is that, in the eyes of most naturalists, the
structure of the embryo is even more important for classification than
that of the adult. For the embryo is the animal in its less modified
state; [449]and in so far it reveals the structure of
its progenitor. In two groups of animals, however much they may at
present differ from each other in structure and habits, if they pass
through the same or similar embryonic stages, we may feel assured that
they have both descended from the same or nearly similar parents, and are
therefore in that degree closely related. Thus, community in embryonic
structure reveals community of descent. It will reveal this community of
descent, however much the structure of the adult may have been modified
and obscured; we have seen, for instance, that cirripedes can at once be
recognised by their larvæ as belonging to the great class of crustaceans.
As the embryonic state of each species and group of species partially
shows us the structure of their less modified ancient progenitors, we can
clearly see why ancient and extinct forms of life should resemble the
embryos of their descendants,—our existing species. Agassiz
believes this to be a law of nature; but I am bound to confess that I
only hope to see the law hereafter proved true. It can be proved true in
those cases alone in which the ancient state, now supposed to be
represented in existing embryos, has not been obliterated, either by the
successive variations in a long course of modification having supervened
at a very early age, or by the variations having been inherited at an
earlier period than that at which they first appeared. It should also be
borne in mind, that the supposed law of resemblance of ancient forms of
life to the embryonic stages of recent forms, may be true, but yet, owing
to the geological record not extending far enough back in time, may
remain for a long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight [450]modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period in
the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises greatly
in interest, when we thus look at the embryo as a picture, more or less
obscured, of the common parent-form of each great class of animals.
Rudimentary, atrophied, or aborted Organs.—Organs or
parts in this strange condition, bearing the stamp of inutility, are
extremely common throughout nature. For instance, rudimentary mammæ are
very general in the males of mammals: I presume that the "bastard-wing"
in birds may be safely considered as a digit in a rudimentary state: in
very many snakes one lobe of the lungs is rudimentary; in other snakes
there are rudiments of the pelvis and hind limbs. Some of the cases of
rudimentary organs are extremely curious; for instance, the presence of
teeth in fœtal whales, which when grown up have not a tooth in
their heads; and the presence of teeth, which never cut through the gums,
in the upper jaws of our unborn calves. It has even been stated on good
authority that rudiments of teeth can be detected in the beaks of certain
embryonic birds. Nothing can be plainer than that wings are formed for
flight, yet in how many insects do we see wings so reduced in size as to
be utterly incapable of flight, and not rarely lying under wing-cases,
firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one of which
will have full-sized wings, and another mere rudiments of membrane; and
here it is impossible to doubt, that the [451]rudiments represent
wings. Rudimentary organs sometimes retain their potentiality, and are
merely not developed: this seems to be the case with the mammæ of male
mammals, for many instances are on record of these organs having become
well developed in full-grown males, and having secreted milk. So again
there are normally four developed and two rudimentary teats in the udders
of the genus Bos, but in our domestic cows the two sometimes become
developed and give milk. In plants of the same species the petals
sometimes occur as mere rudiments, and sometimes in a well-developed
state. In plants with separated sexes, the male flowers often have a
rudiment of a pistil; and Kölreuter found that by crossing such male
plants with an hermaphrodite species, the rudiment of the pistil in the
hybrid offspring was much increased in size; and this shows that the
rudiment and the perfect pistil are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules protected in the ovarium at
its base. The pistil consists of a stigma supported on the style; but in
some Compositæ, the male florets, which of course cannot be fecundated,
have a pistil, which is in a rudimentary state, for it is not crowned
with a stigma; but the style remains well developed, and is clothed with
hairs as in other compositæ, for the purpose of brushing the pollen out
of the surrounding anthers. Again, an organ may become rudimentary for
its proper purpose, and be used for a distinct object: in certain fish
the swim-bladder seems to be nearly rudimentary for its proper function
of giving buoyancy, but has become converted into a [452]nascent
breathing organ or lung. Other similar instances could be given.
Organs, however little developed, if of use, should not be called
rudimentary; they cannot properly be said to be in an atrophied
condition; they may be called nascent, and may hereafter be developed to
any extent by natural selection. Rudimentary organs, on the other hand,
are essentially useless, as teeth which never cut through the gums; in a
still less developed condition, they would be of still less use. They
cannot, therefore, under their present condition, have been formed by
natural selection, which acts solely by the preservation of useful
modifications; they have been retained, as we shall see, by inheritance,
and relate to a former condition of their possessor. It is difficult to
know what are nascent organs; looking to the future, we cannot of course
tell how any part will be developed, and whether it is now nascent;
looking to the past, creatures with an organ in a nascent condition will
generally have been supplanted and exterminated by their successors with
the organ in a more perfect and developed condition. The wing of the
penguin is of high service, and acts as a fin; it may, therefore,
represent the nascent state of the wings of birds; not that I believe
this to be the case, it is more probably a reduced organ, modified for a
new function: the wing of the Apteryx is useless, and is truly
rudimentary. The mammary glands of the Ornithorhynchus may, perhaps, be
considered, in comparison with the udder of a cow, as in a nascent state.
The ovigerous frena of certain cirripedes, which are only slightly
developed and which have ceased to give attachment to the ova, are
nascent branchiæ.
Rudimentary organs in the individuals of the same species are very
liable to vary in degree of development [453]and in other respects.
Moreover, in closely allied species, the degree to which the same organ
has been rendered rudimentary occasionally differs much. This latter fact
is well exemplified in the state of the wings of the female moths in
certain groups. Rudimentary organs may be utterly aborted; and this
implies, that we find in an animal or plant no trace of an organ, which
analogy would lead us to expect to find, and which is occasionally found
in monstrous individuals of the species. Thus in the snapdragon
(antirrhinum) we generally do not find a rudiment of a fifth stamen; but
this may sometimes be seen. In tracing the homologies of the same part in
different members of a class, nothing is more common, or more necessary,
than the use and discovery of rudiments. This is well shown in the
drawings given by Owen of the bones of the leg of the horse, ox, and
rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the embryo,
but afterwards wholly disappear. It is also, I believe, a universal rule,
that a rudimentary part or organ is of greater size relatively to the
adjoining parts in the embryo, than in the adult; so that the organ at
this early age is less rudimentary, or even cannot be said to be in any
degree rudimentary. Hence, also, a rudimentary organ in the adult is
often said to have retained its embryonic condition.
I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment: for
the same reasoning power which tells us plainly that most parts and
organs are exquisitely adapted for certain purposes, tells us with equal
plainness that these rudimentary or atrophied organs, are imperfect and
useless. In works [454]on natural history rudimentary organs are
generally said to have been created "for the sake of symmetry," or in
order "to complete the scheme of nature;" but this seems to me no
explanation, merely a re-statement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic courses round
the sun, satellites follow the same course round the planets, for the
sake of symmetry, and to complete the scheme of nature? An eminent
physiologist accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or injurious to
the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed merely of
cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth, which are subsequently absorbed, can be of any service
to the rapidly growing embryonic calf by the excretion of precious
phosphate of lime? When a man's fingers have been amputated, imperfect
nails sometimes appear on the stumps: I could as soon believe that these
vestiges of nails have appeared, not from unknown laws of growth, but in
order to excrete horny matter, as that the rudimentary nails on the fin
of the manatee were formed for this purpose.
On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions,—as the stump of a tail in tailless
breeds,—the vestige of an ear in earless breeds,—the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals,—and the state of
the whole flower in the cauliflower. We often see rudiments of various
parts in monsters. But I doubt whether any of these cases throw light on
the origin of rudimentary organs in a state of nature, [455]further than
by showing that rudiments can be produced; for I doubt whether species
under nature ever undergo abrupt changes. I believe that disuse has been
the main agency; that it has led in successive generations to the gradual
reduction of various organs, until they have become rudimentary,—as
in the case of the eyes of animals inhabiting dark caverns, and of the
wings of birds inhabiting oceanic islands, which have seldom been forced
to take flight, and have ultimately lost the power of flying. Again, an
organ useful under certain conditions, might become injurious under
others, as with the wings of beetles living on small and exposed islands;
and in this case natural selection would continue slowly to reduce the
organ, until it was rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ
rendered, during changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. Or an organ
might be retained for one alone of its former functions. An organ, when
rendered useless, may well be variable, for its variations cannot be
checked by natural selection. At whatever period of life disuse or
selection reduces an organ, and this will generally be when the being has
come to maturity and to its full powers of action, the principle of
inheritance at corresponding ages will reproduce the organ in its reduced
state at the same age, and consequently will seldom affect or reduce it
in the embryo. Thus we can understand the greater relative size of
rudimentary organs in the embryo, and their lesser relative size in the
adult. But if each step of the process of reduction were to be inherited,
not at the corresponding age, but at an extremely early period of life
(as we have good [456]reason to believe to be possible), the
rudimentary part would tend to be wholly lost, and we should have a case
of complete abortion. The principle, also, of economy, explained in a
former chapter, by which the materials forming any part or structure, if
not useful to the possessor, will be saved as far as is possible, will
probably often come into play; and this will tend to cause the entire
obliteration of a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency in
every part of the organisation, which has long existed, to be
inherited—we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary parts
as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless in
the pronunciation, but which serve as a clue in seeking for its
derivation. On the view of descent with modification, we may conclude
that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far from presenting a strange difficulty, as
they assuredly do on the ordinary doctrine of creation, might even have
been anticipated, and can be accounted for by the laws of
inheritance.
Summary.—In this chapter I have attempted to show, that
the subordination of group to group in all organisms throughout all time;
that the nature of the relationship, by which all living and extinct
beings are united by complex, radiating, and circuitous lines of
affinities into one grand system; the rules followed and the difficulties
encountered by naturalists in their classifications; the value set upon
characters, if constant and prevalent, whether of high vital importance,
or of the most trifling [457]importance, or, as in rudimentary organs,
of no importance; the wide opposition in value between analogical or
adaptive characters, and characters of true affinity; and other such
rules;—all naturally follow on the view of the common parentage of
those forms which are considered by naturalists as allied, together with
their modification through natural selection, with its contingencies of
extinction and divergence of character. In considering this view of
classification, it should be borne in mind that the element of descent
has been universally used in ranking together the sexes, ages, and
acknowledged varieties of the same species, however different they may be
in structure. If we extend the use of this element of descent,—the
only certainly known cause of similarity in organic beings,—we
shall understand what is meant by the natural system: it is genealogical
in its attempted arrangement, with the grades of acquired difference
marked by the terms varieties, species, genera, families, orders, and
classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible,—whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on
the same pattern in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from
each other in structure and function; and the resemblance in different
species of a class of the homologous parts or [458]organs, though fitted
in the adult members for purposes as different as possible. Larvæ are
active embryos, which have become specially modified in relation to their
habits of life, through the principle of modifications being inherited at
corresponding ages. On this same principle—and bearing in mind,
that when organs are reduced in size, either from disuse or selection, it
will generally be at that period of life when the being has to provide
for its own wants, and bearing in mind how strong is the principle of
inheritance—the occurrence of rudimentary organs and their final
abortion, present to us no inexplicable difficulties; on the contrary,
their presence might have been even anticipated. The importance of
embryological characters and of rudimentary organs in classification is
intelligible, on the view that an arrangement is only so far natural as
it is genealogical.
Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera, and families of organic beings, with which this world is
peopled, have all descended, each within its own class or group, from
common parents, and have all been modified in the course of descent, that
I should without hesitation adopt this view, even if it were unsupported
by other facts or arguments.
[459]
CHAPTER XIV.
Recapitulation and Conclusion.
Recapitulation of the difficulties on the theory of Natural
Selection—Recapitulation of the general and special circumstances
in its favour—Causes of the general belief in the immutability of
species—How far the theory of natural selection may be
extended—Effects of its adoption on the study of Natural
history—Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly
recapitulated.
That many and serious objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first can
appear more difficult to believe than that the more complex organs and
instincts should have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor. Nevertheless,
this difficulty, though appearing to our imagination insuperably great,
cannot be considered real if we admit the following propositions,
namely,—that gradations in the perfection of any organ or instinct
which we may consider, either do now exist or could have existed, each
good of its kind,—that all organs and instincts are, in ever so
slight a degree, variable,—and, lastly, that there is a struggle
for existence leading to the preservation of each profitable deviation of
structure or instinct. The truth of these propositions cannot, I think,
be disputed. [460]
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially amongst
broken and failing groups of organic beings; but we see so many strange
gradations in nature, that we ought to be extremely cautious in saying
that any organ or instinct, or any whole being, could not have arrived at
its present state by many graduated steps. There are, it must be
admitted, cases of special difficulty on the theory of natural selection;
and one of the most curious of these is the existence of two or three
defined castes of workers or sterile females in the same community of
ants; but I have attempted to show how this difficulty can be
mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted together; but
that it is incidental on constitutional differences in the reproductive
systems of the intercrossed species. We see the truth of this conclusion
in the vast difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first used as the
father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been [461]produced under domestication; and as
domestication (I do not mean mere confinement) apparently tends to
eliminate sterility, we ought not to expect it also to produce
sterility.
The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds are
rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need not
feel surprise at hybrids being in some degree sterile, for their
constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported
by another parallel, but directly opposite, class of facts; namely, that
the vigour and fertility of all organic beings are increased by slight
changes in their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased vigour
and fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of
life and crosses between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are
now found, they must in the course of successive generations have passed
from some one part to the others. We are often wholly unable [462]even to
conjecture how this could have been effected. Yet, as we have reason to
believe that some species have retained the same specific form for very
long periods, enormously long as measured by years, too much stress ought
not to be laid on the occasional wide diffusion of the same species; for
during very long periods of time there will always have been a good
chance for wide migration by many means. A broken or interrupted range
may often be accounted for by the extinction of the species in the
intermediate regions. It cannot be denied that we are as yet very
ignorant of the full extent of the various climatal and geographical
changes which have affected the earth during modern periods; and such
changes will obviously have greatly facilitated migration. As an example,
I have attempted to show how potent has been the influence of the Glacial
period on the distribution both of the same and of representative species
throughout the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct species of the
same genus inhabiting very distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration
will have been possible during a very long period; and consequently the
difficulty of the wide diffusion of species of the same genus is in some
degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our present varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover directly connecting
[463]links between them, but only between each
and some extinct and supplanted form. Even on a wide area, which has
during a long period remained continuous, and of which the climate and
other conditions of life change insensibly in going from a district
occupied by one species into another district occupied by a closely
allied species, we have no just right to expect often to find
intermediate varieties in the intermediate zone. For we have reason to
believe that only a few species are undergoing change at any one period;
and all changes are slowly effected. I have also shown that the
intermediate varieties which will at first probably exist in the
intermediate zones, will be liable to be supplanted by the allied forms
on either hand; and the latter, from existing in greater numbers, will
generally be modified and improved at a quicker rate than the
intermediate varieties, which exist in lesser numbers; so that the
intermediate varieties will, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does not
every collection of fossil remains afford plain evidence of the gradation
and mutation of the forms of life? We meet with no such evidence, and
this is the most obvious and forcible of the many objections which may be
urged against my theory. Why, again, do whole groups of allied species
appear, though certainly they often falsely appear, to have come in
suddenly on the several geological stages? Why do we not find great piles
of strata beneath the Silurian system, stored with the remains of the
progenitors of the Silurian groups of fossils? For certainly on my theory
such [464]strata must somewhere have been deposited
at these ancient and utterly unknown epochs in the world's history.
I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect. The
number of specimens in all our museums is absolutely as nothing compared
with the countless generations of countless species which certainly have
existed. We should not be able to recognise a species as the parent of
any one or more species if we were to examine them ever so closely,
unless we likewise possessed many of the intermediate links between their
past or parent and present states; and these many links we could hardly
ever expect to discover, owing to the imperfection of the geological
record. Numerous existing doubtful forms could be named which are
probably varieties; but who will pretend that in future ages so many
fossil links will be discovered, that naturalists will be able to decide,
on the common view, whether or not these doubtful forms are varieties? As
long as most of the links between any two species are unknown, if any one
link or intermediate variety be discovered, it will simply be classed as
another and distinct species. Only a small portion of the world has been
geologically explored. Only organic beings of certain classes can be
preserved in a fossil condition, at least in any great number. Widely
ranging species vary most, and varieties are often at first
local,—both causes rendering the discovery of intermediate links
less likely. Local varieties will not spread into other and distant
regions until they are considerably modified and [465]improved; and when they
do spread, if discovered in a geological formation, they will appear as
if suddenly created there, and will be simply classed as new species.
Most formations have been intermittent in their accumulation; and their
duration, I am inclined to believe, has been shorter than the average
duration of specific forms. Successive formations are separated from each
other by enormous blank intervals of time; for fossiliferous formations,
thick enough to resist future degradation, can be accumulated only where
much sediment is deposited on the subsiding bed of the sea. During the
alternate periods of elevation and of stationary level the record will be
blank. During these latter periods there will probably be more
variability in the forms of life; during periods of subsidence, more
extinction.
With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in the
ninth chapter. That the geological record is imperfect all will admit;
but that it is imperfect to the degree which I require, few will be
inclined to admit. If we look to long enough intervals of time, geology
plainly declares that all species have changed; and they have changed in
the manner which my theory requires, for they have changed slowly and in
a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to each
other, than are the fossils from formations distant from each other in
time.
Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them. I
have felt these difficulties far too heavily during many years to [466]doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor
do we know how ignorant we are. We do not know all the possible
transitional gradations between the simplest and the most perfect organs;
it cannot be pretended that we know all the varied means of Distribution
during the long lapse of years, or that we know how imperfect the
Geological Record is. Grave as these several difficulties are, in my
judgment they do not overthrow the theory of descent from a few created
forms with subsequent modification.
Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life;
so that this system, when not rendered impotent, fails to reproduce
offspring exactly like the parent-form. Variability is governed by many
complex laws,—by correlation of growth, by use and disuse, and by
the direct action of the physical conditions of life. There is much
difficulty in ascertaining how much modification our domestic productions
have undergone; but we may safely infer that the amount has been large,
and that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On
the other hand we have evidence that variability, when it has once come
into play, does not wholly cease; for new varieties are still
occasionally produced by our most anciently domesticated productions.
[467]
Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts on
the organisation, and causes variability. But man can and does select the
variations given to him by nature, and thus accumulate them in any
desired manner. He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it unconsciously by
preserving the individuals most useful to him at the time, without any
thought of altering the breed. It is certain that he can largely
influence the character of a breed by selecting, in each successive
generation, individual differences so slight as to be quite inappreciable
by an uneducated eye. This process of selection has been the great agency
in the production of the most distinct and useful domestic breeds. That
many of the breeds produced by man have to a large extent the character
of natural species, is shown by the inextricable doubts whether very many
of them are varieties or aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful and
ever-acting means of selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common to
all organic beings. This high rate of increase is proved by
calculation,—by the rapid increase of many animals and plants
during a succession of peculiar seasons, or when naturalised in a new
country. More individuals are born than can possibly survive. A grain in
the balance will determine which individual shall live and which shall
die,—which variety or species shall increase in number, and which
[468]shall decrease, or finally become extinct.
As the individuals of the same species come in all respects into the
closest competition with each other, the struggle will generally be most
severe between them; it will be almost equally severe between the
varieties of the same species, and next in severity between the species
of the same genus. But the struggle will often be very severe between
beings most remote in the scale of nature. The slightest advantage in one
being, at any age or during any season, over those with which it comes
into competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of defence,
or on the charms of the males; and the slightest advantage will lead to
victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would have
varied under nature, in the same way as they generally have varied under
the changed conditions of domestication. And if there be any variability
under nature, it would be an unaccountable fact if natural selection had
not come into play. It has often been asserted, but the assertion is
quite incapable of proof, that the amount of variation under nature is a
strictly limited quantity. Man, though acting on external characters
alone and often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least individual
differences in species under [469]nature. But, besides such differences, all
naturalists have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No one
can draw any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and sub-species, and
species. Let it be observed how naturalists differ in the rank which they
assign to the many representative forms in Europe and North America.
If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by
patience select variations most useful to himself, should nature fail in
selecting variations useful, under changing conditions of life, to her
living products? What limit can be put to this power, acting during long
ages and rigidly scrutinising the whole constitution, structure, and
habits of each creature,—favouring the good and rejecting the bad?
I can see no limit to this power, in slowly and beautifully adapting each
form to the most complex relations of life. The theory of natural
selection, even if we looked no further than this, seems to me to be in
itself probable. I have already recapitulated, as fairly as I could, the
opposed difficulties and objections: now let us turn to the special facts
and arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation, and
varieties which are acknowledged to have been produced by secondary laws.
On this same [470]view we can understand how it is that in
each region where many species of a genus have been produced, and where
they now flourish, these same species should present many varieties; for
where the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they
differ from each other by a less amount of difference than do the species
of smaller genera. The closely allied species also of the larger genera
apparently have restricted ranges, and in their affinities they are
clustered in little groups round other species—in which respects
they resemble varieties. These are strange relations on the view of each
species having been independently created, but are intelligible if all
species first existed as varieties.
As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by so much the more as they become
diversified in habits and structure, so as to be enabled to seize on many
and widely different places in the economy of nature, there will be a
constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence during a long-continued course of
modification, the slight differences, characteristic of varieties of the
same species, tend to be augmented into the greater differences
characteristic of species of the same genus. New and improved varieties
will inevitably supplant and exterminate the older, less improved and
intermediate varieties; and thus species are rendered to a large extent
defined and distinct objects. Dominant species belonging to the [471]larger
groups tend to give birth to new and dominant forms; so that each large
group tends to become still larger, and at the same time more divergent
in character. But as all groups cannot thus succeed in increasing in
size, for the world would not hold them, the more dominant groups beat
the less dominant. This tendency in the large groups to go on increasing
in size and diverging in character, together with the almost inevitable
contingency of much extinction, explains the arrangement of all the forms
of life, in groups subordinate to groups, all within a few great classes,
which we now see everywhere around us, and which has prevailed throughout
all time. This grand fact of the grouping of all organic beings seems to
me utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification; it
can act only by very short and slow steps. Hence the canon of "Natura non
facit saltum," which every fresh addition to our knowledge tends to make
truer, is on this theory simply intelligible. We can plainly see why
nature is prodigal in variety, though niggard in innovation. But why this
should be a law of nature if each species has been independently created,
no man can explain.
Many other facts are, as it seems to me, explicable on this theory.
How strange it is that a bird, under the form of woodpecker, should have
been created to prey on insects on the ground; that upland geese, which
never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects;
and that a petrel should have been created with habits and structure
fitting it for the life of an auk or grebe! and so on in endless other
cases. But on the view of each [472]species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature,
these facts cease to be strange, or perhaps might even have been
anticipated.
As natural selection acts by competition, it adapts the inhabitants of
each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought we
to marvel if all the contrivances in nature be not, as far as we can
judge, absolutely perfect; and if some of them be abhorrent to our ideas
of fitness. We need not marvel at the sting of the bee causing the bee's
own death; at drones being produced in such vast numbers for one single
act, with the great majority slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred
of the queen bee for her own fertile daughters; at ichneumonidæ feeding
within the live bodies of caterpillars; and at other such cases. The
wonder indeed is, on the theory of natural selection, that more cases of
the want of absolute perfection have not been observed.
The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone,
they occasionally assume some of the characters of the species proper to
that zone. In both varieties and species, use and disuse seem to have
produced some effect; for it is difficult to resist this [473]conclusion
when we look, for instance, at the logger-headed duck, which has wings
incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have
their eyes covered with skin; or when we look at the blind animals
inhabiting the dark caves of America and Europe. In both varieties and
species correlation of growth seems to have played a most important part,
so that when one part has been modified other parts are necessarily
modified. In both varieties and species reversions to long-lost
characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several
species of the horse-genus and in their hybrids! How simply is this fact
explained if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of pigeon
have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently
created, why should the specific characters, or those by which the
species of the same genus differ from each other, be more variable than
the generic characters in which they all agree? Why, for instance, should
the colour of a flower be more likely to vary in any one species of a
genus, if the other species, supposed to have been created independently,
have differently coloured flowers, than if all the species of the genus
have the same coloured flowers? If species are only well-marked
varieties, of which the characters have become in a high degree
permanent, we can understand this fact; for they have already varied
since they branched off from a common progenitor in certain characters,
by which they have come to be specifically distinct from each other; [474]and
therefore these same characters would be more likely still to be variable
than the generic characters which have been inherited without change for
an enormous period. It is inexplicable on the theory of creation why a
part developed in a very unusual manner in any one species of a genus,
and therefore, as we may naturally infer, of great importance to the
species, should be eminently liable to variation; but, on my view, this
part has undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But a
part may be developed in the most unusual manner, like the wing of a bat,
and yet not be more variable than any other structure, if the part be
common to many subordinate forms, that is, if it has been inherited for a
very long period; for in this case it will have been rendered constant by
long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt sometimes comes into
play in modifying instincts; but it certainly is not indispensable, as we
see, in the case of neuter insects, which leave no progeny to inherit the
effects of long-continued habit. On the view of all the species of the
same genus having descended from a common parent, and having inherited
much in common, we can understand how it is that allied species, when
placed under considerably different conditions of life, [475]yet should
follow nearly the same instincts; why the thrush of South America, for
instance, lines her nest with mud like our British species. On the view
of instincts having been slowly acquired through natural selection we
need not marvel at some instincts being apparently not perfect and liable
to mistakes, and at many instincts causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents,—in being
absorbed into each other by successive crosses, and in other such
points,—as do the crossed offspring of acknowledged varieties. On
the other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.
If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change, after equal intervals
of time, is widely different in different groups. The extinction of
species and of whole groups of species, which has played so conspicuous a
part in the history of the organic world, almost inevitably follows on
the principle of natural selection; for old forms will be supplanted by
new and improved forms. Neither single species nor groups of species
reappear when the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to
appear as if they had changed simultaneously throughout the world. The
fact of the fossil remains of each formation being in some degree
intermediate in character between the [476]fossils in the
formations above and below, is simply explained by their intermediate
position in the chain of descent. The grand fact that all extinct organic
beings belong to the same system with recent beings, falling either into
the same or into intermediate groups, follows from the living and the
extinct being the offspring of common parents. As the groups which have
descended from an ancient progenitor have generally diverged in
character, the progenitor with its early descendants will often be
intermediate in character in comparison with its later descendants; and
thus we can see why the more ancient a fossil is, the oftener it stands
in some degree intermediate between existing and allied groups. Recent
forms are generally looked at as being, in some vague sense, higher than
ancient and extinct forms; and they are in so far higher as the later and
more improved forms have conquered the older and less improved organic
beings in the struggle for life. Lastly, the law of the long endurance of
allied forms on the same continent,—of marsupials in Australia, of
edentata in America, and other such cases,—is intelligible, for
within a confined country, the recent and the extinct will naturally be
allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading
facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and
in their geological succession throughout time; for in both cases the
beings have been connected by the bond of ordinary generation, and the
means of [477]modification have been the same. We see
the full meaning of the wonderful fact, which must have struck every
traveller, namely, that on the same continent, under the most diverse
conditions, under heat and cold, on mountain and lowland, on deserts and
marshes, most of the inhabitants within each great class are plainly
related; for they will generally be descendants of the same progenitors
and early colonists. On this same principle of former migration, combined
in most cases with modification, we can understand, by the aid of the
Glacial period, the identity of some few plants, and the close alliance
of many others, on the most distant mountains, under the most different
climates; and likewise the close alliance of some of the inhabitants of
the sea in the northern and southern temperate zones, though separated by
the whole intertropical ocean. Although two areas may present the same
physical conditions of life, we need feel no surprise at their
inhabitants being widely different, if they have been for a long period
completely separated from each other; for as the relation of organism to
organism is the most important of all relations, and as the two areas
will have received colonists from some third source or from each other,
at various periods and in different proportions, the course of
modification in the two areas will inevitably be different.
On this view of migration, with subsequent modification, we can see
why oceanic islands should be inhabited by few species, but of these,
that many should be peculiar. We can clearly see why those animals which
cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
should not inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, which can traverse the ocean, should so often
be found on islands far distant from any continent. Such facts [478]as the
presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation.
The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the same
parents formerly inhabited both areas; and we almost invariably find that
wherever many closely allied species inhabit two areas, some identical
species common to both still exist. Wherever many closely allied yet
distinct species occur, many doubtful forms and varieties of the same
species likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in nearly
all the plants and animals of the Galapagos archipelago, of Juan
Fernandez, and of the other American islands being related in the most
striking manner to the plants and animals of the neighbouring American
mainland; and those of the Cape de Verde archipelago and other African
islands to the African mainland. It must be admitted that these facts
receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group, and
with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see
how it is, that the mutual affinities of the species and genera within
each class are so complex and circuitous. We see why certain characters
are far more serviceable than others for classification;—why
adaptive characters, though of paramount importance to the being, are of
hardly any [479]importance in classification; why
characters derived from rudimentary parts, though of no service to the
being, are often of high classificatory value; and why embryological
characters are the most valuable of all. The real affinities of all
organic beings are due to inheritance or community of descent. The
natural system is a genealogical arrangement, in which we have to
discover the lines of descent by the most permanent characters, however
slight their vital importance may be.
The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,—the same number of
vertebræ forming the neck of the giraffe and of the elephant,—and
innumerable other such facts, at once explain themselves on the theory of
descent with slow and slight successive modifications. The similarity of
pattern in the wing and leg of a bat, though used for such different
purpose,—in the jaws and legs of a crab,—in the petals,
stamens, and pistils of a flower, is likewise intelligible on the view of
the gradual modification of parts or organs, which were alike in the
early progenitor of each class. On the principle of successive variations
not always supervening at an early age, and being inherited at a
corresponding not early period of life, we can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiæ.
Disuse, aided sometimes by natural selection, will often tend to
reduce an organ, when it has become useless by changed habits or under
changed conditions [480]of life; and we can clearly understand on
this view the meaning of rudimentary organs. But disuse and selection
will generally act on each creature, when it has come to maturity and has
to play its full part in the struggle for existence, and will thus have
little power of acting on an organ during early life; hence the organ
will not be much reduced or rendered rudimentary at this early age. The
calf, for instance, has inherited teeth, which never cut through the gums
of the upper jaw, from an early progenitor having well-developed teeth;
and we may believe, that the teeth in the mature animal were reduced,
during successive generations, by disuse or by the tongue and palate
having been better fitted by natural selection to browse without their
aid; whereas in the calf, the teeth have been left untouched by selection
or disuse, and on the principle of inheritance at corresponding ages have
been inherited from a remote period to the present day. On the view of
each organic being and each separate organ having been specially created,
how utterly inexplicable it is that parts, like the teeth in the
embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain
stamp of inutility! Nature may be said to have taken pains to reveal, by
rudimentary organs and by homologous structures, her scheme of
modification, which it seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent, by the preservation or the natural selection of many
successive slight favourable variations. I cannot believe that a false
theory would explain, as it seems to me that the theory of natural
selection does explain, [481]the several large classes of facts above
specified. I see no good reason why the views given in this volume should
shock the religious feelings of any one. A celebrated author and divine
has written to me that "he has gradually learnt to see that it is just as
noble a conception of the Deity to believe that He created a few original
forms capable of self-development into other and needful forms, as to
believe that He required a fresh act of creation to supply the voids
caused by the action of His laws."
Why, it may be asked, have all the most eminent living naturalists and
geologists rejected this view of the mutability of species? It cannot be
asserted that organic beings in a state of nature are subject to no
variation; it cannot be proved that the amount of variation in the course
of long ages is a limited quantity; no clear distinction has been, or can
be, drawn between species and well-marked varieties. It cannot be
maintained that species when intercrossed are invariably sterile, and
varieties invariably fertile; or that sterility is a special endowment
and sign of creation. The belief that species were immutable productions
was almost unavoidable as long as the history of the world was thought to
be of short duration; and now that we have acquired some idea of the
lapse of time, we are too apt to assume, without proof, that the
geological record is so perfect that it would have afforded us plain
evidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long [482]lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the term
of a hundred million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an almost infinite
number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such
expressions as the "plan of creation," "unity of design," &c., and to
think that we give an explanation when we only restate a fact. Any one
whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject my theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt on the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will
be able to view both sides of the question with impartiality. Whoever is
led to believe that species are mutable will do good service by
conscientiously expressing his conviction; for only thus can the load of
prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This
seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately [483]they themselves thought
were special creations, and which are still thus looked at by the
majority of naturalists, and which consequently have every external
characteristic feature of true species,—they admit that these have
been produced by variation, but they refuse to extend the same view to
other and very slightly different forms. Nevertheless they do not pretend
that they can define, or even conjecture, which are the created forms of
life, and which are those produced by secondary laws. They admit
variation as a vera causa in one case, they arbitrarily reject it
in another, without assigning any distinction in the two cases. The day
will come when this will be given as a curious illustration of the
blindness of preconceived opinion. These authors seem no more startled at
a miraculous act of creation than at an ordinary birth. But do they
really believe that at innumerable periods in the earth's history certain
elemental atoms have been commanded suddenly to flash into living
tissues? Do they believe that at each supposed act of creation one
individual or many were produced? Were all the infinitely numerous kinds
of animals and plants created as eggs or seed, or as full grown? and in
the case of mammals, were they created bearing the false marks of
nourishment from the mother's womb? Although naturalists very properly
demand a full explanation of every difficulty from those who believe in
the mutability of species, on their own side they ignore the whole
subject of the first appearance of species in what they consider reverent
silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away
in force. But some arguments of the greatest weight [484]extend very
far. All the members of whole classes can be connected together by chains
of affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very wide
intervals between existing orders. Organs in a rudimentary condition
plainly show that an early progenitor had the organ in a fully developed
state; and this in some instances necessarily implies an enormous amount
of modification in the descendants. Throughout whole classes various
structures are formed on the same pattern, and at an embryonic age the
species closely resemble each other. Therefore I cannot doubt that the
theory of descent with modification embraces all the members of the same
class. I believe that animals have descended from at most only four or
five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison secreted by the
gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed by the Creator.
When the views advanced by me in this volume, and by Mr. Wallace in
the Linnean Journal, or when analogous views on the origin of species are
generally [485]admitted, we can dimly foresee that there
will be a considerable revolution in natural history. Systematists will
be able to pursue their labours as at present; but they will not be
incessantly haunted by the shadowy doubt whether this or that form be in
essence a species. This I feel sure, and I speak after experience, will
be no slight relief. The endless disputes whether or not some fifty
species of British brambles are true species will cease. Systematists
will have only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a far
more essential consideration than it is at present; for differences,
however slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as sufficient to raise both
forms to the rank of species. Hereafter we shall be compelled to
acknowledge that the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to be connected at
the present day by intermediate gradations, whereas species were formerly
thus connected. Hence, without rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall be
led to weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names, as with the primrose and cowslip; and in this case
scientific and common language will come into accordance. In short, we
shall have to treat species in the same manner as those naturalists treat
genera, who admit that genera are merely artificial combinations [486]made
for convenience. This may not be a cheering prospect; but we shall at
least be freed from the vain search for the undiscovered and
undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, &c., will cease to be
metaphorical, and will have a plain signification. When we no longer look
at an organic being as a savage looks at a ship, as at something wholly
beyond his comprehension; when we regard every production of nature as
one which has had a history; when we contemplate every complex structure
and instinct as the summing up of many contrivances, each useful to the
possessor, nearly in the same way as when we look at any great mechanical
invention as the summing up of the labour, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each organic
being, how far more interesting, I speak from experience, will the study
of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects of
use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value. A
new variety raised by man will be a more important and interesting
subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as far as
they can be so made, genealogies; and will then truly give what may be
called the plan of creation. The rules for classifying will no doubt
become simpler when we have a definite object in view. We possess no [487]pedigrees or armorial bearings; and we
have to discover and trace the many diverging lines of descent in our
natural genealogies, by characters of any kind which have long been
inherited. Rudimentary organs will speak infallibly with respect to the
nature of long-lost structures. Species and groups of species, which are
called aberrant, and which may fancifully be called living fossils, will
aid us in forming a picture of the ancient forms of life. Embryology will
reveal to us the structure, in some degree obscured, of the prototypes of
each great class.
When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not very
remote period descended from one parent, and have migrated from some one
birthplace; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the
differences of the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent in
relation to their apparent means of immigration, some light can be thrown
on ancient geography.
The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must not
be looked at as a well-filled museum, but as a poor collection made at
hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between the
successive stages as having been of vast duration. But we shall [488]be able to
gauge with some security the duration of these intervals by a comparison
of the preceding and succeeding organic forms. We must be cautious in
attempting to correlate as strictly contemporaneous two formations, which
include few identical species, by the general succession of their forms
of life. As species are produced and exterminated by slowly acting and
still existing causes, and not by miraculous acts of creation and by
catastrophes; and as the most important of all causes of organic change
is one which is almost independent of altered and perhaps suddenly
altered physical conditions, namely, the mutual relation of organism to
organism,—the improvement of one being entailing the improvement or
the extermination of others; it follows, that the amount of organic
change in the fossils of consecutive formations probably serves as a fair
measure of the lapse of actual time. A number of species, however,
keeping in a body might remain for a long period unchanged, whilst within
this same period, several of these species, by migrating into new
countries and coming into competition with foreign associates, might
become modified; so that we must not overrate the accuracy of organic
change as a measure of time. During early periods of the earth's history,
when the forms of life were probably fewer and simpler, the rate of
change was probably slower; and at the first dawn of life, when very few
forms of the simplest structure existed, the rate of change may have been
slow in an extreme degree. The whole history of the world, as at present
known, although of a length quite incomprehensible by us, will hereafter
be recognised as a mere fragment of time, compared with the ages which
have elapsed since the first creature, the progenitor of innumerable
extinct and living descendants, was created.
In the distant future I see open fields for far more [489]important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the
view that each species has been independently created. To my mind it
accords better with what we know of the laws impressed on matter by the
Creator, that the production and extinction of the past and present
inhabitants of the world should have been due to secondary causes, like
those determining the birth and death of the individual. When I view all
beings not as special creations, but as the lineal descendants of some
few beings which lived long before the first bed of the Silurian system
was deposited, they seem to me to become ennobled. Judging from the past,
we may safely infer that not one living species will transmit its
unaltered likeness to a distant futurity. And of the species now living
very few will transmit progeny of any kind to a far distant futurity; for
the manner in which all organic beings are grouped, shows that the
greater number of species of each genus, and all the species of many
genera, have left no descendants, but have become utterly extinct. We can
so far take a prophetic glance into futurity as to foretel that it will
be the common and widely-spread species, belonging to the larger and
dominant groups, which will ultimately prevail and procreate new and
dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Silurian epoch, we may
feel certain that the ordinary succession by generation has never once
been broken, and that no cataclysm has desolated the whole world. Hence
we may look with some confidence to a secure future of equally
inappreciable length. And as natural selection works [490]solely by and
for the good of each being, all corporeal and mental endowments will tend
to progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different
from each other, and dependent on each other in so complex a manner, have
all been produced by laws acting around us. These laws, taken in the
largest sense, being Growth with Reproduction; Inheritance which is almost
implied by reproduction; Variability from the indirect and direct action
of the external conditions of life, and from use and disuse; a Ratio of
Increase so high as to lead to a Struggle for Life, and as a consequence
to Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of nature, from
famine and death, the most exalted object which we are capable of
conceiving, namely, the production of the higher animals, directly
follows. There is grandeur in this view of life, with its several powers,
having been originally breathed by the Creator into a few forms or into
one; and that, whilst this planet has gone cycling on according to the
fixed law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being, evolved.
[491]
INDEX.
Aberrant groups, 429.
Abyssinia, plants of, 375.
Acclimatisation, 139.
Affinities of extinct species, 329.
—— of organic beings, 411.
Agassiz on Amblyopsis, 139.
—— on groups of species suddenly appearing, 302, 305.
—— on embryological succession, 338.
—— on the glacial period, 366.
—— on embryological characters, 418.
—— on the embryos of vertebrata, 439.
—— on parallelism of embryological development and geological succession, 449.
Algæ of New Zealand, 376.
Alligators, males, fighting, 88.
Amblyopsis, blind fish, 139.
America, North, productions allied to those of Europe, 371.
————, boulders and glaciers of, 373.
——, South, no modern formations on west coast, 290.
Ammonites, sudden extinction of, 321.
Anagallis, sterility of, 247.
Analogy of variations, 159.
Ancylus, 386.
Animals, not domesticated from being variable, 17.
——, domestic, descended from several stocks, 19.
————, acclimatisation of, 141.
—— of Australia, 116.
—— with thicker fur in cold climates, 133.
——, blind, in caves, 137.
——, extinct, of Australia, 339.
Anomma, 240.
Antarctic islands, ancient flora of, 399.
Antirrhinum, 161.
Ants attending aphides, 210.
——, slave-making instinct, 219.
——, neuter, structure of, 236.
Aphides, attended by ants, 210.
Aphis, development of, 442.
Apteryx, 182.
Arab horses, 35.
Aralo-Caspian Sea, 339.
Archaic, M. de, on the succession of species, 325.
Artichoke, Jerusalem, 142.
Ascension, plants of, 389.
Asclepias, pollen of, 193.
Asparagus, 359.
Aspicarpa, 417.
Asses, striped, 163.
Ateuchus, 135.
Audubon on habits of frigate-bird, 185.
—— on variation in birds'-nests, 212.
—— on heron eating seeds, 387.
Australia, animals of, 116.
——. dogs of, 215.
——, extinct animals of, 339.
——, European plants in, 375.
Azara on flies destroying cattle, 72.
Azores, flora of, 363.
Babington, Mr., on British plants, 48.
Balancement of growth, 147.
Bamboo with hooks, 197.
Barberry, flowers of, 98.
Barrande, M., on Silurian colonies, 313.
—— on the succession of species, 325.
—— on parallelism of palæozoic formations, 328.
—— on affinities of ancient species, 330.
Barriers, importance of, 347.
Batrachians on islands, 393.
Bats, how structure acquired, 180.
——, distribution of, 394.
Bear, catching water-insects, 184.
Bee, sting of, 202.
——, queen, killing rivals, 202.
Bees fertilising flowers, 73.
——, hive, not sucking the red clover, 95.
[492]
————, cell-making instinct, 224.
——, humble, cells of, 225.
——, parasitic, 218.
Beetles, wingless, in Madeira, 135.
—— with deficient tarsi, 135.
Bentham, Mr., on British plants, 48.
——, on classification, 419.
Berkeley, Mr., on seeds in salt-water, 358.
Bermuda, birds of, 391.
Birds acquiring fear, 212.
—— annually cross the Atlantic, 364.
——, colour of, on continents, 132.
——, footsteps and remains of, in secondary rocks, 304.
——, fossil, in caves of Brazil, 339.
—— of Madeira, Bermuda, and Galapagos, 391.
——, song of males, 89.
—— transporting seeds, 361.
——, waders, 385.
——, wingless, 134, 182.
——, with traces of embryonic teeth, 450.
Bizcacha, 349.
——, affinities of, 429.
Bladder for swimming in fish, 190.
Blindness of cave animals, 137.
Blyth, Mr., on distinctness of Indian cattle, 18.
——, on striped Hemionus, 163.
——, on crossed geese, 254.
Boar, shoulder-pad of, 88.
Borrow, Mr., on the Spanish pointer, 35.
Bory St. Vincent on Batrachians, 393.
Bosquet, M., on fossil Chthamalus, 305.
Boulders, erratic, on the Azores, 363.
Branchiæ, 190.
Brent, Mr., on house-tumblers, 214.
——, on hawks killing pigeons, 362.
Brewer, Dr., on American cuckoo, 217.
Britain, mammals of, 396.
Bronn on duration of specific forms, 294.
Brown, Robert, on classification, 415.
Buckman on variation in plants, 10.
Buzareingues on sterility of varieties, 270.
Cabbage, varieties of, crossed, 99.
Calceolaria, 251.
Canary-birds, sterility of hybrids, 252.
Cape de Verde islands, 398.
Cape of Good Hope, plants of, 110, 375.
Carrier-pigeons killed by hawks, 362.
Cassini on flowers of compositæ, 145.
Catasetum, 424.
Cats, with blue eyes, deaf, 12.
——, variation in habits of, 91.
—— curling tail when going to spring, 201.
Cattle destroying fir-trees, 72.
—— destroyed by flies in La Plata, 72.
——, breeds of, locally extinct, 111.
——, fertility of Indian and European breeds, 254.
Cave, inhabitants of, blind, 137.
Centres of creation, 352.
Cephalopodæ, development of, 442.
Cervulus, 253.
Cetacea, teeth and hair, 144.
Ceylon, plants of, 375.
Chalk formation, 322.
Characters, divergence of, 111.
——, sexual, variable, 156.
——, adaptive or analogical, 426.
Charlock, 76.
Checks to increase, 67.
—— ——, mutual, 71.
Chickens, instinctive tameness of, 216.
Chthamalinæ, 289.
Chthamalus, cretacean species of, 305.
Circumstances favourable to selection of domestic products, 40.
—— —— to natural selection, 102.
Cirripedes capable of crossing, 101.
——, carapace aborted, 148.
——, their ovigerous frena, 192.
——, fossil, 304.
——, larvæ of, 440.
Classification, 413.
Clift, Mr., on the succession of types, 339.
Climate, effects of, in checking increase of beings, 68.
——, adaptation of, to organisms, 139.
[493]
Cobites, intestine of, 190.
Cockroach, 76.
Collections, palæontological, poor, 288.
Colour, influenced by climate, 132.
——, in relation to attacks by flies, 198.
Columba livia, parent of domestic pigeons, 23.
Colymbetes, 386.
Compensation of growth, 147.
Compositæ, outer and inner florets of, 144.
——, male flowers of, 451.
Conclusion, general, 480.
Conditions, slight changes in, favourable to fertility, 267.
Coot, 185.
Coral-islands, seeds drifted to, 361.
—— reefs, indicating movements of earth, 310.
Corn-crake, 186.
Correlation of growth in domestic productions, 11.
—— of growth, 143, 198.
Cowslip, 49.
Creation, single centres of, 352.
Crinum, 250.
Crosses, reciprocal, 258.
Crossing of domestic animals, importance in altering breeds, 20.
——, advantages of, 96.
—— unfavourable to selection, 102.
Crustacea of New Zealand, 376.
Crustacean, blind, 137.
Cryptocerus, 239.
Ctenomys, blind, 137.
Cuckoo, instinct of, 216.
Currants, grafts of, 262.
Currents of sea, rate of, 360.
Cuvier on conditions of existence, 206.
—— on fossil monkeys, 304.
——, Fred., on instinct, 208.
Dana, Prof., on blind cave-animals, 139.
——, on relations of crustaceans of Japan, 372.
——, on crustaceans of New Zealand, 376.
De Candolle on struggle for existence, 62.
—— on umbelliferæ, 146.
—— on general affinities, 430.
——, Alph., on low plants, widely dispersed, 406.
——, ——, on widely-ranging plants being variable, 53.
——, ——, on naturalisation, 115.
——, ——, on winged seeds, 146.
——, ——, on Alpine species suddenly becoming rare, 175.
——, ——, on distribution of plants with large seeds, 360.
——, ——, on vegetation of Australia, 379.
——, ——, on fresh-water plants, 386.
——, ——, on insular plants, 389.
Degradation of coast-rocks, 282.
Denudation, rate of, 285.
—— of oldest rocks, 308.
Development of ancient forms, 336.
Devonian system, 334.
Dianthus, fertility of crosses, 256.
Dirt on feet of birds, 362.
Dispersal, means of, 356.
—— during glacial period, 365.
Distribution, geographical, 346.
——, means of, 356.
Disuse, effects of, under nature, 134.
Divergence of character, 111.
Division, physiological, of labour, 115.
Dogs, hairless, with imperfect teeth, 12.
—— descended from several wild stocks, 18.
——, domestic instincts of, 213.
——, inherited civilisation of, 215.
——, fertility of breeds together, 254.
——, —— of crosses, 268.
——, proportions of, when young, 444.
Domestication, variation under, 7.
Downing, Mr., on fruit-trees in America, 85.
Downs, North and South, 286.
Dragon-flies, intestines of, 190.
Drift-timber, 360.
Driver-ant, 240.
Drones killed by other bees, 202.
Duck, domestic, wings of, reduced, 11.
——, logger-headed, 182.
[494]
Duckweed, 385.
Dugong, affinities of, 414.
Dung-beetles with deficient tarsi, 135.
Dyticus, 386.
Earl, Mr. W., on the Malay Archipelago, 395.
Ears, drooping, in domestic animals, 11.
——, rudimentary, 454.
Earth, seeds in roots of trees, 361.
Eciton, 238.
Economy of organisation, 147.
Edentata, teeth and hair, 144.
——, fossil species of, 339.
Edwards, Milne, on physiological divisions of labour, 115.
——, on gradations of structure, 194.
——, on embryonical characters, 418.
Eggs, young birds escaping from, 87.
Electric organs, 192.
Elephant, rate of increase, 64.
—— of glacial period, 141.
Embryology, 438.
Existence, struggle for, 60.
——, conditions of, 206.
Extinction, as bearing on natural selection, 109.
—— of domestic varieties, 111,
——, 317.
Eye, structure of, 187.
——, correction for aberration, 202.
Eyes reduced in moles, 137.
Fabre, M. on parasitic sphex, 218.
Falconer, Dr., on naturalisation of plants in India, 65.
—— on fossil crocodile, 313.
—— on elephants and mastodons, 334.
—— and Cautley on mammals of sub-Himalayan beds, 340.
Falkland Island, wolf of, 394.
Faults, 285.
Faunas, marine, 348.
Fear, instinctive, in birds, 212.
Feet of bird, young molluscs adhering to, 385.
Fertility of hybrids, 249.
—— from slight changes in conditions, 267.
—— of crossed varieties, 268.
Fir-trees destroyed by cattle, 72.
—— ——, pollen of, 203.
Fish, flying, 182.
——, teleostean, sudden appearance of, 305.
—— eating seeds, 362, 387.
——, fresh-water, distribution of, 384.
Fishes, ganoid, now confined to fresh water, 107.
——, electric organs of, 192.
——, ganoid, living in fresh water, 321.
—— of southern hemisphere, 376.
Flight, powers of, how acquired, 182.
Flowers, structure of, in relation to crossing, 97.
—— of compositæ and umbelliferæ, 144.
Forbes, E., on colours of shells, 132.
—— on abrupt range of shells in depth, 175.
—— on poorness of palæontological collections, 288.
—— on continuous succession of genera, 316.
—— on continental extensions, 357.
—— on distribution during glacial period, 366.
—— on parallelism in time and space, 409.
Forests, changes in, in America, 74.
Formation, Devonian, 334.
Formations, thickness of, in Britain, 284.
——, intermittent, 290.
Formica rufescens, 219.
—— sanguinea, 219.
—— flava, neuter of, 240.
Frena, ovigerous, of cirripedes, 192.
Fresh-water productions, dispersal of, 383.
Fries on species in large genera being closely allied to other species, 57.
Frigate-bird, 185.
Frogs on islands, 393.
Fruit-trees, gradual improvement of, 37.
—— —— in United States, 85.
—— ——, varieties of, acclimatised in United States, 142.
[495]
Fuci, crossed, 258.
Fur, thicker in cold climates, 133.
Furze, 439.
Galapagos Archipelago, birds of, 390.
——, productions of, 398, 400.
Galeopithecus, 181.
Game, increase of, checked by vermin, 68.
Gärtner on sterility of hybrids, 247, 255.
——, on reciprocal crosses, 258.
——, on crossed maize and verbascum, 270.
——, on comparison of hybrids and mongrels, 272.
Geese, fertility when crossed, 253.
——, upland, 185.
Genealogy important in classification, 425.
Geoffroy St. Hilaire on balancement, 147.
—— —— on homologous organs, 434.
—— ——, Isidore, on variability of repeated parts, 149.
—— ——, on correlation in monstrosities, 11.
—— ——, on correlation, 144.
—— ——, on variable parts being often monstrous, 155.
Geographical distribution, 346.
Geography, ancient, 487.
Geology, future progress of, 487.
——, imperfection of the record, 279.
Giraffe, tail of, 195.
Glacial period, 365.
Gmelin on distribution, 365.
Gnathodon, fossil, 368.
Godwin-Austen, Mr., on the Malay Archipelago, 300.
Goethe on compensation of growth, 147.
Gooseberry, grafts of, 262.
Gould, Dr. A., on land-shells, 397.
——, Mr., on colours of birds, 132.
——, on birds of the Galapagos, 398.
——, on distribution of genera of birds, 404.
Gourds, crossed, 270.
Grafts, capacity of, 261.
Grasses, varieties of, 113.
Gray, Dr. Asa, on trees of United States, 100.
——, on naturalised plants in the United States, 115.
——, on rarity of intermediate varieties, 176.
——, on Alpine plants, 365.
——, Dr. J. E., on striped mule, 165.
Grebe, 185.
Groups, aberrant, 429.
Grouse, colours of, 84.
——, red, a doubtful species, 49.
Growth, compensation of, 147.
——, correlation of, in domestic products, 11.
——, correlation of, 143.
Habit, effect of, under domestication, 11.
——, effect of, under nature, 134.
——, diversified, of same species, 183.
Hair and teeth, correlated, 144.
Harcourt, Mr. E. V., on the birds of Madeira, 391.
Hartung, M. on boulders in the Azores, 363.
Hazel-nuts, 359.
Hearne on habits of bears, 184.
Heath, changes in vegetation, 72.
Heer, O., on plants of Madeira, 107.
Helix pomatia, 397.
Helosciadium, 359.
Hemionus, striped, 163.
Herbert, W., on struggle for existence, 62.
——, on sterility of hybrids, 249.
Hermaphrodites crossing, 96.
Heron eating seed, 387.
Heron, Sir R., on peacocks, 89.
Heusinger on white animals not poisoned by certain plants, 12.
Hewitt, Mr., on sterility of first crosses, 264.
Himalaya, glaciers of, 373.
——, plants of, 375.
Hippeastrum, 250.
Holly-trees, sexes of, 93.
Hollyhock, varieties of, crossed, 271.
Hooker, Dr., on trees of New Zealand, 100.
[496]
——, on acclimatisation of Himalayan trees, 140.
——, on flowers of umbelliferæ, 145.
——, on glaciers of Himalaya, 373.
——, on algæ of New Zealand, 376.
——, on vegetation at the base of the Himalaya, 378.
——, on plants of Tierra del Fuego, 374, 378.
——, on Australian plants, 375, 399.
——, on relations of flora of South America, 379.
——, on flora of the Antarctic lands, 381, 399.
——, on the plants of the Galapagos, 392, 398.
Hooks on bamboos, 197.
—— to seeds on islands, 392.
Horner, Mr., on the antiquity of Egyptians, 18.
Horns, rudimentary, 454.
Horse, fossil, in La Plata, 318.
Horses destroyed by flies in La Plata, 72.
——, striped, 163.
——, proportions of, when young, 444.
Horticulturists, selection applied by, 32.
Huber on cells of bees, 230.
——, P., on reason blended with instinct, 208.
——, on habitual nature of instincts, 208.
——, on slave-making ants, 219.
——, on Melipona domestica, 225.
Humble-bees, cells of, 225.
Hunter, J., on secondary sexual characters, 150.
Hutton, Captain, on crossed geese, 254.
Huxley, Prof., on structure of hermaphrodites, 101.
——, on embryological succession, 338.
——, on homologous organs, 438.
——, on the development of aphis, 442.
Hybrids and mongrels compared, 272.
Hybridism, 245.
Hydra, structure of, 190.
Ibla, 148.
Icebergs transporting seeds, 363.
Increase, rate of, 63.
Individuals, numbers favourable to selection, 102.
——, many, whether simultaneously created, 355.
Inheritance, laws of, 12.
—— at corresponding ages, 14, 86.
Insects, colour of, fitted for habitations, 84.
——, sea-side, colours of, 132.
——, blind, in caves, 138.
——, luminous, 193.
——, neuter, 236.
Instinct, 207.
Instincts, domestic, 213.
Intercrossing, advantages of, 96.
Islands, oceanic, 388.
Isolation favourable to selection, 104.
Japan, productions of, 372.
Java, plants of, 375.
Jones, Mr. J. M., on the birds of Bermuda, 391.
Jussieu on classification, 417.
Kentucky, caves of, 137.
Kerguelen-land, flora of, 381, 399.
Kidney-bean, acclimatisation of, 142.
Kidneys of birds, 144.
Kirby on tarsi deficient in beetles, 135.
Knight, Andrew, on cause of variation, 7.
Kölreuter on the barberry, 98.
—— on sterility of hybrids, 246.
—— on reciprocal crosses, 258.
—— on crossed varieties of nicotiana, 271.
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