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Title: On the Origin of Species by Means of Natural Selection - or the Preservation of Favoured Races in the Struggle for Life. (2nd edition)
Author: Darwin, Charles, 1809-1882
Language: English
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Transcriber's note: A few typographical errors have been corrected: they
are listed at the end of the text.

       *       *       *       *       *


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
oesophagus. 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 oesophagus; 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 oesophagus,--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.

[Illustration]

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
a^1 and m^1. 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 a^1 is supposed in the
diagram to have produced variety a^2, which will, owing to the principle of
divergence, differ more from (A) than did variety a^1. Variety m^1 is
supposed to have produced two varieties, namely m^2 and s^2, 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 a^1 to a^{10}.
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, a^{10}, f^{10}, and m^{10}, 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 a^{14} and m^{14}, 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 (w^{10} and z^{10}) 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
n^{14} to z^{14}, 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 (a^{14} to m^{14}); and
(I) will have been replaced by six (n^{14} to z^{14}) 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
a^{14} and z^{14} 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 a^{14}, q^{14}, p^{14}, will be nearly related from
having recently branched off from a^{10}; b^{14} and f^{14}, from having
diverged at an earlier period from a^5, will be in some degree distinct
from the three first-named species; and lastly, o^{14}, e^{14} and m^{14},
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 F^{14}, 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 (F^{14}) 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 a^{14} to p^{14}, those marked b^{14} and
f^{14}, and those marked o^{14} to m^{14}, 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 coelospermous 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 ×
[root]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 coerulea), 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, a^{14}, q^{14}, p^{14},
will form a small family; b^{14} and f^{14} a closely allied family or
sub-family; and o^{14}, e^{14}, m^{14}, 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 F^{14}.

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 a^1, a^5, a^{10}, f^8, m^3, m^6, m^9, 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, a^{14}, &c, and
b^{14},&c.) would have to be united into one family; and the two other
families (namely, a^{14} to f^{14} now including five genera, and o^{14} to
m^{14}) 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 6¾ 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 (a^{14} to z^{14}) 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 F^{14} 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 foetal 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.

          A.

  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.

          B.

  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.

          C.

  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.

          D.

  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.

          E.

  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.

          F.

  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.

          G.

  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.

          H.

  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.

          I.

  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.

          J.

  Japan, productions of, 372.
  Java, plants of, 375.
  Jones, Mr. J. M., on the birds of Bermuda, 391.
  Jussieu on classification, 417.

          K.

  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.
  ---- on crossing male and hermaphrodite flowers, 451.

          L.

  Lamarck on adaptive characters, 426.
  Land-shells, distribution of, 397.
  ---- of Madeira, naturalised, 403.
  Languages, classification of, 422.
  Lapse, great, of time, 282.
  {497}
  Larvæ, 440.
  Laurel, nectar secreted by the leaves,
  Laws of variation, 131.
  Leech, varieties of, 76.
  Leguminosæ, nectar secreted by glands, 92.
  Lepidosiren, 107, 330.
  Life, struggle for, 60.
  Lingula, Silurian, 307.
  Linnæus, aphorism of, 413.
  Lion, mane of, 88.
  ----, young of, striped, 439.
  Lobelia fulgens, 73, 98.
  Lobelia, sterility of crosses, 250.
  Loess of the Rhine, 384.
  Lowness of structure connected with variability, 149.
  Lowness, related to wide distribution, 406.
  Lubbock, Mr., on the nerves of coccus, 46.
  Lucas, Dr. P., on inheritance, 12.
  ----, on resemblance of child to parent, 275.
  Lund and Clausen on fossils of Brazil, 339.
  Lyell, Sir C, on the struggle for existence, 62.
  ----, on modern changes of the earth, 95.
  ----, on measure of denudation, 284.
  ----, on a carboniferous land-shell, 289.
  ----, on strata beneath Silurian system, 308.
  ----, on the imperfection of the geological record, 311.
  ----, on the appearance of species, 312.
  ----, on Barrande's colonies, 313.
  ----, on tertiary formations of Europe and North America, 323.
  ----, on parallelism of tertiary formations, 328.
  ----, on transport of seeds by icebergs, 363.
  ----, on great alternations of climate, 382.
  ----, on the distribution of fresh-water shells, 385.
  ----, on land-shells of Madeira, 402.
  Lyell and Dawson on fossilized trees in Nova Scotia, 297.

          M.

  Macleay on analogical characters, 426.
  Madeira, plants of, 107.
  ----, beetles of, wingless, 135.
  ----, fossil land-shells of, 339.
  ----, birds of, 390.
  Magpie tame in Norway, 212.
  Maize, crossed, 270.
  Malay Archipelago compared with Europe, 300.
  ----, mammals of, 395.
  Malpighiaceæ, 417.
  Mammæ, rudimentary, 451.
  Mammals, fossil, in secondary formation, 304.
  ----, insular, 394.
  Man, origin of races of, 199.
  Manatee, rudimentary nails of, 454.
  Marsupials of Australia, 116.
  ----, fossil species of, 339.
  Martens, M., experiment on seeds, 360.
  Martin, Mr. W. C., on striped mules, 165.
  Matteucci on the electric organs of rays, 193.
  Matthiola, reciprocal crosses of, 258.
  Means of dispersal, 356.
  Melipona domestica, 225.
  Metamorphism of oldest rocks, 308.
  Mice destroying bees, 74.
  ----, acclimatisation of, 141.
  Migration, bears on first appearance of fossils, 297.
  Miller, Prof., on the cells of bees, 226.
  Mirabilis, crosses of, 258.
  Missel-thrush, 76.
  Misseltoe, complex relations of, 3.
  Mississippi, rate of deposition at mouth, 284.
  Mocking-thrush of the Galapagos, 402.
  Modification of species, how far applicable, 483.
  Moles, blind, 137.
  Mongrels, fertility and sterility of, 268.
  ---- and hybrids compared, 272.
  {498}
  Monkeys, fossil, 304.
  Monocanthus, 424.
  Mons, Van, on the origin of fruit-trees, 29.
  Moquin-Tandon on sea-side plants, 132.
  Morphology, 433.
  Mozart, musical powers of, 209.
  Mud, seeds in, 386.
  Mules, striped, 165.
  Müller, Dr. F., on Alpine Australian plants, 375.
  Murchison, Sir R., on the formations of Russia, 290.
  ----, on azoic formations, 308.
  ----, on extinction, 317.
  Mustela vison, 179.
  Myanthus, 424.
  Myrmecocystus, 239.
  Myrmica, eyes of, 240.

          N.

  Nails, rudimentary, 454.
  Natural history, future progress of, 485.
  ---- selection, 80.
  ---- system, 413.
  Naturalisation of forms distinct from the indigenous species, 115.
  ---- in New Zealand, 201.
  Nautilus, Silurian, 307.
  Nectar of plants, 92.
  Nectaries, how formed, 92.
  Nelumbium luteum, 387.
  Nests, variation in, 211.
  Neuter insects, 236.
  Newman, Mr., on humble-bees, 74.
  New Zealand, productions of, not perfect, 201.
  ----, naturalised products of, 337.
  ----, fossil birds of, 339.
  ----, glacial action in, 373.
  ----, crustaceans of, 376.
  ----, algæ of, 376.
  ----, number of plants of, 389.
  ----, flora of, 399.
  Nicotiana, crossed varieties of, 271.
  ----, certain species very sterile, 257.
  Noble, Mr., on fertility of Rhododendron, 252.
  Nodules, phosphatic, in azoic rocks, 308.

          O.

  Oak, varieties of, 50.
  Onites apelles, 135.
  Orchis, pollen of, 193.
  Organs of extreme perfection, 186.
  ----, electric, of fishes, 192.
  ---- of little importance, 194.
  ----, homologous, 434.
  ----, rudiments of, and nascent, 450.
  Ornithorhynchus, 107, 416.
  Ostrich not capable of flight, 134.
  ----, habit of laying eggs together, 218.
  ----, American, two species of, 349.
  Otter, habits of, how acquired, 179.
  Ouzel, water, 185.
  Owen, Prof., on birds not flying, 134.
  ----, on vegetative repetition, 149.
  ----, on variable length of arms in ourang-outang, 150.
  ----, on the swim-bladder of fishes, 191.
  ----, on electric organs, 192.
  ----, on fossil horse of La Plata, 319.
  ----, on relations of ruminants and pachyderms, 329.
  ----, on fossil birds of New Zealand, 339.
  ----, on succession of types, 339.
  ----, on affinities of the dugong, 414.
  ----, on homologous organs, 434.
  ----, on the metamorphosis of cephalopods and spiders, 442.

          P.

  Pacific Ocean, faunas of, 348.
  Paley on no organ formed to give pain, 201.
  Pallas on the fertility of the wild stocks of domestic animals, 254.
  Paraguay, cattle destroyed by flies, 72.
  Parasites, 217.
  Partridge, dirt on feet, 363.
  Parts greatly developed, variable, 150.
  ----, degrees of utility of, 201.
  Parus major, 184.
  Passiflora, 251.
  Peaches in United States, 85.
  Pear, grafts of, 262.
  {499}
  Pelargonium, flowers of, 145.
  ----, sterility of, 251.
  Pelvis of women, 144.
  Peloria, 145.
  Period, glacial, 365.
  Petrels, habits of, 184.
  Phasianus, fertility of hybrids, 253.
  Pheasant, young, wild, 216.
  Philippi on tertiary species in Sicily, 312.
  Pictet, Prof., on groups of species suddenly appearing, 302, 305.
  ----, on rate of organic change, 313.
  ----, on continuous succession of genera, 316.
  ----, on close alliance of fossils in consecutive formations, 335.
  ----, on embryological succession, 338.
  Pierce, Mr., on varieties of wolves, 91.
  Pigeons with feathered feet and skin between toes, 12.
  ----, breeds described, and origin of, 20.
  ----, breeds of, how produced, 39, 42.
  ----, tumbler, not being able to get out of egg, 87.
  ----, reverting to blue colour, 160.
  ----, instinct of tumbling, 214.
  ----, carriers, killed by hawks, 362.
  ----, young of, 445.
  Pistil, rudimentary, 451.
  Plants, poisonous, not affecting certain coloured animals, 12.
  ----, selection applied to, 32.
  ----, gradual improvement of, 37.
  ---- not improved in barbarous countries, 38.
  ---- destroyed by insects, 67.
  ----, in midst of range, have to struggle with other plants, 77.
  ----, nectar of, 92.
  ----, fleshy, on sea-shores, 132.
  ----, fresh-water, distribution of, 386.
  ----, low in scale, widely distributed, 406.
  Plumage, laws of change in sexes of birds, 89.
  Plums in the United States, 85.
  Pointer dog, origin of, 35.
  ----, habits of, 213.
  Poison not affecting certain coloured animals, 12.
  ----, similar effect of, on animals and plants, 484.
  Pollen of fir-trees, 203.
  Poole, Col., on striped hemionus, 163.
  Potamogeton, 387.
  Prestwich, Mr., on English and French eocene formations, 328.
  Primrose, 49.
  ----, sterility of, 247.
  Primula, varieties of, 49.
  Proteolepas, 148.
  Proteus, 139.
  Psychology, future progress of, 489.

          Q.

  Quagga, striped, 165.
  Quince, grafts of, 262.

          R.

  Rabbit, disposition of young, 215.
  Races, domestic, characters of, 16.
  Race-horses, Arab, 35.
  ----, English, 356.
  Ramond on plants of Pyrenees, 368.
  Ramsay, Prof., on thickness of the British formations, 284.
  ----, on faults, 285.
  Ratio of increase, 63.
  Rats, supplanting each other, 76.
  ----, acclimatisation of, 141.
  ----, blind in cave, 137.
  Rattle-snake, 201.
  Reason and instinct, 208.
  Recapitulation, general, 459.
  Reciprocity of crosses, 258.
  Record, geological, imperfect, 279.
  Rengger on flies destroying cattle, 72.
  Reproduction, rate of, 63.
  Resemblance to parents in mongrels and hybrids, 273.
  Reversion, law of inheritance, 14.
  ---- in pigeons to blue colour, 160.
  Rhododendron, sterility of, 251.
  Richard, Prof., on Aspicarpa, 417.
  Richardson, Sir J., on structure of squirrels, 180.
  ----, on fishes of the southern hemisphere, 376.
  Robinia, grafts of, 262.
  {500}
  Rodents, blind, 137.
  Rudimentary organs, 450.
  Rudiments important for classification, 416.

          S.

  Sagaret on grafts, 262.
  Salmons, males fighting, and hooked jaws of, 88.
  Salt-water, how far injurious to seeds, 358.
  Saurophagus sulphuratus, 183.
  Schiödte on blind insects, 138.
  Schlegel on snakes, 144.
  Sea-water, how far injurious to seeds, 358.
  Sebright, Sir J., on crossed animals, 20.
  ----, on selection of pigeons, 31.
  Sedgwick, Prof., on groups of species suddenly appearing, 302.
  Seedlings destroyed by insects, 67.
  Seeds, nutriment in, 77.
  ----, winged, 146.
  ----, power of resisting salt-water, 358.
  ---- in crops and intestines of birds, 361.
  ---- eaten by fish, 362, 387.
  ---- in mud, 386.
  ----, hooked, on islands, 392.
  Selection of domestic products, 29.
  ----, principle not of recent origin, 33.
  ----, unconscious, 34.
  ----, natural, 80.
  ----, sexual, 87.
  ----, natural, circumstances favourable to, 102.
  Sexes, relations of, 87.
  Sexual characters variable, 156.
  ---- selection, 87.
  Sheep, Merino, their selection, 31.
  ----, two sub-breeds unintentionally produced, 36.
  ----, mountain, varieties of, 76.
  Shells, colours of, 132.
  ----, littoral, seldom embedded, 288.
  ----, fresh-water, dispersal of, 385
  ---- of Madeira, 391.
  ----, land, distribution of, 397.
  Silene, fertility of crosses, 257.
  Silliman, Prof., on blind rat, 137.
  Skulls of young mammals, 197, 436.
  Slave-making instinct, 219.
  Smith, Col. Hamilton, on striped horses, 164.
  ----, Mr. Fred., on slave-making ants, 219.
  ----, on neuter ants, 239.
  ----, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.
  Snap-dragon, 161.
  Somerville, Lord, on selection of sheep, 31.
  Sorbus, grafts of, 262.
  Spaniel, King Charles's breed, 35.
  Species, polymorphic, 46.
  ----, common, variable, 53.
  ---- in large genera variable, 54.
  ----, groups of, suddenly appearing, 302, 307.
  ---- beneath Silurian formations, 307.
  ---- successively appearing, 312.
  ---- changing simultaneously throughout the world, 322.
  Spencer, Lord, on increase in size of cattle, 35.
  Sphex, parasitic, 218.
  Spiders, development of, 442.
  Spitz-dog crossed with fox, 268.
  Sports in plants, 9.
  Sprengel, C. C, on crossing, 98.
  ----, on ray-florets, 145.
  Squirrels, gradations in structure, 180.
  Staffordshire, heath, changes in, 71.
  Stag-beetles, fighting, 88.
  Sterility from changed conditions of life, 9.
  ---- of hybrids, 246.
  ---- ----, laws of, 255.
  ---- ----, causes of, 263.
  ---- from unfavourable conditions, 265.
  ---- of certain varieties, 269.
  St. Helena, productions of, 390.
  St. Hilaire, Aug., on classification, 418.
  St. John, Mr., on habits of cats, 91.
  Sting of bee, 202.
  Stocks, aboriginal, of domestic animals, 18.
  Strata, thickness of, in Britain, 284.
  Stripes on horses, 163.
  {501}
  Structure, degrees of utility of, 201.
  Struggle for existence, 60.
  Succession, geological, 312.
  Succession of types in same areas, 338.
  Swallow, one species supplanting another, 76.
  Swim-bladder, 190.
  System, natural, 413.

          T.

  Tail of giraffe, 195.
  ---- of aquatic animals, 196.
  ----, rudimentary, 454.
  Tarsi deficient, 135.
  Tausch on umbelliferous flowers, 146.
  Teeth and hair correlated, 144.
  ----, embryonic, traces of, in birds, 450.
  ----, rudimentary, in embryonic calf, 450, 480.
  Tegetmeier, Mr., on cells of bees, 228, 233.
  Temminck on distribution aiding classification, 419.
  Thouin on grafts, 262.
  Thrush, aquatic species of, 185.
  ----, mocking, of the Galapagos, 402.
  ----, young of, spotted, 439.
  ----, nest of, 243.
  Thuret, M., on crossed fuci, 258.
  Thwaites, Mr., on acclimatisation, 140.
  Tierra del Fuego, dogs of, 215.
  ----, plants of, 374, 378.
  Timber-drift, 360.
  Time, lapse of, 282.
  Titmouse, 184.
  Toads on islands, 393.
  Tobacco, crossed varieties of, 271.
  Tomes, Mr., on the distribution of bats, 395.
  Transitions in varieties rare, 172.
  Trees on islands belong to peculiar orders, 392.
  ---- with separated sexes, 99.
  Trifolium pratense, 73, 94.
  ---- incarnatum, 94.
  Trigonia, 321.
  Trilobites, 307.
  ----, sudden extinction of, 321.
  Troglodytes, 243.
  Tucutucu, blind, 137.
  Tumbler pigeons, habits of, hereditary, 214.
  ----, young of, 446.
  Turkey-cock, brush of hair on breast, 90.
  Turkey, naked skin on head, 197.
  ----, young, wild, 216.
  Turnip and cabbage, analogous variations of, 159.
  Type, unity of, 206.
  Types, succession of, in same areas, 339.

          U.

  Udders enlarged by use, 11.
  ----, rudimentary, 451.
  Ulex, young leaves of, 439.
  Umbelliferæ, outer and inner florets of, 144.
  Unity of type, 206.
  Use, effects of, under domestication, 11.
  ----, effects of, in a state of nature, 134.
  Utility, how far important in the construction of each part, 199.

          V.

  Valenciennes on fresh-water fish, 384.
  Variability of mongrels and hybrids, 274.
  Variation under domestication, 7.
  ---- caused by reproductive system being affected by conditions of life,
      8.
  ---- under nature, 44.
  ----, laws of, 131.
  Variations appear at corresponding ages, 14, 86.
  ----, analogous in distinct species, 159.
  Varieties, natural, 44.
  ----, struggle between, 75.
  ----, domestic, extinction of, 111.
  ----, transitional, rarity of, 172.
  ----, when crossed, fertile, 268.
  ----, when crossed, sterile, 269.
  ----, classification of, 423.
  Verbascum, sterility of, 251.
  ----, varieties of, crossed, 271.
  Verneuil, M. de, on the succession of species, 325.
  Viola tricolor, 73.
  {502}
  Volcanic islands, denudation of, 285.
  Vulture, naked skin on head, 197.

          W.

  Wading-birds, 386.
  Wallace, Mr., on origin of species, 2.
  ----, on law of geographical distribution, 355.
  ----, on the Malay Archipelago, 395.
  Wasp, sting of, 202.
  Water, fresh, productions of, 383.
  Water-hen, 185.
  Waterhouse, Mr., on Australian marsupials, 116.
  ----, on greatly developed parts being variable, 150.
  ----, on the cells of bees, 225.
  ----, on general affinities, 429.
  Water-ouzel, 185.
  Watson, Mr. H. C, on range of varieties of British plants, 58.
  ----, on acclimatisation, 140.
  ----, on flora of Azores, 363.
  ----, on Alpine plants, 368, 376.
  ----, on rarity of intermediate varieties, 176.
  Weald, denudation of, 285.
  Web of feet in water-birds, 185.
  West Indian islands, mammals of, 396.
  Westwood on species in large genera being closely allied to others, 57.
  ---- on the tarsi of Engidæ, 157.
  ---- on the antennæ of hymenopterous insects, 415.
  Wheat, varieties of, 113.
  White Mountains, flora of, 365.
  Wings, reduction of size, 134.
  ---- of insects homologous with branchiæ, 191.
  ----, rudimentary, in insects, 450.
  Wolf crossed with dog, 214.
  ---- of Falkland Isles, 394.
  Wollaston, Mr., on varieties of insects, 48.
  ----, on fossil varieties of land-shells in Madeira, 52.
  ----, on colours of insects on sea-shore, 132.
  ----, on wingless beetles, 135.
  ----, on rarity of intermediate varieties, 176.
  ----, on insular insects, 389.
  ----, on land-shells of Madeira, naturalised, 402.
  Wolves, varieties of, 90.
  Woodpecker, habits of, 184.
  ----, green colour of, 197.
  Woodward, Mr., on the duration of specific forms, 294.
  ----, on the continuous succession of genera, 316.
  ----, on the succession of types, 339.
  World, species changing simultaneously throughout, 322.
  Wrens, nest of, 243.

          Y.

  Youatt, Mr., on selection, 31.
  ----, on sub-breeds of sheep, 36.
  ----, on rudimentary horns in young cattle, 454.

          Z.

  Zebra, stripes on, 163.

THE END.

       *       *       *       *       *

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING
CROSS.

       *       *       *       *       *


Corrections made to printed original.

p. 133. "the slightest use to a being": 'slighest' in original.

p. 193. "as Matteucci asserts": 'Matteucei' in original (the index
correctly has Matteucci).

p. 201. "deposited in the living bodies of other insects": 'depo-sisted'
(across page break) in original.

p. 315. "the newly-formed fantail": 'faintail' in original.

p. 398. "the volcanic nature of the soil": 'volanic' in original.

p. 403. "Madeira and the adjoining islet": 'Maderia' in original; and so in
"from Porto Santo to Madeira".

p. 442. "the same individual embryo": 'indivividual' in original.

p. 458. "innumerable species, genera, and families": 'inumerable' in
original.

p. 490. "Inheritance which is almost implied by reproduction":
'Inheritrnce' in original.





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