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Title: Darwinism (1889)

Author: Alfred Russel Wallace

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    DARWINISM

    AN EXPOSITION OF THE

    THEORY OF NATURAL SELECTION

    WITH SOME OF ITS APPLICATIONS

    BY

    ALFRED RUSSEL WALLACE

    LL.D., F.L.S., ETC.


    WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS

    MACMILLAN AND CO.
    LONDON AND NEW YORK
    [Second Edition] 1889


       *       *       *       *       *

[Illustration: Alfred R. Wallace]

       *       *       *       *       *






PREFACE TO SECOND EDITION


The present edition is a reprint of the first, with a few verbal
corrections and the alteration of some erroneous or doubtful statements.
Of these latter the following are the most important:--

_P._ 30. The statement as to the fulmar petrel, which Professor A.
Newton assures me is erroneous, has been modified.

_P._ 34. A note is added as to Darwin's statement about the missel and
song-thrushes in Scotland.

_P._ 172. An error as to the differently-coloured herds of cattle in the
Falkland Islands, is corrected.


    PARKSTONE, DORSET
    _August, 1889_.







PREFACE TO FIRST EDITION


The present work treats the problem of the Origin of Species on the same
general lines as were adopted by Darwin; but from the standpoint reached
after nearly thirty years of discussion, with an abundance of new facts
and the advocacy of many new or old theories.

While not attempting to deal, even in outline, with the vast subject of
evolution in general, an endeavour has been made to give such an account
of the theory of Natural Selection as may enable any intelligent reader
to obtain a clear conception of Darwin's work, and to understand
something of the power and range of his great principle.

Darwin wrote for a generation which had not accepted evolution, and
which poured contempt on those who upheld the derivation of species from
species by any natural law of descent. He did his work so well that
"descent with modification" is now universally accepted as the order of
nature in the organic world; and the rising generation of naturalists
can hardly realise the novelty of this idea, or that their fathers
considered it a scientific heresy to be condemned rather than seriously
discussed.

The objections now made to Darwin's theory apply, solely, to the
particular means by which the change of species has been brought about,
not to the fact of that change. The objectors seek to minimise the
agency of natural selection and to subordinate it to laws of variation,
of use and disuse, of intelligence, and of heredity. These views and
objections are urged with much force and more confidence, and for the
most part by the modern school of laboratory naturalists, to whom the
peculiarities and distinctions of species, as such, their distribution
and their affinities, have little interest as compared with the problems
of histology and embryology, of physiology and morphology. Their work in
these departments is of the greatest interest and of the highest
importance, but it is not the kind of work which, by itself, enables one
to form a sound judgment on the questions involved in the action of the
law of natural selection. These rest mainly on the external and vital
relations of species to species in a state of nature--on what has been
well termed by Semper the "physiology of organisms," rather than on the
anatomy or physiology of organs.

       *       *       *       *       *

It has always been considered a weakness in Darwin's work that he based
his theory, primarily, on the evidence of variation in domesticated
animals and cultivated plants. I have endeavoured to secure a firm
foundation for the theory in the variations of organisms in a state of
nature; and as the exact amount and precise character of these
variations is of paramount importance in the numerous problems that
arise when we apply the theory to explain the facts of nature, I have
endeavoured, by means of a series of diagrams, to exhibit to the eye the
actual variations as they are found to exist in a sufficient number of
species. By doing this, not only does the reader obtain a better and
more precise idea of variation than can be given by any number of
tabular statements or cases of extreme individual variation, but we
obtain a basis of fact by which to test the statements and objections
usually put forth on the subject of specific variability; and it will be
found that, throughout the work, I have frequently to appeal to these
diagrams and the facts they illustrate, just as Darwin was accustomed to
appeal to the facts of variation among dogs and pigeons.

I have also made what appears to me an important change in the
arrangement of the subject. Instead of treating first the comparatively
difficult and unfamiliar details of variation, I commence with the
Struggle for Existence, which is really the fundamental phenomenon on
which natural selection depends, while the particular facts which
illustrate it are comparatively familiar and very interesting. It has
the further advantage that, after discussing variation and the effects
of artificial selection, we proceed at once to explain how natural
selection acts.

Among the subjects of novelty or interest discussed in this volume, and
which have important bearings on the theory of natural selection, are:
(1) A proof that all _specific_ characters are (or once have been)
either useful in themselves or correlated with useful characters (Chap.
VI); (2) a proof that natural selection can, in certain cases, increase
the sterility of crosses (Chap. VII); (3) a fuller discussion of the
colour relations of animals, with additional facts and arguments on the
origin of sexual differences of colour (Chaps. VIII-X); (4) an attempted
solution of the difficulty presented by the occurrence of both very
simple and very complex modes of securing the cross-fertilisation of
plants (Chap. XI); (5) some fresh facts and arguments on the
wind-carriage of seeds, and its bearing on the wide dispersal of many
arctic and alpine plants (Chap. XII); (6) some new illustrations of the
non-heredity of acquired characters, and a proof that the effects of use
and disuse, even if inherited, must be overpowered by natural selection
(Chap. XIV); and (7) a new argument as to the nature and origin of the
moral and intellectual faculties of man (Chap. XV).

       *       *       *       *       *

Although I maintain, and even enforce, my differences from some of
Darwin's views, my whole work tends forcibly to illustrate the
overwhelming importance of Natural Selection over all other agencies in
the production of new species. I thus take up Darwin's earlier
position, from which he somewhat receded in the later editions of his
works, on account of criticisms and objections which I have endeavoured
to show are unsound. Even in rejecting that phase of sexual selection
depending on female choice, I insist on the greater efficacy of natural
selection. This is pre-eminently the Darwinian doctrine, and I therefore
claim for my book the position of being the advocate of pure Darwinism.

I wish to express my obligation to Mr. Francis Darwin for lending me
some of his father's unused notes, and to many other friends for facts
or information, which have, I believe, been acknowledged either in the
text or footnotes. Mr. James Sime has kindly read over the proofs and
given me many useful suggestions; and I have to thank Professor Meldola,
Mr. Hemsley, and Mr. E.B. Poulton for valuable notes or corrections in
the later chapters in which their special subjects are touched upon.

GODALMING, _March 1889_.




CONTENTS


CHAPTER I

WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"


    Definition of species--Special creation--The early
    transmutationists--Scientific opinion before Darwin--The problem
    before Darwin--The change of opinion effected by Darwin--The
    Darwinian theory--Proposed mode of treatment of the subject



CHAPTER II

THE STRUGGLE FOR EXISTENCE


    Its importance--The struggle among plants--Among
    animals--Illustrative cases--Succession of trees in forests of
    Denmark--The struggle for existence on the Pampas--Increase of
    organisms in a geometrical ratio--Examples of rapid increase of
    animals--Rapid increase and wide spread of plants--Great
    fertility not essential to rapid increase--Struggle between
    closely allied species most severe--The ethical aspect of the
    struggle for existence



CHAPTER III

THE VARIABILITY OF SPECIES IN A STATE OF NATURE


   Importance of variability--Popular ideas regarding
   it--Variability of the lower animals--The variability of
   insects--Variation among lizards--Variation among
   birds--Diagrams of bird-variation--Number of varying
   individuals--Variation in the mammalia--Variation in internal
   organs--Variations in the skull--Variations in the habits of
   animals--The variability of plants--Species which vary
   little--Concluding remarks



CHAPTER IV

VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS


    The facts of variation and artificial selection--Proofs of the
    generality of variation--Variations of apples and
    melons--Variations of flowers--Variations of domestic
    animals--Domestic pigeons--Acclimatisation--Circumstances
    favourable to selection by man--Conditions favourable to
    variation--Concluding remarks



CHAPTER V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST


    Effect of struggle for existence under unchanged conditions--The
    effect under change of conditions--Divergence of character--In
    insects--In birds--In mammalia--Divergence leads to a maximum of
    life in each area--Closely allied species inhabit distinct
    areas--Adaptation to conditions at various periods of life--The
    continued existence of low forms of life--Extinction of low
    types among the higher animals--Circumstances favourable to the
    origin of new species--Probable origin of the dippers--The
    importance of isolation--On the advance of organisation by
    natural selection--Summary of the first five chapters



CHAPTER VI

DIFFICULTIES AND OBJECTIONS


    Difficulty as to smallness of variations--As to the right
    variations occurring when required--The beginnings of important
    organs--The mammary glands--The eyes of flatfish--Origin of the
    eye--Useless or non-adaptive characters--Recent extension of the
    region of utility in plants--The same in animals--Uses of
    tails--Of the horns of deer--Of the scale-ornamentation of
    reptiles--Instability of non-adaptive characters--Delboeuf's
    law--No "specific" character proved to be useless--The swamping
    effects of intercrossing--Isolation as preventing
    intercrossing--Gulick on the effects of isolation--Cases in
    which isolation is ineffective



CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING


    Statement of the problem--Extreme susceptibility of the
    reproductive functions--Reciprocal crosses--Individual
    differences in respect to cross-fertilisation--Dimorphism and
    trimorphism among plants--Cases of the fertility of hybrids and
    of the infertility of mongrels--The effects of close
    interbreeding--Mr. Huth's objections--Fertile hybrids among
    animals--Fertility of hybrids among plants--Cases of sterility
    of mongrels--Parallelism between crossing and change of
    conditions--Remarks on the facts of hybridity--Sterility due to
    changed conditions and usually correlated with other
    characters--Correlation of colour with constitutional
    peculiarities--The isolation of varieties by selective
    association--The influence of natural selection upon sterility
    and fertility--Physiological selection--Summary and concluding
    remarks



CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS


    The Darwinian theory threw new light on organic colour--The
    problem to be solved--The constancy of animal colour indicates
    utility--Colour and environment--Arctic animals
    white--Exceptions prove the rule--Desert, forest, nocturnal, and
    oceanic animals--General theories of animal colour--Variable
    protective colouring--Mr. Poulton's experiments--Special or
    local colour adaptations--Imitation of particular objects--How
    they have been produced--Special protective colouring of
    butterflies--Protective resemblance among marine
    animals--Protection by terrifying enemies--Alluring
    coloration--The coloration of birds' eggs--Colour as a means of
    recognition--Summary of the preceding exposition--Influence of
    locality or of climate on colour--Concluding remarks



CHAPTER IX

WARNING COLORATION AND MIMICRY


    The skunk as an example of warning coloration--Warning colours
    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
    has been produced--Heliconidae--Perfection of the
    imitation--Other cases of mimicry among Lepidoptera--Mimicry
    among protected groups--Its explanation--Extension of the
    principle--Mimicry in other orders of insects--Mimicry among the
    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
    birds--Objections to the theory of mimicry--Concluding remarks
    on warning colours and mimicry



CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX


    Sex colours in the mollusca and crustacea--In insects--In
    butterflies and moths--Probable causes of these colours--Sexual
    selection as a supposed cause--Sexual coloration of birds--Cause
    of dull colours of female birds--Relation of sex colour to
    nesting habits--Sexual colours of other vertebrates--Sexual
    selection by the struggles of males--Sexual characters due to
    natural selection--Decorative plumage of males and its effect on
    the females--Display of decorative plumage by the males--A
    theory of animal coloration--The origin of accessory
    plumes--Development of accessory plumes and their display--The
    effect of female preference will be neutralised by natural
    selection--General laws of animal coloration--Concluding remarks



CHAPTER XI

THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE


    The general colour relations of plants--Colours of fruits--The
    meaning of nuts--Edible or attractive fruits--The colours of
    flowers--Modes of securing cross-fertilisation--The
    interpretation of the facts--Summary of additional facts
    bearing on insect fertilisation--Fertilisation of flowers by
    birds--Self-fertilisation of flowers--Difficulties and
    contradictions--Intercrossing not necessarily
    advantageous--Supposed evil results of close interbreeding--How
    the struggle for existence acts among flowers--Flowers the
    product of insect agency--Concluding remarks on colour in nature



CHAPTER XII

THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS


    The facts to be explained--The conditions which have determined
    distribution--The permanence of oceans--Oceanic and continental
    areas--Madagascar and New Zealand--The teachings of the
    thousand-fathom line--The distribution of marsupials--The
    distribution of tapirs--Powers of dispersal as illustrated by
    insular organisms--Birds and insects at sea--Insects at great
    altitudes--The dispersal of plants--Dispersal of seeds by the
    wind--Mineral matter carried by the wind--Objections to the
    theory of wind-dispersal answered--Explanation of north
    temperate plants in the southern hemisphere--No proof of
    glaciation in the tropics--Lower temperature not needed to
    explain the facts--Concluding remarks



CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION


    What we may expect--The number of known species of extinct
    animals--Causes of the imperfection of the geological
    record--Geological evidences of
    evolution--Shells--Crocodiles--The rhinoceros tribe--The
    pedigree of the horse tribe--Development of deer's horns--Brain
    development--Local relations of fossil and living animals--Cause
    of extinction of large animals--Indications of general progress
    in plants and animals--The progressive development of
    plants--Possible cause of sudden late appearance of
    exogens--Geological distribution of insects--Geological
    succession of vertebrata--Concluding remarks



CHAPTER XIV

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY


    Fundamental difficulties and objections--Mr. Herbert Spencer's
    factors of organic evolution--Disuse and effects of withdrawal
    of natural selection--Supposed effects of disuse among wild
    animals--Difficulty as to co-adaptation of parts by variation
    and selection--Direct action of the environment--The American
    school of evolutionists--Origin of the feet of the
    ungulates--Supposed action of animal intelligence--Semper on the
    direct influence of the environment--Professor Geddes's theory
    of variation in plants--Objections to the theory--On the origin
    of spines--Variation and selection overpower the effects of use
    and disuse--Supposed action of the environment in imitating
    variations--Weismann's theory of heredity--The cause of
    variation--The non-heredity of acquired characters--The theory
    of instinct--Concluding remarks



CHAPTER XV

DARWINISM APPLIED TO MAN


    General identity of human and animal structure--Rudiments and
    variations showing relation of man to other mammals--The
    embryonic development of man and other mammalia--Diseases common
    to man and the lower animals--The animals most nearly allied to
    man--The brains of man and apes--External differences of man and
    apes--Summary of the animal characteristics of man--The
    geological antiquity of man--The probable birthplace of man--The
    origin of the moral and intellectual nature of man--The argument
    from continuity--The origin of the mathematical faculty--The
    origin of the musical and artistic faculties--Independent proof
    that these faculties have not been developed by natural
    selection--The interpretation of the facts--Concluding remarks





LIST OF ILLUSTRATIONS



    PORTRAIT OF AUTHOR
    MAP SHOWING THE 1000-FATHOM LINE
    1.  DIAGRAM OF VARIATIONS OF LACERTA MURALIS
    2.     "       VARIATION OF LIZARDS
    3.     "       VARIATION OF WINGS AND TAIL OF BIRDS
    4.     "       VARIATION OF DOLICHONYX ORYZIVORUS
    5.     "       VARIATION OF AGELAEUS PHOENICEUS
    6.     "       VARIATION OF CARDINALIS VIRGINIANUS
    7.     "       VARIATION OF TARSUS AND TOES
    8.     "       VARIATION OF BIRDS IN LEYDEN MUSEUM
    9.     "       VARIATION OF ICTERUS BALTIMORE
    10.    "       VARIATION OF AGELAEUS PHOENICEUS
    11.    "       CURVES OF VARIATION
    12.    "       VARIATION OF CARDINALIS VIRGINIANUS
    13.    "       VARIATION OF SCIURUS CAROLINENSIS
    14.    "       VARIATION OF SKULLS OF WOLF
    15.    "       VARIATION OF SKULLS OF URSUS LABIATUS
    16.    "       VARIATION OF SKULLS OF SUS CRISTATUS
    17. PRIMULA VERIS (Cowslip). From Darwin's _Forms of Flowers_
    18. GAZELLA SOEMMERRINGI (to show recognition marks)
    19. RECOGNITION MARKS OF AFRICAN PLOVERS
          (from Seebohm's _Charadriadae_)
    20. RECOGNITION OF OEDICNEMUS VERMICULATUS AND OE. SENEGALENSIS
          (from Seebohm's _Charadriadae_)
    21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS
          (from Seebohm's _Charadriadae_)
    22. RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA
          (from Seebohm's _Charadriadae_)
    23. METHONA PSIDII AND LEPTALIS ORISE
    24. OPTHALMIS LINCEA AND ARTAXA SIMULANS
          (from the Official _Narrative of the Voyage of the Challenger_)
    25. WINGS OF ITUNA ILIONE AND THYRIDIA MEGISTO
          (from _Proceedings of the Entomological Society_)
    26. MYGNIMIA AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS
    27. MIMICKING INSECTS FROM THE PHILIPPINES
          (from Semper's _Animal Life_)
    28. MALVA SYLVESTRIS AND M. ROTUNDIFOLIA
          (from Lubbock's _British Wild Flowers in Relation to Insects_)
    29. LYTHRUM SALICARIA, THREE FORMS OF
          (from Lubbock's _British Wild Flowers in Relation to Insects_)
    30. ORCHIS PYRAMIDALIS (from Darwin's _Fertilisation of Orchids_)
    31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES
    32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS
    33. GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE
          (from Huxley's _American Addresses_)
    34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF PLANTS
          (from Ward's _Sketch of Palaeobotany_)
    35. TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII
          (from Semper's _Animal Life_)
    36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA
          (from Semper's _Animal Life_)
    37. CHIMPANZEE (TROGLODYTES NIGER)





CHAPTER I

WHAT ARE "SPECIES," AND WHAT IS MEANT BY THEIR "ORIGIN"


    Definition of species--Special creation--The early
    Transmutationists--Scientific opinion before Darwin--The problem
    before Darwin--The change of opinion effected by Darwin--The
    Darwinian theory--Proposed mode of treatment of the subject.



The title of Mr. Darwin's great work is--_On the Origin of Species by
means of Natural Selection and the Preservation of Favoured Races in the
Struggle for Life_. In order to appreciate fully the aim and object of
this work, and the change which it has effected not only in natural
history but in many other sciences, it is necessary to form a clear
conception of the meaning of the term "species," to know what was the
general belief regarding them at the time when Mr. Darwin's book first
appeared, and to understand what he meant, and what was generally meant,
by discovering their "origin." It is for want of this preliminary
knowledge that the majority of educated persons who are not naturalists
are so ready to accept the innumerable objections, criticisms, and
difficulties of its opponents as proofs that the Darwinian theory is
unsound, while it also renders them unable to appreciate, or even to
comprehend, the vast change which that theory has effected in the whole
mass of thought and opinion on the great question of evolution.

The term "species" was thus defined by the celebrated botanist De
Candolle: "A species is a collection of all the individuals which
resemble each other more than they resemble anything else, which can by
mutual fecundation produce fertile individuals, and which reproduce
themselves by generation, in such a manner that we may from analogy
suppose them all to have sprung from one single individual." And the
zoologist Swainson gives a somewhat similar definition: "A species, in
the usual acceptation of the term, is an animal which, in a state of
nature, is distinguished by certain peculiarities of form, size, colour,
or other circumstances, from another animal. It propagates, 'after its
kind,' individuals perfectly resembling the parent; its peculiarities,
therefore, are permanent."[1]

To illustrate these definitions we will take two common English birds,
the rook (Corvus frugilegus) and the crow (Corvus corone). These are
distinct _species_, because, in the first place, they always differ from
each other in certain slight peculiarities of structure, form, and
habits, and, in the second place, because rooks always produce rooks,
and crows produce crows, and they do not interbreed. It was therefore
concluded that all the rooks in the world had descended from a single
pair of rooks, and the crows in like manner from a single pair of crows,
while it was considered impossible that crows could have descended from
rooks or _vice versā_. The "origin" of the first pair of each kind was a
mystery. Similar remarks may be applied to our two common plants, the
sweet violet (Viola odorata) and the dog violet (Viola canina). These
also produce their like and never produce each other or intermingle, and
they were therefore each supposed to have sprung from a single
individual whose "origin" was unknown. But besides the crow and the rook
there are about thirty other kinds of birds in various parts of the
world, all so much like our species that they receive the common name of
crows; and some of them differ less from each other than does our crow
from our rook. These are all _species_ of the genus Corvus, and were
therefore believed to have been always as distinct as they are now,
neither more nor less, and to have each descended from one pair of
ancestral crows of the same identical species, which themselves had an
unknown "origin." Of violets there are more than a hundred different
kinds in various parts of the world, all differing very slightly from
each other and forming distinct _species_ of the genus Viola. But, as
these also each produce their like and do not intermingle, it was
believed that every one of them had always been as distinct from all the
others as it is now, that all the individuals of each kind had descended
from one ancestor, but that the "origin" of these hundred slightly
differing ancestors was unknown. In the words of Sir John Herschel,
quoted by Mr. Darwin, the origin of such species was "the mystery of
mysteries."


_The Early Transmutationists_.

A few great naturalists, struck by the very slight difference between
many of these species, and the numerous links that exist between the
most different forms of animals and plants, and also observing that a
great many species do vary considerably in their forms, colours, and
habits, conceived the idea that they might be all produced one from the
other. The most eminent of these writers was a great French naturalist,
Lamarck, who published an elaborate work, the _Philosophie Zoologique_,
in which he endeavoured to prove that all animals whatever are descended
from other species of animals. He attributed the change of species
chiefly to the effect of changes in the conditions of life--such as
climate, food, etc.--and especially to the desires and efforts of the
animals themselves to improve their condition, leading to a modification
of form or size in certain parts, owing to the well-known physiological
law that all organs are strengthened by constant use, while they are
weakened or even completely lost by disuse. The arguments of Lamarck did
not, however, satisfy naturalists, and though a few adopted the view
that closely allied species had descended from each other, the general
belief of the educated public was, that each species was a "special
creation" quite independent of all others; while the great body of
naturalists equally held, that the change from one species to another by
any known law or cause was impossible, and that the "origin of species"
was an unsolved and probably insoluble problem. The only other important
work dealing with the question was the celebrated _Vestiges of
Creation_, published anonymously, but now acknowledged to have been
written by the late Robert Chambers. In this work the action of general
laws was traced throughout the universe as a system of growth and
development, and it was argued that the various species of animals and
plants had been produced in orderly succession from each other by the
action of unknown laws of development aided by the action of external
conditions. Although this work had a considerable effect in influencing
public opinion as to the extreme improbability of the doctrine of the
independent "special creation" of each species, it had little effect
upon naturalists, because it made no attempt to grapple with the problem
in detail, or to show in any single case how the allied species of a
genus could have arisen, and have preserved their numerous slight and
apparently purposeless differences from each other. No clue whatever was
afforded to a law which should produce from any one species one or more
slightly differing but yet permanently distinct species, nor was any
reason given why such slight yet constant differences should exist at
all.


_Scientific Opinion before Darwin._

In order to show how little effect these writers had upon the public
mind, I will quote a few passages from the writings of Sir Charles
Lyell, as representing the opinions of the most advanced thinkers in the
period immediately preceding that of Darwin's work. When recapitulating
the facts and arguments in favour of the invariability and permanence of
species, he says: "The entire variation from the original type which any
given kind of change can produce may usually be effected in a brief
period of time, after which no further deviation can be obtained by
continuing to alter the circumstances, though ever so gradually,
indefinite divergence either in the way of improvement or deterioration
being prevented, and the least possible excess beyond the defined limits
being fatal to the existence of the individual." In another place he
maintains that "varieties of some species may differ more than other
species do from each other without shaking our confidence in the reality
of species." He further adduces certain facts in geology as being, in
his opinion, "fatal to the theory of progressive development," and he
explains the fact that there are so often distinct species in countries
of similar climate and vegetation by "special creations" in each
country; and these conclusions were arrived at after a careful study of
Lamarck's work, a full abstract of which is given in the earlier
editions of the _Principles of Geology_.[2]

Professor Agassiz, one of the greatest naturalists of the last
generation, went even further, and maintained not only that each species
was specially created, but that it was created in the proportions and in
the localities in which we now find it to exist. The following extract
from his very instructive book on Lake Superior explains this view:
"There are in animals peculiar adaptations which are characteristic of
their species, and which cannot be supposed to have arisen from
subordinate influences. Those which live in shoals cannot be supposed to
have been created in single pairs. Those which are made to be the food
of others cannot have been created in the same proportions as those
which live upon them. Those which are everywhere found in innumerable
specimens must have been introduced in numbers capable of maintaining
their normal proportions to those which live isolated and are
comparatively and constantly fewer. For we know that this harmony in the
numerical proportions between animals is one of the great laws of
nature. The circumstance that species occur within definite limits where
no obstacles prevent their wider distribution leads to the further
inference that these limits were assigned to them from the beginning,
and so we should come to the final conclusion that the order which
prevails throughout nature is intentional, that it is regulated by the
limits marked out on the first day of creation, and that it has been
maintained unchanged through ages with no other modifications than those
which the higher intellectual powers of man enable him to impose on some
few animals more closely connected with him."[3]

These opinions of some of the most eminent and influential writers of
the pre-Darwinian age seem to us, now, either altogether obsolete or
positively absurd; but they nevertheless exhibit the mental condition of
even the most advanced section of scientific men on the problem of the
nature and origin of species. They render it clear that,
notwithstanding the vast knowledge and ingenious reasoning of Lamarck,
and the more general exposition of the subject by the author of the
_Vestiges of Creation_, the first step had not been taken towards a
satisfactory explanation of the derivation of any one species from any
other. Such eminent naturalists as Geoffroy Saint Hilaire, Dean Herbert,
Professor Grant, Von Buch, and some others, had expressed their belief
that species arose as simple varieties, and that the species of each
genus were all descended from a common ancestor; but none of them gave a
clue as to the law or the method by which the change had been effected.
This was still "the great mystery." As to the further question--how far
this common descent could be carried; whether distinct families, such as
crows and thrushes, could possibly have descended from each other; or,
whether all birds, including such widely distinct types as wrens,
eagles, ostriches, and ducks, could all be the modified descendants of a
common ancestor; or, still further, whether mammalia, birds, reptiles,
and fishes, could all have had a common origin;--these questions had
hardly come up for discussion at all, for it was felt that, while the
very first step along the road of "transmutation of species" (as it was
then called) had not been made, it was quite useless to speculate as to
how far it might be possible to travel in the same direction, or where
the road would ultimately lead to.


_The Problem before Darwin_.

It is clear, then, that what was understood by the "origin" or the
"transmutation" of species before Darwin's work appeared, was the
comparatively simple question whether the allied species of each genus
had or had not been derived from one another and, remotely, from some
common ancestor, by the ordinary method of reproduction and by means of
laws and conditions still in action and capable of being thoroughly
investigated. If any naturalist had been asked at that day whether,
supposing it to be clearly shown that all the different species of each
genus had been derived from some one ancestral species, and that a full
and complete explanation were to be given of how each minute difference
in form, colour, or structure might have originated, and how the
several peculiarities of habit and of geographical distribution might
have been brought about--whether, if this were done, the "origin of
species" would be discovered, the great mystery solved, he would
undoubtedly have replied in the affirmative. He would probably have
added that he never expected any such marvellous discovery to be made in
his lifetime. But so much as this assuredly Mr. Darwin has done, not
only in the opinion of his disciples and admirers, but by the admissions
of those who doubt the completeness of his explanations. For almost all
their objections and difficulties apply to those larger differences
which separate genera, families, and orders from each other, not to
those which separate one species from the species to which it is most
nearly allied, and from the remaining species of the same genus. They
adduce such difficulties as the first development of the eye, or of the
milk-producing glands of the mammalia; the wonderful instincts of bees
and of ants; the complex arrangements for the fertilisation of orchids,
and numerous other points of structure or habit, as not being
satisfactorily explained. But it is evident that these peculiarities had
their origin at a very remote period of the earth's history, and no
theory, however complete, can do more than afford a probable conjecture
as to how they were produced. Our ignorance of the state of the earth's
surface and of the conditions of life at those remote periods is very
great; thousands of animals and plants must have existed of which we
have no record; while we are usually without any information as to the
habits and general life-history even of those of which we possess some
fragmentary remains; so that the truest and most complete theory would
not enable us to solve _all_ the difficult problems which the whole
course of the development of life upon our globe presents to us.

What we may expect a true theory to do is to enable us to comprehend and
follow out in some detail those changes in the form, structure, and
relations of animals and plants which are effected in short periods of
time, geologically speaking, and which are now going on around us. We
may expect it to explain satisfactorily most of the lesser and
superficial differences which distinguish one species from another. We
may expect it to throw light on the mutual relations of the animals and
plants which live together in any one country, and to give some rational
account of the phenomena presented by their distribution in different
parts of the world. And, lastly, we may expect it to explain many
difficulties and to harmonise many incongruities in the excessively
complex affinities and relations of living things. All this the
Darwinian theory undoubtedly does. It shows us how, by means of some of
the most universal and ever-acting laws in nature, new species are
necessarily produced, while the old species become extinct; and it
enables us to understand how the continuous action of these laws during
the long periods with which geology makes us acquainted is calculated to
bring about those greater differences presented by the distinct genera,
families, and orders into which all living things are classified by
naturalists. The differences which these present are all of the same
_nature_ as those presented by the species of many large genera, but
much greater in _amount_; and they can all be explained by the action of
the same general laws and by the extinction of a larger or smaller
number of intermediate species. Whether the distinctions between the
higher groups termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differences which
separate the mammals, birds, reptiles, and fishes from each other,
though vast, yet seem of the same nature as those which distinguish a
mouse from an elephant or a swallow from a goose. But the vertebrate
animals, the mollusca, and the insects, are so radically distinct in
their whole organisation and in the very plan of their structure, that
objectors may not unreasonably doubt whether they can all have been
derived from a common ancestor by means of the very same laws as have
sufficed for the differentiation of the various species of birds or of
reptiles.


_The Change of Opinion effected by Darwin_.

The point I wish especially to urge is this. Before Darwin's work
appeared, the great majority of naturalists, and almost without
exception the whole literary and scientific world, held firmly to the
belief that _species_ were realities, and had not been derived from
other species by any process accessible to us; the different species of
crow and of violet they are now, and to have originated by some totally
unknown process so far removed from ordinary reproduction that it was
usually spoken of as "special creation." There was, then, no question of
the origin of families, orders, and classes, because the very first step
of all, the "origin of species," was believed to be an insoluble
problem. But now this is all changed. The whole scientific and literary
world, even the whole educated public, accepts, as a matter of common
knowledge, the origin of species from other allied species by the
ordinary process of natural birth. The idea of special creation or any
altogether exceptional mode of production is absolutely extinct! Yet
more: this is held also to apply to many higher groups as well as to the
species of a genus, and not even Mr. Darwin's severest critics venture
to suggest that the primeval bird, reptile, or fish must have been
"specially created." And this vast, this totally unprecedented change in
public opinion has been the result of the work of one man, and was
brought about in the short space of twenty years! This is the answer to
those who continue to maintain that the "origin of species" is not yet
discovered; that there are still doubts and difficulties; that there are
divergencies of structure so great that we cannot understand how they
had their beginning. We may admit all this, just as we may admit that
there are enormous difficulties in the way of a complete comprehension
of the origin and nature of all the parts of the solar system and of the
stellar universe. But we claim for Darwin that he is the Newton of
natural history, and that, just so surely as that the discovery and
demonstration by Newton of the law of gravitation established order in
place of chaos and laid a sure foundation for all future study of the
starry heavens, so surely has Darwin, by his discovery of the law of
natural selection and his demonstration of the great principle of the
preservation of useful variations in the struggle for life, not only
thrown a flood of light on the process of development of the whole
organic world, but also established a firm foundation for all future
study of nature.

In order to show the view Darwin took of his own work, and what it was
that he alone claimed to have done, the concluding passage of the
introduction to the _Origin of_ _Species_ should be carefully
considered. It is as follows: "Although much remains obscure, and will
long remain obscure, I can entertain no doubt, after the most deliberate
and dispassionate judgment of which I am capable, that the view which
most naturalists until recently entertained 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 most important, but not the exclusive, means of
modification."

It should be especially noted that all which is here claimed is now
almost universally admitted, while the criticisms of Darwin's works
refer almost exclusively to those numerous questions which, as he
himself says, "will long remain obscure."


_The Darwinian Theory_.

As it will be necessary, in the following chapters, to set forth a
considerable body of facts in almost every department of natural
history, in order to establish the fundamental propositions on which the
theory of natural selection rests, I propose to give a preliminary
statement of what the theory really is, in order that the reader may
better appreciate the necessity for discussing so many details, and may
thus feel a more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be appreciated
by every one who takes an interest in nature, but unless the need of
them is clearly seen it may be thought that time is being wasted on mere
curious details and strange facts which have little bearing on the
question at issue.

The theory of natural selection rests on two main classes of facts which
apply to all organised beings without exception, and which thus take
rank as fundamental principles or laws. The first is, the power of rapid
multiplication in a geometrical progression; the second, that the
offspring always vary slightly from the parents, though generally very
closely resembling them. From the first fact or law there follows,
necessarily, a constant struggle for existence; because, while the
offspring always exceed the parents in number, generally to an enormous
extent, yet the total number of living organisms in the world does not,
and cannot, increase year by year. Consequently every year, on the
average, as many die as are born, plants as well as animals; and the
majority die premature deaths. They kill each other in a thousand
different ways; they starve each other by some consuming the food that
others want; they are destroyed largely by the powers of nature--by cold
and heat, by rain and storm, by flood and fire. There is thus a
perpetual struggle among them which shall live and which shall die; and
this struggle is tremendously severe, because so few can possibly remain
alive--one in five, one in ten, often only one in a hundred or even one
in a thousand.

Then comes the question, Why do some live rather than others? If all the
individuals of each species were exactly alike in every respect, we
could only say it is a matter of chance. But they are not alike. We find
that they vary in many different ways. Some are stronger, some swifter,
some hardier in constitution, some more cunning. An obscure colour may
render concealment more easy for some, keener sight may enable others to
discover prey or escape from an enemy better than their fellows. Among
plants the smallest differences may be useful or the reverse. The
earliest and strongest shoots may escape the slug; their greater vigour
may enable them to flower and seed earlier in a wet autumn; plants best
armed with spines or hairs may escape being devoured; those whose
flowers are most conspicuous may be soonest fertilised by insects. We
cannot doubt that, on the whole, any beneficial variations will give the
possessors of it a greater probability of living through the tremendous
ordeal they have to undergo. There may be something left to chance, but
on the whole _the fittest will survive_.

Then we have another important fact to consider, the principle of
heredity or transmission of variations. If we grow plants from seed or
breed any kind of animals year after year, consuming or giving away all
the increase we do not wish to keep just as they come to hand, our
plants or animals will continue much the same; but if every year we
carefully save the best seed to sow and the finest or brightest
coloured animals to breed from, we shall soon find that an improvement
will take place, and that the average quality of our stock will be
raised. This is the way in which all our fine garden fruits and
vegetables and flowers have been produced, as well as all our splendid
breeds of domestic animals; and they have thus become in many cases so
different from the wild races from which they originally sprang as to be
hardly recognisable as the same. It is therefore proved that if any
particular kind of variation is preserved and bred from, the variation
itself goes on increasing in amount to an enormous extent; and the
bearing of this on the question of the origin of species is most
important. For if in each generation of a given animal or plant the
fittest survive to continue the breed, then whatever may be the special
peculiarity that causes "fitness" in the particular case, that
peculiarity will go on increasing and strengthening _so long as it is
useful to the species_. But the moment it has reached its maximum of
usefulness, and some other quality or modification would help in the
struggle, then the individuals which vary in the new direction will
survive; and thus a species may be gradually modified, first in one
direction, then in another, till it differs from the original parent
form as much as the greyhound differs from any wild dog or the
cauliflower from any wild plant. But animals or plants which thus differ
in a state of nature are always classed as distinct species, and thus we
see how, by the continuous survival of the fittest or the preservation
of favoured races in the struggle for life, new species may be
originated.

This self-acting process which, by means of a few easily demonstrated
groups of facts, brings about change in the organic world, and keeps
each species in harmony with the conditions of its existence, will
appear to some persons so clear and simple as to need no further
demonstration. But to the great majority of naturalists and men of
science endless difficulties and objections arise, owing to the
wonderful variety of animal and vegetable forms, and the intricate
relations of the different species and groups of species with each
other; and it was to answer as many of these objections as possible, and
to show that the more we know of nature the more we find it to
harmonise with the development hypothesis, that Darwin devoted the whole
of his life to collecting facts and making experiments, the record of a
portion of which he has given us in a series of twelve masterly volumes.


_Proposed Mode of Treatment of the Subject_.

It is evidently of the most vital importance to any theory that its
foundations should be absolutely secure. It is therefore necessary to
show, by a wide and comprehensive array of facts, that animals and
plants _do_ perpetually vary in the manner and to the amount requisite;
and that this takes place in wild animals as well as in those which are
domesticated. It is necessary also to prove that all organisms _do_ tend
to increase at the great rate alleged, and that this increase actually
occurs, under favourable conditions. We have to prove, further, that
variations of all kinds can be increased and accumulated by selection;
and that the struggle for existence to the extent here indicated
actually occurs in nature, and leads to the continued preservation of
favourable variations.

These matters will be discussed in the four succeeding chapters, though
in a somewhat different order--the struggle for existence and the power
of rapid multiplication, which is its cause, occupying the first place,
as comprising those facts which are the most fundamental and those which
can be perfectly explained without any reference to the less generally
understood facts of variation. These chapters will be followed by a
discussion of certain difficulties, and of the vexed question of
hybridity. Then will come a rather full account of the more important of
the complex relations of organisms to each other and to the earth
itself, which are either fully explained or greatly elucidated by the
theory. The concluding chapter will treat of the origin of man and his
relations to the lower animals.

FOOTNOTES:

[Footnote 1: _Geography and Classification of Animals_, p. 350.]

[Footnote 2: These expressions occur in Chapter IX. of the earlier
editions (to the ninth) of the _Principles of Geology_.]

[Footnote 3: L. Agassiz, _Lake Superior_, p. 377.]




CHAPTER II

THE STRUGGLE FOR EXISTENCE


    Its importance--The struggle among plants--Among
    animals--Illustrative cases--Succession of trees in forests of
    Denmark--The struggle for existence on the Pampas--Increase of
    organisms in a geometrical ratio--Examples of great powers of
    increase of animals--Rapid increase and wide spread of
    plants--Great fertility not essential to rapid
    increase--Struggle between closely allied species most
    severe--The ethical aspect of the struggle for existence.



There is perhaps no phenomenon of nature that is at once so important,
so universal; and so little understood, as the struggle for existence
continually going on among all organised beings. To most persons nature
appears calm, orderly, and peaceful. They see the birds singing in the
trees, the insects hovering over the flowers, the squirrel climbing
among the tree-tops, and all living things in the possession of health
and vigour, and in the enjoyment of a sunny existence. But they do not
see, and hardly ever think of, the means by which this beauty and
harmony and enjoyment is brought about. They do not see the constant and
daily search after food, the failure to obtain which means weakness or
death; the constant effort to escape enemies; the ever-recurring
struggle against the forces of nature. This daily and hourly struggle,
this incessant warfare, is nevertheless the very means by which much of
the beauty and harmony and enjoyment in nature is produced, and also
affords one of the most important elements in bringing about the origin
of species. We must, therefore, devote some time to the consideration of
its various aspects and of the many curious phenomena to which it gives
rise.

It is a matter of common observation that if weeds are allowed to grow
unchecked in a garden they will soon destroy a number of the flowers.
It is not so commonly known that if a garden is left to become
altogether wild, the weeds that first take possession of it, often
covering the whole surface of the ground with two or three different
kinds, will themselves be supplanted by others, so that in a few years
many of the original flowers and of the earliest weeds may alike have
disappeared. This is one of the very simplest cases of the struggle for
existence, resulting in the successive displacement of one set of
species by another; but the exact causes of this displacement are by no
means of such a simple nature. All the plants concerned may be perfectly
hardy, all may grow freely from seed, yet when left alone for a number
of years, each set is in turn driven out by a succeeding set, till at
the end of a considerable period--a century or a few centuries
perhaps--hardly one of the plants which first monopolised the ground
would be found there.

Another phenomenon of an analogous kind is presented by the different
behaviour of introduced wild plants or animals into countries apparently
quite as well suited to them as those which they naturally inhabit.
Agassiz, in his work on Lake Superior, states that the roadside weeds of
the northeastern United States, to the number of 130 species, are all
European, the native weeds having disappeared westwards; and in New
Zealand there are no less than 250 species of naturalised European
plants, more than 100 species of which have spread widely over the
country, often displacing the native vegetation. On the other hand, of
the many hundreds of hardy plants which produce seed freely in our
gardens, very few ever run wild, and hardly any have become common. Even
attempts to naturalise suitable plants usually fail; for A. de Candolle
states that several botanists of Paris, Geneva, and especially of
Montpellier, have sown the seeds of many hundreds of species of hardy
exotic plants in what appeared to be the most favourable situations, but
that, in hardly a single case, has any one of them become
naturalised.[4] Even a plant like the potato--so widely cultivated, so
hardy, and so well adapted to spread by means of its many-eyed
tubers--has not established itself in a wild state in any part of
Europe. It would be thought that Australian plants would easily run
wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr.
Bidwell habitually scattered Australian seeds during his extensive
travels in New Zealand, yet only two or three Australian plants appear
to have established themselves in that country, and these only in
cultivated or newly moved soil.

These few illustrations sufficiently show that all the plants of a
country are, as De Candolle says, at war with each other, each one
struggling to occupy ground at the expense of its neighbour. But,
besides this direct competition, there is one not less powerful arising
from the exposure of almost all plants to destruction by animals. The
buds are destroyed by birds, the leaves by caterpillars, the seeds by
weevils; some insects bore into the trunk, others burrow in the twigs
and leaves; slugs devour the young seedlings and the tender shoots,
wire-worms gnaw the roots. Herbivorous mammals devour many species
bodily, while some uproot and devour the buried tubers.

In animals, it is the eggs or the very young that suffer most from their
various enemies; in plants, the tender seedlings when they first appear
above the ground. To illustrate this latter point Mr. Darwin cleared and
dug a piece of ground three feet long and two feet wide, and then marked
all the seedlings of weeds and other plants which came up, noting what
became of them. The total number was 357, and out of these no less than
295 were destroyed by slugs and insects. The direct strife of plant with
plant is almost equally fatal when the stronger are allowed to smother
the weaker. When turf is mown or closely browsed by animals, a number of
strong and weak plants live together, because none are allowed to grow
much beyond the rest; but Mr. Darwin found that when the plants which
compose such turf are allowed to grow up freely, the stronger kill the
weaker. In a plot of turf three feet by four, twenty distinct species of
plants were found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up to their full
size.[5]

But besides having to protect themselves against competing plants and
against destructive animals, there is a yet deadlier enemy in the
forces of inorganic nature. Each species can sustain a certain amount of
heat and cold, each requires a certain amount of moisture at the right
season, each wants a proper amount of light or of direct sunshine, each
needs certain elements in the soil; the failure of a due proportion in
these inorganic conditions causes weakness, and thus leads to speedy
death. The struggle for existence in plants is, therefore, threefold in
character and infinite in complexity, and the result is seen in their
curiously irregular distribution over the face of the earth. Not only
has each country its distinct plants, but every valley, every hillside,
almost every hedgerow, has a different set of plants from its adjacent
valley, hillside, or hedgerow--if not always different in the actual
species yet very different in comparative abundance, some which are rare
in the one being common in the other. Hence it happens that slight
changes of conditions often produce great changes in the flora of a
country. Thus in 1740 and the two following years the larva of a moth
(Phalaena graminis) committed such destruction in many of the meadows of
Sweden that the grass was greatly diminished in quantity, and many
plants which were before choked by the grass sprang up, and the ground
became variegated with a multitude of different species of flowers. The
introduction of goats into the island of St. Helena led to the entire
destruction of the native forests, consisting of about a hundred
distinct species of trees and shrubs, the young plants being devoured by
the goats as fast as they grew up. The camel is a still greater enemy to
woody vegetation than the goat, and Mr. Marsh believes that forests
would soon cover considerable tracts of the Arabian and African deserts
if the goat and the camel were removed from them.[6] Even in many parts
of our own country the existence of trees is dependent on the absence of
cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in
Surrey, a few clumps of old Scotch firs, but no young trees over
hundreds of acres. Some portions of the heath had, however, been
enclosed a few years before, and these enclosures were crowded with
young fir-trees growing too close together for all to live; and these
were not sown or planted, nothing having been done to the ground beyond
enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in the
unenclosed parts, and there he found multitudes of little trees and
seedlings which had been perpetually browsed down by the cattle. In one
square yard, at a point about a hundred yards from one of the old clumps
of firs, he counted thirty-two little trees, and one of them had
twenty-six rings of growth, showing that it had for many years tried to
raise its head above the stems of the heather and had failed. Yet this
heath was very extensive and very barren, and, as Mr. Darwin remarks, no
one would ever have imagined that cattle would have so closely and so
effectually searched it for food.

In the case of animals, the competition and struggle are more obvious.
The vegetation of a given district can only support a certain number of
animals, and the different kinds of plant-eaters will compete together
for it. They will also have insects for their competitors, and these
insects will be kept down by birds, which will thus assist the mammalia.
But there will also be carnivora destroying the herbivora; while small
rodents, like the lemming and some of the field-mice, often destroy so
much vegetation as materially to affect the food of all the other groups
of animals. Droughts, floods, severe winters, storms and hurricanes will
injure these in various degrees, but no one species can be diminished in
numbers without the effect being felt in various complex ways by all the
rest. A few illustrations of this reciprocal action must be given.


_Illustrative Cases of the Struggle for Life_.

Sir Charles Lyell observes that if, by the attacks of seals or other
marine foes, salmon are reduced in numbers, the consequence will be that
otters, living far inland, will be deprived of food and will then
destroy many young birds or quadrupeds, so that the increase of a marine
animal may cause the destruction of many land animals hundreds of miles
away. Mr. Darwin carefully observed the effects produced by planting a
few hundred acres of Scotch fir, in Staffordshire, on part of a very
extensive heath which had never been cultivated. After the planted
portion was about twenty-five years old he observed that the change in
the native vegetation was greater than is often seen in passing from
one quite different soil to another. Besides a great change in the
proportional numbers of the native heath-plants, twelve species which
could not be found on the heath flourished in the plantations. The
effect on the insect life must have been still greater, for six
insectivorous birds which were very common in the plantations were not
to be seen on the heath, which was, however, frequented by two or three
different species of insectivorous birds. It would have required
continued study for several years to determine all the differences in
the organic life of the two areas, but the facts stated by Mr. Darwin
are sufficient to show how great a change may be effected by the
introduction of a single kind of tree and the keeping out of cattle.

The next case I will give in Mr. Darwin's own words: "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 numbers, 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 other parasitic insects. Hence, if
certain insectivorous birds were to decrease in Paraguay, the parasitic
insects would probably increase; and this would lessen the number of the
navel-frequenting flies--then cattle and horses would become feral, and
this would greatly alter (as indeed I have observed in parts of South
America) the vegetation: this again would largely affect the insects,
and this, as we have just seen in Staffordshire, the insectivorous
birds, and so onward in ever-increasing circles of complexity. Not that
under nature the relations will ever be as simple as this. Battle within
battle must be continually recurring with varying success; and yet in
the long run the forces are so nicely balanced, that the face of nature
remains for a long time uniform, though assuredly the merest trifle
would give the victory to one organic being over another."[7]

Such cases as the above may perhaps be thought exceptional, but there
is good reason to believe that they are by no means rare, but are
illustrations of what is going on in every part of the world, only it is
very difficult for us to trace out the complex reactions that are
everywhere occurring. The general impression of the ordinary observer
seems to be that wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and surroundings,
and therefore having no difficulty in maintaining itself. Before showing
that this view is, everywhere and always, demonstrably untrue, we will
consider one other case of the complex relations of distinct organisms
adduced by Mr. Darwin, and often quoted for its striking and almost
eccentric character. It is now well known that many flowers require to
be fertilised by insects in order to produce seed, and this
fertilisation can, in some cases, only be effected by one particular
species of insect to which the flower has become specially adapted. Two
of our common plants, the wild heart's-ease (Viola tricolor) and the red
clover (Trifolium pratense), are thus fertilised by humble-bees almost
exclusively, and if these insects are prevented from visiting the
flowers, they produce either no seed at all or exceedingly few. Now it
is known that field-mice destroy the combs and nests of humble-bees, and
Colonel Newman, who has paid great attention to these insects, believes
that more than two-thirds of all the humble-bees' nests in England are
thus destroyed. But the number of mice depends a good deal on the number
of cats; and the same observer says that near villages and towns he has
found the nests of humble-bees more numerous than elsewhere, which he
attributes to the number of cats that destroy the mice. Hence it
follows, that the abundance of red clover and wild heart's-ease in a
district will depend on a good supply of cats to kill the mice, which
would otherwise destroy and keep down the humble-bees and prevent them
from fertilising the flowers. A chain of connection has thus been found
between such totally distinct organisms as flesh-eating mammalia and
sweet-smelling flowers, the abundance or scarcity of the one closely
corresponding to that of the other!

The following account of the struggle between trees in the forests of
Denmark, from the researches of M. Hansten-Blangsted, strikingly
illustrates our subject.[8] The chief combatants are the beech and the
birch, the former being everywhere successful in its invasions. Forests
composed wholly of birch are now only found in sterile, sandy tracts;
everywhere else the trees are mixed, and wherever the soil is favourable
the beech rapidly drives out the birch. The latter loses its branches at
the touch of the beech, and devotes all its strength to the upper part
where it towers above the beech. It may live long in this way, but it
succumbs ultimately in the fight--of old age if of nothing else, for the
life of the birch in Denmark is shorter than that of the beech. The
writer believes that light (or rather shade) is the cause of the
superiority of the latter, for it has a greater development of its
branches than the birch, which is more open and thus allows the rays of
the sun to pass through to the soil below, while the tufted, bushy top
of the beech preserves a deep shade at its base. Hardly any young plants
can grow under the beech except its own shoots; and while the beech can
nourish under the shade of the birch, the latter dies immediately under
the beech. The birch has only been saved from total extermination by the
facts that it had possession of the Danish forests long before the beech
ever reached the country, and that certain districts are unfavourable to
the growth of the latter. But wherever the soil has been enriched by the
decomposition of the leaves of the birch the battle begins. The birch
still flourishes on the borders of lakes and other marshy places, where
its enemy cannot exist. In the same way, in the forests of Zeeland, the
fir forests are disappearing before the beech. Left to themselves, the
firs are soon displaced by the beech. The struggle between the latter
and the oak is longer and more stubborn, for the branches and foliage of
the oak are thicker, and offer much resistance to the passage of light.
The oak, also, has greater longevity; but, sooner or later, it too
succumbs, because it cannot develop in the shadow of the beech. The
earliest forests of Denmark were mainly composed of aspens, with which
the birch was apparently associated; gradually the soil was raised, and
the climate grew milder; then the fir came and formed large forests.
This tree ruled for centuries, and then ceded the first place to the
holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak,
and beech appear to be the steps in the struggle for the survival of the
fittest among the forest-trees of Denmark.

It may be added that in the time of the Romans the beech was the
principal forest-tree of Denmark as it is now, while in the much earlier
bronze age, represented by the later remains found in the peat bogs,
there were no beech-trees, or very few, the oak being the prevailing
tree, while in the still earlier stone period the fir was the most
abundant. The beech is a tree essentially of the temperate zone, having
its northern limit considerably southward of the oak, fir, birch, or
aspen, and its entrance into Denmark was no doubt due to the
amelioration of the climate after the glacial epoch had entirely passed
away. We thus see how changes of climate, which are continually
occurring owing either to cosmical or geographical causes, may initiate
a struggle among plants which may continue for thousands of years, and
which must profoundly modify the relations of the animal world, since
the very existence of innumerable insects, and even of many birds and
mammals, is dependent more or less completely on certain species of
plants.


_The Struggle for Existence on the Pampas_.

Another illustration of the struggle for existence, in which both plants
and animals are implicated, is afforded by the pampas of the southern
part of South America. The absence of trees from these vast plains has
been imputed by Mr. Darwin to the supposed inability of the tropical and
sub-tropical forms of South America to thrive on them, and there being
no other source from which they could obtain a supply; and that
explanation was adopted by such eminent botanists as Mr. Ball and
Professor Asa Gray. This explanation has always seemed to me
unsatisfactory, because there are ample forests both in the temperate
regions of the Andes and on the whole west coast down to Terra del
Fuego; and it is inconsistent with what we know of the rapid variation
and adaptation of species to new conditions. What seems a more
satisfactory explanation has been given by Mr. Edwin Clark, a civil
engineer, who resided nearly two years in the country and paid much
attention to its natural history. He says: "The peculiar characteristics
of these vast level plains which descend from the Andes to the great
river basin in unbroken monotony, are the absence of rivers or
water-storage, and the periodical occurrence of droughts, or 'siccos,'
in the summer months. These conditions determine the singular character
both of its flora and fauna.

"The soil is naturally fertile and favourable for the growth of trees,
and they grow luxuriantly wherever they are protected. The eucalyptus is
covering large tracts wherever it is enclosed, and willows, poplars, and
the fig surround every estancia when fenced in.

"The open plains are covered with droves of horses and cattle, and
overrun by numberless wild rodents, the original tenants of the pampas.
During the long periods of drought, which are so great a scourge to the
country, these animals are starved by thousands, destroying, in their
efforts to live, every vestige of vegetation. In one of these 'siccos,'
at the time of my visit, no less than 50,000 head of oxen and sheep and
horses perished from starvation and thirst, after tearing deep out of
the soil every trace of vegetation, including the wiry roots of the
pampas-grass. Under such circumstances the existence of an unprotected
tree is impossible. The only plants that hold their own, in addition to
the indestructible thistles, grasses, and clover, are a little
herbaceous oxalis, producing viviparous buds of extraordinary vitality,
a few poisonous species, such as the hemlock, and a few tough, thorny
dwarf-acacias and wiry rushes, which even a starving rat refuses.

"Although the cattle are a modern introduction, the numberless
indigenous rodents must always have effectually prevented the
introduction of any other species of plants; large tracts are still
honeycombed by the ubiquitous biscacho, a gigantic rabbit; and numerous
other rodents still exist, including rats and mice, pampas-hares, and
the great nutria and carpincho (capybara) on the river banks."[9]

Mr. Clark further remarks on the desperate struggle for existence which
characterises the bordering fertile zones, where rivers and marshy
plains permit a more luxuriant and varied vegetable and animal life.
After describing how the river sometimes rose 30 feet in eight hours,
doing immense destruction, and the abundance of the larger carnivora and
large reptiles on its banks, he goes on: "But it was among the flora
that the principle of natural selection was most prominently displayed.
In such a district--overrun with rodents and escaped cattle, subject to
floods that carried away whole islands of botany, and especially to
droughts that dried up the lakes and almost the river itself--no
ordinary plant could live, even on this rich and watered alluvial
debris. The only plants that escaped the cattle were such as were either
poisonous, or thorny, or resinous, or indestructibly tough. Hence we had
only a great development of solanums, talas, acacias, euphorbias, and
laurels. The buttercup is replaced by the little poisonous yellow oxalis
with its viviparous buds; the passion-flowers, asclepiads, bignonias,
convolvuluses, and climbing leguminous plants escape both floods and
cattle by climbing the highest trees and towering overhead in a flood of
bloom. The ground plants are the portulacas, turneras, and cenotheras,
bitter and ephemeral, on the bare rock, and almost independent of any
other moisture than the heavy dews. The pontederias, alismas, and
plantago, with grasses and sedges, derive protection from the deep and
brilliant pools; and though at first sight the 'monte' doubtless
impresses the traveller as a scene of the wildest confusion and ruin,
yet, on closer examination, we found it far more remarkable as a
manifestation of harmony and law, and a striking example of the
marvellous power which plants, like animals, possess, of adapting
themselves to the local peculiarities of their habitat, whether in the
fertile shades of the luxuriant 'monte' or on the arid, parched-up
plains of the treeless pampas."

A curious example of the struggle between plants has been communicated
to me by Mr. John Ennis, a resident in New Zealand. The English
water-cress grows so luxuriantly in that country as to completely choke
up the rivers, sometimes leading to disastrous floods, and necessitating
great outlay to keep the stream open. But a natural remedy has now been
found in planting willows on the banks. The roots of these trees
penetrate the bed of the stream in every direction, and the water-cress,
unable to obtain the requisite amount of nourishment, gradually
disappears.

_Increase of Organisms in a Geometrical Ratio_.

The facts which have now been adduced, sufficiently prove that there is
a continual competition, and struggle, and war going on in nature, and
that each species of animal and plant affects many others in complex and
often unexpected ways. We will now proceed to show the fundamental cause
of this struggle, and to prove that it is ever acting over the whole
field of nature, and that no single species of animal or plant can
possibly escape from it. This results from the fact of the rapid
increase, in a geometrical ratio, of all the species of animals and
plants. In the lower orders this increase is especially rapid, a single
flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so
quickly that they reach their full size in five days; hence the great
Swedish naturalist, Linnaeus, asserted that a dead horse would be
devoured by three of these flies as quickly as by a lion. Each of these
larvae remains in the pupa state about five or six days, so that each
parent fly may be increased ten thousand-fold in a fortnight. Supposing
they went on increasing at this rate during only three months of summer,
there would result one hundred millions of millions of millions for each
fly at the commencement of summer,--a number greater probably than
exists at any one time in the whole world. And this is only one species,
while there are thousands of other species increasing also at an
enormous rate; so that, if they were unchecked, the whole atmosphere
would be dense with flies, and all animal food and much of animal life
would be destroyed by them. To prevent this tremendous increase there
must be incessant war against these insects, by insectivorous birds and
reptiles as well as by other insects, in the larva as well as in the
perfect state, by the action of the elements in the form of rain, hail,
or drought, and by other unknown causes; yet we see nothing of this
ever-present war, though by its means alone, perhaps, we are saved from
famine and pestilence.

Let us now consider a less extreme and more familiar case. We possess a
considerable number of birds which, like the redbreast, sparrow, the
four common titmice, the thrush, and the blackbird, stay with us all the
year round These lay on an average six eggs, but, as several of them
have two or more broods a year, ten will be below the average of the
year's increase. Such birds as these often live from fifteen to twenty
years in confinement, and we cannot suppose them to live shorter lives
in a state of nature, if unmolested; but to avoid possible exaggeration
we will take only ten years as the average duration of their lives. Now,
if we start with a single pair, and these are allowed to live and breed,
unmolested, till they die at the end of ten years,--as they might do if
turned loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds--their
numbers would amount to more than twenty millions. But we know very well
that our bird population is no greater, on the average, now than it was
ten years ago. Year by year it may fluctuate a little according as the
winters are more or less severe, or from other causes, but on the whole
there is no increase. What, then, becomes of the enormous surplus
population annually produced? It is evident they must all die or be
killed, somehow; and as the increase is, on the average, about five to
one, it follows that, if the average number of birds of all kinds in our
islands is taken at ten millions--and this is probably far under the
mark--then about fifty millions of birds, including eggs as possible
birds, must annually die or be destroyed. Yet we see nothing, or almost
nothing, of this tremendous slaughter of the innocents going on all
around us. In severe winters a few birds are found dead, and a few
feathers or mangled remains show us where a wood-pigeon or some other
bird has been destroyed by a hawk, but no one would imagine that five
times as many birds as the total number in the country in early spring
die every year. No doubt a considerable proportion of these do not die
here but during or after migration to other countries, but others which
are bred in distant countries come here, and thus balance the account.
Again, as the average number of young produced is four or five times
that of the parents, we ought to have at least five times as many birds
in the country at the end of summer as at the beginning, and there is
certainly no such enormous disproportion as this. The fact is, that the
destruction commences, and is probably most severe, with nestling birds,
which are often killed by heavy rains or blown away by severe storms, or
left to die of hunger if either of the parents is killed; while they
offer a defenceless prey to jackdaws, jays, and magpies, and not a few
are ejected from their nests by their foster-brothers the cuckoos. As
soon as they are fledged and begin to leave the nest great numbers are
destroyed by buzzards, sparrow-hawks, and shrikes. Of those which
migrate in autumn a considerable proportion are probably lost at sea or
otherwise destroyed before they reach a place of safety; while those
which remain with us are greatly thinned by cold and starvation during
severe winters. Exactly the same thing goes on with every species of
wild animal and plant from the lowest to the highest. All breed at such
a rate, that in a few years the progeny of any one species would, if
allowed to increase unchecked, alone monopolise the land; but all alike
are kept within bounds by various destructive agencies, so that, though
the numbers of each may fluctuate, they can never permanently increase
except at the expense of some others, which must proportionately
decrease.


_Cases showing the Great Powers of Increase of Animals._

As the facts now stated are the very foundation of the theory we are
considering, and the enormous increase and perpetual destruction
continually going on require to be kept ever present in the mind, some
direct evidence of actual cases of increase must be adduced. That even
the larger animals, which breed comparatively slowly, increase
enormously when placed under favourable conditions in new countries, is
shown by the rapid spread of cattle and horses in America. Columbus, in
his second voyage, left a few black cattle at St. Domingo, and these ran
wild and increased so much that, twenty-seven years afterwards, herds of
from 4000 to 8000 head were not uncommon. Cattle were afterwards taken
from this island to Mexico and to other parts of America, and in 1587,
sixty-five years after the conquest of Mexico, the Spaniards exported
64,350 hides from that country and 35,444 from St. Domingo, an
indication of the vast numbers of these animals which must then have
existed there, since those captured and killed could have been only a
small portion of the whole. In the pampas of Buenos Ayres there were, at
the end of the last century, about twelve million cows and three million
horses, besides great numbers in all other parts of America where open
pastures offered suitable conditions. Asses, about fifty years after
their introduction, ran wild and multiplied so amazingly in Quito, that
the Spanish traveller Ulloa describes them as being a nuisance. They
grazed together in great herds, defending themselves with their mouths,
and if a horse strayed among them they all fell upon him and did not
cease biting and kicking till they left him dead. Hogs were turned out
in St. Domingo by Columbus in 1493, and the Spaniards took them to other
places where they settled, the result being, that in about half a
century these animals were found in great numbers over a large part of
America, from 25° north to 40° south latitude. More recently, in New
Zealand, pigs have multiplied so greatly in a wild state as to be a
serious nuisance and injury to agriculture. To give some idea of their
numbers, it is stated that in the province of Nelson there were killed
in twenty months 25,000 wild pigs.[10] Now, in the case of all these
animals, we know that in their native countries, and even in America at
the present time, they do not increase at all in numbers; therefore the
whole normal increase must be kept down, year by year, by natural or
artificial means of destruction.


_Rapid Increase and Wide Spread of Plants_.

In the case of plants, the power of increase is even greater and its
effects more distinctly visible. Hundreds of square miles of the plains
of La Plata are now covered with two or three species of European
thistle, often to the exclusion of almost every other plant; but in the
native countries of these thistles they occupy, except in cultivated or
waste ground, a very subordinate part in the vegetation. Some American
plants, like the cotton-weed (Asclepias cuiussayica), have now become
common weeds over a large portion of the tropics. White clover
(Trifolium repens) spreads over all the temperate regions of the world,
and in New Zealand is exterminating many native species, including even
the native flax (Phormium tenax), a large plant with iris-like leaves 5
or 6 feet high. Mr. W.L. Travers has paid much attention to the effects
of introduced plants in New Zealand, and notes the following species as
being especially remarkable. The common knotgrass (Polygonum aviculare)
grows most luxuriantly, single plants covering a space 4 or 5 feet in
diameter, and sending their roots 3 or 4 feet deep. A large sub-aquatic
dock (Rumex obtusifolius) abounds in every river-bed, even far up among
the mountains. The common sow-thistle (Sonchus oleraceus) grows all over
the country up to an elevation of 6000 feet. The water-cress (Nasturtium
officinale) grows with amazing vigour in many of the rivers, forming
stems 12 feet long and 3/4 inch in diameter, and completely choking them
up. It cost £300 a year to keep the Avon at Christchurch free from it.
The sorrel (Rumex acetosella) covers hundreds of acres with a sheet of
red. It forms a dense mat, exterminating other plants, and preventing
cultivation. It can, however, be itself exterminated by sowing the
ground with red clover, which will also vanquish the Polygonum
aviculare. The most noxious weed in New Zealand appears, however, to be
the Hypochaeris radicata, a coarse yellow-flowered composite not
uncommon in our meadows and waste places. This has been introduced with
grass seeds from England, and is very destructive. It is stated that
excellent pasture was in three years destroyed by this weed, which
absolutely displaced every other plant on the ground. It grows in every
kind of soil, and is said even to drive out the white clover, which is
usually so powerful in taking possession of the soil.

In Australia another composite plant, called there the Cape-weed
(Cryptostemma calendulaceum), did much damage, and was noticed by Baron
Von Hugel in 1833 as "an unexterminable weed"; but, after forty years'
occupation, it was found to give way to the dense herbage formed by
lucerne and choice grasses.

In Ceylon we are told by Mr. Thwaites, in his _Enumeration of Ceylon
Plants_, that a plant introduced into the island less than fifty years
ago is helping to alter the character of the vegetation up to an
elevation of 3000 feet. This is the Lantana mixta, a verbenaceous plant
introduced from the West Indies, which appears to have found in Ceylon
a soil and climate exactly suited to it. It now covers thousands of
acres with its dense masses of foliage, taking complete possession of
land where cultivation has been neglected or abandoned, preventing the
growth of any other plants, and even destroying small trees, the tops of
which its subscandent stems are able to reach. The fruit of this plant
is so acceptable to frugivorous birds of all kinds that, through their
instrumentality, it is spreading rapidly, to the complete exclusion of
the indigenous vegetation where it becomes established.


_Great Fertility not essential to Rapid Increase_.

The not uncommon circumstance of slow-breeding animals being very
numerous, shows that it is usually the amount of destruction which an
animal or plant is exposed to, not its rapid multiplication, that
determines its numbers in any country. The passenger-pigeon (Ectopistes
migratorius) is, or rather was, excessively abundant in a certain area
in North America, and its enormous migrating flocks darkening the sky
for hours have often been described; yet this bird lays only two eggs.
The fulmar petrel exists in myriads at St. Kilda and other haunts of the
species, yet it lays only one egg. On the other hand the great shrike,
the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many
other birds, lay from four to six or seven eggs, and yet are never
abundant. So in plants, the abundance of a species bears little or no
relation to its seed-producing power. Some of the grasses and sedges,
the wild hyacinth, and many buttercups occur in immense profusion over
extensive areas, although each plant produces comparatively few seeds;
while several species of bell-flowers, gentians, pinks, and mulleins,
and even some of the composite, which produce an abundance of minute
seeds, many of which are easily scattered by the wind, are yet rare
species that never spread beyond a very limited area.

The above-mentioned passenger-pigeon affords such an excellent example
of an enormous bird-population kept up by a comparatively slow rate of
increase, and in spite of its complete helplessness and the great
destruction which it suffers from its numerous enemies, that the
following account of one of its breeding-places and migrations by the
celebrated American naturalist, Alexander Wilson, will be read with
interest:--

"Not far from Shelbyville, in the State of Kentucky, about five years
ago, there was one of these breeding-places, which stretched through the
woods in nearly a north and south direction, was several miles in
breadth, and was said to be upwards of 40 miles in extent. In this tract
almost every tree was furnished with nests wherever the branches could
accommodate them. The pigeons made their first appearance there about
the 10th of April, and left it altogether with their young before the
25th of May. As soon as the young were fully grown and before they left
the nests, numerous parties of the inhabitants from all parts of the
adjacent country came with waggons, axes, beds, cooking utensils, many
of them accompanied by the greater part of their families, and encamped
for several days at this immense nursery. Several of them informed me
that the noise was so great as to terrify their horses, and that it was
difficult for one person to hear another without bawling in his ear. The
ground was strewed with broken limbs of trees, eggs, and young squab
pigeons, which had been precipitated from above, and on which herds of
hogs were fattening. Hawks, buzzards, and eagles were sailing about in
great numbers, and seizing the squabs from the nests at pleasure; while,
from 20 feet upwards to the top of the trees, the view through the woods
presented a perpetual tumult of crowding and fluttering multitudes of
pigeons, their wings roaring like thunder, mingled with the frequent
crash of falling timber; for now the axemen were at work cutting down
those trees that seemed most crowded with nests, and contrived to fell
them in such a manner, that in their descent they might bring down
several others; by which means the falling of one large tree sometimes
produced 200 squabs little inferior in size to the old birds, and almost
one heap of fat. On some single trees upwards of a hundred nests were
found, each containing one squab only; a circumstance in the history of
the bird not generally known to naturalists.[11] It was dangerous to
walk under these flying and fluttering millions, from the frequent fall
of large branches, broken down by the weight of the multitudes above,
and which in their descent often destroyed numbers of the birds
themselves; while the clothes of those engaged in traversing the woods
were completely covered with the excrements of the pigeons.

"These circumstances were related to me by many of the most respectable
part of the community in that quarter, and were confirmed in part by
what I myself witnessed. I passed for several miles through this same
breeding-place, where every tree was spotted with nests, the remains of
those above described. In many instances I counted upwards of ninety
nests on a single tree; but the pigeons had abandoned this place for
another, 60 or 80 miles off, towards Green River, where they were said
at that time to be equally numerous. From the great numbers that were
constantly passing over our heads to or from that quarter, I had no
doubt of the truth of this statement. The mast had been chiefly consumed
in Kentucky; and the pigeons, every morning a little before sunrise, set
out for the Indiana territory, the nearest part of which was about sixty
miles distant. Many of these returned before ten o'clock, and the great
body generally appeared on their return a little after noon. I had left
the public road to visit the remains of the breeding-place near
Shelbyville, and was traversing the woods with my gun, on my way to
Frankfort, when about ten o'clock the pigeons which I had observed
flying the greater part of the morning northerly, began to return in
such immense numbers as I never before had witnessed. Coming to an
opening by the side of a creek, where I had a more uninterrupted view, I
was astonished at their appearance: they were flying with great
steadiness and rapidity, at a height beyond gunshot, in several strata
deep, and so close together that, could shot have reached them, one
discharge could not have failed to bring down several individuals. From
right to left, as far as the eye could reach, the breadth of this vast
procession extended, seeming everywhere equally crowded. Curious to
determine how long this appearance would continue, I took out my watch
to note the time, and sat down to observe them. It was then half-past
one; I sat for more than an hour, but instead of a diminution of this
prodigious procession, it seemed rather to increase, both in numbers and
rapidity; and anxious to reach Frankfort before night, I rose and went
on. About four o'clock in the afternoon I crossed Kentucky River, at the
town of Frankfort, at which time the living torrent above my head seemed
as numerous and as extensive as ever. Long after this I observed them in
large bodies that continued to pass for six or eight minutes, and these
again were followed by other detached bodies, all moving in the same
south-east direction, till after six o'clock in the evening. The great
breadth of front which this mighty multitude preserved would seem to
intimate a corresponding breadth of their breeding-place, which, by
several gentlemen who had lately passed through part of it, was stated
to me at several miles."

From these various observations, Wilson calculated that the number of
birds contained in the mass of pigeons which he saw on this occasion was
at least two thousand millions, while this was only one of many similar
aggregations known to exist in various parts of the United States. The
picture here given of these defenceless birds, and their still more
defenceless young, exposed to the attacks of numerous rapacious enemies,
brings vividly before us one of the phases of the unceasing struggle for
existence ever going on; but when we consider the slow rate of increase
of these birds, and the enormous population they are nevertheless able
to maintain, we must be convinced that in the case of the majority of
birds which multiply far more rapidly, and yet are never able to attain
such numbers, the struggle against their numerous enemies and against
the adverse forces of nature must be even more severe or more
continuous.


_Struggle for Life between, closely allied Animals and Plants often the
most severe._

The struggle we have hitherto been considering has been mainly that
between an animal or plant and its direct enemies, whether these enemies
are other animals which devour it, or the forces of nature which destroy
it. But there is another kind of struggle often going on at the same
time between closely related species, which almost always terminates in
the destruction of one of them. As an example of what is meant, Darwin
states that the recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush.[12] The black rat
(Mus rattus) was the common rat of Europe till, in the beginning of the
eighteenth century, the large brown rat (Mus decumanus) appeared on the
Lower Volga, and thence spread more or less rapidly till it overran all
Europe, and generally drove out the black rat, which in most parts is
now comparatively rare or quite extinct. This invading rat has now been
carried by commerce all over the world, and in New Zealand has
completely extirpated a native rat, which the Maoris allege they brought
with them from their home in the Pacific; and in the same country a
native fly is being supplanted by the European house-fly. In Russia the
small Asiatic cockroach has driven away a larger native species; and in
Australia the imported hive-bee is exterminating the small stingless
native bee.

The reason why this kind of struggle goes on is apparent if we consider
that the allied species fill nearly the same place in the economy of
nature. They require nearly the same kind of food, are exposed to the
same enemies and the same dangers. Hence, if one has ever so slight an
advantage over the other in procuring food or in avoiding danger, in its
rapidity of multiplication or its tenacity of life, it will increase
more rapidly, and by that very fact will cause the other to decrease and
often become altogether extinct. In some cases, no doubt, there is
actual war between the two, the stronger killing the weaker; but this is
by no means necessary, and there may be cases in which the weaker
species, physically, may prevail, by its power of more rapid
multiplication, its better withstanding vicissitudes of climates, or its
greater cunning in escaping the attacks of the common enemies. The same
principle is seen at work in the fact that certain mountain varieties of
sheep will starve out other mountain varieties, so that the two cannot
be kept together. In plants the same thing occurs. If several distinct
varieties of wheat are sown together, and the mixed seed resown, some of
the varieties which best suit the soil and climate, or are naturally the
most fertile, will beat the others and so yield more seed, and will
consequently in a few years supplant the other varieties.

As an effect of this principle, we seldom find closely allied species
of animals or plants living together, but often in distinct though
adjacent districts where the conditions of life are somewhat different.
Thus we may find cowslips (Primula veris) growing in a meadow, and
primroses (P. vulgaris) in an adjoining wood, each in abundance, but not
often intermingled. And for the same reason the old turf of a pasture or
heath consists of a great variety of plants matted together, so much so
that in a patch little more than a yard square Mr. Darwin found twenty
distinct species, belonging to eighteen distinct genera and to eight
natural orders, thus showing their extreme diversity of organisation.
For the same reason a number of distinct grasses and clovers are sown in
order to make a good lawn instead of any one species; and the quantity
of hay produced has been found to be greater from a variety of very
distinct grasses than from any one species of grass.

It may be thought that forests are an exception to this rule, since in
the north-temperate and arctic regions we find extensive forests of
pines or of oaks. But these are, after all, exceptional, and
characterise those regions only where the climate is little favourable
to forest vegetation. In the tropical and all the warm temperate parts
of the earth, where there is a sufficient supply of moisture, the
forests present the same variety of species as does the turf of our old
pastures; and in the equatorial virgin forests there is so great a
variety of forms, and they are so thoroughly intermingled, that the
traveller often finds it difficult to discover a second specimen of any
particular species which he has noticed. Even the forests of the
temperate zones, in all favourable situations, exhibit a considerable
variety of trees of distinct genera and families, and it is only when we
approach the outskirts of forest vegetation, where either drought or
winds or the severity of the winter is adverse to the existence of most
trees, that we find extensive tracts monopolised by one or two species.
Even Canada has more than sixty different forest trees and the Eastern
United States a hundred and fifty; Europe is rather poor, containing
about eighty trees only; while the forests of Eastern Asia, Japan, and
Manchuria are exceedingly rich, about a hundred and seventy species
being already known. And in all these countries the trees grow
intermingled, so that in every extensive forest we have a considerable
variety, as may be seen in the few remnants of our primitive woods in
some parts of Epping Forest and the New Forest.

Among animals the same law prevails, though, owing to their constant
movements and power of concealment, it is not so readily observed. As
illustrations we may refer to the wolf, ranging over Europe and Northern
Asia, while the jackal inhabits Southern Asia and Northern Africa; the
tree-porcupines, of which there are two closely allied species, one
inhabiting the eastern, the other the western half of North America; the
common hare (Lepus timidus) in Central and Southern Europe, while all
Northern Europe is inhabited by the variable hare (Lepus variabilis);
the common jay (Garrulus glandarius) inhabiting all Europe, while
another species (Garrulus Brandti) is found all across Asia from the
Urals to Japan; and many species of birds in the Eastern United States
are replaced by closely allied species in the west. Of course there are
also numbers of closely related species in the same country, but it will
almost always be found that they frequent different stations and have
somewhat different habits, and so do not come into direct competition
with each other; just as closely allied plants may inhabit the same
districts, when one prefers meadows the other woods, one a chalky soil
the other sand, one a damp situation the other a dry one. With plants,
fixed as they are to the earth, we easily note these peculiarities of
station; but with wild animals, which we see only on rare occasions, it
requires close and long-continued observation to detect the
peculiarities in their mode of life which may prevent all direct
competition between closely allied species dwelling in the same area.


_The Ethical Aspect of the Struggle for Existence_.

Our exposition of the phenomena presented by the struggle for existence
may be fitly concluded by a few remarks on its ethical aspect. Now that
the war of nature is better known, it has been dwelt upon by many
writers as presenting so vast an amount of cruelty and pain as to be
revolting to our instincts of humanity, while it has proved a
stumbling-block in the way of those who would fain believe in an
all-wise and benevolent ruler of the universe. Thus, a brilliant writer
says: "Pain, grief, disease, and death, are these the inventions of a
loving God? That no animal shall rise to excellence except by being
fatal to the life of others, is this the law of a kind Creator? It is
useless to say that pain has its benevolence, that massacre has its
mercy. Why is it so ordained that bad should be the raw material of
good? Pain is not the less pain because it is useful; murder is not less
murder because it is conducive to development. Here is blood upon the
hand still, and all the perfumes of Arabia will not sweeten it."[13]

Even so thoughtful a writer as Professor Huxley adopts similar views. In
a recent article on "The Struggle for Existence" he speaks of the
myriads of generations of herbivorous animals which "have been tormented
and devoured by carnivores"; of the carnivores and herbivores alike
"subject to all the miseries incidental to old age, disease, and
over-multiplication"; and of the "more or less enduring suffering,"
which is the meed of both vanquished and victor. And he concludes that,
since thousands of times a minute, were our ears sharp enough, we should
hear sighs and groans of pain like those heard by Dante at the gate of
hell, the world cannot be governed by what we call benevolence.[14]

Now there is, I think, good reason to believe that all this is greatly
exaggerated; that the supposed "torments" and "miseries" of animals have
little real existence, but are the reflection of the imagined sensations
of cultivated men and women in similar circumstances; and that the
amount of actual suffering caused by the struggle for existence among
animals is altogether insignificant. Let us, therefore, endeavour to
ascertain what are the real facts on which these tremendous accusations
are founded.

In the first place, we must remember that animals are entirely spared
the pain we suffer in the anticipation of death--a pain far greater, in
most cases, than the reality. This leads, probably, to an almost
perpetual enjoyment of their lives; since their constant watchfulness
against danger, and even their actual flight from an enemy, will be the
enjoyable exercise of the powers and faculties they possess, unmixed
with any serious dread. There is, in the next place, much evidence to
show that violent deaths, if not too prolonged, are painless and easy;
even in the case of man, whose nervous system is in all probability much
more susceptible to pain than that of most animals. In all cases in
which persons have escaped after being seized by a lion or tiger, they
declare that they suffered little or no pain, physical or mental. A
well-known instance is that of Livingstone, who thus describes his
sensations when seized by a lion: "Starting and looking half round, I
saw the lion just in the act of springing on me. I was upon a little
height; he caught my shoulder as he sprang, and we both came to the
ground below together. Growling horribly close to my ear, he shook me as
a terrier-dog does a rat. The shock produced a stupor similar to that
which seems to be felt by a mouse after the first shake of the cat. It
causes a sort of dreaminess, _in which there was no sense of pain or
feeling of terror_, though I was quite conscious of all that was
happening. It was like what patients partially under the influence of
chloroform describe, who see all the operation, but feel not the knife.
This singular condition was not the result of any mental process. The
shake annihilated fear, and allowed no sense of horror in looking round
at the beast."

This absence of pain is not peculiar to those seized by wild beasts, but
is equally produced by any accident which causes a general shock to the
system. Mr. Whymper describes an accident to himself during one of his
preliminary explorations of the Matterhorn, when he fell several hundred
feet, bounding from rock to rock, till fortunately embedded in a
snow-drift near the edge of a tremendous precipice. He declares that
while falling and feeling blow after blow, he neither lost consciousness
nor suffered pain, merely thinking, calmly, that a few more blows would
finish him. We have therefore a right to conclude, that when death
follows soon after any great shock it is as easy and painless a death as
possible; and this is certainly what happens when an animal is seized by
a beast of prey. For the enemy is one which hunts for food, not for
pleasure or excitement; and it is doubtful whether any carnivorous
animal in a state of nature begins to seek after prey till driven to do
so by hunger. When an animal is caught, therefore, it is very soon
devoured, and thus the first shock is followed by an almost painless
death. Neither do those which die of cold or hunger suffer much. Cold is
generally severest at night and has a tendency to produce sleep and
painless extinction. Hunger, on the other hand, is hardly felt during
periods of excitement, and when food is scarce the excitement of seeking
for it is at its greatest. It is probable, also, that when hunger
presses, most animals will devour anything to stay their hunger, and
will die of gradual exhaustion and weakness not necessarily painful, if
they do not fall an earlier prey to some enemy or to cold.[15]

Now let us consider what are the enjoyments of the lives of most
animals. As a rule they come into existence at a time of year when food
is most plentiful and the climate most suitable, that is in the spring
of the temperate zone and at the commencement of the dry season in the
tropics. They grow vigorously, being supplied with abundance of food;
and when they reach maturity their lives are a continual round of
healthy excitement and exercise, alternating with complete repose. The
daily search for the daily food employs all their faculties and
exercises every organ of their bodies, while this exercise leads to the
satisfaction of all their physical needs. In our own case, we can give
no more perfect definition of happiness, than this exercise and this
satisfaction; and we must therefore conclude that animals, as a rule,
enjoy all the happiness of which they are capable. And this normal state
of happiness is not alloyed, as with us, by long periods--whole lives
often--of poverty or ill-health, and of the unsatisfied longing for
pleasures which others enjoy but to which we cannot attain. Illness, and
what answers to poverty in animals--continued hunger--are quickly
followed by unanticipated and almost painless extinction. Where we err
is, in giving to animals feelings and emotions which they do not
possess. To us the very sight of blood and of torn or mangled limbs is
painful, while the idea of the suffering implied by it is heartrending.
We have a horror of all violent and sudden death, because we think of
the life full of promise cut short, of hopes and expectations
unfulfilled, and of the grief of mourning relatives. But all this is
quite out of place in the case of animals, for whom a violent and a
sudden death is in every way the best. Thus the poet's picture of


    "Nature red in tooth and claw
         With ravine"


is a picture the evil of which is read into it by our imaginations, the
reality being made up of full and happy lives, usually terminated by the
quickest and least painful of deaths.

On the whole, then, we conclude that the popular idea of the struggle
for existence entailing misery and pain on the animal world is the very
reverse of the truth. What it really brings about, is, the maximum of
life and of the enjoyment of life with the minimum of suffering and
pain. Given the necessity of death and reproduction--and without these
there could have been no progressive development of the organic
world,--and it is difficult even to imagine a system by which a greater
balance of happiness could have been secured. And this view was
evidently that of Darwin himself, who thus concludes his chapter on the
struggle for existence: "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."

FOOTNOTES:

[Footnote 4: _Géographic Botanique_, p. 798.]

[Footnote 5: _The Origin of Species_, p. 53.]

[Footnote 6: _The Earth as Modified by Human Action_, p. 51.]

[Footnote 7: _The Origin of Species_, p. 56.]

[Footnote 8: See _Nature_, vol. xxxi. p. 63.]

[Footnote 9: _A Visit to South America_, 1878; also _Nature_, vol. xxxi.
pp. 263-339.]

[Footnote 10: Still more remarkable is the increase of rabbits both in
New Zealand and Australia. No less than seven millions of rabbit-skins
have been exported from the former country in a single year, their value
being £67,000. In both countries, sheep-runs have been greatly
deteriorated in value by the abundance of rabbits, which destroy the
herbage; and in some cases they have had to be abandoned altogether.]

[Footnote 11: Later observers have proved that two eggs are laid and
usually two young produced, but it may be that in most cases only one of
these comes to maturity.]

[Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however,
informs me that these species do not interfere with one another in the
way here stated.]

[Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.]

[Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.]

[Footnote 15: The Kestrel, which usually feeds on mice, birds, and
frogs, sometimes stays its hunger with earthworms, as do some of the
American buzzards. The Honey-buzzard sometimes eats not only earthworms
and slugs, but even corn; and the Buteo borealis of North America, whose
usual food is small mammals and birds, sometimes eats crayfish.]




CHAPTER III

THE VARIABILITY OF SPECIES IN A STATE OF NATURE


    Importance of variability--Popular ideas regarding
    it--Variability of the lower animals--The variability of
    insects--Variation among lizards--Variation among
    birds--Diagrams of bird-variation--Number of varying
    individuals--Variation in the mammalia--Variation in internal
    organs--Variations in the skull--Variations in the habits of
    Animals--The Variability of plants--Species which vary
    little--Concluding remarks.



The foundation of the Darwinian theory is the variability of species,
and it is quite useless to attempt even to understand that theory, much
less to appreciate the completeness of the proof of it, unless we first
obtain a clear conception of the nature and extent of this variability.
The most frequent and the most misleading of the objections to the
efficacy of natural selection arise from ignorance of this subject, an
ignorance shared by many naturalists, for it is only since Mr. Darwin
has taught us their importance that varieties have been systematically
collected and recorded; and even now very few collectors or students
bestow upon them the attention they deserve. By the older naturalists,
indeed, varieties--especially if numerous, small, and of frequent
occurrence--were looked upon as an unmitigated nuisance, because they
rendered it almost impossible to give precise definitions of species,
then considered the chief end of systematic natural history. Hence it
was the custom to describe what was supposed to be the "typical form" of
species, and most collectors were satisfied if they possessed this
typical form in their cabinets. Now, however, a collection is valued in
proportion as it contains illustrative specimens of all the varieties
that occur in each species, and in some cases these have been carefully
described, so that we possess a considerable mass of information on the
subject. Utilising this information we will now endeavour to give some
idea of the nature and extent of variation in the species of animals and
plants.

It is very commonly objected that the widespread and constant
variability which is admitted to be a characteristic of domesticated
animals and cultivated plants is largely due to the unnatural conditions
of their existence, and that we have no proof of any corresponding
amount of variation occurring in a state of nature. Wild animals and
plants, it is said, are usually stable, and when variations occur these
are alleged to be small in amount and to affect superficial characters
only; or if larger and more important, to occur so rarely as not to
afford any aid in the supposed formation of new species.

This objection, as will be shown, is utterly unfounded; but as it is one
which goes to the very root of the problem, it is necessary to enter at
some length into the various proofs of variation in a state of nature.
This is the more necessary because the materials collected by Mr. Darwin
bearing on this question have never been published, and comparatively
few of them have been cited in _The Origin of Species_; while a
considerable body of facts has been made known since the publication of
the last edition of that work.


_Variability of the Lower Animals_.

Among the lowest and most ancient marine organisms are the Foraminifera,
little masses of living jelly, apparently structureless, but which
secrete beautiful shelly coverings, often perfectly symmetrical, as
varied in form as those of the mollusca and far more complicated. These
have been studied with great care by many eminent naturalists, and the
late Dr. W.B. Carpenter in his great work--the _Introduction to the
Study of the Foraminifera_--thus refers to their variability: "There is
not a single species of plant or animal of which the range of variation
has been studied by the collocation and comparison of so large a number
of specimens as have passed under the review of Messrs. Williamson,
Parker, Rupert Jones, and myself in our studies of the types of this
group;" and he states as the result of this extensive comparison of
specimens: "The range of variation is so great among the Foraminifera
as to include not merely those differential characters which have been
usually accounted _specific_, but also those upon which the greater part
of the _genera_, of this group have been founded, and even in some
instances those of its _orders_."[16]

Coming now to a higher group--the Sea-Anemones--Mr. P.H. Gosse and other
writers on these creatures often refer to variations in size, in the
thickness and length of the tentacles, the form of the disc and of the
mouth, and the character of surface of the column, while the colour
varies enormously in a great number of the species. Similar variations
occur in all the various groups of marine invertebrata, and in the great
sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S.P.
Woodward states that many present a most perplexing amount of variation,
resulting (as he supposes) from supply of food, variety of depth and of
saltness of the water; but we know that many variations are quite
independent of such causes, and we will now consider a few cases among
the land-mollusca in which they have been more carefully studied.

In the small forest region of Oahu, one of the Sandwich Islands, there
have been found about 175 species of land-shells represented by 700 or
800 varieties; and we are told by the Rev. J.T. Gulick, who studied them
carefully, that "we frequently find a genus represented in several
successive valleys by allied species, sometimes feeding on the same,
sometimes on different plants. In every such case the valleys that are
nearest to each other furnish the most nearly allied forms; _and a full
set of the varieties of each species presents a minute gradation of
forms between the more divergent types found in the more widely
separated localities_."

In most land-shells there is a considerable amount of variation in
colour, markings, size, form, and texture or striation of the surface,
even in specimens collected in the same locality. Thus, a French author
has enumerated no less than 198 varieties of the common wood-snail
(Helix nemoralis), while of the equally common garden-snail (Helix
hortensis) ninety varieties have been described. Fresh-water shells are
also subject to great variation, so that there is much uncertainty as
to the number of species; and variations are especially frequent in the
Planorbidae, which exhibit many eccentric deviations from the usual form
of the species--deviations which must often affect the form of the
living animal. In Mr. Ingersoll's Report on the Recent Mollusca of
Colorado many of these extraordinary variations are referred to, and it
is stated that a shell (Helisonia trivolvis) abundant in some small
ponds and lakes, had scarcely two specimens alike, and many of them
closely resembled other and altogether distinct species.[17]


_The Variability of Insects_.

Among Insects there is a large amount of variation, though very few
entomologists devote themselves to its investigation. Our first examples
will be taken from the late Mr. T. Vernon Wollaston's book, _On the
Variation of Species_, and they must be considered as indications of
very widespread though little noticed phenomena. He speaks of the
curious little carabideous beetles of the genus Notiophilus as being
"extremely unstable both in their sculpture and hue;" of the common
Calathus mollis as having "the hind wings at one time ample, at another
rudimentary, and at a third nearly obsolete;" and of the same
irregularity as to the wings being characteristic of many Orthoptera and
of the Homopterous Fulgoridae. Mr. Westwood in his _Modern
Classification of Insects_ states that "the species of Gerris,
Hydrometra, and Velia are mostly found perfectly apterous, though
occasionally with full-sized wings."

It is, however, among the Lepidoptera (butterflies and moths) that the
most numerous cases of variation have been observed, and every good
collection of these insects affords striking examples. I will first
adduce the testimony of Mr. Bates, who speaks of the butterflies of the
Amazon valley exhibiting innumerable local varieties or races, while
some species showed great individual variability. Of the beautiful
Mechanitis Polymnia he says, that at Ega on the Upper Amazons, "it
varies not only in general colour and pattern, but also very
considerably in the shape of the wings, especially in the male sex."
Again, at St. Paulo, Ithomia Orolina exhibits four distinct varieties,
all occurring together, and these differ not only in colour but in form,
one variety being described as having the fore wings much elongated in
the male, while another is much larger and has "the hind wings in the
male different in shape." Of Heliconius Numata Mr. Bates says: "This
species is so variable that it is difficult to find two examples exactly
alike," while "it varies in structure as well as in colours. The wings
are sometimes broader, sometimes narrower; and their edges are simple in
some examples and festooned in others." Of another species of the same
genus, H. melpomene, ten distinct varieties are described all more or
less connected by intermediate forms, and four of these varieties were
obtained at one locality, Serpa on the north bank of the Amazon.
Ceratina Ninonia is another of these very unstable species exhibiting
many local varieties which are, however, incomplete and connected by
intermediate forms; while the several species of the genus Lycorea all
vary to such an extent as almost to link them together, so that Mr.
Bates thinks they might all fairly be considered as varieties of one
species only.

Turning to the Eastern Hemisphere we have in Papilio Severus a species
which exhibits a large amount of simple variation, in the presence or
absence of a pale patch on the upper wings, in the brown submarginal
marks on the lower wings, in the form and extent of the yellow band, and
in the size of the specimens. The most extreme forms, as well as the
intermediate ones, are often found in one locality and in company with
each other. A small butterfly (Terias hecabe) ranges over the whole of
the Indian and Malayan regions to Australia, and everywhere exhibits
great variations, many of which have been described as distinct species;
but a gentleman in Australia bred two of these distinct forms (T. hecabe
and T. Aesiope), with several intermediates, from one batch of
caterpillars found feeding together on the same plant.[18] It is
therefore very probable that a considerable number of supposed distinct
species are only individual varieties.

Cases of variation similar to those now adduced among butterflies might
be increased indefinitely, but it is as well to note that such important
characters as the neuration of the wings, on which generic and family
distinctions are often established, are also subject to variation. The
Rev. R.P. Murray, in 1872, laid before the Entomological Society
examples of such variation in six species of butterflies, and other
cases have been since described. The larvae of butterflies and moths are
also very variable, and one observer recorded in the _Proceedings of the
Entomological Society for_ 1870 no less than sixteen varieties of the
caterpillar of the bedstraw hawk-moth (Deilephela galii).


_Variation among Lizards_.

Passing on from the lower animals to the vertebrata, we find more
abundant and more definite evidence as to the extent and amount of
individual variation. I will first give a case among the Reptilia from
some of Mr. Darwin's unpublished MSS., which have been kindly lent me by
Mr. Francis Darwin.

"M. Milne Edwards (_Annales des Sci. Nat._, I ser., tom. xvi. p. 50) has
given a curious table of measurements of fourteen specimens of Lacerta
muralis; and, taking the length of the head as a standard, he finds the
neck, trunk, tail, front and hind legs, colour, and femoral pores, all
varying wonderfully; and so it is more or less with other species. So
apparently trifling a character as the scales on the head affording
almost the only constant characters."

[Illustration: FIG. 1.--Variations of Lacerta muralis.]

[Illustration: FIG. 2.--Variation of Lizards.]

As the table of measurements above referred to would give no clear
conception of the nature and amount of the variation without a laborious
study and comparison of the figures, I have endeavoured to find a method
of presenting the facts to the eye, so that they may be easily grasped
and appreciated. In the diagram opposite, the comparative variations of
the different organs of this species are given by means of variously
bent lines. The head is represented by a straight line because it
presented (apparently) no variation. The body is next given, the
specimens being arranged in the order of their size from No. 1, the
smallest, to No. 14, the largest, the actual lengths being laid down
from a base line at a suitable distance below, in this case two inches
below the centre, the mean length of the body of the fourteen specimens
being two inches. The respective lengths of the neck, legs, and toe of
each specimen are then laid down in the same manner at convenient
distances apart for comparison; and we see that their variations bear no
definite relation to those of the body, and not much to those of each
other. With the exception of No. 5, in which all the parts agree in
being large, there is a marked independence of each part, shown by the
lines often curving in opposite directions; which proves that in those
specimens one part is large while the other is small. The actual amount
of the variation is very great, ranging from one-sixth of the mean
length in the neck to considerably more than a fourth in the hind leg,
and this among only fourteen examples which happen to be in a particular
museum.

To prove that this is not an isolated case, Professor Milne Edwards also
gives a table showing the amount of variation in the museum specimens of
six common species of lizards, also taking the head as the standard, so
that the comparative variation of each part to the head is given. In the
accompanying diagram (Fig. 2) the variations are exhibited by means of
lines of varying length. It will be understood that, however much the
specimens varied in _size_, if they had kept the same _proportions_, the
variation line would have been in every case reduced to a point, as in
the neck of L. velox which exhibits no variation. The different
proportions of the variation lines for each species may show a distinct
mode of variation, or may be merely due to the small and differing
number of specimens; for it is certain that whatever amount of variation
occurs among a few specimens will be greatly increased when a much
larger number of specimens are examined. That the amount of variation is
large, may be seen by comparing it with the actual length of the head
(given below the diagram) which was used as a standard in determining
the variation, but which itself seems not to have varied.[19]


_Variation among Birds_.

Coming now to the class of Birds, we find much more copious evidence of
variation. This is due partly to the fact that Ornithology has perhaps a
larger body of devotees than any other branch of natural history (except
entomology); to the moderate size of the majority of birds; and to the
circumstance that the form and dimensions of the wings, tail, beak, and
feet offer the best generic and specific characters and can all be
easily measured and compared. The most systematic observations on the
individual variation of birds have been made by Mr. J.A. Allen, in his
remarkable memoir: "On the Mammals and Winter Birds of East Florida,
with an examination of certain assumed specific characters in Birds, and
a sketch of the Bird Faunae of Eastern North America," published in the
_Bulletin of the Museum of Comparative Zoology_ at Harvard College,
Cambridge, Massachusetts, in 1871. In this work exact measurements are
given of all the chief external parts of a large number of species of
common American birds, from twenty to sixty or more specimens of each
species being measured, so that we are able to determine with some
precision the nature and extent of the variation that usually occurs.
Mr. Allen says: "The facts of the case show that a variation of from 15
to 20 per cent in general size, and an equal degree of variation in the
relative size of different parts, may be ordinarily expected among
specimens of the same species and sex, taken at the same locality, while
in some cases the variation is even greater than this." He then goes on
to show that each part varies to a considerable extent independently of
the other parts; so that when the size varies, the proportions of all
the parts vary, often to a much greater amount. The wing and tail, for
example, besides varying in length, vary in the proportionate length of
each feather, and this causes their outline to vary considerably in
shape. The bill also varies in length, width, depth, and curvature. The
tarsus varies in length, as does each toe separately and independently;
and all this not to a minute degree requiring very careful measurement
to detect it at all, but to an amount easily seen without any
measurement, as it averages one-sixth of the whole length and often
reaches one-fourth. In twelve species of common perching birds the wing
varied (in from twenty-five to thirty specimens) from 14 to 21 per cent
of the mean length, and the tail from 13.8 to 23.4 per cent. The
variation of the form of the wing can be very easily tested by noting
which feather is longest, which next in length, and so on, the
respective feathers being indicated by the numbers 1, 2, 3, etc.,
commencing with the outer one. As an example of the irregular variation
constantly met with, the following occurred among twenty-five specimens
of Dendroeca coronata. Numbers bracketed imply that the corresponding
feathers were of equal length.[20]


    RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
    DENDROECA CORONATA.
    ---------+-----------+----------+-----------+----------+----------
    Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
             |  Length.  |  Length. |  Length.  |  Length. |  Length.
    ---------+-----------+----------+-----------+----------+----------
    2        |     3     |     1    |     4     |     5    |     6
    3        |     2     |     4    |     1     |     5    |     6
             |  /  2     |          |           |          |
    3        | {         |     1    |     5     |     6    |     7
             |  \  4     |          |           |          |
    2    \   |           |          |           |          |
          }  |     4     |     1    |     5     |     6    |     7
    3    /   |           |          |           |          |
    2    \   |           |          |           |          |
    1     |  |           |          |           |          |
           } |     5     |     6    |     7     |     8    |     9
    3     |  |           |          |           |          |
    4    /   |           |          |           |          |
    ---------+-----------+----------+-----------+----------+----------


Here we have five very distinct proportionate lengths of the wing
feathers, any one of which is often thought sufficient to characterise a
distinct species of bird; and though this is rather an extreme case, Mr.
Allen assures us that "the comparison, extended in the table to only a
few species, has been carried to scores of others with similar results."

Along with this variation in size and proportions there occurs a large
amount of variation in colour and markings. "The difference in intensity
of colour between the extremes of a series of fifty or one hundred
specimens of any species, collected at a single locality, and nearly at
the same season of the year, is often as great as occurs between truly
distinct species." But there is also a great amount of individual
variability in the markings of the same species. Birds having the
plumage varied with streaks and spots differ exceedingly in different
individuals of the same species in respect to the size, shape, and
number of these marks, and in the general aspect of the plumage
resulting from such variations. "In the common song sparrow (Melospiza
melodia), the fox-coloured sparrow (Passerella iliaca), the swamp
sparrow (Melospiza palustris), the black and white creeper (Mniotilta
varia), the water-wagtail (Seiurus novaeboracencis), in Turdus
fuscescens and its allies, the difference in the size of the streaks is
often very considerable. In the song sparrow they vary to such an extent
that in some cases they are reduced to narrow lines; in others so
enlarged as to cover the greater part of the breast and sides of the
body, sometimes uniting on the middle of the breast into a nearly
continuous patch."

Mr. Allen then goes on to particularise several species in which such
variations occur, giving cases in which two specimens taken at the same
place on the same day exhibited the two extremes of coloration. Another
set of variations is thus described: "The white markings so common on
the wings and tails of birds, as the bars formed by the white tips of
the greater wing-coverts, the white patch occasionally present at the
base of the primary quills, or the white band crossing them, and the
white patch near the end of the outer tail-feathers are also extremely
liable to variation in respect to their extent and the number of
feathers to which, in the same species, these markings extend." It is to
be especially noted that all these varieties are distinct from those
which depend on season, on age, or on sex, and that they are such as
have in many other species been considered to be of specific value.

These variations of colour could not be presented to the eye without a
series of carefully engraved plates, but in order to bring Mr. Allen's
_measurements_, illustrating variations of size and proportion, more
clearly before the reader, I have prepared a series of diagrams
illustrating the more important facts and their bearings on the
Darwinian theory.

The first of these is intended, mainly, to show the actual amount of the
variation, as it gives the true length of the wing and tail in the
extreme cases among thirty specimens of each of three species. The
shaded portion shows the minimum length, the unshaded portion the
additional length in the maximum. The point to be specially noted here
is, that in each of these common species there is about the same amount
of variation, and that it is so great as to be obvious at a glance.

[Illustration: FIG. 3.--Variation of Wings and Tail.]

There is here no question of "minute" or "infinitesimal" variation,
which many people suppose to be the only kind of variation that exists.
It cannot even be called small; yet from all the evidence we now possess
it seems to be the amount which characterises most of the common species
of birds.

It may be said, however, that these are the extreme variations, and only
occur in one or two individuals, while the great majority exhibit little
or no difference. Other diagrams will show that this is not the case;
but even if it were so, it would be no objection at all, because these
are the extremes among thirty specimens only. We may safely assume that
these thirty specimens, taken by chance, are not, in the case of all
these species, exceptional lots, and therefore we might expect at least
two similarly varying specimens in each additional thirty. But the
number of individuals, even in a very rare species, is probably thirty
thousand or more, and in a common species thirty, or even three hundred,
millions. Even one individual in each thirty, varying to the amount
shown in the diagram, would give at least a million in the total
population of any common bird, and among this million many would vary
much more than the extreme among thirty only. We should thus have a vast
body of individuals varying to a large extent in the length of the wings
and tail, and offering ample material for the modification of these
organs by natural selection. We will now proceed to show that other
parts of the body vary, simultaneously, but independently, to an equal
amount.

[Illustration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]

[Illustration: FIG. 5.--Agelaeus phoeniceus. 40 Males.]

The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx
oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven
important characters in twenty male adult specimens.[21] These
characters are--the lengths of the body, wing, tail, tarsus, middle toe,
outer toe, and hind toe, being as many as can be conveniently exhibited
in one diagram. The length of the body is not given by Mr. Allen, but as
it forms a convenient standard of comparison, it has been obtained by
deducting the length of the tail from the total length of the birds as
given by him. The diagram has been constructed as follows:--The twenty
specimens are first arranged in a series according to the body-lengths
(which may be considered to give the size of the bird), from the
shortest to the longest, and the same number of vertical lines are
drawn, numbered from one to twenty. In this case (and wherever
practicable) the body-length is measured from the lower line of the
diagram, so that the actual length of the bird is exhibited as well as
the actual variations of length. These can be well estimated by means of
the horizontal line drawn at the mean between the two extremes, and it
will be seen that one-fifth of the total number of specimens taken on
either side exhibits a very large amount of variation, which would of
course be very much greater if a hundred or more specimens were
compared. The lengths of the wing, tail, and other parts are then laid
down, and the diagram thus exhibits at a glance the comparative
variation of these parts in every specimen as well as the actual amount
of variation in the twenty specimens; and we are thus enabled to arrive
at some important conclusions.

We note, first, that the variations of none of the parts follow the
variations of the body, but are sometimes almost in an opposite
direction. Thus the longest wing corresponds to a rather small body, the
longest tail to a medium body, while the longest leg and toes belong to
only a moderately large body. Again, even related parts do not
constantly vary together but present many instances of independent
variation, as shown by the want of parallelism in their respective
variation-lines. In No. 5 (see Fig. 4) the wing is very long, the tail
moderately so; while in No. 6 the wing is much shorter while the tail is
considerably longer. The tarsus presents comparatively little variation;
and although the three toes may be said to vary in general together,
there are many divergencies; thus, in passing from No. 9 to No. 10, the
outer toe becomes longer, while the hind toe becomes considerably
shorter; while in Nos. 3 and 4 the middle toe varies in an opposite way
to the outer and the hind toes.

[Illustration: FIG. 6.--Cardinalis virginianus. 31 Males.]

In the next diagram (Fig. 5) we have the variations in forty males of
the Red-winged Blackbird (Agelaeus phoeniceus), and here we see the same
general features. One-fifth of the whole number of specimens offer a
large amount of variation either below or above the mean; while the
wings, tail, and head vary quite independently of the body. The wing and
tail too, though showing some amount of correlated variation, yet in
no less than nine cases vary in opposite directions as compared with the
preceding species.

The next diagram (Fig. 6), showing the variations of thirty-one males of
the Cardinal bird (Cardinalis virginianus), exhibits these features much
more strongly. The amount of variation in proportion to the size of the
bird is very much greater; while the variations of the wing and tail not
only have no correspondence with that of the body but very little with
each other. In no less than twelve or thirteen instances they vary in
opposite directions, while even where they correspond in direction the
amount of the variation is often very disproportionate.

As the proportions of the tarsi and toes of birds have great influence
on their mode of life and habits and are often used as specific or even
generic characters, I have prepared a diagram (Fig. 7) to show the
variation in these parts only, among twenty specimens of each of four
species of birds, four or five of the most variable alone being given.
The extreme divergence of each of the lines in a vertical direction
shows the actual amount of variation; and if we consider the small
length of the toes of these small birds, averaging about three-quarters
of an inch, we shall see that the variation is really very large; while
the diverging curves and angles show that each part varies, to a great
extent, independently. It is evident that if we compared some thousands
of individuals instead of only twenty, we should have an amount of
independent variation occurring each year which would enable almost any
modification of these important organs to be rapidly effected.

[Illustration: FIG. 7.--Variation of Tarsus and Toes.]

[Illustration: FIG. 8.--Variation of Birds in Leyden Museum.]

In order to meet the objection that the large amount of variability here
shown depends chiefly on the observations of one person and on the birds
of a single country, I have examined Professor Schlegel's Catalogue of
the Birds in the Leyden Museum, in which he usually gives the range of
variation of the specimens in the museum (which are commonly less than a
dozen and rarely over twenty) as regards some of their more important
dimensions. These fully support the statement of Mr. Allen, since they
show an equal amount of variability when the numbers compared are
sufficient, which, however, is not often the case. The accompanying
diagram exhibits the actual differences of size in five organs which
occur in five species taken almost at random from this catalogue. Here,
again, we perceive that the variation is decidedly large, even among a
very small number of specimens; while the facts all show that there is
no ground whatever for the common assumption that natural species
consist of individuals which are nearly all alike, or that the
variations which occur are "infinitesimal" or even "small."


_The proportionate Number of Individuals which present a considerable
amount of Variation._

The notion that variation is a comparatively exceptional phenomenon, and
that in any case considerable variations occur very rarely in proportion
to the number of individuals which do not vary, is so deeply rooted that
it is necessary to show by every possible method of illustration how
completely opposed it is to the facts of nature. I have therefore
prepared some diagrams in which each of the individual birds measured is
represented by a spot, placed at a proportionate distance, right and
left, from the median line accordingly as it varies in excess or defect
of the mean length as regards the particular part compared. As the
object in this set of diagrams is to show the number of individuals
which vary considerably in proportion to those which vary little or not
at all, the scale has been enlarged in order to allow room for placing
the spots without overlapping each other.

In the diagram opposite twenty males of Icterus Baltimore are
registered, so as to exhibit to the eye the proportionate number of
specimens which vary, to a greater or less amount, in the length of the
tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed
that there is usually no very great accumulation of dots about the
median line which shows the average dimensions, but that a considerable
number are spread at varying distances on each side of it.

In the next diagram (Fig. 10), showing the variation among forty males
of Agelaeeus phoeniceus, this approach to an equable spreading of the
variations is still more apparent; while in Fig. 12, where fifty-eight
specimens of Cardinalis virginianus are registered, we see a remarkable
spreading out of the spots, showing in some of the characters a tendency
to segregation into two or more groups of individuals, each varying
considerably from the mean.

[Illustration: FIG. 9]

[Illustration: FIG. 10.]

[Illustration: FIG. 11.]

In order fully to appreciate the teaching of these diagrams, we must
remember, that, whatever kind and amount of variations are exhibited by
the few specimens here compared, would be greatly extended and brought
into symmetrical form if large numbers--thousands or millions--were
subjected to the same process of measurement and registration. We know,
from the general law which governs variations from a mean value, that
with increasing numbers the range of variation of each part would
increase also, at first rather rapidly and then more slowly; while gaps
and irregularities would be gradually filled up, and at length the
distribution of the dots would indicate a tolerably regular curve of
double curvature like those shown in Fig. 11. The great divergence of
the dots, when even a few specimens are compared, shows that the curve,
with high numbers, would be a flat one like the lower curve in the
illustration here given. This being the case it would follow that a very
large proportion of the total number of individuals constituting a
species would diverge considerably from its average condition as regards
each part or organ; and as we know from the previous diagrams of
variation (Figs. 1 to 7) that each part varies to a considerable extent,
_independently_, the materials constantly ready for natural selection to
act upon are abundant in quantity and very varied in kind. Almost any
combination of variations of distinct parts will be available, where
required; and this, as we shall see further on, obviates one of the most
weighty objections which have been urged against the efficiency of
natural selection in producing new species, genera, and higher groups.

[Illustration: FIG. 12.]


_Variation in the Mammalia._

Owing to the generally large size of this class of animals, and the
comparatively small number of naturalists who study them, large series
of specimens are only occasionally examined and compared, and thus the
materials for determining the question of their variability in a state
of nature are comparatively scanty. The fact that our domestic animals
belonging to this group, especially dogs, present extreme varieties not
surpassed even by pigeons and poultry among birds, renders it almost
certain that an equal amount of variability exists in the wild state;
and this is confirmed by the example of a species of squirrel (Sciurus
carolinensis), of which sixteen specimens, all males and all taken in
Florida, were measured and tabulated by Mr. Allen. The diagram here
given shows, that, both the general amount of the variation and the
independent variability of the several members of the body, accord
completely with the variations so common in the class of birds; while
their amount and their independence of each other are even greater than
usual.


_Variation in the Internal Organs of Animals._

In case it should be objected that the cases of variation hitherto
adduced are in the external parts only, and that there is no proof that
the internal organs vary in the same manner, it will be advisable to
show that such varieties also occur. It is, however, impossible to
adduce the same amount of evidence in this class of variation, because
the great labour of dissecting large numbers of specimens of the same
species is rarely undertaken, and we have to trust to the chance
observations of anatomists recorded in their regular course of study.

It must, however, be noted that a very large proportion of the
variations already recorded in the external parts of animals necessarily
imply corresponding internal variations. When feet and legs vary in
size, it is because the bones vary; when the head, body, limbs, and tail
change their proportions, the bony skeleton must also change; and even
when the wing or tail feathers of birds become longer or more numerous,
there is sure to be a corresponding change in the bones which support
and the muscles which move them. I will, however, give a few cases of
variations which have been directly observed.

[Illustration: FIG. 13.--Sciurus carolinensis. 32 specimens. Florida.]

Mr. Frank E. Beddard has kindly communicated to me some remarkable
variations he has observed in the internal organs of a species of
earthworm (Perionyx excavatus). The normal characters of this species
are--


    Setae forming a complete row round each segment.

    Two pairs of spermathecae--spherical pouches without
    diverticulae--in segments 8 and 9.

    Two pairs of testes in segments 11 and 12.

    Ovaries, a single pair in segment 13.

    Oviducts open by a common pore in the middle of segment 14.

    Vasa deferentia open separately in segment 18, each furnished at
    its termination with a large prostate gland.


Between two and three hundred specimens were examined, and among them
thirteen specimens exhibited the following marked variations:--


    (1) The number of the spermathecae varied from two to three or
    four pairs, their position also varying.

    (2) There were occasionally two pairs of ovaries, each with its
    own oviduct; the external apertures of these varied in position,
    being upon segments 13 and 14, 14 and 15, or 15 and 16.
    Occasionally when there was only the normal single oviduct pore
    present it varied in position, once occurring on the 10th, and
    once on the 11th segment.

    (3) The male generative pores varied in position from segments
    14 to 20. In one instance there were two pairs instead of the
    normal single pair, and in this case each of the four apertures
    had its own prostate gland.


Mr. Beddard remarks that all, or nearly all, the above variations are
found _normally_ in other genera and species.

When we consider the enormous number of earthworms and the comparatively
very small number of individuals examined, we may be sure, not only that
such variations as these occur with considerable frequency, but also
that still more extraordinary deviations from the normal structure may
often exist.

The next example is taken from Mr. Darwin's unpublished MSS.


    "In some species of Shrews (Sorex) and in some field-mice
    (Arvicola), the Rev. L. Jenyns (_Ann. Nat. Hist._, vol. vii. pp.
    267, 272) found the proportional length of the intestinal canal
    to vary considerably. He found the same variability in the
    number of the caudal vertebrae. In three specimens of an
    Arvicola he found the gall-bladder having a very different
    degree of development, and there is reason to believe it is
    sometimes absent. Professor Owen has shown that this is the case
    with the gall-bladder of the giraffe."


Dr. Crisp (_Proc. Zool. Soc._, 1862, p. 137) found the gall-bladder
present in some specimens of Cervus superciliaris while absent in
others; and he found it to be absent in three giraffes which he
dissected. A double gall-bladder was found in a sheep, and in a small
mammal preserved in the Hunterian Museum there are three distinct
gall-bladders.

The length of the alimentary canal varies greatly. In three adult
giraffes described by Professor Owen it was from 124 to 136 feet long;
one dissected in France had this canal 211 feet long; while Dr. Crisp
measured one of the extraordinary length of 254 feet, and similar
variations are recorded in other animals.[22]

The number of ribs varies in many animals. Mr. St. George Mivart says:
"In the highest forms of the Primates, the number of true ribs is seven,
but in Hylobates there are sometimes eight pairs. In Semnopithecus and
Colobus there are generally seven, but sometimes eight pairs of true
ribs. In the Cebidae there are generally seven or eight pairs, but in
Ateles sometimes nine" (_Proc. Zool. Soc._, 1865, p. 568). In the same
paper it is stated that the number of dorsal vertebrae in man is
normally twelve, very rarely thirteen. In the Chimpanzee there are
normally thirteen dorsal vertebrae, but occasionally there are fourteen
or only twelve.


_Variations in the Skull._

[Illustration: FIG. 14.--Variation of Skull of Wolf. 10 specimens.]

Among the nine adult male Orang-utans, collected by myself in Borneo,
the skulls differed remarkably in size and proportions. The orbits
varied in width and height, the cranial ridge was either single or
double, either much or little developed, and the zygomatic aperture
varied considerably in size. I noted particularly that these
variations bore no necessary relation to each other, so that a large
temporal muscle and zygomatic aperture might exist either with a large
or a small cranium; and thus was explained the curious difference
between the single-crested and the double-crested skulls, which had been
supposed to characterise distinct species. As an instance of the amount
of variation in the skulls of fully adult male orangs, I found the width
between the orbits externally to be only 4 inches in one specimen and
fully 5 inches in another.

Exact measurements of large series of comparable skulls of the mammalia
are not easily found, but from those available I have prepared three
diagrams (Figs. 14, 15, and 16), in order to exhibit the facts of
variation in this very important organ. The first shows the variation in
ten specimens of the common wolf (Canis lupus) from one district in
North America, and we see that it is not only large in amount, but that
each part exhibits a considerable independent variability.[23]

In Diagram 15 we have the variations of eight skulls of the Indian
Honey-bear (Ursus labiatus), as tabulated by the late Dr. J.E. Gray of
the British Museum. For such a small number of specimens the amount of
variation is very large--from one-eighth to one-fifth of the mean
size,--while there are an extraordinary number of instances of
independent variability. In Diagram 16 we have the length and width of
twelve skulls of adult males of the Indian wild boar (Sus cristatus),
also given by Dr. Gray, exhibiting in both sets of measurements a
variation of more than one-sixth, combined with a very considerable
amount of independent variability.[24]

[Illustration: FIG. 15.--Variation of 8 skulls (Ursus labiatus).]

[Illustration: FIG. 16.]

The few facts now given, as to variations of the internal parts of
animals, might be multiplied indefinitely by a search through the
voluminous writings of comparative anatomists. But the evidence already
adduced, taken in conjunction with the much fuller evidence of variation
in all external organs, leads us to the conclusion that wherever
variations are looked for among a considerable number of individuals of
the more common species they are sure to be found; that they are
everywhere of considerable amount, often reaching 20 per cent of the
size of the part implicated; and that they are to a great extent
independent of each other, and thus afford almost any combination of
variations that may be needed.

It must be particularly noticed that the whole series of
variation-diagrams here given (except the three which illustrate the
number of varying individuals) in every case represent the actual amount
of the variation, not on any reduced or enlarged scale, but as it were
life-size. Whatever number of inches or decimals of an inch the species
varies in any of its parts is marked on the diagrams, so that with the
help of an ordinary divided rule or a pair of compasses the variation of
the different parts can be ascertained and compared just as if the
specimens themselves were before the reader, but with much greater ease.

In my lectures on the Darwinian theory in America and in this country I
used diagrams constructed on a different plan, equally illustrating the
large amount of independent variability, but less simple and less
intelligible. The present method is a modification of that used by Mr.
Francis Galton in his researches on the theory of variability, the upper
line (showing the variability of the body) in Diagrams 4, 5, 6, and 13,
being laid down on the method he has used in his experiments with
sweet-peas and in pedigree moth-breeding.[25] I believe, after much
consideration, and many tedious experiments in diagram-making, that no
better method can be adopted for bringing before the eye, both the
amount and the peculiar features of individual variability.


_Variations of the Habits of Animals._

Closely connected with those variations of internal and external
structure which have been already described, are the changes of habits
which often occur in certain individuals or in whole species, since
these must necessarily depend upon some corresponding change in the
brain or in other parts of the organism; and as these changes are of
great importance in relation to the theory of instinct, a few examples
of them will be now adduced.

The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain
ranges of the Middle Island of New Zealand. It belongs to the family of
Brush-tongued parrots, and naturally feeds on the honey of flowers and
the insects which frequent them, together with such fruits or berries as
are found in the region. Till quite recently this comprised its whole
diet, but since the country it inhabits has become occupied by Europeans
it has developed a taste for a carnivorous diet, with alarming results.
It began by picking the sheepskins hung out to dry or the meat in
process of being cured. About 1868 it was first observed to attack
living sheep, which had frequently been found with raw and bleeding
wounds on their backs. Since then it is stated that the bird actually
burrows into the living sheep, eating its way down to the kidneys, which
form its special delicacy. As a natural consequence, the bird is being
destroyed as rapidly as possible, and one of the rare and curious
members of the New Zealand fauna will no doubt shortly cease to exist.
The case affords a remarkable instance of how the climbing feet and
powerful hooked beak developed for one set of purposes can be applied to
another altogether different purpose, and it also shows how little real
stability there may be in what appear to us the most fixed habits of
life. A somewhat similar change of diet has been recorded by the Duke of
Argyll, in which a goose, reared by a golden eagle, was taught by its
foster-parent to eat flesh, which it continued to do regularly and
apparently with great relish.[26]

Change of habits appears to be often a result of imitation, of which Mr.
Tegetmeier gives some good examples. He states that if pigeons are
reared exclusively with small grain, as wheat or barley, they will
starve before eating beans. But when they are thus starving, if a
bean-eating pigeon is put among them, they follow its example, and
thereafter adopt the habit. So fowls sometimes refuse to eat maize, but
on seeing others eat it, they do the same and become excessively fond of
it. Many persons have found that their yellow crocuses were eaten by
sparrows, while the blue, purple, and white coloured varieties were left
untouched; but Mr. Tegetmeier, who grows only these latter colours,
found that after two years the sparrows began to attack them, and
thereafter destroyed them quite as readily as the yellow ones; and he
believes it was merely because some bolder sparrow than the rest set the
example. On this subject Mr. Charles C. Abbott well remarks: "In
studying the habits of our American birds--and I suppose it is true of
birds everywhere--it must at all times be remembered that there is less
stability in the habits of birds than is usually supposed; and no
account of the habits of any one species will exactly detail the various
features of its habits as they really are, in every portion of the
territory it inhabits."[27]

Mr. Charles Dixon has recorded a remarkable change in the mode of
nest-building of some common chaffinches which were taken to New Zealand
and turned out there. He says: "The cup of the nest is small, loosely
put together, apparently lined with feathers, and the walls of the
structure are prolonged for about 18 inches, and hang loosely down the
side of the supporting branch. The whole structure bears some
resemblance to the nests of the hangnests (Icteridae), with the
exception that the cavity is at the top. Clearly these New Zealand
chaffinches were at a loss for a design when fabricating their nest.
They had no standard to work by, no nests of their own kind to copy, no
older birds to give them any instruction, and the result is the abnormal
structure I have just described."[28]

These few examples are sufficient to show that both the habits and
instincts of animals are subject to variation; and had we a sufficient
number of detailed observations we should probably find that these
variations were as numerous, as diverse in character, as large in
amount, and as independent of each other as those which we have seen to
characterise their bodily structure.


_The Variability of Plants._

The variability of plants is notorious, being proved not only by the
endless variations which occur whenever a species is largely grown by
horticulturists, but also by the great difficulty that is felt by
botanists in determining the limits of species in many large genera. As
examples we may take the roses, the brambles, and the willows as well
illustrating this fact. In Mr. Baker's _Revision of the British Roses_
(published by the Linnean Society in 1863), he includes under the single
species, Rosa canina--the common dog-rose--no less than twenty-eight
named _varieties_ distinguished by more or less constant characters and
often confined to special localities, and to these are referred about
seventy of the _species_ of British and continental botanists. Of the
genus Rubus or bramble, _five_ British species are given in Bentham's
_Handbook of the British Flora_, while in the fifth edition of
Babington's _Manual of British Botany_, published about the same time,
no less than _forty-five_ species are described. Of willows (Salix) the
same two works enumerate _fifteen_ and _thirty-one_ species
respectively. The hawkweeds (Hieracium) are equally puzzling, for while
Mr. Bentham admits only seven British species, Professor Babington
describes no less than thirty-two, besides several named varieties.

A French botanist, Mons. A. Jordan, has collected numerous forms of a
common little plant, the spring whitlow-grass (Draba verna); he has
cultivated these for several successive years, and declares that they
preserve their peculiarities unchanged; he also says that they each come
true from seed, and thus possess all the characteristics of true
species. He has described no less than fifty-two such species or
permanent varieties, all found in the south of France; and he urges
botanists to follow his example in collecting, describing, and
cultivating all such varieties as may occur in their respective
districts. Now, as the plant is very common almost all over Europe and
ranges from North America to the Himalayas, the number of similar forms
over this wide area would probably have to be reckoned by hundreds if
not by thousands.

The class of facts now adduced must certainly be held to prove that in
many large genera and in some single species there is a very large
amount of variation, which renders it quite impossible for experts to
agree upon the limits of species. We will now adduce a few striking
cases of individual variation.

The distinguished botanist, Alp. de Candolle, made a special study of
the oaks of the whole world, and has stated some remarkable facts as to
their variability. He declares that on the same branch of oak he has
noted the following variations: (1) In the length of the petiole, as one
to three; (2) in the form of the leaf, being either elliptical or
obovoid; (3) in the margin being entire, or notched, or even pinnatifid;
(4) in the extremity being acute or blunt; (5) in the base being sharp,
blunt, or cordate; (6) in the surface being pubescent or smooth; (7) the
perianth varies in depth and lobing; (8) the stamens vary in number,
independently; (9) the anthers are mucronate or blunt; (10) the fruit
stalks vary greatly in length, often as one to three; (11) the number of
fruits varies; (12) the form of the base of the cup varies; (13) the
scales of the cup vary in form; (14) the proportions of the acorns vary;
(15) the times of the acorns ripening and falling vary.

Besides this, many species exhibit well-marked varieties which have been
described and named, and these are most numerous in the best-known
species. Our British oak (Quercus robur) has twenty-eight varieties;
Quercus Lusitanica has eleven; Quercus calliprinos has ten; and Quercus
coccifera eight.

A most remarkable case of variation in the parts of a common flower has
been given by Dr. Hermann Müller. He examined two hundred flowers of
Myosurus minimus, among which he found _thirty-five_ different
proportions of the sepals, petals, and anthers, the first varying from
four to seven, the second from two to five, and the third from two to
ten. Five sepals occurred in one hundred and eighty-nine out of the two
hundred, but of these one hundred and five had three petals, forty-six
had four petals, and twenty-six had five petals; but in each of these
sets the anthers varied in number from three to eight, or from two to
nine. We have here an example of the same amount of "independent
variability" that, as we have seen, occurs in the various dimensions of
birds and mammals; and it may be taken as an illustration of the kind
and degree of variability that may be expected to occur among small and
little specialised flowers.[29]

In the common wind-flower (Anemone nemorosa) an almost equal amount of
variation occurs; and I have myself gathered in one locality flowers
varying from 7/8 inch to 1-3/4 inch in diameter; the bracts varying from
1-1/2 inch to 4 inches across; and the petaloid sepals either broad or
narrow, and varying in number from five to ten. Though generally pure
white on their upper surface, some specimens are a full pink, while
others have a decided bluish tinge.

Mr. Darwin states that he carefully examined a large number of plants of
Geranium phaeum and G. pyrenaicum (not perhaps truly British but
frequently found wild), which had escaped from cultivation, and had
spread by seed in an open plantation; and he declares that "the
seedlings varied in almost every single character, both in their flowers
and foliage, to a degree which I have never seen exceeded; yet they
could not have been exposed to any great change of their
conditions."[30]

The following examples of variation in important parts of plants were
collected by Mr. Darwin and have been copied from his unpublished
MSS.:--

"De Candolle (_Mem. Soc. Phys. de Genčve_, tom. ii. part ii. p. 217)
states that Papaver bracteatum and P. orientale present indifferently
two sepals and four petals, or three sepals and six petals, which is
sufficiently rare with other species of the genus."

"In the Primulacae and in the great class to which this family belongs
the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de
Genčve_, tom. ii. p. 406) has often found individuals in Cyclamen
hederaefolium, in which the base of the ovary was connected for a third
part of its length with the inferior part of the calyx."

"M. Aug. St. Hilaire (Sur la Gynobase, _Mem. des Mus. d'Hist. Nat._,
tom. x. p. 134), speaking of some bushes of the Gomphia oleaefolia,
which he at first thought formed a quite distinct species, says: 'Voilą
donc dans un mźme individu des loges et un style qui se rattachent
tantōt a un axe vertical, et tantōt a un gynobase; donc celui-ci n'est
qu'un axe veritable; mais cet axe est deprimé au lieu d'źtre vertical."
He adds (p. 151), 'Does not all this indicate that nature has tried, in
a manner, in the family of Rutaceae to produce from a single
multilocular ovary, one-styled and symmetrical, several unilocular
ovaries, each with its own style.' And he subsequently shows that, in
Xanthoxylum monogynum, 'it often happens that on the same plant, on the
same panicle, we find flowers with one or with two ovaries;' and that
this is an important character is shown by the Rutaceae (to which
Xanthoxylum belongs), being placed in a group of natural orders
characterised by having a solitary ovary."

"De Candolle has divided the Cruciferae into five sub-orders in
accordance with the position of the radicle and cotyledons, yet Mons. T.
Gay (_Ann. des Scien. Nat._, ser. i. tom. vii. p. 389) found in sixteen
seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that
he could not tell whether it ought to be placed in the sub-orders
'Pleurorhizée' or 'Notor-hizée'; so again (p. 400) in Cochlearia
saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were
vigorously 'pleurorhizées,' nine had characters intermediate between
pleuro-and notor-hizées, and four were pure notor-hizées."

"M. Raspail asserts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440)
that a grass (Nostus Borbonicus) is so eminently variable in its floral
organisation, that the varieties might serve to make a family with
sufficiently numerous genera and tribes--a remark which shows that
important organs must be here variable."


_Species which vary little._

The preceding statements, as to the great amount of variation occurring
in animals and plants, do not prove that all species vary to the same
extent, or even vary at all, but, merely, that a considerable number of
species in every class, order, and family do so vary. It will have been
observed that the examples of great variability have all been taken from
common species, or species which have a wide range and are abundant in
individuals. Now Mr. Darwin concludes, from an elaborate examination of
the floras and faunas of several distinct regions, that common, wide
ranging species, as a rule, vary most, while those that are confined to
special districts and are therefore comparatively limited in number of
individuals vary least. By a similar comparison it is shown that species
of large genera vary more than species of small genera. These facts
explain, to some extent, why the opinion has been so prevalent that
variation is very limited in amount and exceptional in character. For
naturalists of the old school, and all mere collectors, were interested
in species in proportion to their rarity, and would often have in their
collections a larger number of specimens of a rare species than of a
species that was very common. Now as these rare species do really vary
much less than the common species, and in many cases hardly vary at all,
it was very natural that a belief in the fixity of species should
prevail. It is not, however, as we shall see presently, the rare, but
the common and widespread species which become the parents of new forms,
and thus the non-variability of any number of rare or local species
offers no difficulty whatever in the way of the theory of evolution.


_Concluding Remarks._

We have now shown in some detail, at the risk of being tedious, that
individual variability is a general character of all common and
widespread species of animals or plants; and, further, that this
variability extends, so far as we know, to every part and organ, whether
external or internal, as well as to every mental faculty. Yet more
important is the fact that each part or organ varies to a considerable
extent independently of other parts. Again, we have shown, by abundant
evidence, that the variation that occurs is very large in
amount--usually reaching 10 or 20, and sometimes even 25 per cent of the
average size of the varying part; while not one or two only, but from 5
to 10 per cent of the specimens examined exhibit nearly as large an
amount of variation. These facts have been brought clearly before the
reader by means of numerous diagrams, drawn to scale and exhibiting the
actual variations in inches, so that there can be no possibility of
denying either their generality or their amount. The importance of this
full exposition of the subject will be seen in future chapters, when we
shall frequently have to refer to the facts here set forth, especially
when we deal with the various theories of recent writers and the
criticisms that have been made of the Darwinian theory.

A full exposition of the facts of variation among wild animals and
plants is the more necessary, because comparatively few of them were
published in Mr. Darwin's works, while the more important have only been
made known since the last edition of _The Origin of Species_ was
prepared; and it is clear that Mr. Darwin himself did not fully
recognise the enormous amount of variability that actually exists. This
is indicated by his frequent reference to the extreme slowness of the
changes for which variation furnishes the materials, and also by his use
of such expressions as the following: "A variety when once formed must
again, _perhaps after a long interval of time_, vary or present
individual differences of the same favourable nature as before"
(_Origin_, p. 66). And again, after speaking of changed conditions
"affording a better chance of the occurrence of favourable variations,"
he adds: "_Unless such occur natural selection can do nothing_"
(_Origin_, p. 64). These expressions are hardly consistent with the fact
of the constant and large amount of variation, of every part, in all
directions, which evidently occurs in each generation of all the more
abundant species, and which must afford an ample supply of favourable
variations whenever required; and they have been seized upon and
exaggerated by some writers as proofs of the extreme difficulties in the
way of the theory. It is to show that such difficulties do not exist,
and in the full conviction that an adequate knowledge of the facts of
variation affords the only sure foundation for the Darwinian theory of
the origin of species, that this chapter has been written.

FOOTNOTES:

[Footnote 16: _Foraminifera_, preface, p. x.]

[Footnote 17: _United States Geological Survey of the Territories_,
1874.]

[Footnote 18: _Proceedings of the Entomological Society of London_,
1875, p. vii.]

[Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.]

[Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.]

[Footnote 21: See Table I, p. 211, of Allen's _Winter Birds of
Florida_.]

[Footnote 22: _Proc. Zool. Soc._, 1864, p. 64.]

[Footnote 23: J.A. Allen, on Geographical Variation among North American
Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).]

[Footnote 24: _Proc. Zool. Soc. Lond._, 1864, p. 700, and 1868, p. 28.]

[Footnote 25: See _Trans. Entomological Society of London_, 1887, p.
24.]

[Footnote 26: _Nature_, vol. xix. p. 554.]

[Footnote 27: _Nature_, vol. xvi. p. 163; and vol. xi. p. 227.]

[Footnote 28: _Ibid._, vol. xxxi. (1885), p. 533.]

[Footnote 29: _Nature_, vol. xxvi. p. 81.]

[Footnote 30: _Animals and Plants under Domestication_, vol. ii. p.
258.]




CHAPTER IV

VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS


    The facts of variation and artificial selection--Proofs of the
    generality of variation--Variations of apples and
    melons--Variations of flowers--Variations of domestic
    animals--Domestic pigeons--Acclimatisation--Circumstances
    favourable to selection by man--Conditions favourable to
    variation--Concluding remarks.



Having so fully discussed variation under nature it will be unnecessary
to devote so much space to domesticated animals and cultivated plants,
especially as Mr. Darwin has published two remarkable volumes on the
subject where those who desire it may obtain ample information. A
general sketch of the more important facts will, however, be given, for
the purpose of showing how closely they correspond with those described
in the preceding chapter, and also to point out the general principles
which they illustrate. It will also be necessary to explain how these
variations have been increased and accumulated by artificial selection,
since we are thereby better enabled to understand the action of natural
selection, to be discussed in the succeeding chapter.


_The facts of Variation and Artificial Selection._

Every one knows that in each litter of kittens or of puppies no two are
alike. Even in the case in which several are exactly alike in colours,
other differences are always perceptible to those who observe them
closely. They will differ in size, in the proportions of their bodies
and limbs, in the length or texture of their hairy covering, and notably
in their disposition. They each possess, too, an individual
countenance, almost as varied when closely studied as that of a human
being; not only can a shepherd distinguish every sheep in his flock, but
we all know that each kitten in the successive families of our old
favourite cat has a face of its own, with an expression and
individuality distinct from all its brothers and sisters. Now this
individual variability exists among all creatures whatever, which we can
closely observe, even when the two parents are very much alike and have
been matched in order to preserve some special breed. The same thing
occurs in the vegetable kingdom. All plants raised from seed differ more
or less from each other. In every bed of flowers or of vegetables we
shall find, if we look closely, that there are countless small
differences, in the size, in the mode of growth, in the shape or colour
of the leaves, in the form, colour, or markings of the flowers, or in
the size, form, colour, or flavour of the fruit. These differences are
usually small, but are yet easily seen, and in their extremes are very
considerable; and they have this important quality, that they have a
tendency to be reproduced, and thus by careful breeding any particular
variation or group of variations can be increased to an enormous
extent--apparently to any extent not incompatible with the life, growth,
and reproduction of the plant or animal.

The way this is done is by artificial selection, and it is very
important to understand this process and its results. Suppose we have a
plant with a small edible seed, and we want to increase the size of that
seed. We grow as large a quantity of it as possible, and when the crop
is ripe we carefully choose a few of the very largest seeds, or we may
by means of a sieve sort out a quantity of the largest seeds. Next year
we sow only these large seeds, taking care to give them suitable soil
and manure, and the result is found to be that the _average_ size of the
seeds is larger than in the first crop, and that the largest seeds are
now somewhat larger and more numerous. Again sowing these, we obtain a
further slight increase of size, and in a very few years we obtain a
greatly improved race, which will always produce larger seeds than the
unimproved race, even if cultivated without any special care. In this
way all our fine sorts of vegetables, fruits, and flowers have been
obtained, all our choice breeds of cattle or of poultry, our wonderful
race-horses, and our endless varieties of dogs. It is a very common but
mistaken idea that this improvement is due to crossing and feeding in
the case of animals, and to improved cultivation in the case of plants.
Crossing is occasionally used in order to obtain a combination of
qualities found in two distinct breeds, and also because it is found to
increase the constitutional vigour; but every breed possessing any
exceptional quality is the result of the selection of variations
occurring year after year and accumulated in the manner just described.
Purity of breed, with repeated selection of the best varieties of that
breed, is the foundation of all improvement in our domestic animals and
cultivated plants.


_Proofs of the Generality of Variation._

Another very common error is, that variation is the exception, and
rather a rare exception, and that it occurs only in one direction at a
time--that is, that only one or two of the numerous possible modes of
variation occur at the same time. The experience of breeders and
cultivators, however, proves that variation is the rule instead of the
exception, and that it occurs, more or less, in almost every direction.
This is shown by the fact that different species of plants and animals
have required different _kinds_ of modification to adapt them to our
use, and we have never failed to meet with variation _in that particular
direction_, so as to enable us to accumulate it and so to produce
ultimately a large amount of change in the required direction. Our
gardens furnish us with numberless examples of this property of plants.
In the cabbage and lettuce we have found variation in the size and mode
of growth of the leaf, enabling us to produce by selection the almost
innumerable varieties, some with solid heads of foliage quite unlike any
plant in a state of nature, others with curiously wrinkled leaves like
the savoy, others of a deep purple colour used for pickling. From the
very same species as the cabbage (Brassica oleracea) have arisen the
broccoli and cauliflower, in which the leaves have undergone little
alteration, while the branching heads of flowers grow into a compact
mass forming one of our most delicate vegetables. The brussels sprouts
are another form of the same plant, in which the whole mode of growth
has been altered, numerous little heads of leaves being produced on the
stem. In other varieties the ribs of the leaves are thickened so as to
become themselves a culinary vegetable; while, in the Kohlrabi, the stem
grows into a turnip-like mass just above ground. Now all these
extraordinarily distinct plants come from one original species which
still grows wild on our coasts; and it must have varied in all these
directions, otherwise variations could not have been accumulated to the
extent we now see them. The flowers and seeds of all these plants have
remained nearly stationary, because no attempt has been made to
accumulate the slight variations that no doubt occur in them.

If now we turn to another set of plants, the turnips, radishes, carrots,
and potatoes, we find that the roots or underground tubers have been
wonderfully enlarged and improved, and also altered in shape and colour,
while the stems, leaves, flowers, and fruits have remained almost
unchanged. In the various kinds of peas and beans it is the pod or fruit
and the seed that has been subjected to selection, and therefore greatly
modified; and it is here very important to notice that while all these
plants have undergone cultivation in a great variety of soils and
climates, with different manures and under different systems, yet the
flowers have remained but little altered, those of the broad bean, the
scarlet-runner, and the garden-pea, being nearly the same in all the
varieties. This shows us how little change is produced by mere
cultivation, or even by variety of soil and climate, if there is no
_selection_ to preserve and accumulate the small variations that are
continually occurring. When, however, a great amount of modification has
been effected in one country, change to another country produces a
decided effect. Thus it has been found that some of the numerous
varieties of maize produced and cultivated in the United States change
considerably, not only in their size and colour, but even in the shape
of the seed when grown for a few successive years in Germany.[31] In all
our cultivated fruit trees the fruits vary immensely in shape, size,
colour, flavour, time of ripening, and other qualities, while the leaves
and flowers usually differ so little that they are hardly
distinguishable except to a very close observer.


_Variations of Apples and of Melons._

The most remarkable varieties are afforded by the apple and the melon,
and some account of these will be given as illustrating the effects of
slight variations accumulated by selection. All our apples are known to
have descended from the common crab of our hedges (Pyrus malus), and
from this at least a thousand distinct varieties have been produced.
These differ greatly in the size and form of the fruit, in its colour,
and in the texture of the skin. They further differ in the time of
ripening, in their flavour, and in their keeping properties; but apple
trees also differ in many other ways. The foliage of the different
varieties can often be distinguished by peculiarities of form and
colour, and it varies considerably in the time of its appearance; in
some hardly a leaf appears till the tree is in full bloom, while others
produce their leaves so early as almost to hide the flowers. The flowers
differ in size and colour, and in one case in structure also, that of
the St. Valery apple having a double calyx with ten divisions, and
fourteen styles with oblique stigmas, but without stamens or corolla.
The flowers, therefore, have to be fertilised with the pollen from other
varieties in order to produce fruit. The pips or seeds differ also in
shape, size, and colour; some varieties are liable to canker more than
others, while the Winter Majetin and one or two others have the strange
constitutional peculiarity of never being attacked by the mealy bug even
when all the other trees in the same orchard are infested with it.

All the cucumbers and gourds vary immensely, but the melon (Cucumis
melo) exceeds them all. A French botanist, M. Naudin, devoted six years
to their study. He found that previous botanists had described thirty
distinct species, as they thought, which were really only varieties of
melons. They differ chiefly in their fruits, but also very much in
foliage and mode of growth. Some melons are only as large as small
plums, others weigh as much as sixty-six pounds. One variety has a
scarlet fruit. Another is not more than an inch in diameter, but
sometimes more than a yard in length, twisting about in all directions
like a serpent. Some melons are exactly like cucumbers; and an Algerian
variety, when ripe, cracks and falls to pieces, just as occurs in a
wild gourd (C. momordica).[32]


_Variations of Flowers._

Turning to flowers, we find that in the same genus as our currant and
gooseberry, which we have cultivated for their fruits, there are some
ornamental species, as the Ribes sanguinea, and in these the flowers
have been selected so as to produce deep red, pink, or white varieties.
When any particular flower becomes fashionable and is grown in large
quantities, variations are always met with sufficient to produce great
varieties of tint or marking, as shown by our roses, auriculas, and
geraniums. When varied leaves are required, it is found that a number of
plants vary sufficiently in this direction also, and we have zonal
geraniums, variegated ivies, gold and silver marked hollies, and many
others.


_Variations of Domestic Animals._

Coming now to our domesticated animals, we find still more extraordinary
cases; and it appears as if any special quality or modification in an
animal can be obtained if we only breed it in sufficient quantity, watch
carefully for the required variations, and carry on selection with
patience and skill for a sufficiently long period. Thus, in sheep we
have enormously increased the wool, and have obtained the power of
rapidly forming flesh and fat; in cows we have increased the production
of milk; in horses we have obtained strength, endurance, or speed, and
have greatly modified size, form, and colour; in poultry we have secured
various colours of plumage, increase of size, and almost perpetual
egg-laying. But it is in dogs and pigeons that the most marvellous
changes have been effected, and these require our special attention.

Our various domestic dogs are believed to have originated from several
distinct wild species, because in every part of the world the native
dogs resemble some wild dogs or wolves of the same country. Thus perhaps
several species of wolves and jackals were domesticated in very early
times, and from breeds derived from these, crossed and improved by
selection, our existing dogs have descended. But this intermixture of
distinct species will go a very little way in accounting for the
peculiarities of the different breeds of dogs, many of which are totally
unlike any wild animal. Such is the case with greyhounds, bloodhounds,
bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and
many others; and these differ so greatly in size, shape, colour, and
habits, as well as in the form and proportions of all the different
parts of the body, that it seems impossible that they could have
descended from any of the known wild dogs, wolves, or allied animals,
none of which differ nearly so much in size, form, and proportions. We
have here a remarkable proof that variation is not confined to
superficial characters--to the colour, hair, or external appendages,
when we see how the entire skeletons of such forms as the greyhound and
the bulldog have been gradually changed in opposite directions till they
are both completely unlike that of any known wild animal, recent or
extinct. These changes have been the result of some thousands of years
of domestication and selection, different breeds being used and
preserved for different purposes; but some of the best breeds are known
to have been improved and perfected in modern times. About the middle of
the last century a new and improved kind of foxhound was produced; the
greyhound was also greatly improved at the end of the last century,
while the true bulldog was brought to perfection about the same period.
The Newfoundland dog has been so much changed since it was first
imported that it is now quite unlike any existing native dog in that
island.[33]


_Domestic Pigeons._

The most remarkable and instructive example of variation produced by
human selection is afforded by the various races and breeds of domestic
pigeons, not only because the variations produced are often most
extraordinary in amount and diverse in character, but because in this
case there is no doubt whatever that all have been derived from one wild
species, the common rock-pigeon (Columba livia). As this is a very
important point it is well to state the evidence on which the belief is
founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a
dark band across the end, the wings have two black bands, and the outer
tail-feathers are edged with white at the base. No other wild pigeon in
the world has this combination of characters. Now in every one of the
domestic varieties, even the most extreme, all the above marks, even to
the white edging of the outer tail-feathers, are sometimes found
perfectly developed. When birds belonging to two distinct breeds are
crossed one or more times, neither of the parents being blue, or having
any of the above-named marks, the mongrel offspring are very apt to
acquire some of these characters. Mr. Darwin gives instances which he
observed himself. He crossed some white fantails with some black barbs,
and the mongrels were black, brown, or mottled. He also crossed a barb
with a spot, which is a white bird with a red tail and red spot on the
forehead, and the mongrel offspring were dusky and mottled. On now
crossing these two sets of mongrels with each other, he obtained a bird
of a beautiful blue colour, with the barred and white edged tail, and
double-banded wings, so as almost exactly to resemble a wild
rock-pigeon. This bird was descended in the second generation from a
pure white and pure black bird, both of which when unmixed breed their
kind remarkably true. These facts, well known to experienced
pigeon-fanciers, together with the habits of the birds, which all like
to nest in holes, or dovecots, not in trees like the great majority of
wild pigeons, have led to the general belief in the single origin of all
the different kinds.

In order to afford some idea of the great differences which exist among
domesticated pigeons, it will be well to give a brief abstract of Mr.
Darwin's account of them. He divides them into eleven distinct races,
most of which have several sub-races.

RACE I. _Pouters_.--These are especially distinguished by the enormously
enlarged crop, which can be so inflated in some birds as almost to
conceal the beak. They are very long in the body and legs and stand
almost upright, so as to present a very distinct appearance. Their
skeleton has become modified, the ribs being broader and the vertebrae
more numerous than in other pigeons.

RACE II. _Carriers_.--These are large, long-necked birds, with a long
pointed beak, and the eyes surrounded with a naked carunculated skin or
wattle, which is also largely developed at the base of the beak. The
opening of the mouth is unusually wide. There are several sub-races, one
being called Dragons.

RACE III. _Runts_.--These are very large-bodied, long-beaked pigeons,
with naked skin round the eyes. The wings are usually very long, the
legs long, and the feet large, and the skin of the neck is often red.
There are several sub-races, and these differ very much, forming a
series of links between the wild rock-pigeon and the carrier.

RACE IV. _Barbs_.--These are remarkable for their very short and thick
beak, so unlike that of most pigeons that fanciers compare it with that
of a bullfinch. They have also a naked carunculated skin round the eyes,
and the skin over the nostrils swollen.

RACE V. _Fantails_.--Short-bodied and rather small-beaked pigeons, with
an enormously developed tail, consisting usually of from fourteen to
forty feathers instead of twelve, the regular number in all other
pigeons, wild and tame. The tail spreads out like a fan and is usually
carried erect, and the bird bends back its slender neck, so that in
highly-bred varieties the head touches the tail. The feet are small, and
they walk stiffly.

RACE VI. _Turbits and Owls_.--These are characterised by the feathers of
the middle of neck and breast in front spreading out irregularly so as
to form a frill. The Turbits also have a crest on the head, and both
have the beak exceedingly short.

RACE VII. _Tumblers_.--- These have a small body and short beak, but
they are specially distinguished by the singular habit of tumbling over
backwards during flight. One of the sub-races, the Indian Lotan or
Ground tumbler, if slightly shaken and placed on the ground, will
immediately begin tumbling head over heels until taken up and soothed.
If not taken up, some of them will go on tumbling till they die. Some
English tumblers are almost equally persistent. A writer, quoted by Mr.
Darwin, says that these birds generally begin to tumble almost as soon
as they can fly; "at three months old they tumble well, but still fly
strong; at five or six months they tumble excessively; and in the second
year they mostly give up flying, on account of their tumbling so much
and so close to the ground. Some fly round with the flock, throwing a
clean summersault every few yards till they are obliged to settle from
giddiness and exhaustion. These are called Air-tumblers, and they
commonly throw from twenty to thirty summersaults in a minute, each
clear and clean. I have one red cock that I have on two or three
occasions timed by my watch, and counted forty summersaults in the
minute. At first they throw a single summersault, then it is double,
till it becomes a continuous roll, which puts an end to flying, for if
they fly a few yards over they go, and roll till they reach the ground.
Thus I had one kill herself, and another broke his leg. Many of them
turn over only a few inches from the ground, and will tumble two or
three times in flying across their loft. These are called House-tumblers
from tumbling in the house. The act of tumbling seems to be one over
which they have no control, an involuntary movement which they seem to
try to prevent. I have seen a bird sometimes in his struggles fly a yard
or two straight upwards, the impulse forcing him backwards while he
struggles to go forwards."[34]

The Short-faced tumblers are an improved sub-race which have almost lost
the power of tumbling, but are valued for possessing some other
characteristics in an extreme degree. They are very small, have almost
globular heads, and a very minute beak, so that fanciers say the head of
a perfect bird should resemble a cherry with a barleycorn stuck in it.
Some of these weigh less than seven ounces, whereas the wild rock-pigeon
weighs about fourteen ounces. The feet, too, are very short and small,
and the middle toe has twelve or thirteen instead of fourteen or fifteen
scutellae. They have often only nine primary wing-feathers instead of
ten as in all other pigeons.

RACE VIII. _Indian Frill-back_.--In these birds the beak is very short,
and the feathers of the whole body are reversed or turn backwards.

RACE IX. _Jacobin_.--These curious birds have a hood of feathers almost
enclosing the head and meeting in front of the neck. The wings and tail
are unusually long.

RACE X. _Trumpeter_.--Distinguished by a tuft of feathers curling
forwards over the beak, and the feet very much feathered. They obtain
their name from the peculiar voice unlike that of any other pigeon. The
coo is rapidly repeated, and is continued for several minutes. The feet
are covered with feathers so large as often to appear like little wings.

RACE XI. comprises _Laughers_, _Frill-backs_, _Nuns_, _Spots_, _and
Swallows_.--They are all very like the common rock-pigeon, but have each
some slight peculiarity. The Laughers have a peculiar voice, supposed to
resemble a laugh. The Nuns are white, with the head, tail, and primary
wing-feathers black or red. The Spots are white, with the tail and a
spot on the forehead red. The Swallows are slender, white in colour,
with the head and wings of some darker colour.

Besides these races and sub-races a number of other kinds have been
described, and about one hundred and fifty varieties can be
distinguished. It is interesting to note that almost every part of the
bird, whose variations can be noted and selected, has led to variations
of a considerable extent, and many of these have necessitated changes in
the plumage and in the skeleton quite as great as any that occur in the
numerous distinct species of large genera. The form of the skull and
beak varies enormously, so that the skulls of the Short-faced tumbler
and some of the Carriers differ more than any wild pigeons, even those
classed in distinct genera. The breadth and number of the ribs vary, as
well as the processes on them; the number of the vertebrae and the
length of the sternum also vary; and the perforations in the sternum
vary in size and shape. The oil gland varies in development, and is
sometimes absent. The number of the wing-feathers varies, and those of
the tail to an enormous extent. The proportions of the leg and feet and
the number of the scutellae also vary. The eggs also vary somewhat in
size and shape; and the amount of downy clothing on the young bird, when
first hatched, differs very considerably. Finally, the attitude of the
body, the manner of walking, the mode of flight, and the voice, all
exhibit modifications of the most remarkable kind.[35]


_Acclimatisation_.

A very important kind of variation is that constitutional change termed
acclimatisation, which enables any organism to become gradually adapted
to a different climate from the parent stock. As closely allied species
often inhabit different countries possessing very different climates, we
should expect to find cases illustrating this change among our
domesticated animals and cultivated plants. A few examples will
therefore be adduced showing that such constitutional variation does
occur.

Among animals the cases are not numerous, because no systematic attempt
has been made to select varieties for this special quality. It has,
however, been observed that, though no European dogs thrive well in
India, the Newfoundland dog, originating from a severe climate, can
hardly be kept alive. A better case, perhaps, is furnished by merino
sheep, which, when imported directly from England, do not thrive, while
those which have been bred in the intermediate climate of the Cape of
Good Hope do much better. When geese were first introduced into Bogota,
they laid few eggs at long intervals, and few of the young survived. By
degrees, however, the fecundity improved, and in about twenty years
became equal to what it is in Europe. According to Garcilaso, when fowls
were first introduced into Peru they were not fertile, whereas now they
are as much so as in Europe.

Plants furnish much more important evidence. Our nurserymen distinguish
in their catalogues varieties of fruit-trees which are more or less
hardy, and this is especially the case in America, where certain
varieties only will stand the severe climate of Canada. There is one
variety of pear, the Forelle, which both in England and France withstood
frosts that killed the flowers and buds of all other kinds of pears.
Wheat, which is grown over so large a portion of the world, has become
adapted to special climates. Wheat imported from India and sown in good
wheat soil in England produced the most meagre ears; while wheat taken
from France to the West Indian Islands produced either wholly barren
spikes or spikes furnished with two or three miserable seeds, while West
Indian seed by its side yielded an enormous harvest. The orange was very
tender when first introduced into Italy, and continued so as long as it
was propagated by grafts, but when trees were raised from seed many of
these were found to be hardier, and the orange is now perfectly
acclimatised in Italy. Sweet-peas (Lathyrus odoratus) imported from
England to the Calcutta Botanic Gardens produced few blossoms and no
seed; those from France flowered a little better, but still produced no
seed, but plants raised from seed brought from Darjeeling in the
Himalayas, but originally derived from England, flower and seed
profusely in Calcutta.[36]

An observation by Mr. Darwin himself is perhaps even more instructive.
He says: "On 24th May 1864 there was a severe frost in Kent, and two
rows of scarlet runners (Phaseolus multiflorus) in my garden, containing
390 plants of the same age and equally exposed, were all blackened and
killed except about a dozen plants. In an adjoining row of Fulmer's
dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more
severe frost occurred four days afterwards, and of the dozen plants
which had previously escaped only three survived; these were not taller
or more vigorous than the other young plants, but they escaped
completely, with not even the tips of their leaves browned. It was
impossible to behold these three plants, with their blackened, withered,
and dead brethren all around them, and not see at a glance that they
differed widely in their constitutional power of resisting frost."

The preceding sketch of the variation that occurs among domestic animals
and cultivated plants shows how wide it is in range and how great in
amount; and we have good reason to believe that similar variation
extends to all organised beings. In the class of fishes, for example, we
have one kind which has been long domesticated in the East, the gold
and silver carps; and these present great variation, not only of colour
but in the form and structure of the fins and other external organs. In
like manner, the only domesticated insects, hive bees and silkworm
moths, present numbers of remarkable varieties which have been produced
by the selection of chance variations just as in the case of plants and
the higher animals.


_Circumstances favourable to Selection by Man._

It may be supposed, that the systematic selection which has been
employed for the purpose of improving the races of animals or plants
useful to man is of comparatively recent origin, though some of the
different races are known to have been in existence in very early times.
But Mr. Darwin has pointed out, that unconscious selection must have
begun to produce an effect as soon as plants were cultivated or animals
domesticated by man. It would have been very soon observed that animals
and plants produced their like, that seed of early wheat produced early
wheat, that the offspring of very swift dogs were also swift, and as
every one would try to have a good rather than a bad sort this would
necessarily lead to the slow but steady improvement of all useful plants
and animals subject to man's care. Soon there would arise distinct
breeds, owing to the varying uses to which the animals and plants were
put. Dogs would be wanted chiefly to hunt one kind of game in one part
of the country and another kind elsewhere; for one purpose scent would
be more important, for another swiftness, for another strength and
courage, for yet another watchfulness and intelligence, and this would
soon lead to the formation of very distinct races. In the case of
vegetables and fruits, different varieties would be found to succeed
best in certain soils and climates; some might be preferred on account
of the quantity of food they produced, others for their sweetness and
tenderness, while others might be more useful on account of their
ripening at a particular season, and thus again distinct varieties would
be established. An instance of unconscious selection leading to distinct
results in modern times is afforded by two flocks of Leicester sheep
which both originated from the same stock, and were then bred pure for
upwards of fifty years by two gentlemen, Mr. Buckley and Mr. Burgess.
Mr. Youatt, one of the greatest authorities on breeding domestic
animals, says: "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
original 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." In this case there was no desire to deviate
from the original breed, and the difference must have arisen from some
slight difference of taste or judgment in selecting, each year, the
parents for the next year's stock, combined perhaps with some direct
effect of the slight differences of climate and soil on the two farms.

Most of our domesticated animals and cultivated plants have come to us
from the earliest seats of civilisation in Western Asia or Egypt, and
have therefore been the subjects of human care and selection for some
thousands of years, the result being that, in many cases, we do not know
the wild stock from which they originally sprang. The horse, the camel,
and the common bull and cow are nowhere found in a wild state, and they
have all been domesticated from remote antiquity. The original of the
domestic fowl is still wild in India and the Malay Islands, and it was
domesticated in India and China before 1400 B.C. It was introduced into
Europe about 600 B.C. Several distinct breeds were known to the Romans
about the commencement of the Christian era, and they have since spread
all over the civilised world and been subjected to a vast amount of
conscious and unconscious selection, to many varieties of climate and to
differences of food; the result being seen in the wonderful diversity of
breeds which differ quite as remarkably as do the different races of
pigeons already described.

In the vegetable kingdom, most of the cereals--wheat, barley, etc.--are
unknown as truly wild plants; and the same is the case with many
vegetables, for De Candolle states that out of 157 useful cultivated
plants thirty-two are quite unknown in a wild state, and that forty more
are of doubtful origin. It is not improbable that most of these do exist
wild, but they have been so profoundly changed by thousands of years of
cultivation as to be quite unrecognisable. The peach is unknown in a
wild state, unless it is derived from the common almond, on which point
there is much difference of opinion among botanists and horticulturists.

The immense antiquity of most of our cultivated plants sufficiently
explains the apparent absence of such useful productions in Australia
and the Cape of Good Hope, notwithstanding that they both possess an
exceedingly rich and varied flora. These countries having been, until a
comparatively recent period, inhabited only by uncivilised men, neither
cultivation nor selection has been carried on for a sufficiently long
time. In North America, however, where there was evidently a very
ancient if low form of civilisation, as indicated by the remarkable
mounds, earthworks, and other prehistoric remains, maize was cultivated,
though it was probably derived from Peru; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.


_Conditions favourable to the production of Variations._

In order that plants and animals may be improved and modified to any
considerable extent, it is of course essential that suitable variations
should occur with tolerable frequency. There seem to be three conditions
which are especially favourable to the production of variations: (1)
That the particular species or variety should be kept in very large
numbers; (2) that it should be spread over a wide area and thus
subjected to a considerable diversity of physical conditions; and (3)
that it should be occasionally crossed with some distinct but closely
allied race. The first of these conditions is perhaps the most
important, the chance of variations of any particular kind being
increased in proportion to the quantity of the original stock and of its
annual offspring. It has been remarked that only those breeders who keep
large flocks can effect much improvement; and it is for the same reason
that pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great a number of
persons, have produced such strange and numerous varieties. In like
manner, nurserymen who grow fruit and flowers in large quantities have a
great advantage over private amateurs in the production of new
varieties.

Although I believe, for reasons which will be given further on, that
some amount of variability is a constant and necessary property of all
organisms, yet there appears to be good evidence to show that changed
conditions of life tend to increase it, both by a direct action on the
organisation and by indirectly affecting the reproductive system. Hence
the extension of civilisation, by favouring domestication under altered
conditions, facilitates the process of modification. Yet this change
does not seem to be an essential condition, for nowhere has the
production of extreme varieties of plants and flowers been carried
farther than in Japan, where careful selection continued for many
generations must have been the chief factor. The effect of occasional
crosses often results in a great amount of variation, but it also leads
to instability of character, and is therefore very little employed in
the production of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired qualities can be increased by
selection. It is for this reason that among savage peoples, whose
animals run half wild, little improvement takes place; and the
difficulty of isolation also explains why distinct and pure breeds of
cats are so rarely met with. The wide distribution of useful animals and
plants from a very remote epoch has, no doubt, been a powerful cause of
modification, because the particular breed first introduced into each
country has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also usually have
been selected for a somewhat different purpose in each locality, and
thus very distinct races would soon originate.

The important physiological effects of crossing breeds or strains, and
the part this plays in the economy of nature, will be explained in a
future chapter.


_Concluding Remarks._

The examples of variation now adduced--and these might have been almost
indefinitely increased--will suffice to show that there is hardly an
organ or a quality in plants or animals which has not been observed to
vary; and further, that whenever any of these variations have been
useful to man he has been able to increase them to a marvellous extent
by the simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller amount
occasionally appear, sometimes in external, sometimes in internal
characters, the very bones of the skeleton often changing slightly in
form, size, or number; but as these secondary characters have been of no
use to man, and have not been specially selected by him, they have,
usually, not been developed to any great amount except when they have
been closely dependent on those external characters which he has largely
modified.

As man has considered only utility to himself, or the satisfaction of
his love of beauty, of novelty, or merely of something strange or
amusing, the variations he has thus produced have something of the
character of monstrosities. Not only are they often of no use to the
animals or plants themselves, but they are not unfrequently injurious to
them. In the Tumbler pigeons, for instance, the habit of tumbling is
sometimes so excessive as to injure or kill the bird; and many of our
highly-bred animals have such delicate constitutions that they are very
liable to disease, while their extreme peculiarities of form or
structure would often render them quite unfit to live in a wild state.
In plants, many of our double flowers, and some fruits, have lost the
power of producing seed, and the race can thus be continued only by
means of cuttings or grafts. This peculiar character of domestic
productions distinguishes them broadly from wild species and varieties,
which, as will be seen by and by, are necessarily adapted in every part
of their organisation to the conditions under which they have to live.
Their importance for our present inquiry depends on their demonstrating
the occurrence of incessant slight variations in all parts of an
organism, with the transmission to the offspring of the special
characteristics of the parents; and also, that all such slight
variations are capable of being accumulated by selection till they
present very large and important divergencies from the ancestral stock.

We thus see, that the evidence as to variation afforded by animals and
plants under domestication strikingly accords with that which we have
proved to exist in a state of nature. And it is not at all surprising
that it should be so, since all the species were in a state of nature
when first domesticated or cultivated by man, and whatever variations
occur must be due to purely natural causes. Moreover, on comparing the
variations which occur in any one generation of domesticated animals
with those which we know to occur in wild animals, we find no evidence
of greater individual variation in the former than in the latter. The
results of man's selection are more striking to us because we have
always considered the varieties of each domestic animal to be
essentially identical, while those which we observe in a wild state are
held to be essentially diverse. The greyhound and the spaniel seem
wonderful, as varieties of one animal produced by man's selection; while
we think little of the diversities of the fox and the wolf, or the horse
and the zebra, because we have been accustomed to look upon them as
radically distinct animals, not as the results of nature's selection of
the varieties of a common ancestor.

FOOTNOTES:

[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
p. 322.]

[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
275, 330.]

[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
vol. i. pp. 40-42.]

[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
151.]

[Footnote 35: This account of domestic pigeons is greatly condensed from
Mr. Darwin's work already referred to.]

[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
307-311.]




CHAPTER V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST


    Effect of struggle for existence under unchanged conditions--The
    effect under change of conditions--Divergence of character--In
    insects--In birds--In mammalia--Divergence leads to a maximum of
    life in each area--Closely allied species inhabit distinct
    areas--Adaptation to conditions at various periods of life--The
    continued existence of low forms of life--Extinction of low
    types among the higher animals--Circumstances favourable to the
    origin of new species--Probable origin of the dippers--The
    importance of isolation--On the advance of organisation by
    natural selection--Summary of the first five chapters.



In the preceding chapters we have accumulated a body of facts and
arguments which will enable us now to deal with the very core of our
subject--the formation of species by means of natural selection. We have
seen how tremendous is the struggle for existence always going on in
nature owing to the great powers of increase of all organisms; we have
ascertained the fact of variability extending to every part and organ,
each of which varies simultaneously and for the most part independently;
and we have seen that this variability is both large in its amount in
proportion to the size of each part, and usually affects a considerable
proportion of the individuals in the large and dominant species. And,
lastly, we have seen how similar variations, occurring in cultivated
plants and domestic animals, are capable of being perpetuated and
accumulated by artificial selection, till they have resulted in all the
wonderful varieties of our fruits, flowers, and vegetables, our domestic
animals and household pets, many of which differ from each other far
more in external characters, habits, and instincts than do species in a
state of nature. We have now to inquire whether there is any analogous
process in nature, by which wild animals and plants can be permanently
modified and new races or new species produced.


_Effect of Struggle for Existence under Unchanged Conditions._

Let us first consider what will be the effect of the struggle for
existence upon the animals and plants which we see around us, under
conditions which do not perceptibly vary from year to year or from
century to century. We have seen that every species is exposed to
numerous and varied dangers throughout its entire existence, and that it
is only by means of the exact adaptation of its organisation--including
its instincts and habits--to its surroundings that it is enabled to live
till it produces offspring which may take its place when it ceases to
exist. We have seen also that, of the whole annual increase only a very
small fraction survives; and though the survival in individual cases may
sometimes be due rather to accident than to any real superiority, yet we
cannot doubt that, in the long run, those survive which are best fitted
by their perfect organisation to escape the dangers that surround them.
This "survival of the fittest" is what Darwin termed "natural
selection," because it leads to the same results in nature as are
produced by man's selection among domestic animals and cultivated
plants. Its primary effect will, clearly, be to keep each species in the
most perfect health and vigour, with every part of its organisation in
full harmony with the conditions of its existence. It prevents any
possible deterioration in the organic world, and produces that
appearance of exuberant life and enjoyment, of health and beauty, that
affords us so much pleasure, and which might lead a superficial observer
to suppose that peace and quietude reigned throughout nature.


_The Effect under changed Conditions._

But the very same process which, so long as conditions remain
substantially the same, secures the continuance of each species of
animal or plant in its full perfection, will usually, under changed
conditions, bring about whatever change of structure or habits may be
necessitated by them. The changed conditions to which we refer are such
as we know have occurred throughout all geological time and in every
part of the world. Land and water have been continually shifting their
positions; some regions are undergoing subsidence with diminution of
area, others elevation with extension of area; dry land has been
converted into marshes, while marshes have been drained or have even
been elevated into plateaux. Climate too has changed again and again,
either through the elevation of mountains in high latitudes leading to
the accumulation of snow and ice, or by a change in the direction of
winds and ocean currents produced by the subsidence or elevation of
lands which connected continents and divided oceans. Again, along with
all these changes have come not less important changes in the
distribution of species. Vegetation has been greatly modified by changes
of climate and of altitude; while every union of lands before separated
has led to extensive migrations of animals into new countries,
disturbing the balance that before existed among its forms of life,
leading to the extermination of some species and the increase of others.

When such physical changes as these have taken place, it is evident that
many species must either become modified or cease to exist. When the
vegetation has changed in character the herbivorous animals must become
able to live on new and perhaps less nutritious food; while the change
from a damp to a dry climate may necessitate migration at certain
periods to escape destruction by drought. This will expose the species
to new dangers, and require special modifications of structure to meet
them. Greater swiftness, increased cunning, nocturnal habits, change of
colour, or the power of climbing trees and living for a time on their
foliage or fruit, may be the means adopted by different species to bring
themselves into harmony with the new conditions; and by the continued
survival of those individuals, only, which varied sufficiently in the
right direction, the necessary modifications of structure or of function
would be brought about, just as surely as man has been able to breed the
greyhound to hunt by sight and the foxhound by scent, or has produced
from the same wild plant such distinct forms as the cauliflower and the
brussels sprouts.

We will now consider the special characteristics of the changes in
species that are likely to be effected, and how far they agree with what
we observe in nature.


_Divergence of Character._

In species which have a wide range the struggle for existence will often
cause some individuals or groups of individuals to adopt new habits in
order to seize upon vacant places in nature where the struggle is less
severe. Some, living among extensive marshes, may adopt a more aquatic
mode of life; others, living where forests abound, may become more
arboreal. In either case we cannot doubt that the changes of structure
needed to adapt them to their new habits would soon be brought about,
because we know that variations in all the external organs and all their
separate parts are very abundant and are also considerable in amount.
That such divergence of character has actually occurred we have some
direct evidence. Mr. Darwin informs us that in the Catskill Mountains in
the United States there are two varieties of wolves, one with a light
greyhound-like form which pursues deer, the other more bulky with
shorter legs, which more frequently attacks sheep.[37] Another good
example is that of the insects in the island of Madeira, many of which
have either lost their wings or have had them so much reduced as to be
useless for flight, while the very same species on the continent of
Europe possess fully developed wings. In other cases the wingless
Madeira species are distinct from, but closely allied to, winged species
of Europe. The explanation of this change is, that Madeira, like many
oceanic islands in the temperate zone, is much exposed to sudden gales
of wind, and as most of the fertile land is on the coast, insects which
flew much would be very liable to be blown out to sea and lost. Year
after year, therefore, those individuals which had shorter wings, or
which used them least, were preserved; and thus, in time, terrestrial,
wingless, or imperfectly winged races or species have been produced.
That this is the true explanation of this singular fact is proved by
much corroborative evidence. There are some few flower-frequenting
insects in Madeira to whom wings are essential, and in these the wings
are somewhat larger than in the same species on the mainland. We thus
see that there is no general tendency to the abortion of wings in
Madeira, but that it is simply a case of adaptation to new conditions.
Those insects to whom wings were not absolutely essential escaped a
serious danger by not using them, and the wings therefore became reduced
or were completely lost. But when they were essential they were enlarged
and strengthened, so that the insect could battle against the winds and
save itself from destruction at sea. Many flying insects, not varying
fast enough, would be destroyed before they could establish themselves,
and thus we may explain the total absence from Madeira of several whole
families of winged insects which must have had many opportunities of
reaching the islands. Such are the large groups of the tiger-beetles
(Cicindelidae), the chafers (Melolonthidae), the click-beetles
(Elateridae), and many others.

But the most curious and striking confirmation of this portion of Mr.
Darwin's theory is afforded by the case of Kerguelen Island. This island
was visited by the _Transit of Venus_ expedition. It is one of the
stormiest places on the globe, being subject to almost perpetual gales,
while, there being no wood, it is almost entirely without shelter. The
Rev. A.E. Eaton, an experienced entomologist, was naturalist to the
expedition, and he assiduously collected the few insects that were to be
found. All were incapable of flight, and most of them entirely without
wings. They included a moth, several flies, and numerous beetles. As
these insects could hardly have reached the islands in a wingless state,
even if there were any other known land inhabited by them--which there
is not--we must assume that, like the Madeiran insects, they were
originally winged, and lost their power of flight because its possession
was injurious to them.

It is no doubt due to the same cause that some butterflies on small and
exposed islands have their wings reduced in size, as is strikingly the
case with the small tortoise-shell butterfly (Vanessa urticae)
inhabiting the Isle of Man, which is only about half the size of the
same species in England or Ireland; and Mr. Wollaston notes that Vanessa
callirhoe--a closely allied South European form of our red-admiral
butterfly--is permanently smaller in the small and bare island of Porto
Santo than in the larger and more wooded adjacent island of Madeira.

A very good example of comparatively recent divergence of character, in
accordance with new conditions of life, is afforded by our red grouse.
This bird, the Lagopus scoticus of naturalists, is entirely confined to
the British Isles. It is, however, very closely allied to the willow
grouse (Lagopus albus), a bird which ranges all over Europe, Northern
Asia, and North America, but which, unlike our species, changes to white
in winter. No difference in form or structure can be detected between
the two birds, but as they differ so decidedly in colour--our species
being usually rather darker in winter than in summer, while there are
also slight differences in the call-note and in habits,--the two species
are generally considered to be distinct. The differences, however, are
so clearly adaptations to changed conditions that we can hardly doubt
that, during the early part of the glacial period, when our islands were
united to the continent, our grouse was identical with that of the rest
of Europe. But when the cold passed away and our islands became
permanently separated from the mainland, with a mild and equable climate
and very little snow in winter, the change to white at that season
became hurtful, rendering the birds more conspicuous instead of serving
as a means of concealment. The colour was, therefore, gradually changed
by the process of variation and natural selection; and as the birds
obtained ample shelter among the heather which clothes so many of our
moorlands, it became useful for them to assimilate with its brown and
dusky stems and withered flowers rather than with the snow of the higher
mountains. An interesting confirmation of this change having really
occurred is afforded by the occasional occurrence in Scotland of birds
with a considerable amount of white in the winter plumage. This is
considered to be a case of reversion to the ancestral type, just as the
slaty colours and banded wings of the wild rock-pigeon sometimes
reappear in our fancy breeds of domestic pigeons.[38]

The principle of "divergence of character" pervades all nature from the
lowest groups to the highest, as may be well seen in the class of birds.
Among our native species we see it well marked in the different species
of titmice, pipits, and chats. The great titmouse (Parus major) by its
larger size and stronger bill is adapted to feed on larger insects, and
is even said sometimes to kill small and weak birds. The smaller and
weaker coal titmouse (Parus ater) has adopted a more vegetarian diet,
eating seeds as well as insects, and feeding on the ground as well as
among trees. The delicate little blue titmouse (Parus coeruleus), with
its very small bill, feeds on the minutest insects and grubs which it
extracts from crevices of bark and from the buds of fruit-trees. The
marsh titmouse, again (Parus palustris), has received its name from the
low and marshy localities it frequents; while the crested titmouse
(Parus cristatus) is a northern bird frequenting especially pine
forests, on the seeds of which trees it partially feeds. Then, again,
our three common pipits--the tree-pipit (Anthus arboreus), the
meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus
obscurus) have each occupied a distinct place in nature to which they
have become specially adapted, as indicated by the different form and
size of the hind toe and claw in each species. So, the stone-chat
(Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
oenanthe) are more or less divergent forms of one type, with
modifications in the shape of the wing, feet, and bill adapting them to
slightly different modes of life. The whin-chat is the smallest, and
frequents furzy commons, fields, and lowlands, feeding on worms,
insects, small molluscs, and berries; the stone-chat is next in size,
and is especially active and lively, frequenting heaths and uplands, and
is a permanent resident with us, the two other species being migrants;
while the larger and more conspicuous wheat-ear, besides feeding on
grubs, beetles, etc., is able to capture flying insects on the wing,
something after the manner of true flycatchers.

These examples sufficiently indicate how divergence of character has
acted, and has led to the adaptation of numerous allied species, each to
a more or less special mode of life, with the variety of food, of
habits, and of enemies which must necessarily accompany such diversity.
And when we extend our inquiries to higher groups we find the same
indications of divergence and special adaptation, often to a still more
marked extent. Thus we have the larger falcons, which prey upon birds,
while some of the smaller species, like the hobby (Falco subbuteo), live
largely on insects. The true falcons capture their prey in the air,
while the hawks usually seize it on or near the ground, feeding on
hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and
buzzards, on the other hand, seize their prey upon the ground, and the
former feed largely on reptiles and offal as well as on birds and
quadrupeds. Others have adopted fish as their chief food, and the osprey
snatches its prey from the water with as much facility as a gull or a
petrel; while the South American caracaras (Polyborus) have adopted the
habits of vultures and live altogether on carrion. In every great group
there is the same divergence of habits. There are ground-pigeons,
rock-pigeons, and wood-pigeons,--seed-eating pigeons and fruit-eating
pigeons; there are carrion-eating, insect-eating, and fruit-eating
crows. Even kingfishers are, some aquatic, some terrestrial in their
habits; some live on fish, some on insects, some on reptiles. Lastly,
among the primary divisions of birds we find a purely terrestrial
group--the Ratitae, including the ostriches, cassowaries, etc.; other
great groups, including the ducks, cormorants, gulls, penguins, etc.,
are aquatic; while the bulk of the Passerine birds are aerial and
arboreal. The same general facts can be detected in all other classes of
animals. In the mammalia, for example, we have in the common rat a
fish-eater and flesh-eater as well as a grain-eater, which has no doubt
helped to give it the power of spreading over the world and driving away
the native rats of other countries. Throughout the Rodent tribe we find
everywhere aquatic, terrestrial, and arboreal forms. In the weasel and
cat tribes some live more in trees, others on the ground; squirrels have
diverged into terrestrial, arboreal, and flying species; and finally, in
the bats we have a truly aerial, and in the whales a truly aquatic order
of mammals. We thus see that, beginning with different varieties of the
same species, we have allied species, genera, families, and orders, with
similarly divergent habits, and adaptations to different modes of life,
indicating some general principle in nature which has been operative in
the development of the organic world. But in order to be thus operative
it must be a generally useful principle, and Mr. Darwin has very clearly
shown us in what this utility consists.


_Divergence leads to a Maximum of Organic Forms in each Area._

Divergence of character has a double purpose and use. In the first place
it enables a species which is being overcome by rivals, or is in
process of extinction by enemies, to save itself by adopting new habits
or by occupying vacant places in nature. This is the immediate and
obvious effect of all the numerous examples of divergence of character
which we have pointed out. But there is another and less obvious result,
which is, that the greater the diversity in the organisms inhabiting a
country or district the greater will be the total amount of life that
can be supported there. Hence the continued action of the struggle for
existence will tend to bring about more and more diversity in each area,
which may be shown to be the case by several kinds of evidence. As an
example, a piece of turf, three feet by four in size, was found by Mr.
Darwin to contain twenty species of plants, and these twenty species
belonged to eighteen genera and to eight orders, showing how greatly
they differed from each other. Farmers find that a greater quantity of
hay is obtained from ground sown with a variety of genera of grasses,
clover, etc., than from similar land sown with one or two species only;
and the same principle applies to rotation of crops, plants differing
very widely from each other giving the best results. So, in small and
uniform islands, and in small ponds of fresh water, the plants and
insects, though few in number, are found to be wonderfully varied in
character.

The same principle is seen in the naturalisation of plants and animals
by man's agency in distant lands, for the species that thrive best and
establish themselves permanently are not only very varied among
themselves but differ greatly from the native inhabitants. Thus, in the
Northern United States there are, according to Dr. Asa Gray, 260
naturalised flowering plants which belong to no less than 162 genera;
and of these, 100 genera are not natives of the United States. So, in
Australia, the rabbit, though totally unlike any native animal, has
increased so much that it probably outnumbers in individuals all the
native mammals of the country; and in New Zealand the rabbit and the pig
have equally multiplied. Darwin remarks that this "advantage of
diversification of structure 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 body. No physiologist doubts that a stomach adapted
to digest vegetable matter alone, or flesh alone, draws more 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."[39]


_The most closely allied Species inhabit distinct Areas._

One of the curious results of the general action of this principle in
nature is, that the most closely allied species--those whose differences
though often real and important are hardly perceptible to any one but a
naturalist--are usually not found in the same but in widely separated
countries. Thus, the nearest allies to our European golden plover are
found in North America and East Asia; the nearest ally of our European
jay is found in Japan, although there are several other species of jays
in Western Asia and North Africa; and though we have several species of
titmice in England they are not very closely allied to each other. The
form most akin to our blue tit is the azure tit of Central Asia (Parus
azureus); the Parus ledouci of Algeria is very near our coal tit, and
the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to
our marsh tit. So, our four species of wild pigeons--the ring-dove,
stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each
other, but each of them belongs, according to some ornithologists, to a
separate genus or subgenus, and has its nearest relatives in distant
parts of Asia and Africa. In mammalia the same thing occurs. Each
mountain region of Europe and Asia has usually its own species of wild
sheep and goat, and sometimes of antelope and deer; so that in each
region there is found the greatest diversity in this class of animals,
while the closest allies inhabit quite distinct and often distant areas.
In plants we find the same phenomenon prevalent. Distinct species of
columbine are found in Central Europe (Aguilegia vulgaris), in Eastern
Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the
Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in
Corsica (A. Bernardi), but rarely are two species found in the same
area. So, each part of the world has its own peculiar forms of pines,
firs, and cedars, but the closely allied species or varieties are in
almost every case inhabitants of distinct areas. Examples are the deodar
of the Himalayas, the cedar of Lebanon, and that of North Africa, all
very closely allied but confined to distinct areas; and the numerous
closely allied species of true pine (genus Pinus), which almost always
inhabit different countries or occupy different stations. We will now
consider some other modes in which natural selection will act, to adapt
organisms to changed conditions.


_Adaptation to Conditions at Various Periods of Life._

It is found, that, in domestic animals and cultivated plants, variations
occurring at any one period of life reappear in the offspring at the
same period, and can be perpetuated and increased by selection without
modifying other parts of the organisation. Thus, variations in the
caterpillar or the cocoon of the silkworm, in the eggs of poultry, and
in the seeds or young shoots of many culinary vegetables, have been
accumulated till those parts have become greatly modified and, for man's
purposes, improved. Owing to this fact it is easy for organisms to
become so modified as to avoid dangers that occur at any one period of
life. Thus it is that so many seeds have become adapted to various modes
of dissemination or protection. Some are winged, or have down or hairs
attached to them, so as to enable them to be carried long distances in
the air; others have curious hooks and prickles, which cause them to be
attached firmly to the fur of mammals or the feathers of birds; while
others are buried within sweet or juicy and brightly coloured fruits,
which are seen and devoured by birds, the hard smooth seeds passing
through their bodies in a fit state for germination. In the struggle for
existence it must benefit a plant to have increased means of dispersing
its seeds, and of thus having young plants produced in a greater variety
of soils, aspects, and surroundings, with a greater chance of some of
them escaping their numerous enemies and arriving at maturity. The
various differences referred to would, therefore, be brought about by
variation and survival of the fittest, just as surely as the length and
quality of cotton on the seed of the cotton-plant have been increased
by man's selection.

The larvae of insects have thus been wonderfully modified in order to
escape the numerous enemies to whose attacks they are exposed at this
period of their existence. Their colours and markings have become
marvellously adapted to conceal them among the foliage of the plant they
live upon, and this colour often changes completely after the last
moult, when the creature has to descend to the ground for its change to
the pupa state, during which period a brown instead of a green colour is
protective. Others have acquired curious attitudes and large ocelli,
which cause them to resemble the head of some reptile, or they have
curious horns or coloured ejectile processes which frighten away
enemies; while a great number have acquired secretions which render them
offensive to the taste of their enemies, and these are always adorned
with very conspicuous markings or brilliant colours, which serve as a
sign of inedibility and prevent their being needlessly attacked. This,
however, is a portion of the very large subject of organic colour and
marking, which will be fully discussed and illustrated in a separate
chapter.

In this way every possible modification of an animal or plant, whether
in colour, form, structure, or habits, which would be serviceable to it
or to its progeny at any period of its existence, may be readily brought
about. There are some curious organs which are used only once in a
creature's life, but which are yet essential to its existence, and thus
have very much the appearance of design by an intelligent designer. Such
are, the great jaws possessed by some insects, used exclusively for
opening the cocoon, and the hard tip to the beak of unhatched birds used
for breaking the eggshell. The increase in thickness or hardness of the
cocoons or the eggs being useful for protection against enemies or to
avoid accidents, it is probable that the change has been very gradual,
because it would be constantly checked by the necessity for a
corresponding change in the young insects or birds enabling them to
overcome the additional obstacle of a tougher cocoon or a harder
eggshell. As we have seen, however, that every part of the organism
appears to be varying independently, at the same time, though to
different amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the required change
from taking place.


_The Continued Existence of Low Forms of Life._

Since species are continually undergoing modifications giving them some
superiority over other species or enabling them to occupy fresh places
in nature, it may be asked--Why do any low forms continue to exist? Why
have they not long since been improved and developed into higher forms?
The answer, probably, is, that these low forms occupy places in nature
which cannot be filled by higher forms, and that they have few or no
competitors; they therefore continue to exist. Thus, earthworms are
adapted to their mode of life better than they would be if more highly
organised. So, in the ocean, the minute foraminifera and infusoria, and
the larger sponges and corals, occupy places which more highly developed
creatures could not fill. They form, as it were, the base of the great
structure of animal life, on which the next higher forms rest; and
though in the course of ages they may undergo some changes, and
diversification of form and structure, in accordance with changed
conditions, their essential nature has probably remained the same from
the very dawn of life on the earth. The low aquatic diatomaceae and
confervae, together with the lowest fungi and lichens, occupy a similar
position in the vegetable kingdom, filling places in nature which would
be left vacant if only highly organised plants existed. There is,
therefore, no motive power to destroy or seriously to modify them; and
they have thus probably persisted, under slightly varying forms, through
all geological time.


_Extinction of Lower Types among the Higher Animals._

So soon; however, as we approach the higher and more fully developed
groups, we see indications of the often repeated extinction of lower by
higher forms. This is shown by the great gaps that separate the
mammalia, birds, reptiles, and fishes from each other; while the lowest
forms of each are always few in number and confined to limited areas.
Such are the lowest mammals--the echidna and ornithorhynchus of
Australia; the lowest birds--the apteryx of New Zealand and the
cassowaries of the New Guinea region; while the lowest fish--the
amphioxus or lancelet, is completely isolated, and has apparently
survived only by its habit of burrowing in the sand. The great
distinctness of the carnivora, ruminants, rodents, whales, bats, and
other orders of mammalia; of the accipitres, pigeons, and parrots, among
birds; and of the beetles, bees, flies, and moths, among insects, all
indicate an enormous amount of extinction among the comparatively low
forms by which, on any theory of evolution, these higher and more
specialised groups must have been preceded.


_Circumstances favourable to the Origin of New Species by Natural
Selection._

We have already seen that, when there is no change in the physical or
organic conditions of a country, the effect of natural selection is to
keep all the species inhabiting it in a state of perfect health and full
development, and to preserve the balance that already exists between the
different groups of organisms. But, whenever the physical or organic
conditions change, to however small an extent, some corresponding change
will be produced in the flora and fauna, since, considering the severe
struggle for existence and the complex relations of the various
organisms, it is hardly possible that the change should not be
beneficial to some species and hurtful to others. The most common
effect, therefore, will be that some species will increase and others
will diminish; and in cases where a species was already small in numbers
a further diminution might lead to extinction. This would afford room
for the increase of other species, and thus a considerable readjustment
of the proportions of the several species might take place. When,
however, the change was of a more important character, directly
affecting the existence of many species so as to render it difficult for
them to maintain themselves without some considerable change in
structure or habits, that change would, in some cases, be brought about
by variation and natural selection, and thus new varieties or new
species might be formed. We have to consider, then, which are the
species that would be most likely to be so modified, while others, not
becoming modified, would succumb to the changed conditions and become
extinct.

The most important condition of all is, undoubtedly, that variations
should occur of sufficient amount, of a sufficiently diverse character,
and in a large number of individuals, so as to afford ample materials
for natural selection to act upon; and this, we have seen, does occur in
most, if not in all, large, wide-ranging, and dominant species. From
some of these, therefore, the new species adapted to the changed
conditions would usually be derived; and this would especially be the
case when the change of conditions was rather rapid, and when a
correspondingly rapid modification could alone save some species from
extinction. But when the change was very gradual, then even less
abundant and less widely distributed species might become modified into
new forms, more especially if the extinction of many of the rarer
species left vacant places in the economy of nature.


_Probable Origin of the Dippers._

An excellent example of how a limited group of species has been able to
maintain itself by adaptation to one of these "vacant places" in nature,
is afforded by the curious little birds called dippers or water-ouzels,
forming the genus Cinclus and the family Cinclidae of naturalists. These
birds are something like small thrushes, with very short wings and tail,
and very dense plumage. They frequent, exclusively, mountain torrents in
the northern hemisphere, and obtain their food entirely in the water,
consisting, as it does, of water-beetles, caddis-worms and other
insect-larvae, as well as numerous small freshwater shells. These birds,
although not far removed in structure from thrushes and wrens, have the
extraordinary power of flying under water; for such, according to the
best observers, is their process of diving in search of their prey,
their dense and somewhat fibrous plumage retaining so much air that the
water is prevented from touching their bodies or even from wetting their
feathers to any great extent. Their powerful feet and long curved claws
enable them to hold on to stones at the bottom, and thus to retain their
position while picking up insects, shells, etc. As they frequent
chiefly the most rapid and boisterous torrents, among rocks, waterfalls,
and huge boulders, the water is never frozen over, and they are thus
able to live during the severest winters. Only a very few species of
dipper are known, all those of the old world being so closely allied to
our British bird that some ornithologists consider them to be merely
local races of one species; while in North America and the northern
Andes there are two other species.

Here then we have a bird, which, in its whole structure, shows a close
affinity to the smaller typical perching birds, but which has departed
from all its allies in its habits and mode of life, and has secured for
itself a place in nature where it has few competitors and few enemies.
We may well suppose, that, at some remote period, a bird which was
perhaps the common and more generalised ancestor of most of our
thrushes, warblers, wrens, etc., had spread widely over the great
northern continent, and had given rise to numerous varieties adapted to
special conditions of life. Among these some took to feeding on the
borders of clear streams, picking out such larvae and molluscs as they
could reach in shallow water. When food became scarce they would attempt
to pick them out of deeper and deeper water, and while doing this in
cold weather many would become frozen and starved. But any which
possessed denser and more hairy plumage than usual, which was able to
keep out the water, would survive; and thus a race would be formed which
would depend more and more on this kind of food. Then, following up the
frozen streams into the mountains, they would be able to live there
during the winter; and as such places afforded them much protection from
enemies and ample shelter for their nests and young, further adaptations
would occur, till the wonderful power of diving and flying under water
was acquired by a true land-bird.

That such habits might be acquired under stress of need is rendered
highly probable by the facts stated by the well-known American
naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.)
all wade in water, and often, seeing minute mollusca on the bottom of
the stream, plunge both head and neck beneath the surface, so that
often, for several seconds, a large part of the body is submerged. Now
these birds still have the plumage pervious to water, and so are liable
to be drenched and sodden; but they have also the faculty of giving
these drenched feathers such a good shaking that flight is practicable a
moment after leaving the water. Certainly the water-thrushes (Seiurus
ludovicianus, S. auricapillus, and S. noveboracensis) have taken many
preliminary steps to becoming as aquatic as the dipper; and the
winter-wren, and even the Maryland yellow-throat are not far
behind."[40]

Another curious example of the way in which species have been modified
to occupy new places in nature, is afforded by the various animals which
inhabit the water-vessels formed by the leaves of many epiphytal species
of Bromelia. Fritz Müller has described a caddis-fly larva which lives
among these leaves, and which has been modified in the pupa state in
accordance with its surroundings. The pupae of caddis-flies inhabiting
streams have fringes of hair on the tarsi to enable them to reach the
surface on leaving their cases. But in the species inhabiting bromelia
leaves there is no need for swimming, and accordingly we find the tarsi
entirely bare. In the same plants are found curious little Entomostraca,
very abundant there but found nowhere else. These form a new genus, but
are most nearly allied to Cythere, a marine type. It is believed that
the transmission of this species from one tree to another must be
effected by the young crustacea, which are very minute, clinging to
beetles, many of which, both terrestrial and aquatic, also inhabit the
bromelia leaves; and as some water-beetles are known to frequent the
sea, it is perhaps by these means that the first emigrants established
themselves in this strange new abode. Bromeliae are often very abundant
on trees growing on the water's edge, and this would facilitate the
transition from a marine to an arboreal habitat. Fritz Müller has also
found, among the bromelia leaves, a small frog bearing its eggs on its
back, and having some other peculiarities of structure. Several
beautiful little aquatic plants of the genus Utricularia or bladder-wort
also inhabit bromelia leaves; and these send runners out to neighbouring
plants and thus spread themselves with great rapidity.


_The Importance of Isolation._

Isolation is no doubt an important aid to natural selection, as shown by
the fact that islands so often present a number of peculiar species; and
the same thing is seen on the two sides of a great mountain range or on
opposite coasts of a continent. The importance of isolation is twofold.
In the first place, it leads to a body of individuals of each species
being limited in their range and thus subjected to uniform conditions
for long spaces of time. Both the direct action of the environment and
the natural selection of such varieties only as are suited to the
conditions, will, therefore, be able to produce their full effect. In
the second place, the process of change will not be interfered with by
intercrossing with other individuals which are becoming adapted to
somewhat different conditions in an adjacent area. But this question of
the swamping effects of intercrossing will be considered in another
chapter.

Mr. Darwin was of opinion that, on the whole, the largeness of the area
occupied by a species was of more importance than isolation, as a factor
in the production of new species, and in this I quite agree with him. It
must, too, be remembered, that isolation will often be produced in a
continuous area whenever a species becomes modified in accordance with
varied conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become modified
in one direction, and in the southern part in another direction; and
though for a long time an intermediate form may continue to exist in the
intervening area, this will be likely soon to die out, both because its
numbers will be small, and it will be more or less pressed upon in
varying seasons by the modified varieties, each better able to endure
extremes of climate. So, when one portion of a terrestrial species takes
to a more arboreal or to a more aquatic mode of life, the change of
habit itself leads to the isolation of each portion. Again, as will be
more fully explained in a future chapter, any difference of habits or of
haunts usually leads to some modification of colour or marking, as a
means of concealment from enemies; and there is reason to believe that
this difference will be intensified by natural selection as a means of
identification and recognition by members of the same variety or
incipient species. It has also been observed that each differently
coloured variety of wild animals, or of domesticated animals which have
run wild, keep together, and refuse to pair with individuals of the
other colours; and this must of itself act to keep the races separate as
completely as physical isolation.


_On the Advance of Organisation by Natural Selection._

As natural selection acts solely by the preservation of useful
variations, or those which are beneficial to the organism under the
conditions to which it is exposed, the result must necessarily be that
each species or group tends to become more and more improved in relation
to its conditions. Hence we should expect that the larger groups in each
class of animals and plants--those which have persisted and have been
abundant throughout geological ages--would, almost necessarily, have
arrived at a high degree of organisation, both physical and mental.
Illustrations of this are to be seen everywhere. Among mammalia we have
the carnivora, which from Eocene times have been becoming more and more
specialised, till they have culminated in the cat and dog tribes, which
have reached a degree of perfection both in structure and intelligence
fully equal to that of any other animals. In another line of
development, the herbivora have been specialised for living solely on
vegetable food till they have culminated in the sheep, the cattle, the
deer, and the antelopes. The horse tribe, commencing with an early
four-toed ancestor in the Eocene age, has increased in size and in
perfect adaptation of feet and teeth to a life on open plains, and has
reached its highest perfection in the horse, the ass, and the zebra. In
birds, also, we see an advance from the imperfect tooth-billed and
reptile-tailed birds of the secondary epoch, to the wonderfully
developed falcons, crows, and swallows of our time. So, the ferns,
lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic
rocks, have developed into the marvellous wealth of forms of the higher
dicotyledons that now adorn the earth.

But this remarkable advance in the higher and larger groups does not
imply any universal law of progress in organisation, because we have at
the same time numerous examples (as has been already pointed out) of the
persistence of lowly organised forms, and also of absolute degradation
or degeneration. Serpents, for example, have been developed from some
lizard-like type which has lost its limbs; and though this loss has
enabled them to occupy fresh places in nature and to increase and
flourish to a marvellous extent, yet it must be considered to be a
retrogression rather than an advance in organisation. The same remark
will apply to the whale tribe among mammals; to the blind amphibia and
insects of the great caverns; and among plants to the numerous cases in
which flowers, once specially adapted to be fertilised by insects, have
lost their gay corollas and their special adaptations, and have become
degraded into wind-fertilised forms. Such are our plantains, our meadow
burnet, and even, as some botanists maintain, our rushes, sedges, and
grasses. The causes which have led to this degeneration will be
discussed in a future chapter; but the facts are undisputed, and they
show us that although variation and the struggle for existence may lead,
on the whole, to a continued advance of organisation; yet they also lead
in many cases to a retrogression, when such retrogression may aid in the
preservation of any form under new conditions. They also lead to the
persistence, with slight modifications, of numerous lowly organised
forms which are suited to places which higher forms could not fully
occupy, or to conditions under which they could not exist. Such are the
ocean depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc.; and it is in such places as these, as well as
in some oceanic islands which competing higher forms have not been able
to reach, that we find many curious relics of an earlier world, which,
in the free air and sunlight and in the great continents, have long
since been driven out or exterminated by higher types.


_Summary of the first Five Chapters._

We have now passed in review, in more or less detail, the main facts on
which the theory of "the origin of species by means of natural
selection" is founded. In future chapters we shall have to deal mainly
with the application of the theory to explain the varied and complex
phenomena presented by the organic world; and, also, to discuss some of
the theories put forth by modern writers, either as being more
fundamental than that of Darwin or as supplementary to it. Before doing
this, however, it will be well briefly to summarise the facts and
arguments already set forth, because it is only by a clear comprehension
of these that the full importance of the theory can be appreciated and
its further applications understood.

The theory itself is exceedingly simple, and the facts on which it
rests--though excessively numerous individually, and coextensive with
the entire organic world--yet come under a few simple and easily
understood classes. These facts are,--first, the enormous powers of
increase in geometrical progression possessed by all organisms, and the
inevitable struggle for existence among them; and, in the second place,
the occurrence of much individual variation combined with the hereditary
transmission of such variations. From these two great classes of facts,
which are universal and indisputable, there necessarily arises, as
Darwin termed it, the "preservation of favoured races in the struggle
for life," the continuous action of which, under the ever-changing
conditions both of the inorganic and organic universe, necessarily leads
to the formation or development of new species.

But, although this general statement is complete and indisputable, yet
to see its applications under all the complex conditions that actually
occur in nature, it is necessary always to bear in mind the tremendous
power and universality of the agencies at work. We must never for an
instant lose sight of the fact of the enormously rapid increase of all
organisms, which has been illustrated by actual cases, given in our
second chapter, no less than by calculations of the results of unchecked
increase for a few years. Then, never forgetting that the animal and
plant population of any country is, on the whole, stationary, we must be
always trying to realise the ever-recurring destruction of the enormous
annual increase, and asking ourselves what determines, in each
individual case, the death of the many, the survival of the few. We must
think over all the causes of destruction to each organism,--to the seed,
the young shoot, the growing plant, the full-grown tree, or shrub, or
herb, and again the fruit and seed; and among animals, to the egg or
new-born young, to the youthful, and to the adults. Then, we must always
bear in mind that what goes on in the case of the individual or family
group we may observe or think of, goes on also among the millions and
scores of millions of individuals which are comprised in almost every
species; and must get rid of the idea that _chance_ determines which
shall live and which die. For, although in many individual cases death
may be due to chance rather than to any inferiority in those which die
first, yet we cannot possibly believe that this can be the case on the
large scale on which nature works. A plant, for instance, cannot be
increased unless there are suitable vacant places its seeds can grow in,
or stations where it can overcome other less vigorous and healthy
plants. The seeds of all plants, by their varied modes of dispersal, may
be said to be seeking out such places in which to grow; and we cannot
doubt that, in the long run, those individuals whose seeds are the most
numerous, have the greatest powers of dispersal, and the greatest vigour
of growth, will leave more descendants than the individuals of the same
species which are inferior in all these respects, although now and then
some seed of an inferior individual may _chance_ to be carried to a spot
where it can grow and survive. The same rule will apply to every period
of life and to every danger to which plants or animals are exposed. The
best organised, or the most healthy, or the most active, or the best
protected, or the most intelligent, will inevitably, in the long run,
gain an advantage over those which are inferior in these qualities; that
is, _the fittest will survive_, the fittest being, in each particular
case, those which are superior in the special qualities on which safety
depends. At one period of life, or to escape one kind of danger,
concealment may be necessary; at another time, to escape another danger,
swiftness; at another, intelligence or cunning; at another, the power to
endure rain or cold or hunger; and those which possess all these
faculties in the fullest perfection will generally survive.

Having fully grasped these facts in all their fulness and in their
endless and complex results, we have next to consider the phenomena of
variation, discussed in the third and fourth chapters; and it is here
that perhaps the greatest difficulty will be felt in appreciating the
full importance of the evidence as set forth. It has been so generally
the practice to speak of variation as something exceptional and
comparatively rare--as an abnormal deviation from the uniformity and
stability of the characters of a species--and so few even among
naturalists have ever compared, accurately, considerable numbers of
individuals, that the conception of variability as a general
characteristic of all dominant and widespread species, large in its
amount and affecting, not a few, but considerable masses of the
individuals which make up the species, will be to many entirely new.
Equally important is the fact that the variability extends to every
organ and every structure, external and internal; while perhaps most
important of all is the independent variability of these several parts,
each one varying without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such correlation
in the differences that exist between species and species--more
developed wings usually accompanying smaller feet and _vice versā_--but
this is, generally, a useful adaptation which has been brought about by
natural selection, and does not apply to the individual variability
which occurs within the species.

It is because these facts of variation are so important and so little
understood, that they have been discussed in what will seem to some
readers wearisome and unnecessary detail. Many naturalists, however,
will hold that even more evidence is required; and more, to almost any
amount, could easily have been given. The character and variety of that
already adduced will, however, I trust, convince most readers that the
facts are as stated; while they have been drawn from a sufficiently wide
area to indicate a general principle throughout nature.

If, now, we fully realise these facts of variation, along with those of
rapid multiplication and the struggle for existence, most of the
difficulties in the way of comprehending how species have originated
through natural selection will disappear. For whenever, through changes
of climate, or of altitude, or of the nature of the soil, or of the area
of the country, any species are exposed to new dangers, and have to
maintain themselves and provide for the safety of their offspring under
new and more arduous conditions, then, in the variability of all parts,
organs, and structures, no less than of habits and intelligence, we have
the means of producing modifications which will certainly bring the
species into harmony with its new conditions. And if we remember that
all such physical changes are slow and gradual in their operation, we
shall see that the amount of variation which we know occurs in every new
generation will be quite sufficient to enable modification and
adaptation to go on at the same rate. Mr. Darwin was rather inclined to
exaggerate the necessary slowness of the action of natural selection;
but with the knowledge we now possess of the great amount and range of
individual variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied species,
sometimes taking place in less than a century, should any rapid change
of conditions necessitate an equally rapid adaptation. This may often
have occurred, either to immigrants into a new land, or to residents
whose country has been cut off by subsidence from a larger and more
varied area over which they had formerly roamed. When no change of
conditions occurs, species may remain unchanged for very long periods,
and thus produce that appearance of stability of species which is even
now often adduced as an argument against evolution by natural selection,
but which is really quite in harmony with it.

On the principles, and by the light of the facts, now briefly
summarised, we have been able, in the present chapter, to indicate how
natural selection acts, how divergence of character is set up, how
adaptation to conditions at various periods of life has been effected,
how it is that low forms of life continue to exist, what kind of
circumstances are most favourable to the formation of new species, and,
lastly, to what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to consider some of
the more important objections and difficulties which have been advanced
by eminent naturalists.

FOOTNOTES:

[Footnote 37: _Origin of Species_, p. 71.]

[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
77.]

[Footnote 39: _Origin of Species_, p. 89.]

[Footnote 40: _Nature_, vol. xxx. p. 30.]




CHAPTER VI

DIFFICULTIES AND OBJECTIONS


    Difficulty as to smallness of variations--As to the right
    variations occurring when required--The beginnings of important
    organs--The mammary glands--The eyes of flatfish--Origin of the
    eye--Useless or non-adaptive characters--Recent extension of the
    region of utility in plants--The same in animals--Uses of
    tails--Of the horns of deer--Of the scale-ornamentation of
    reptiles--Instability of non-adaptive characters--Delboeuf's
    law--No "specific" character proved to be useless--The swamping
    effects of intercrossing--Isolation as preventing
    intercrossing--Gulick on the effects of isolation--Cases in
    which isolation is ineffective.



In the present chapter I propose to discuss the more obvious and often
repeated objections to Darwin's theory, and to show how far they affect
its character as a true and sufficient explanation of the origin of
species. The more recondite difficulties, affecting such fundamental
questions as the causes and laws of variability, will be left for a
future chapter, after we have become better acquainted with the
applications of the theory to the more important adaptations and
correlations of animal and plant life.

One of the earliest and most often repeated objections was, that it was
difficult "to imagine a reason why variations tending in an
infinitesimal degree in any special direction should be preserved," or
to believe that the complex adaptation of living organisms could have
been produced "by infinitesimal beginnings." Now this term
"infinitesimal," used by a well-known early critic of the _Origin of
Species_, was never made use of by Darwin himself, who spoke only of
variations being "slight," and of the "small amount" of the variations
that might be selected. Even in using these terms he undoubtedly
afforded grounds for the objection above made, that such small and
slight variations could be of no real use, and would not determine the
survival of the individuals possessing them. We have seen, however, in
our third chapter, that even Darwin's terms were hardly justified; and
that the variability of many important species is of considerable
amount, and may very often be properly described as large. As this is
found to be the case both in animals and plants, and in all their chief
groups and subdivisions, and also to apply to all the separate parts and
organs that have been compared, we must take it as proved that the
average _amount_ of variability presents no difficulty whatever in the
way of the action of natural selection. It may be here mentioned that,
up to the time of the preparation of the last edition of _The Origin of
Species_, Darwin had not seen the work of Mr. J.A. Allen of Harvard
University (then only just published), which gave us the first body of
accurate comparisons and measurements demonstrating this large amount of
variability. Since then evidence of this nature has been accumulating,
and we are, therefore, now in a far better position to appreciate the
facilities for natural selection, in this respect, than was Mr. Darwin
himself.

Another objection of a similar nature is, that the chances are immensely
against the right variation or combination of variations occurring just
when required; and further, that no variation can be perpetuated that is
not accompanied by several concomitant variations of dependent
parts--greater length of a wing in a bird, for example, would be of
little use if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong one so long
as it was supposed that variations occurred singly and at considerable
intervals; but it ceases to have any weight now we know that they occur
simultaneously in various parts of the organism, and also in a large
proportion of the individuals which make up the species. A considerable
number of individuals will, therefore, every year possess the required
combination of characters; and it may also be considered probable that
when the two characters are such that they always _act_ together, there
will be such a correlation between them that they will frequently _vary_
together. But there is another consideration that seems to show that
this coincident variation is not essential. All animals in a state of
nature are kept, by the constant struggle for existence and the survival
of the fittest, in such a state of perfect health and usually
superabundant vigour, that in all ordinary circumstances they possess a
surplus power in every important organ--a surplus only drawn upon in
cases of the direst necessity when their very existence is at stake. It
follows, therefore, that _any_ additional power given to one of the
component parts of an organ must be useful--an increase, for example,
either in the wing muscles or in the form or length of the wing might
give _some_ increased powers of flight; and thus alternate
variations--in one generation in the muscles, in another generation in
the wing itself--might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals. On either
supposition, however, this objection appears to have little weight if we
take into consideration the large amount of coincident variability that
has been shown to exist.


_The Beginnings of Important Organs._

We now come to an objection which has perhaps been more frequently urged
than any other, and which Darwin himself felt to have much weight--the
first beginnings of important organs, such, for example, as wings, eyes,
mammary glands, and numerous other structures. It is urged, that it is
almost impossible to conceive how the first rudiments of these could
have been of any use, and, if not of use they could not have been
preserved and further developed by natural selection.

Now, the first remark to be made on objections of this nature is, that
they are really outside the question of the origin of all existing
species from allied species not very far removed from them, which is all
that Darwin undertook to _prove_ by means of his theory. Organs and
structures such as those above mentioned all date back to a very remote
past, when the world and its inhabitants were both very different from
what they are now. To ask of a new theory that it shall reveal to us
exactly what took place in remote geological epochs, and how it took
place, is unreasonable. The most that should be asked is, that some
probable or possible mode of origination should be pointed out in some
at least of these difficult cases, and this Mr. Darwin has done. One or
two of these may be briefly given here, but the whole series should be
carefully read by any one who wishes to see how many curious facts and
observations have been required in order to elucidate them; whence we
may conclude that further knowledge will probably throw light on any
difficulties that still remain.[41]

In the case of the mammary glands Mr. Darwin remarks that it is admitted
that the ancestral mammals were allied to the marsupials. Now in the
very earliest mammals, almost before they really deserved that name, the
young may have been nourished by a fluid secreted by the interior
surface of the marsupial sack, as is believed to be the case with the
fish (Hippocampus) whose eggs are hatched within a somewhat similar
sack. This being the case, those individuals which secreted a more
nutritious fluid, and those whose young were able to obtain and swallow
a more constant supply by suction, would be more likely to live and come
to a healthy maturity, and would therefore be preserved by natural
selection.

In another case which has been adduced as one of special difficulty, a
more complete explanation is given. Soles, turbots, and other flatfish
are, as is well known, unsymmetrical. They live and move on their sides,
the under side being usually differently coloured from that which is
kept uppermost. Now the eyes of these fish are curiously distorted in
order that both eyes may be on the upper side, where alone they would be
of any use. It was objected by Mr. Mivart that a sudden transformation
of the eye from one side to the other was inconceivable, while, if the
transit were gradual the first step could be of no use, since this would
not remove the eye from the lower side. But, as Mr. Darwin shows by
reference to the researches of Malm and others, the young of these fish
are quite symmetrical, and during their growth exhibit to us the whole
process of change. This begins by the fish (owing to the increasing
depth of the body) being unable to maintain the vertical position, so
that it falls on one side. It then twists the lower eye as much as
possible towards the upper side; and, the whole bony structure of the
head being at this time soft and flexible, the constant repetition of
this effort causes the eye gradually to move round the head till it
comes to the upper side. Now if we suppose this process, which in the
young is completed in a few days or weeks, to have been spread over
thousands of generations during the development of these fish, those
usually surviving whose eyes retained more and more of the position into
which the young fish tried to twist them, the change becomes
intelligible; though it still remains one of the most extraordinary
cases of degeneration, by which symmetry--which is so universal a
characteristic of the higher animals--is lost, in order that the
creature may be adapted to a new mode of life, whereby it is enabled the
better to escape danger and continue its existence.

The most difficult case of all, that of the eye--the thought of which
even to the last, Mr. Darwin says, "gave him a cold shiver"--is
nevertheless shown to be not unintelligible; granting of course the
sensitiveness to light of some forms of nervous tissue. For he shows
that there are, in several of the lower animals, rudiments of eyes,
consisting merely of pigment cells covered with a translucent skin,
which may possibly serve to distinguish light from darkness, but nothing
more. Then we have an optic nerve and pigment cells; then we find a
hollow filled with gelatinous substance of a convex form--the first
rudiment of a lens. Many of the succeeding steps are lost, as would
necessarily be the case, owing to the great advantage of each
modification which gave increased distinctness of vision, the creatures
possessing it inevitably surviving, while those below them became
extinct. But we can well understand how, after the first step was taken,
every variation tending to more complete vision would be preserved till
we reached the perfect eye of birds and mammals. Even this, as we know,
is not absolutely, but only relatively, perfect. Neither the chromatic
nor the spherical aberration is absolutely corrected; while long-and
short-sightedness, and the various diseases and imperfections to which
the eye is liable, may be looked upon as relics of the imperfect
condition from which the eye has been raised by variation and natural
selection.

These few examples of difficulties as to the origin of remarkable or
complex organs must suffice here; but the reader who wishes further
information on the matter may study carefully the whole of the sixth
and seventh chapters of the last edition of _The Origin of Species_, in
which these and many other cases are discussed in considerable detail.


_Useless or non-adaptive Characters._

Many naturalists seem to be of opinion that a considerable number of the
characters which distinguish species are of no service whatever to their
possessors, and therefore cannot have been produced or increased by
natural selection. Professors Bronn and Broca have urged this objection
on the continent. In America, Dr. Cope, the well-known palaeontologist,
has long since put forth the same objection, declaring that non-adaptive
characters are as numerous as those which are adaptive; but he differs
completely from most who hold the same general opinion in considering
that they occur chiefly "in the characters of the classes, orders,
families, and other higher groups;" and the objection, therefore, is
quite distinct from that in which it is urged that "specific characters"
are mostly useless. More recently, Professor G.J. Romanes has urged this
difficulty in his paper on "Physiological Selection" (_Journ. Linn.
Soc._, vol. xix. pp. 338, 344). He says that the characters "which serve
to distinguish allied species are frequently, if not usually, of a kind
with which natural selection can have had nothing to do," being without
any utilitarian significance. Again he speaks of "the enormous number,"
and further on of "the innumerable multitude" of specific peculiarities
which are useless; and he finally declares that the question needs no
further arguing, "because in the later editions of his works Mr. Darwin
freely acknowledges that a large proportion of specific distinctions
must be conceded to be useless to the species presenting them."

I have looked in vain in Mr. Darwin's works to find any such
acknowledgment, and I think Mr. Romanes has not sufficiently
distinguished between "useless characters" and "useless specific
distinctions." On referring to all the passages indicated by him I find
that, in regard to specific characters, Mr. Darwin is very cautious in
admitting inutility. His most pronounced "admissions" on this question
are the following: "But when, from the nature of the organism and of the
conditions, modifications have been induced which are unimportant for
the welfare of the species, they may be, and apparently often have been,
transmitted in nearly the same state _to numerous, otherwise modified,
descendants_" (_Origin_, p. 175). The words I have here italicised
clearly show that such characters are usually not "specific," in the
sense that they are such as distinguish species from each other, but are
found in numerous allied species. Again: "Thus a large yet undefined
extension may safely be given to the direct and indirect results of
natural selection; but I now admit, after reading the essay of Nägeli on
plants, and the remarks by various authors with respect to animals, more
especially those recently made by Professor Broca, that in the earlier
editions of my _Origin of Species_ I perhaps attributed too much to the
action of natural selection or the survival of the fittest. I have
altered the fifth edition of the _Origin_ so as to confine my remarks to
adaptive changes of structure, _but I am convinced, from the light
gained during even the last few years, that very many structures which
now appear to us useless, will hereafter be proved to be useful, and
will therefore come within the range of natural selection_. Nevertheless
I did not formerly consider sufficiently the existence of structures
which, _as far as we can at present judge_, are neither beneficial nor
injurious; and this I believe to be one of the greatest oversights as
yet detected in my work." Now it is to be remarked that neither in these
passages nor in any of the other less distinct expressions of opinion on
this question, does Darwin ever admit that "specific characters"--that
is, the particular characters which serve to distinguish one species
from another--are ever useless, much less that "a large proportion of
them" are so, as Mr. Romanes makes him "freely acknowledge." On the
other hand, in the passage which I have italicised he strongly expresses
his view that much of what we suppose to be useless is due to our
ignorance; and as I hold myself that, as regards many of the supposed
useless characters, this is the true explanation, it may be well to give
a brief sketch of the progress of knowledge in transferring characters
from the one category to the other.

We have only to go back a single generation, and not even the most acute
botanist could have suggested a reasonable use, for each species of
plant, of the infinitely varied forms, sizes, and colours of the
flowers, the shapes and arrangement of the leaves, and the numerous
other external characters of the whole plant. But since Mr. Darwin
showed that plants gained both in vigour and in fertility by being
crossed with other individuals of the same species, and that this
crossing was usually effected by insects which, in search of nectar or
pollen, carried the pollen from one plant to the flowers of another
plant, almost every detail is found to have a purpose and a use. The
shape, the size, and the colour of the petals, even the streaks and
spots with which they are adorned, the position in which they stand, the
movements of the stamens and pistil at various times, especially at the
period of, and just after, fertilisation, have been proved to be
strictly adaptive in so many cases that botanists now believe that all
the external characters of flowers either are or have been of use to the
species.

It has also been shown, by Kerner and other botanists, that another set
of characteristics have relation to the prevention of ants, slugs, and
other animals from reaching the flowers, because these creatures would
devour or injure them without effecting fertilisation. The spines,
hairs, or sticky glands on the stem or flower-stalk, the curious hairs
or processes shutting up the flower, or sometimes even the extreme
smoothness and polish of the outside of the petals so that few insects
can hang to the part, have been shown to be related to the possible
intrusion of these "unbidden guests."[42] And, still more recently,
attempts have been made by Grant Allen and Sir John Lubbock to account
for the innumerable forms, textures, and groupings of leaves, by their
relation to the needs of the plants themselves; and there can be little
doubt that these attempts will be ultimately successful. Again, just as
flowers have been adapted to secure fertilisation or
cross-fertilisation, fruits have been developed to assist in the
dispersal of seeds; and their forms, sizes, juices, and colours can be
shown to be specially adapted to secure such dispersal by the agency of
birds and mammals; while the same end is secured in other cases by
downy seeds to be wafted through the air, or by hooked or sticky
seed-vessels to be carried away, attached to skin, wool, or feathers.

Here, then, we have an enormous extension of the region of utility in
the vegetable kingdom, and one, moreover, which includes almost all the
specific characters of plants. For the species of plants are usually
characterised either by differences in the form, size, and colour of the
flowers, or of the fruits; or, by peculiarities in the shape, size,
dentation, or arrangement of the leaves; or by peculiarities in the
spines, hairs, or down with which various parts of the plant are
clothed. In the case of plants it must certainly be admitted that
"specific" characters are pre-eminently adaptive; and though there may
be some which are not so, yet all those referred to by Darwin as having
been adduced by various botanists as useless, either pertain to genera
or higher groups, or are found in some plants of a species only--that
is, are individual variations not specific characters.

In the case of animals, the most recent wide extension of the sphere of
utility has been in the matter of their colours and markings. It was of
course always known that certain creatures gained protection by their
resemblance to their normal surroundings, as in the case of white arctic
animals, the yellow or brown tints of those living in deserts, and the
green hues of many birds and insects surrounded by tropical vegetation.
But of late years these cases have been greatly increased both in number
and variety, especially in regard to those which closely imitate special
objects among which they live; and there are other kinds of coloration
which long appeared to have no use. Large numbers of animals, more
especially insects, are gaudily coloured, either with vivid hues or with
striking patterns, so as to be very easily seen. Now it has been found,
that in almost all these cases the creatures possess some special
quality which prevents their being attacked by the enemies of their kind
whenever the peculiarity is known; and the brilliant or conspicuous
colours or markings serve as a warning or signal flag against attack.
Large numbers of insects thus coloured are nauseous and inedible;
others, like wasps and bees, have stings; others are too hard to be
eaten by small birds; while snakes with poisonous fangs often have some
characteristic either of rattle, hood, or unusual colour, which
indicates that they had better be left alone.

But there is yet another form of coloration, which consists in special
markings--bands, spots, or patches of white, or of bright colour, which
vary in every species, and are often concealed when the creature is at
rest but displayed when in motion,--as in the case of the bands and
spots so frequent on the wings and tails of birds. Now these specific
markings are believed, with good reason, to serve the purpose of
enabling each species to be quickly recognised, even at a distance, by
its fellows, especially the parents by their young and the two sexes by
each other; and this recognition must often be an important factor in
securing the safety of individuals, and therefore the wellbeing and
continuance of the species. These interesting peculiarities will be more
fully described in a future chapter, but they are briefly referred to
here in order to show that the most common of all the characters by
which species are distinguished from each other--their colours and
markings--can be shown to be adaptive or utilitarian in their nature.

But besides colour there are almost always some structural characters
which distinguish species from species, and, as regards many of these
also, an adaptive character can be often discerned. In birds, for
instance, we have differences in the size or shape of the bill or the
feet, in the length of the wing or the tail, and in the proportions of
the several feathers of which these organs are composed. All these
differences in the organs on which the very existence of birds depends,
which determine the character of flight, facility for running or
climbing, for inhabiting chiefly the ground or trees, and the kind of
food that can be most easily obtained for themselves and their
offspring, must surely be in the highest degree utilitarian; although in
each individual case we, in our ignorance of the minutiae of their
life-history, may be quite unable to see the use. In mammalia specific
differences other than colour usually consist in the length or shape of
the ears and tail, in the proportions of the limbs, or in the length and
quality of the hair on different parts of the body. As regards the ears
and tail, one of the objections by Professor Bronn relates to this very
point. He states that the length of these organs differ in the various
species of hares and of mice, and he considers that this difference can
be of no service whatever to their possessors. But to this objection
Darwin replies, that it has been shown by Dr. Schöbl that the ears of
mice "are supplied in an extraordinary manner with nerves, so that they
no doubt serve as tactile organs." Hence, when we consider the life of
mice, either nocturnal or seeking their food in dark and confined
places, the length of the ears may be in each case adapted to the
particular habits and surroundings of the species. Again, the tail, in
the larger mammals, often serves the purpose of driving off flies and
other insects from the body; and when we consider in how many parts of
the world flies are injurious or even fatal to large mammals, we see
that the peculiar characteristics of this organ may in each case have
been adapted to its requirements in the particular area where the
species was developed. The tail is also believed to have some use as a
balancing organ, which assists an animal to turn easily and rapidly,
much as our arms are used when running; while in whole groups it is a
prehensile organ, and has become modified in accordance with the habits
and needs of each species. In the case of mice it is thus used by the
young. Darwin informs us that the late Professor Henslow kept some
harvest-mice in confinement, and observed that they frequently curled
their tails round the branches of a bush placed in the cage, and thus
aided themselves in climbing; while Dr. Günther has actually seen a
mouse suspend itself by the tail (_Origin_, p. 189).

Again, Mr. Lawson Tait has called attention to the use of the tail in
the cat, squirrel, yak, and many other animals as a means of preserving
the heat of the body during the nocturnal and the winter sleep. He says,
that in cold weather animals with long or bushy tails will be found
lying curled up, with their tails carefully laid over their feet like a
rug, and with their noses buried in the fur of the tail, which is thus
used exactly in the same way and for the same purpose as we use
respirators.[43]

Another illustration is furnished by the horns of deer which, especially
when very large, have been supposed to be a danger to the animal in
passing rapidly through dense thickets. But Sir James Hector states,
that the wapiti, in North America, throws back its head, thus placing
the horns along the sides of the back, and is then enabled to rush
through the thickest forest with great rapidity. The brow-antlers
protect the face and eyes, while the widely spreading horns prevent
injury to the neck or flanks. Thus an organ which was certainly
developed as a sexual weapon, has been so guided and modified during its
increase in size as to be of use in other ways. A similar use of the
antlers of deer has been observed in India.[44]

The various classes of facts now referred to serve to show us that, in
the case of the two higher groups--mammalia and birds--almost all the
characters by which species are distinguished from each other are, or
may be, adaptive. It is these two classes of animals which have been
most studied and whose life-histories are supposed to be most fully
known, yet even here the assertion of inutility, by an eminent
naturalist, in the case of two important organs, has been sufficiently
met by minute details either in the anatomy or in the habits of the
groups referred to. Such a fact as this, together with the extensive
series of characters already enumerated which have been of late years
transferred from the "useless" to the "useful" class, should convince
us, that the assertion of "inutility" in the case of any organ or
peculiarity which is not a rudiment or a correlation, is not, and can
never be, the statement of a fact, but merely an expression of our
ignorance of its purpose or origin.[45]


_Instability of Non-adaptive Characters._

One very weighty objection to the theory that _specific_ characters can
ever be wholly useless (or wholly unconnected with useful organs by
correlation of growth) appears to have been overlooked by those who have
maintained the frequency of such characters, and that is, their almost
necessary instability. Darwin has remarked on the extreme variability of
secondary sexual characters--such as the horns, crests, plumes, etc.,
which are found in males only,--the reason being, that, although of some
use, they are not of such direct and vital importance as those adaptive
characters on which the wellbeing and very existence of the animals
depend. But in the case of wholly useless structures, which are not
rudiments of once useful organs, we cannot see what there is to ensure
any amount of constancy or stability. One of the cases on which Mr.
Romanes lays great stress in his paper on "Physiological Selection"
(_Journ. Linn. Soc._, vol. xix. p. 384) is that of the fleshy appendages
on the corners of the jaw of Normandy pigs and of some other breeds. But
it is expressly stated that they are not constant; they appear
"frequently," or "occasionally," they are "not strictly inherited, for
they occur or fail in animals of the same litter;" and they are not
always symmetrical, sometimes appearing on one side of the face alone.
Now whatever may be the cause or explanation of these anomalous
appendages they cannot be classed with "specific characters," the most
essential features of which are, that they _are_ symmetrical, that they
_are_ inherited, and that they _are_ constant. Admitting that this
peculiar appendage is (as Mr. Romanes says rather confidently, "we
happen to know it to be") wholly useless and meaningless, the fact would
be rather an argument against specific characters being also
meaningless, because the latter never have the characteristics which
this particular variation possesses.

These useless or non-adaptive characters are, apparently, of the same
nature as the "sports" that arise in our domestic productions, but
which, as Mr. Darwin says, without the aid of selection would soon
disappear; while some of them may be correlations with other characters
which are or have been useful. Some of these correlations are very
curious. Mr. Tegetmeier informed Mr. Darwin that the young of white,
yellow, or dun-coloured pigeons are born almost naked, whereas other
coloured pigeons are born well clothed with down. Now, if this
difference occurred between wild species of different colours, it might
be said that the nakedness of the young could not be of any use. But the
colour with which it is correlated might, as has been shown, be useful
in many ways. The skin and its various appendages, as horns, hoofs,
hair, feathers, and teeth, are homologous parts, and are subject to very
strange correlations of growth. In Paraguay, horses with curled hair
occur, and these always have hoofs exactly like those of a mule, while
the hair of the mane and tail is much shorter than usual. Now, if any
one of these characters were useful, the others correlated with it might
be themselves useless, but would still be tolerably constant because
dependent on a useful organ. So the tusks and the bristles of the boar
are correlated and vary in development together, and the former only may
be useful, or both may be useful in unequal degrees.

The difficulty as to how individual differences or sports can become
fixed and perpetuated, if altogether useless, is evaded by those who
hold that such characters are exceedingly common. Mr. Romanes says that,
upon his theory of physiological selection, "it is quite intelligible
that when a varietal form is differentiated from its parent form by the
bar of sterility, any little meaningless peculiarities of structure or
of instinct _should at first be allowed to arise_, and that they should
then _be allowed to perpetuate themselves_ by heredity," until they are
finally eliminated by disuse. But this is entirely begging the
question. Do meaningless peculiarities, which we admit often arise as
spontaneous variations, ever perpetuate themselves in all the
individuals constituting a variety or race, without selection either
human or natural? Such characters present themselves as unstable
variations, and as such they remain, unless preserved and accumulated by
selection; and they can therefore never become "specific" characters
unless they are strictly correlated with some useful and important
peculiarities.

As bearing upon this question we may refer to what is termed Delboeuf's
law, which has been thus briefly stated by Mr. Murphy in his work on
_Habit and Intelligence_, p. 241.


    "If, in any species, a number of individuals, bearing a ratio
    not infinitely small to the entire number of births, are in
    every generation born with a particular variation which is
    neither beneficial nor injurious, and if it is not counteracted
    by reversion, then the proportion of the new variety to the
    original form will increase till it approaches indefinitely near
    to equality."


It is not impossible that some definite varieties, such as the melanic
form of the jaguar and the bridled variety of the guillemot are due to
this cause; but from their very nature such varieties are unstable, and
are continually reproduced in varying proportions from the parent forms.
They can, therefore, never constitute species unless the variation in
question becomes beneficial, when it will be fixed by natural selection.
Darwin, it is true, says--"There can be little doubt that the tendency
to vary in the same manner has often been so strong that all the
individuals of the same species have been similarly modified without the
aid of any form of selection."[46] But no proof whatever is offered of
this statement, and it is so entirely opposed to all we know of the
facts of variation as given by Darwin himself, that the important word
"all" is probably an oversight.

On the whole, then, I submit, not only has it not been proved that an
"enormous number of specific peculiarities" are useless, and that, as a
logical result, natural selection is "not a theory of the origin of
species," but only of the origin of adaptations which are usually
common to many species, or, more commonly, to genera and families; but,
I urge further, it has not even been proved that any truly "specific"
characters--those which either singly or in combination distinguish each
species from its nearest allies--are entirely unadaptive, useless, and
meaningless; while a great body of facts on the one hand, and some
weighty arguments on the other, alike prove that specific characters
have been, and could only have been, developed and fixed by natural
selection because of their utility. We may admit, that among the great
number of variations and sports which continually arise many are
altogether useless without being hurtful; but no cause or influence has
been adduced adequate to render such characters fixed and constant
throughout the vast number of individuals which constitute any of the
more dominant species.[47]


_The Swamping Effects of Intercrossing._

This supposed insuperable difficulty was first advanced in an article in
the _North British Review_ in 1867, and much attention has been
attracted to it by the acknowledgment of Mr. Darwin that it proved to
him that "single variations," or what are usually termed "sports," could
very rarely, if ever, be perpetuated in a state of nature, as he had at
first thought might occasionally be the case. But he had always
considered that the chief part, and latterly the whole, of the materials
with which natural selection works, was afforded by individual
variations, or that amount of ever fluctuating variability which exists
in all organisms and in all their parts. Other writers have urged the
same objection, even as against individual variability, apparently in
total ignorance of its amount and range; and quite recently Professor
G.J. Romanes has adduced it as one of the difficulties which can alone
be overcome by his theory of physiological selection. He urges, that the
same variation does not occur simultaneously in a number of individuals
inhabiting the same area, and that it is mere assumption to say it does;
while he admits that "if the assumption were granted there would be an
end of the present difficulty; for if a sufficient number of individuals
were thus simultaneously and similarly modified, there need be no longer
any danger of the variety becoming swamped by intercrossing." I must
again refer my readers to my third chapter for the proof that such
simultaneous variability is not an assumption but a fact; but, even
admitting this to be proved, the problem is not altogether solved, and
there is so much misconception regarding variation, and the actual
process of the origin of new species is so obscure, that some further
discussion and elucidation of the subject are desirable.

In one of the preliminary chapters of Mr. Seebohm's recent work on the
_Charadriidae_, he discusses the differentiation of species; and he
expresses a rather widespread view among naturalists when, speaking of
the swamping effects of intercrossing, he adds: "This is unquestionably
a very grave difficulty, to my mind an absolutely fatal one, to the
theory of accidental variation." And in another passage he says: "The
simultaneous appearance, and its repetition in successive generations,
of a beneficial variation, in a large number of individuals in the same
locality, cannot possibly be ascribed to chance." These remarks appear
to me to exhibit an entire misconception of the facts of variation as
they actually occur, and as they have been utilised by natural selection
in the modification of species. I have already shown that every part of
the organism, in common species, does vary to a very considerable
amount, in a large number of individuals, and in the same locality; the
only point that remains to be discussed is, whether any or most of these
variations are "beneficial." But every one of these variations consists
either in increase or diminution of size or power of the organ or
faculty that varies; they can all be divided into a more effective and a
less effective group--that is, into one that is more beneficial or less
beneficial. If less size of body would be beneficial, then, as half the
variations in size are above and half below the mean or existing
standard of the species, there would be ample beneficial variations; if
a darker colour or a longer beak or wing were required, there are always
a considerable number of individuals darker and lighter in colour than
the average, with longer or with shorter beaks and wings, and thus the
beneficial variation must always be present. And so with every other
part, organ, function, or habit; because, as variation, so far as we
know, is and always must be in the two directions of excess and defect
in relation to the mean amount, whichever kind of variation is wanted is
always present in some degree, and thus the difficulty as to
"beneficial" variations occurring, as if they were a special and rare
class, falls to the ground. No doubt some organs may vary in three or
perhaps more directions, as in the length, breadth, thickness, or
curvature of the bill. But these may be taken as separate variations,
each of which again occurs as "more" or "less"; and thus the "right" or
"beneficial" or "useful" variation must always be present so long as any
variation at all occurs; and it has not yet been proved that in any
large or dominant species, or in any part, organ, or faculty of such
species, there is no variation. And even were such a case found it would
prove nothing, so long as in numerous other species variation was shown
to exist; because we know that great numbers of species and groups
throughout all geological time have died out, leaving no descendants;
and the obvious and sufficient explanation of this fact is, that they
did _not_ vary enough at the time when variation was required to bring
them into harmony with changed conditions. The objection as to the
"right" or "beneficial" variation occurring when required, seems
therefore to have no weight in view of the actual facts of variation.


_Isolation to prevent Intercrossing._

Most writers on the subject consider the isolation of a portion of a
species a very important factor in the formation of new species, while
others maintain it to be absolutely essential. This latter view has
arisen from an exaggerated opinion as to the power of intercrossing to
keep down any variety or incipient species, and merge it in the parent
stock. But it is evident that this can only occur with varieties which
are not useful, or which, if useful, occur in very small numbers; and
from this kind of variations it is clear that new species do not arise.
Complete isolation, as in an oceanic island, will no doubt enable
natural selection to act more rapidly, for several reasons. In the first
place, the absence of competition will for some time allow the new
immigrants to increase rapidly till they reach the limits of
subsistence. They will then struggle among themselves, and by survival
of the fittest will quickly become adapted to the new conditions of
their environment. Organs which they formerly needed, to defend
themselves against, or to escape from, enemies, being no longer
required, would be encumbrances to be got rid of, while the power of
appropriating and digesting new and varied food would rise in
importance. Thus we may explain the origin of so many flightless and
rather bulky birds in oceanic islands, as the dodo, the cassowary, and
the extinct moas. Again, while this process was going on, the complete
isolation would prevent its being checked by the immigration of new
competitors or enemies, which would be very likely to occur in a
continuous area; while, of course, any intercrossing with the original
unmodified stock would be absolutely prevented. If, now, before this
change has gone very far, the variety spreads into adjacent but rather
distant islands, the somewhat different conditions in each may lead to
the development of distinct forms constituting what are termed
representative species; and these we find in the separate islands of the
Galapagos, the West Indies, and other ancient groups of islands.

But such cases as these will only lead to the production of a few
peculiar species, descended from the original settlers which happened to
reach the islands; whereas, in wide areas, and in continents, we have
variation and adaptation on a much larger scale; and, whenever important
physical changes demand them, with even greater rapidity. The far
greater complexity of the environment, together with the occurrence of
variations in constitution and habits, will often allow of effective
isolation, even here, producing all the results of actual physical
isolation. As we have already explained, one of the most frequent modes
in which natural selection acts is, by adapting some individuals of a
species to a somewhat different mode of life, whereby they are able to
seize upon unappropriated places in nature, and in so doing they become
practically isolated from their parent form. Let us suppose, for
example, that one portion of a species usually living in forests ranges
into the open plains, and finding abundance of food remains there
permanently. So long as the struggle for existence is not exceptionally
severe, these two portions of the species may remain almost unchanged;
but suppose some fresh enemies are attracted to the plains by the
presence of these new immigrants, then variation and natural selection
would lead to the preservation of those individuals best able to cope
with the difficulty, and thus the open country form would become
modified into a marked variety or into a distinct species; and there
would evidently be little chance of this modification being checked by
intercrossing with the parent form which remained in the forest.

Another mode of isolation is brought about by the variety--either owing
to habits, climate, or constitutional change--breeding at a slightly
different time from the parent species. This is known to produce
complete isolation in the case of many varieties of plants. Yet another
mode of isolation is brought about by changes of colour, and by the fact
that in a wild state animals of similar colours prefer to keep together
and refuse to pair with individuals of another colour. The probable
reason and utility of this habit will be explained in another chapter,
but the fact is well illustrated by the cattle which have run wild in
the Falkland Islands. These are of several different colours, but each
colour keeps in a separate herd, often restricted to one part of the
island; and one of these varieties--the mouse-coloured--is said to breed
a month earlier than the others; so that if this variety inhabited a
larger area it might very soon be established as a distinct race or
species.[48] Of course where the change of habits or of station is still
greater, as when a terrestrial animal becomes sub-aquatic, or when
aquatic animals come to live in tree-tops, as with the frogs and
Crustacea described at p. 118, the danger of intercrossing is reduced to
a minimum.

Several writers, however, not content with the indirect effects of
isolation here indicated, maintain that it is in itself a cause of
modification, and ultimately of the origination of new species. This
was the keynote of Mr. Vernon Wollaston's essay on "Variation of
Species," published in 1856, and it is adopted by the Rev. J.G. Gulick
in his paper on "Diversity of Evolution under one Set of External
Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems
to be that there is an inherent tendency to variation in certain
divergent lines, and that when one portion of a species is isolated,
even though under identical conditions, that tendency sets up a
divergence which carries that portion farther and farther away from the
original species. This view is held to be supported by the case of the
land shells of the Sandwich Islands, which certainly present some very
remarkable phenomena. In this comparatively small area there are about
300 species of land shells, almost all of which belong to one family (or
sub-family), the Achatinellidae, found nowhere else in the world. The
interesting point is the extreme restriction of the species and
varieties. The average range of each species is only five or six miles,
while some are restricted to but one or two square miles, and only a
very few range over a whole island. The forest region that extends over
one of the mountain-ranges of the island of Oahu, is about forty miles
in length and five or six miles in breadth; and this small territory
furnishes about 175 species, represented by 700 or 800 varieties. Mr.
Gulick states, that the vegetation of the different valleys on the same
side of this range is much the same, yet each has a molluscan fauna
differing in some degree from that of any other. "We frequently find a
genus represented in several successive valleys by allied species,
sometimes feeding on the same, sometimes on different plants. In every
such case the valleys that are nearest to each other furnish the most
nearly allied forms; and a full set of the varieties of each species
presents a minute gradation of forms between the more divergent types
found in the more widely separated localities." He urges, that these
constant differences cannot be attributed to natural selection, because
they occur in different valleys on the same side of the mountain, where
food, climate, and enemies are the same; and also, because there is no
greater difference in passing from the rainy to the dry side of the
mountains than in passing from one valley to another on the same side
an equal distance apart. In a very lengthy paper, presented to the
Linnean Society last year, on "Divergent Evolution through Cumulative
Segregation," Mr. Gulick endeavours to work out his views into a
complete theory, the main point of which may perhaps be indicated by the
following passage: "No two portions of a species possess exactly the
same average character, and the initial differences are for ever
reacting on the environment and on each other in such a way as to ensure
increasing divergence in each successive generation as long as the
individuals of the two groups are kept from intercrossing."[49]

It need hardly be said that the views of Mr. Darwin and myself are
inconsistent with the notion that, if the environment were absolutely
similar for the two isolated portions of the species, any such necessary
and constant divergence would take place. It is an error to assume that
what seem to us identical conditions are really identical to such small
and delicate organisms as these land molluscs, of whose needs and
difficulties at each successive stage of their existence, from the
freshly-laid egg up to the adult animal, we are so profoundly ignorant.
The exact proportions of the various species of plants, the numbers of
each kind of insect or of bird, the peculiarities of more or less
exposure to sunshine or to wind at certain critical epochs, and other
slight differences which to us are absolutely immaterial and
unrecognisable, may be of the highest significance to these humble
creatures, and be quite sufficient to require some slight adjustments of
size, form, or colour, which natural selection will bring about. All we
know of the facts of variation leads us to believe that, without this
action of natural selection, there would be produced over the whole area
a series of inconstant varieties mingled together, not a distinct
segregation of forms each confined to its own limited area.

Mr. Darwin has shown that, in the distribution and modification of
species, the biological is of more importance than the physical
environment, the struggle with other organisms being often more severe
than that with the forces of nature. This is particularly evident in the
case of plants, many of which, when protected from competition, thrive
in a soil, climate, and atmosphere widely different from those of their
native habitat. Thus, many alpine plants only found near perpetual snow
thrive well in our gardens at the level of the sea; as do the tritomas
from the sultry plains of South Africa, the yuccas from the arid hills
of Texas and Mexico, and the fuchsias from the damp and dreary shores of
the Straits of Magellan. It has been well said that plants do not live
where they like, but where they can; and the same remark will apply to
the animal world. Horses and cattle run wild and thrive both in North
and South America; rabbits, once confined to the south of Europe, have
established themselves in our own country and in Australia; while the
domestic fowl, a native of tropical India, thrives well in every part of
the temperate zone.

If, then, we admit that when one portion of a species is separated from
the rest, there will necessarily be a slight difference in the average
characters of the two portions, it does not follow that this difference
has much if any effect upon the characteristics that are developed by a
long period of isolation. In the first place, the difference itself will
necessarily be very slight unless there is an exceptional amount of
variability in the species; and in the next place, if the average
characters of the species are the expression of its exact adaptation to
its whole environment, then, given a precisely similar environment, and
the isolated portion will inevitably be brought back to the same average
of characters. But, as a matter of fact, it is impossible that the
environment of the isolated portion can be exactly like that of the bulk
of the species. It cannot be so physically, since no two separated areas
can be absolutely alike in climate and soil; and even if these are the
same, the geographical features, size, contour, and relation to winds,
seas, and rivers, would certainly differ. Biologically, the differences
are sure to be considerable. The isolated portion of a species will
almost always be in a much smaller area than that occupied by the
species as a whole, hence it is at once in a different position as
regards its own kind. The proportions of all the other species of
animals and plants are also sure to differ in the two areas, and some
species will almost always be absent in the smaller which are present in
the larger country. These differences will act and react on the
isolated portion of the species. The struggle for existence will differ
in its severity and in its incidence from that which affects the bulk of
the species. The absence of some one insect or other creature inimical
to the young animal or plant may cause a vast difference in its
conditions of existence, and may necessitate a modification of its
external or internal characters in quite a different direction from that
which happened to be present in the average of the individuals which
were first isolated.

On the whole, then, we conclude that, while isolation is an important
factor in effecting some modification of species, it is so, not on
account of any effect produced, or influence exerted by isolation _per
se_, but because it is always and necessarily accompanied by a change of
environment, both physical and biological. Natural selection will then
begin to act in adapting the isolated portion to its new conditions, and
will do this the more quickly and the more effectually because of the
isolation. We have, however, seen reason to believe that geographical or
local isolation is by no means essential to the differentiation of
species, because the same result is brought about by the incipient
species acquiring different habits or frequenting a different station;
and also by the fact that different varieties of the same species are
known to prefer to pair with their like, and thus to bring about a
physiological isolation of the most effective kind. This part of the
subject will be again referred to when the very difficult problems
presented by hybridity are discussed.[50]


_Cases in which Isolation is Ineffective._

One objection to the views of those who, like Mr. Gulick, believe
isolation itself to be a cause of modification of species deserves
attention, namely, the entire absence of change where, if this were a
_vera causa_, we should expect to find it. In Ireland we have an
excellent test case, for we know that it has been separated from Britain
since the end of the glacial epoch, certainly many thousand years. Yet
hardly one of its mammals, reptiles, or land molluscs has undergone the
slightest change, even although there is certainly a distinct difference
in the environment both inorganic and organic. That changes have not
occurred through natural selection, is perhaps due to the less severe
struggle for existence owing to the smaller number of competing species;
but, if isolation itself were an efficient cause, acting continuously
and cumulatively, it is incredible that a decided change should not have
been produced in thousands of years. That no such change has occurred in
this, and many other cases of isolation, seems to prove that it is not
in itself a cause of modification.

There yet remain a number of difficulties and objections relating to the
question of hybridity, which are so important as to require a separate
chapter for their adequate discussion.

FOOTNOTES:

[Footnote 41: See _Origin of Species_, pp. 176-198.]

[Footnote 42: See Kerner's _Flowers and their Unbidden Guests_ for
numerous other structures and peculiarities of plants which are shown to
be adaptive and useful.]

[Footnote 43: _Nature_, vol. xx. p. 603.]

[Footnote 44: _Nature_, vol. xxxviii. p. 328.]

[Footnote 45: A very remarkable illustration of function in an
apparently useless ornament is given by Semper. He says, "It is known
that the skin of reptiles encloses the body with scales. These scales
are distinguished by very various sculpturings, highly characteristic of
the different species. Irrespective of their systematic significance
they appear to be of no value in the life of the animal; indeed, they
are viewed as ornamental without regard to the fact that they are
microscopic and much too delicate to be visible to other animals of
their own species. It might, therefore, seem hopeless to show the
necessity for their existence on Darwinian principles, and to prove that
they are physiologically active organs. Nevertheless, recent
investigations on this point have furnished evidence that this is
possible.

"It is known that many reptiles, and above all the snakes, cast off the
whole skin at once, whereas human beings do so by degrees. If by any
accident they are prevented doing so, they infallibly die, because the
old skin has grown so tough and hard that it hinders the increase in
volume which is inseparable from the growth of the animal. The casting
of the skin is induced by the formation on the surface of the inner
epidermis, of a layer of very fine and equally distributed hairs, which
evidently serve the purpose of mechanically raising the old skin by
their rigidity and position. These hairs then may be designated as
_casting hairs_. That they are destined and calculated for this end is
evident to me from the fact established by Dr. Braun, that the casting
of the shells of the river crayfish is induced in exactly the same
manner by the formation of a coating of hairs which mechanically loosens
the old skin or shell from the new. Now the researches of Braun and
Cartier have shown that these casting hairs--which serve the same
purpose in two groups of animals so far apart in the systematic
scale--after the casting, are partly transformed into the concentric
stripes, sharp spikes, ridges, or warts which ornament the outer edges
of the skin-scales of reptiles or the carapace of crabs."[1] Professor
Semper adds that this example, with many others that might be quoted,
shows that we need not abandon the hope of explaining morphological
characters on Darwinian principles, although their nature is often
difficult to understand.

During a recent discussion of this question in the pages of _Nature_,
Mr. St. George Mivart adduces several examples of what he deems useless
specific characters. Among them are the aborted index finger of the
lemurine Potto, and the thumbless hands of Colobus and Ateles, the
"life-saving action" of either of which he thinks incredible. These
cases suggest two remarks. In the first place, they involve _generic_,
not _specific_, characters; and the three genera adduced are somewhat
isolated, implying considerable antiquity and the extinction of many
allied forms. This is important, because it affords ample time for great
changes of conditions since the structures in question originated; and
without a knowledge of these changes we can never safely assert that any
detail of structure could not have been useful. In the second place, all
three are cases of aborted or rudimentary organs; and these are admitted
to be explained by non-use, leading to diminution of size, a further
reduction being brought about by the action of the principle of economy
of growth. But, when so reduced, the rudiment might be inconvenient or
even hurtful, and then natural selection would aid in its complete
abortion; in other words, the abortion of the part would be _useful_,
and would therefore be subject to the law of survival of the fittest.
The genera Ateles and Colobus are two of the most purely arboreal types
of monkeys, and it is not difficult to conceive that the constant use of
the elongated fingers for climbing from tree to tree, and catching on to
branches while making great leaps, might require all the nervous energy
and muscular growth to be directed to the fingers, the small thumb
remaining useless. The case of the Potto is more difficult, both because
it is, presumably, a more ancient type, and its actual life-history and
habits are completely unknown. These cases are, therefore, not at all to
the point as proving that positive specific characters--not mere
rudiments characterising whole genera--are in any case useless.

Mr. Mivart further objects to the alleged rigidity of the action of
natural selection, because wounded or malformed animals have been found
which had evidently lived a considerable time in their imperfect
condition. But this simply proves that they were living under a
temporarily favourable environment, and that the real struggle for
existence, in their case, had not yet taken place. We must surely admit
that, when the pinch came, and when perfectly formed stoats were dying
for want of food, the one-footed animal, referred to by Mr. Mivart,
would be among the first to succumb; and the same remark will apply to
his abnormally toothed hares and rheumatic monkeys, which might,
nevertheless, get on very well under favourable conditions. The struggle
for existence, under which all animals and plants have been developed,
is intermittent, and exceedingly irregular in its incidence and
severity. It is most severe and fatal to the young; but when an animal
has once reached maturity, and especially when it has gained experience
by several years of an eventful existence, it may be able to maintain
itself under conditions which would be fatal to a young and
inexperienced creature of the same species. The examples adduced by Mr.
Mivart do not, therefore, in any way impugn the hardness of nature as a
taskmaster, or the extreme severity of the recurring struggle for
existence. (See _Nature_, vol. xxxix. p. 127.)]

[Footnote 46: _Origin of Species,_ p. 72.]

[Footnote 47: Darwin's latest expression of opinion on this question is
interesting, since it shows that he was inclined to return to his
earlier view of the general, or universal, utility of specific
characters. In a letter to Semper (30th Nov. 1878) he writes: "As our
knowledge advances, very slight differences, considered by systematists
as of no importance in structure, are continually found to be
functionally important; and I have been especially struck with this fact
in the case of plants, to which my observations have, of late years,
been confined. Therefore it seems to me rather rash to consider slight
differences between representative species, for instance, those
inhabiting the different islands of the same archipelago, as of no
functional importance, and as not in any way due to natural selection"
_(Life of Darwin_, vol. iii. p. 161).]

[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]

[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
215.]

[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,
vol. xx. pp. 189-274) he discusses the various forms of isolation above
referred to, under no less than thirty-eight different divisions and
subdivisions, with an elaborate terminology, and he argues that these
will frequently bring about divergent evolution without any change in
the environment or any action of natural selection. The discussion of
the problem here given will, I believe, sufficiently expose the fallacy
of his contention; but his illustration of the varied and often
recondite modes by which practical isolation may be brought about, may
help to remove one of the popular difficulties in the way of the action
of natural selection in the origination of species.]




CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING


    Statement of the problem--Extreme susceptibility of the
    reproductive functions--Reciprocal crosses--Individual
    differences in respect to cross-fertilisation--Dimorphism and
    trimorphism among plants--Cases of the fertility of hybrids and
    of the infertility of mongrels--The effects of close
    interbreeding--Mr. Huth's objections--Fertile hybrids among
    animals--Fertility of hybrids among plants--Cases of sterility
    of mongrels--Parallelism between crossing and change of
    conditions--Remarks on the facts of hybridity--Sterility due to
    changed conditions and usually correlated with other
    characters--Correlation of colour with constitutional
    peculiarities--The isolation of varieties by selective
    association--The influence of natural selection upon sterility
    and fertility--Physiological selection--Summary and concluding
    remarks.



One of the greatest, or perhaps we may say the greatest, of all the
difficulties in the way of accepting the theory of natural selection as
a complete explanation of the origin of species, has been the remarkable
difference between varieties and species in respect of fertility when
crossed. Generally speaking, it may be said that the varieties of any
one species, however different they may be in external appearance, are
perfectly fertile when crossed, and their mongrel offspring are equally
fertile when bred among themselves; while distinct species, on the other
hand, however closely they may resemble each other externally, are
usually infertile when crossed, and their hybrid offspring absolutely
sterile. This used to be considered a fixed law of nature, constituting
the absolute test and criterion of a _species_ as distinct from a
_variety_; and so long as it was believed that species were separate
creations, or at all events had an origin quite distinct from that of
varieties, this law could have no exceptions, because, if any two
species had been found to be fertile when crossed and their hybrid
offspring to be also fertile, this fact would have been held to prove
them to be not _species_ but _varieties_. On the other hand, if two
varieties had been found to be infertile, or their mongrel offspring to
be sterile, then it would have been said: These are not varieties but
true species. Thus the old theory led to inevitable reasoning in a
circle; and what might be only a rather common fact was elevated into a
law which had no exceptions.

The elaborate and careful examination of the whole subject by Mr.
Darwin, who has brought together a vast mass of evidence from the
experience of agriculturists and horticulturists, as well as from
scientific experimenters, has demonstrated that there is no such fixed
law in nature as was formerly supposed. He shows us that crosses between
some varieties are infertile or even sterile, while crosses between some
species are quite fertile; and that there are besides a number of
curious phenomena connected with the subject which render it impossible
to believe that sterility is anything more than an incidental property
of species, due to the extreme delicacy and susceptibility of the
reproductive powers, and dependent on physiological causes we have not
yet been able to trace. Nevertheless, the fact remains that most species
which have hitherto been crossed produce sterile hybrids, as in the
well-known case of the mule; while almost all domestic varieties, when
crossed, produce offspring which are perfectly fertile among themselves.
I will now endeavour to give such a sketch of the subject as may enable
the reader to see something of the complexity of the problem, referring
him to Mr. Darwin's works for fuller details.


_Extreme Susceptibility of the Reproductive Functions._

One of the most interesting facts, as showing how susceptible to changed
conditions or to slight constitutional changes are the reproductive
powers of animals, is the very general difficulty of getting those which
are kept in confinement to breed; and this is frequently the only bar to
domesticating wild species. Thus, elephants, bears, foxes, and numbers
of species of rodents, very rarely breed in confinement; while other
species do so more or less freely. Hawks, vultures, and owls hardly ever
breed in confinement; neither did the falcons kept for hawking ever
breed. Of the numerous small seed-eating birds kept in aviaries, hardly
any breed, neither do parrots. Gallinaceous birds usually breed freely
in confinement, but some do not; and even the guans and curassows, kept
tame by the South American Indians, never breed. This shows that change
of climate has nothing to do with the phenomenon; and, in fact, the same
species that refuse to breed in Europe do so, in almost every case, when
tamed or confined in their native countries. This inability to reproduce
is not due to ill-health, since many of these creatures are perfectly
vigorous and live very long.

With our true domestic animals, on the other hand, fertility is perfect,
and is very little affected by changed conditions. Thus, we see the
common fowl, a native of tropical India, living and multiplying in
almost every part of the world; and the same is the case with our
cattle, sheep, and goats, our dogs and horses, and especially with
domestic pigeons. It therefore seems probable, that this facility for
breeding under changed conditions was an original property of the
species which man has domesticated--a property which, more than any
other, enabled him to domesticate them. Yet, even with these, there is
evidence that great changes of conditions affect the fertility. In the
hot valleys of the Andes sheep are less fertile; while geese taken to
the high plateau of Bogota were at first almost sterile, but after some
generations recovered their fertility. These and many other facts seem
to show that, with the majority of animals, even a slight change of
conditions may produce infertility or sterility; and also that after a
time, when the animal has become thoroughly acclimatised, as it were, to
the new conditions, the infertility is in some cases diminished or
altogether ceases. It is stated by Bechstein that the canary was long
infertile, and it is only of late years that good breeding birds have
become common; but in this case no doubt selection has aided the change.

As showing that these phenomena depend on deep-seated causes and are of
a very general nature, it is interesting to note that they occur also
in the vegetable kingdom. Allowing for all the circumstances which are
known to prevent the production of seed, such as too great luxuriance of
foliage, too little or too much heat, or the absence of insects to
cross-fertilise the flowers, Mr. Darwin shows that many species which
grow and flower with us, apparently in perfect health, yet never produce
seed. Other plants are affected by very slight changes of conditions,
producing seed freely in one soil and not in another, though apparently
growing equally well in both; while, in some cases, a difference of
position even in the same garden produces a similar result.[51]


_Reciprocal Crosses._

Another indication of the extreme delicacy of the adjustment between the
sexes, which is necessary to produce fertility, is afforded by the
behaviour of many species and varieties when reciprocally crossed. This
will be best illustrated by a few of the examples furnished us by Mr.
Darwin. The two distinct species of plants, Mirabilis jalapa and M.
longiflora, can be easily crossed, and will produce healthy and fertile
hybrids when the pollen of the latter is applied to the stigma of the
former plant. But the same experimenter, Kölreuter, tried in vain, more
than two hundred times during eight years, to cross them by applying the
pollen of M. jalapa to the stigma of M. longiflora. In other cases two
plants are so closely allied that some botanists class them as varieties
(as with Matthiola annua and M. glabra), and yet there is the same great
difference in the result when they are reciprocally crossed.


_Individual Differences in respect to Cross-Fertilisation._

A still more remarkable illustration of the delicate balance of
organisation needful for reproduction, is afforded by the individual
differences of animals and plants, as regards both their power of
intercrossing with other individuals or other species, and the fertility
of the offspring thus produced. Among domestic animals, Darwin states
that it is by no means rare to find certain males and females which will
not breed together, though both are known to be perfectly fertile with
other males and females. Cases of this kind have occurred among horses,
cattle, pigs, dogs, and pigeons; and the experiment has been tried so
frequently that there can be no doubt of the fact. Professor G.J.
Romanes states that he has a number of additional cases of this
individual incompatibility, or of absolute sterility, between two
individuals, each of which is perfectly fertile with other individuals.

During the numerous experiments that have been made on the hybridisation
of plants similar peculiarities have been noticed, some individuals
being capable, others incapable, of being crossed with a distinct
species. The same individual peculiarities are found in varieties,
species, and genera. Kölreuter crossed five varieties of the common
tobacco (Nicotiana tabacum) with a distinct species, Nicotiana
glutinosa, and they all yielded very sterile hybrids; but those raised
from one variety were less sterile, in all the experiments, than the
hybrids from the four other varieties. Again, most of the species of the
genus Nicotiana have been crossed, and freely produce hybrids; but one
species, N. acuminata, not particularly distinct from the others, could
neither fertilise, nor be fertilised by, any of the eight other species
experimented on. Among genera we find some--such as Hippeastrum, Crinum,
Calceolaria, Dianthus--almost all the species of which will fertilise
other species and produce hybrid offspring; while other allied genera,
as Zephyranthes and Silene, notwithstanding the most persevering
efforts, have not produced a single hybrid even between the most closely
allied species.


_Dimorphism and Trimorphism._

Peculiarities in the reproductive system affecting individuals of the
same species reach their maximum in what are called heterostyled, or
dimorphic and trimorphic flowers, the phenomena presented by which form
one of the most remarkable of Mr. Darwin's many discoveries. Our common
cowslip and primrose, as well as many other species of the genus
Primula, have two kinds of flowers in about equal proportions. In one
kind the stamens are short, being situated about the middle of the tube
of the corolla, while the style is long, the globular stigma appearing
just in the centre of the open flower. In the other kind the stamens are
long, appearing in the centre or throat of the flower, while the style
is short, the stigma being situated halfway down the tube at the same
level as the stamens in the other form. These two forms have long been
known to florists as the "pin-eyed" and the "thrum-eyed," but they are
called by Darwin the long-styled and short-styled forms (see woodcut).

[Illustration: FIG. 17.--Primula veris (Cowslip).]

The meaning and use of these different forms was quite unknown till
Darwin discovered, first, that cowslips and primroses are absolutely
barren if insects are prevented from visiting them, and then, what is
still more extraordinary, that each form is almost sterile when
fertilised by its own pollen, and comparatively infertile when crossed
with any other plant of its own form, but is perfectly fertile when the
pollen of a long-styled is carried to the stigma of a short-styled
plant, or _vice versā_. It will be seen, by the figures, that the
arrangement is such that a bee visiting the flowers will carry the
pollen from the long anthers of the short-styled form to the stigma of
the long-styled form, while it would never reach the stigma of another
plant of the short-styled form. But an insect visiting, first, a
long-styled plant, would deposit the pollen on the stigma of another
plant of the same kind if it were next visited; and this is probably the
reason why the wild short-styled plants were found to be almost always
most productive of seed, since they must be all fertilised by the other
form, whereas the long-styled plants might often be fertilised by their
own form. The whole arrangement, however, ensures cross-fertilisation;
and this, as Mr. Darwin has shown by copious experiments, adds both to
the vigour and fertility of almost all plants as well as animals.

Besides the primrose family, many other plants of several distinct
natural orders present similar phenomena, one or two of the most curious
of which must be referred to. The beautiful crimson flax (Linum
grandiflorum) has also two forms, the styles only differing in length;
and in this case Mr. Darwin found by numerous experiments, which have
since been repeated and confirmed by other observers, that each form is
absolutely sterile with pollen from another plant of its own form, but
abundantly fertile when crossed with any plant of the other form. In
this case the pollen of the two forms cannot be distinguished under the
microscope (whereas that of the two forms of Primula differs in size and
shape), yet it has the remarkable property of being absolutely powerless
on the stigmas of half the plants of its own species. The crosses
between the opposite forms, which are fertile, are termed by Mr. Darwin
"legitimate," and those between similar forms, which are sterile,
"illegitimate"; and he remarks that we have here, within the limits of
the same species, a degree of sterility which rarely occurs except
between plants or animals not only of different _species_ but of
different _genera_.

But there is another set of plants, the trimorphic, in which the styles
and stamens have each three forms--long, medium, and short, and in these
it is possible to have eighteen different crosses. By an elaborate
series of experiments it was shown that the six legitimate unions--that
is, when a plant was fertilised by pollen from stamens of length
corresponding to that of its style in the two other forms--were all
abundantly fertile; while the twelve illegitimate unions, when a plant
was fertilised by pollen from stamens of a different length from its
own style, in any of the three forms, were either comparatively or
wholly sterile.[52]

We have here a wonderful amount of constitutional difference of the
reproductive organs within a single species, greater than usually occurs
within the numerous distinct species of a genus or group of genera; and
all this diversity appears to have arisen for a purpose which has been
obtained by many other, and apparently simpler, changes of structure or
of function, in other plants. This seems to show us, in the first place,
that variations in the mutual relations of the reproductive organs of
different individuals must be as frequent as structural variations have
been shown to be; and, also, that sterility in itself can be no test of
specific distinctness. But this point will be better considered when we
have further illustrated and discussed the complex phenomena of
hybridity.


_Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._

I now propose to adduce a few cases in which it has been proved, by
experiment, that hybrids between two distinct species are fertile _inter
se_; and then to consider why it is that such cases are so few in
number.

The common domestic goose (Anser ferns) and the Chinese goose (A.
cygnoides) are very distinct species, so distinct that some naturalists
have placed them in different genera; yet they have bred together, and
Mr. Eyton raised from a pair of these hybrids a brood of eight. This
fact was confirmed by Mr. Darwin himself, who raised several fine birds
from a pair of hybrids which were sent him.[53] In India, according to
Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are
kept in various parts of the country where neither of the pure parent
species exists, and as they are kept for profit they must certainly be
fully fertile.

Another equally striking case is that of the Indian humped and the
common cattle, species which differ osteologically, and also in habits,
form, voice, and constitution, so that they are by no means closely
allied; yet Mr. Darwin assures us that he has received decisive
evidence that the hybrids between these are perfectly fertile _inter
se_.

Dogs have been frequently crossed with wolves and with jackals, and
their hybrid offspring have been found to be fertile _inter se_ to the
third or fourth generation, and then usually to show some signs of
sterility or of deterioration. The wolf and dog may be originally the
same species, but the jackal is certainly distinct; and the appearance
of infertility or of weakness is probably due to the fact that, in
almost all these experiments, the offspring of a single pair--themselves
usually from the same litter--- were bred in-and-in, and this alone
sometimes produces the most deleterious effects. Thus, Mr. Low in his
great work on the _Domesticated Animals of Great Britain_, says: "If we
shall breed a pair of dogs from the same litter, and unite again the
offspring of this pair, we shall produce at once a feeble race of
creatures; and the process being repeated for one or two generations
more, the family will die out, or be incapable of propagating their
race. A gentleman of Scotland made the experiment on a large scale with
certain foxhounds, and he found that the race actually became monstrous
and perished utterly." The same writer tells us that hogs have been made
the subject of similar experiments: "After a few generations the victims
manifest the change induced in the system. They become of diminished
size; the bristles are changed into hairs; the limbs become feeble and
short; the litters diminish in frequency, and in the number of the young
produced; the mother becomes unable to nourish them, and, if the
experiment be carried as far as the case will allow, the feeble, and
frequently monstrous offspring, will be incapable of being reared up,
and the miserable race will utterly perish."[54]

These precise statements, by one of the greatest authorities on our
domesticated animals, are sufficient to show that the fact of
infertility or degeneracy appearing in the offspring of hybrids after a
few generations need not be imputed to the fact of the first parents
being distinct species, since exactly the same phenomena appear when
individuals of the same species are bred under similar adverse
conditions. But in almost all the experiments that have hitherto been
made in crossing distinct species, no care has been taken to avoid close
interbreeding by securing several hybrids from quite distinct stocks to
start with, and by having two or more sets of experiments carried on at
once, so that crosses between the hybrids produced may be occasionally
made. Till this is done no experiments, such as those hitherto tried,
can be held to prove that hybrids are in all cases infertile _inter se_.

It has, however, been denied by Mr. A.H. Huth, in his interesting work
on _The Marriage of Near Kin_, that any amount of breeding in-and-in is
in itself hurtful; and he quotes the evidence of numerous breeders whose
choicest stocks have always been so bred, as well as cases like the
Porto Santo rabbits, the goats of Juan Fernandez, and other cases in
which animals allowed to run wild have increased prodigiously and
continued in perfect health and vigour, although all derived from a
single pair. But in all these cases there has been rigid selection by
which the weak or the infertile have been eliminated, and with such
selection there is no doubt that the ill effects of close interbreeding
can be prevented for a long time; but this by no means proves that no
ill effects are produced. Mr. Huth himself quotes M. Allié, M. Aubé,
Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
other eminent breeders, as finding from experience that close
interbreeding _does_ produce bad effects; and it cannot be supposed that
there would be such a consensus of opinion on this point if the evil
were altogether imaginary. Mr. Huth argues, that the evil results which
do occur do not depend on the close interbreeding itself, but on the
tendency it has to perpetuate any constitutional weakness or other
hereditary taints; and he attempts to prove this by the argument that
"if crosses act by virtue of being a cross, and not by virtue of
removing an hereditary taint, then the greater the difference between
the two animals crossed the more beneficial will that act be." He then
shows that, the wider the difference the less is the benefit, and
concludes that a cross, as such, has no beneficial effect. A parallel
argument would be, that change of air, as from inland to the sea-coast,
or from a low to an elevated site, is not beneficial in itself, because,
if so, a change to the tropics or to the polar regions should be more
beneficial. In both these cases it may well be that no benefit would
accrue to a person in perfect health; but then there is no such thing
as "perfect health" in man, and probably no such thing as absolute
freedom from constitutional taint in animals. The experiments of Mr.
Darwin, showing the great and immediate good effects of a cross between
distinct strains in plants, cannot be explained away; neither can the
innumerable arrangements to secure cross-fertilisation by insects, the
real use and purport of which will be discussed in our eleventh chapter.
On the whole, then, the evidence at our command proves that, whatever
may be its ultimate cause, close interbreeding _does_ usually produce
bad results; and it is only by the most rigid selection, whether natural
or artificial, that the danger can be altogether obviated.


_Fertile Hybrids among Animals._

One or two more cases of fertile hybrids may be given before we pass on
to the corresponding experiments in plants. Professor Alfred Newton
received from a friend a pair of hybrid ducks, bred from a common duck
(Anas boschas), and a pintail (Dafila acuta). From these he obtained
four ducklings, but these latter, when grown up, proved infertile, and
did not breed again. In this case we have the results of close
interbreeding, with too great a difference between the original species,
combining to produce infertility, yet the fact of a hybrid from such a
pair producing healthy offspring is itself noteworthy.

Still more extraordinary is the following statement of Mr. Low: "It has
been long known to shepherds, though questioned by naturalists, that the
progeny of the cross between the sheep and goat is fertile. Breeds of
this mixed race are numerous in the north of Europe."[55] Nothing
appears to be known of such hybrids either in Scandinavia or in Italy;
but Professor Giglioli of Florence has kindly given me some useful
references to works in which they are described. The following extract
from his letter is very interesting: "I need not tell you that there
being such hybrids is now generally accepted as a fact. Buffon
(_Supplements_, tom. iii. p. 7, 1756) obtained one such hybrid in 1751
and eight in 1752. Sanson (_La Culture_, vol. vi. p. 372, 1865) mentions
a case observed in the Vosges, France. Geoff. St. Hilaire (_Hist. Nat.
Gén. des reg. org._, vol. iii. p. 163) was the first to mention, I
believe, that in different parts of South America the ram is more
usually crossed with the she-goat than the sheep with the he-goat. The
well-known 'pellones' of Chile are produced by the second and third
generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466,
_Agriculture_, 1862). Hybrids bred from goat and sheep are called
'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are
called 'carneros lanudos'; their breeding _inter se_ appears to be not
always successful, and often the original cross has to be recommenced to
obtain the proportion of three-eighths of he-goat and five-eighths of
sheep, or of three-eighths of ram and five-eighths of she-goat; such
being the reputed best hybrids."

With these numerous facts recorded by competent observers we can hardly
doubt that races of hybrids between these very distinct species have
been produced, and that such hybrids are fairly fertile _inter se_; and
the analogous facts already given lead us to believe that whatever
amount of infertility may at first exist could be eliminated by careful
selection, if the crossed races were bred in large numbers and over a
considerable area of country. This case is especially valuable, as
showing how careful we should be in assuming the infertility of hybrids
when experiments have been made with the progeny of a single pair, and
have been continued only for one or two generations.

Among insects one case only appears to have been recorded. The hybrids
of two moths (Bombyx cynthia and B. arrindia) were proved in Paris,
according to M. Quatrefages, to be fertile _inter se_ for eight
generations.


_Fertility of Hybrids among Plants._

Among plants the cases of fertile hybrids are more numerous, owing, in
part, to the large scale on which they are grown by gardeners and
nurserymen, and to the greater facility with which experiments can be
made. Darwin tells us that Kölreuter found ten cases in which two plants
considered by botanists to be distinct species were quite fertile
together, and he therefore ranked them all as varieties of each other.
In some cases these were grown for six to ten successive generations,
but after a time the fertility decreased, as we saw to be the case in
animals, and presumably from the same cause, too close interbreeding.

Dean Herbert, who carried on experiments with great care and skill for
many years, found numerous cases of hybrids which were perfectly fertile
_inter se_. Crinum capense, fertilised by three other species--C.
pedunculatum, C. canaliculatum, or C. defixum--all very distinct from
it, produced perfectly fertile hybrids; while other species less
different in appearance were quite sterile with the same C. capense.

All the species of the genus Hippeastrum produce hybrid offspring which
are invariably fertile. Lobelia syphylitica and L. fulgens, two very
distinct species, have produced a hybrid which has been named Lobelia
speciosa, and which reproduces itself abundantly. Many of the beautiful
pelargoniums of our greenhouses are hybrids, such as P. ignescens from a
cross between P. citrinodorum and P. fulgidum, which is quite fertile,
and has become the parent of innumerable varieties of beautiful plants.
All the varied species of Calceolaria, however different in appearance,
intermix with the greatest readiness, and the hybrids are all more or
less fertile. But the most remarkable case is that of two species of
Petunia, of which Dean Herbert says: "It is very remarkable that,
although there is a great difference in the form of the flower,
especially of the tube, of P. nyctanigenaeflora and P. phoenicea the
mules between them are not only fertile, but I have found them seed much
more freely with me than either parent.... From a pod of the
above-mentioned mule, to which no pollen but its own had access, I had a
large batch of seedlings in which there was no variability or difference
from itself; and it is evident that the mule planted by itself, in a
congenial climate, would reproduce itself as a species; at least as much
deserving to be so considered as the various Calceolarias of different
districts of South America."[56]

Darwin was informed by Mr. C. Noble that he raises stocks for grafting
from a hybrid between Rhododendron ponticum and R. catawbiense, and that
this hybrid seeds as freely as it is possible to imagine. He adds that
horticulturists raise large beds of the same hybrid, and such alone are
fairly treated; for, by insect agency, the several individuals are
freely crossed with each other, and the injurious influence of close
interbreeding is thus prevented. Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive generation, as
Gartner believed to be the case, the fact would have been notorious to
nurserymen.[57]


_Cases of Sterility of Mongrels._

The reverse phenomenon to the fertility of hybrids, the sterility of
mongrels or of the crosses between _varieties_ of the same species, is a
comparatively rare one, yet some undoubted cases have occurred. Gartner,
who believed in the absolute distinctness of species and varieties, had
two varieties of maize--one dwarf with yellow seeds, the other taller
with red seeds; yet they never naturally crossed, and, when fertilised
artificially, only a single head produced any seeds, and this one only
five grains. Yet these few seeds were fertile; so that in this case the
first cross was almost sterile, though the hybrid when at length
produced was fertile. In like manner, dissimilarly coloured varieties of
Verbascum or mullein have been found by two distinct observers to be
comparatively infertile. The two pimpernels (Anagallis arvensis and A.
coerulea), classed by most botanists as varieties of one species, have
been found, after repeated trials, to be perfectly sterile when crossed.

No cases of this kind are recorded among animals; but this is not to be
wondered at, when we consider how very few experiments have been made
with natural varieties; while there is good reason for believing that
domestic varieties are exceptionally fertile, partly because one of the
conditions of domestication was fertility under changed conditions, and
also because long continued domestication is believed to have the effect
of increasing fertility and eliminating whatever sterility may exist.
This is shown by the fact that, in many cases, domestic animals are
descended from two or more distinct species. This is almost certainly
the case with the dog, and probably with the hog, the ox, and the sheep;
yet the various breeds are now all perfectly fertile, although we have
every reason to suppose that there would be some degree of infertility
if the several aboriginal species were crossed together for the first
time.


_Parallelism between Crossing and Change of Conditions._

In the whole series of these phenomena, from the beneficial effects of
the crossing of different stocks and the evil effects of close
interbreeding, up to the partial or complete sterility induced by
crosses between species belonging to different genera, we have, as Mr.
Darwin points out, a curious parallelism with the effects produced by
change of physical conditions. It is well known that slight changes in
the conditions of life are beneficial to all living things. Plants, if
constantly grown in one soil and locality from their own seeds, are
greatly benefited by the importation of seed from some other locality.
The same thing happens with animals; and the benefit we ourselves
experience from "change of air" is an illustration of the same
phenomenon. But the amount of the change which is beneficial has its
limits, and then a greater amount is injurious. A change to a climate a
few degrees warmer or colder may be good, while a change to the tropics
or to the arctic regions might be injurious.

Thus we see that, both slight changes of conditions and a slight amount
of crossing, are beneficial; while extreme changes, and crosses between
individuals too far removed in structure or constitution, are injurious.
And there is not only a parallelism but an actual connection between the
two classes of facts, for, as we have already shown, many species of
animals and plants are rendered infertile, or altogether sterile, by the
change from their natural conditions which occurs in confinement or in
cultivation; while, on the other hand, the increased vigour or fertility
which is invariably produced by a judicious cross may be also effected
by a judicious change of climate and surroundings. We shall see in a
subsequent chapter, that this interchangeability of the beneficial
effects of crossing and of new conditions, serves to explain some very
puzzling phenomena in the forms and economy of flowers.


_Remarks on the Facts of Hybridity._

The facts that have now been adduced, though not very numerous, are
sufficiently conclusive to prove that the old belief, of the universal
sterility of hybrids and fertility of mongrels, is incorrect. The
doctrine that such a universal law existed was never more than a
plausible generalisation, founded on a few inconclusive facts derived
from domesticated animals and cultivated plants. The facts were, and
still are, inconclusive for several reasons. They are founded,
primarily, on what occurs among animals in domestication; and it has
been shown that domestication both tends to increase fertility, and was
itself rendered possible by the fertility of those particular species
being little affected by changed conditions. The exceptional fertility
of all the varieties of domesticated animals does not prove that a
similar fertility exists among natural varieties. In the next place, the
generalisation is founded on too remote crosses, as in the case of the
horse and the ass, the two most distinct and widely separated species of
the genus Equus, so distinct indeed that they have been held by some
naturalists to form distinct genera. Crosses between the two species of
zebra, or even between the zebra and the quagga, or the quagga and the
ass, might have led to a very different result. Again, in pre-Darwinian
times it was so universally the practice to argue in a circle, and
declare that the fertility of the offspring of a cross proved the
identity of species of the parents, that experiments in hybridity were
usually made between very remote species and even between species of
different genera, to avoid the possibility of the reply: "They are both
really the same species;" and the sterility of the hybrid offspring of
such remote crosses of course served to strengthen the popular belief.

Now that we have arrived at a different standpoint, and look upon a
species, not as a distinct entity due to special creation, but as an
assemblage of individuals which have become somewhat modified in
structure, form, and constitution so as to adapt them to slightly
different conditions of life; which can be differentiated from other
allied assemblages; which reproduce their like, and which usually breed
together--we require a fresh set of experiments calculated to determine
the matter of fact,--whether such species crossed with their near allies
do always produce offspring which are more or less sterile _inter se_.
Ample materials for such experiments exist, in the numerous
"representative species" inhabiting distinct areas on a continent or
different islands of a group; or even in those found in the same area
but frequenting somewhat different stations.

To carry out these experiments with any satisfactory result, it will be
necessary to avoid the evil effects of confinement and of too close
interbreeding. If birds are experimented with, they should be allowed as
much liberty as possible, a plot of ground with trees and bushes being
enclosed with wire netting overhead so as to form a large open aviary.
The species experimented with should be obtained in considerable
numbers, and by two separate persons, each making the opposite
reciprocal cross, as explained at p. 155. In the second generation these
two stocks might be themselves crossed to prevent the evil effects of
too close interbreeding. By such experiments, carefully carried out with
different groups of animals and plants, we should obtain a body of facts
of a character now sadly wanting, and without which it is hopeless to
expect to arrive at a complete solution of this difficult problem. There
are, however, some other aspects of the question that need to be
considered, and some theoretical views which require to be carefully
examined, having done which we shall be in a condition to state the
general conclusions to which the facts and reasonings at our command
seem to point.


_Sterility due to changed Conditions and usually correlated with other
Characters, especially with Colour._

The evidence already adduced as to the extreme susceptibility of the
reproductive system, and the curious irregularity with which infertility
or sterility appears in the crosses between some varieties or species
while quite absent in those between others, seem to indicate that
sterility is a characteristic which has a constant tendency to appear,
either by itself or in correlation with other characters. It is known to
be especially liable to occur under changed conditions of life; and, as
such change is usually the starting-point and cause of the development
of new species, we have already found a reason why it should so often
appear when species become fully differentiated.

In almost all the cases of infertility or sterility between varieties or
species, we have some external differences with which it is correlated;
and though these differences are sometimes slight, and the amount of the
infertility is not always, or even usually, proportionate to the
external difference between the two forms crossed, we must believe that
there is some connection between the two classes of facts. This is
especially the case as regards colour; and Mr. Darwin has collected a
body of facts which go far to prove that colour, instead of being an
altogether trifling and unimportant character, as was supposed by the
older naturalists, is really one of great significance, since it is
undoubtedly often correlated with important constitutional differences.
Now colour is one of the characters that most usually distinguishes
closely allied species; and when we hear that the most closely allied
species of plants are infertile together, while those more remote are
fertile, the meaning usually is that the former differ chiefly in the
_colour_ of their flowers, while the latter differ in the form of the
flowers or foliage, in habit, or in other structural characters.

It is therefore a most curious and suggestive fact, that in all the
recorded cases, in which a decided infertility occurs between varieties
of the same species, those varieties are distinguished by a difference
of colour. The infertile varieties of Verbascum were white and yellow
flowered respectively; the infertile varieties of maize were red and
yellow seeded; while the infertile pimpernels were the red and the blue
flowered varieties. So, the differently coloured varieties of
hollyhocks, though grown close together, each reproduce their own colour
from seed, showing that they are not capable of freely intercrossing.
Yet Mr. Darwin assures us that the agency of bees is necessary to carry
the pollen from one plant to another, because in each flower the pollen
is shed before the stigma is ready to receive it. We have here,
therefore, either almost complete sterility between varieties of
different colours, or a prepotent effect of pollen from a flower of the
same colour, bringing about the same result.

Similar phenomena have not been recorded among animals; but this is not
to be wondered at when we consider that most of our pure and valued
domestic breeds are characterised by definite colours which constitute
one of their distinctive marks, and they are, therefore, seldom crossed
with these of another colour; and even when they are so crossed, no
notice would be taken of any slight diminution of fertility, since this
is liable to occur from many causes. We have also reason to believe that
fertility has been increased by long domestication, in addition to the
fact of the original stocks being exceptionally fertile; and no
experiments have been made on the differently coloured varieties of wild
animals. There are, however, a number of very curious facts showing that
colour in animals, as in plants, is often correlated with constitutional
differences of a remarkable kind, and as these have a close relation to
the subject we are discussing, a brief summary of them will be here
given.


_Correlation of Colour with Constitutional Peculiarities._

The correlation of a white colour and blue eyes in male cats with
deafness, and of the tortoise-shell marking with the female sex of the
same animal, are two well-known but most extraordinary cases. Equally
remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that
white, yellow, pale blue, or dun pigeons, of all breeds, have the young
birds born naked, while in all other colours they are well covered with
down. Here we have a case in which colour seems of more physiological
importance than all the varied structural differences between the
varieties and breeds of pigeons. In Virginia there is a plant called the
paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours
their bones pink, and causes the hoofs of all but the black varieties to
drop off; so that black pigs only can be kept in the district.[58]
Buckwheat in flower is also said to be injurious to white pigs but not
to black. In the Tarentino, black sheep are not injured by eating the
Hypericum crispum--a species of St. John's-wort--which kills white
sheep. White terriers suffer most from distemper; white chickens from
the gapes. White-haired horses or cattle are subject to cutaneous
diseases from which the dark coloured are free; while, both in Thuringia
and the West Indies, it has been noticed that white or pale coloured
cattle are much more troubled by flies than are those which are brown or
black. The same law even extends to insects, for it is found that
silkworms which produce white cocoons resist the fungus disease much
better than do those which produce yellow cocoons.[59] Among plants, we
have in North America green and yellow-fruited plums not affected by a
disease that attacked the purple-fruited varieties. Yellow-fleshed
peaches suffer more from disease than white-fleshed kinds. In Mauritius,
white sugar-canes were attacked by a disease from which the red canes
were free. White onions and verbenas are most liable to mildew; and
red-flowered hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds.[60]

These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render it
probable that the correlation of colour with infertility, which has been
detected in several cases in plants, may also extend to animals in a
state of nature; and if so, the fact is of the highest importance as
throwing light on the origin of the infertility of many allied species.
This will be better understood after considering the facts which will be
now described.


_The Isolation of Varieties by Selective Association._

In the last chapter I have shown that the importance of geographical
isolation for the formation of new species by natural selection has been
greatly exaggerated, because the very change of conditions, which is
the initial power in starting such new forms, leads also to a local or
stational segregation of the forms acted upon. But there is also a very
powerful cause of isolation in the mental nature--the likes and
dislikes--of animals; and to this is probably due the fact of the
comparative rarity of hybrids in a state of nature. The differently
coloured herds of cattle in the Falkland Islands, each of which keeps
separate, have been already mentioned; and it may be added, that the
mouse-coloured variety seem to have already developed a physiological
peculiarity in breeding a month earlier than the others. Similar facts
occur, however, among our domestic animals and are well known to
breeders. Professor Low, one of the greatest authorities on our
domesticated animals, says: "The female of the dog, when not under
restraint, makes selection of her mate, the mastiff selecting the
mastiff, the terrier the terrier, and so on." And again: "The Merino
sheep and Heath sheep of Scotland, if two flocks are mixed together,
each will breed with its own variety." Mr. Darwin has collected many
facts illustrating this point. One of the chief pigeon-fanciers in
England informed him that, if free to choose, each breed would prefer
pairing with its own kind. Among the wild horses in Paraguay those of
the same colour and size associate together; while in Circassia there
are three races of horses which have received special names, and which,
when living a free life, almost always refuse to mingle and cross, and
will even attack one another. On one of the Faroe Islands, not more than
half a mile in diameter, the half-wild native black sheep do not readily
mix with imported white sheep. In the Forest of Dean, and in the New
Forest, the dark and pale coloured herds of fallow deer have never been
known to mingle; and even the curious Ancon sheep of quite modern origin
have been observed to keep together, separating themselves from the rest
of the flock when put into enclosures with other sheep. The same rule
applies to birds, for Darwin was informed by the Rev. W.D. Fox that his
flocks of white and Chinese geese kept distinct.[61]

This constant preference of animals for their like, even in the case of
slightly different varieties of the same species, is evidently a fact
of great importance in considering the origin of species by natural
selection, since it shows us that, so soon as a slight differentiation
of form or colour has been effected, isolation will at once arise by the
selective association of the animals themselves; and thus the great
stumbling-block of "the swamping effects of intercrossing," which has
been so prominently brought forward by many naturalists, will be
completely obviated.

If now we combine with this fact the correlation of colour with
important constitutional peculiarities, and, in some cases, with
infertility; and consider, further, the curious parallelism that has
been shown to exist between the effects of changed conditions and the
intercrossing of varieties in producing either an increase or a decrease
of fertility, we shall have obtained, at all events, a starting-point
for the production of that infertility which is so characteristic a
feature of distinct species when intercrossed. All we need, now, is some
means of increasing or accumulating this initial tendency; and to a
discussion of this problem we will therefore address ourselves.


_The Influence of Natural Selection upon Sterility and Fertility._

It will occur to many persons that, as the infertility or sterility of
incipient species would be useful to them when occupying the same or
adjacent areas, by neutralising the effects of intercrossing, this
infertility might have been increased by the action of natural
selection; and this will be thought the more probable if we admit, as we
have seen reason to do, that variations in fertility occur, perhaps as
frequently as other variations. Mr. Darwin tells us that, at one time,
this appeared to him probable, but he found the problem to be one of
extreme complexity; and he was also influenced against the view by many
considerations which seemed to render such an origin of the sterility or
infertility of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come in contact
with each other, are often sterile when crossed, is one of the
difficulties; but this may perhaps be overcome by the consideration
that, though now isolated, they may, and often must, have been in
contact at their origination. More important is the objection that
natural selection could not possibly have produced the difference that
often occurs between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different amounts of
infertility or sterility between different species of the same genus,
the infertility often bearing no proportion to the difference between
the species crossed, is also an important objection. But none of these
objections would have much weight if it could be clearly shown that
natural selection _is_ able to increase the infertility variations of
incipient species, as it is certainly able to increase and develop all
useful variations of form, structure, instincts, or habits. Ample causes
of infertility have been shown to exist, in the nature of the organism
and the laws of correlation; the agency of natural selection is only
needed to accumulate the effects produced by these causes, and to render
their final results more uniform and more in accordance with the facts
that exist.

About twenty years ago I had much correspondence and discussion with Mr.
Darwin on this question. I then believed that I was able to demonstrate
the action of natural selection in accumulating infertility; but I could
not convince him, owing to the extreme complexity of the process under
the conditions which he thought most probable. I have recently returned
to the question; and, with the fuller knowledge of the facts of
variation we now possess, I think it may be shown that natural selection
_is_, in some probable cases at all events, able to accumulate
variations in infertility between incipient species.

The simplest case to consider, will be that in which two forms or
varieties of a species, occupying an extensive area, are in process of
adaptation to somewhat different modes of life within the same area. If
these two forms freely intercross with each other, and produce mongrel
offspring which are quite fertile _inter se_, then the further
differentiation of the forms into two distinct species will be retarded,
or perhaps entirely prevented; for the offspring of the crossed unions
will be, perhaps, more vigorous on account of the cross, although less
perfectly adapted to the conditions of existence than either of the pure
breeds; and this would certainly establish a powerful antagonistic
influence to the further differentiation of the two forms.

Now, let us suppose that a partial sterility of the hybrids between the
two forms arises, in correlation with the different modes of life and
the slight external or internal peculiarities that exist between them,
both of which we have seen to be real causes of infertility. The result
will be that, even if the hybrids between the two forms are still freely
produced, these hybrids will not themselves increase so rapidly as the
two pure forms; and as these latter are, by the terms of the problem,
better suited to their conditions of life than are the hybrids between
them, they will not only increase more rapidly, but will also tend to
supplant the hybrids altogether whenever the struggle for existence
becomes exceptionally severe. Thus, the more complete the sterility of
the hybrids the more rapidly will they die out and leave the two parent
forms pure. Hence it will follow that, if there is greater infertility
between the two forms in one part of the area than the other, these
forms will be kept more pure wherever this greater infertility prevails,
will therefore have an advantage at each recurring period of severe
struggle for existence, and will thus ultimately supplant the less
infertile or completely fertile forms that may exist in other portions
of the area. It thus appears that, in such a case as here supposed,
natural selection would preserve those portions of the two breeds which
were most infertile with each other, or whose hybrid offspring were most
infertile; and would, therefore, if variations in fertility continued to
arise, tend to increase that infertility. It must particularly be noted
that this effect would result, not by the preservation of the infertile
variations on account of their infertility, but by the inferiority of
the hybrid offspring, both as being fewer in numbers, less able to
continue their race, and less adapted to the conditions of existence
than either of the pure forms. It is this inferiority of the hybrid
offspring that is the essential point; and as the number of these
hybrids will be permanently less where the infertility is greatest,
therefore those portions of the two forms in which infertility is
greatest will have the advantage, and will ultimately survive in the
struggle for existence.

The differentiation of the two forms into distinct species, with the
increase of infertility between them, would be greatly assisted by two
other important factors in the problem. It has already been shown that,
with each modification of form and habits, and especially with
modifications of colour, there arises a disinclination of the two forms
to pair together; and this would produce an amount of isolation which
would greatly assist the specialisation of the forms in adaptation to
their different conditions of life. Again, evidence has been adduced
that change of conditions or of mode of life is a potent cause of
disturbance of the reproductive system, and, consequently, of
infertility. We may therefore assume that, as the two forms adopted more
and more different modes of life, and perhaps acquired also decided
peculiarities of form and coloration, the infertility between them would
increase or become more general; and as we have seen that every such
increase of infertility would give that portion of the species in which
it arose an advantage over the remaining portions in which the two
varieties were more fertile together, all this induced infertility would
maintain itself, and still further increase the general infertility
between the two forms of the species.

It follows, then, that specialisation to separate conditions of life,
differentiation of external characters, disinclination to cross-unions,
and the infertility of the hybrid produce of these unions, would all
proceed _pari passu_, and would ultimately lead to the production of two
distinct forms having all the characteristics, physiological as well as
structural, of true species.

In the case now discussed it has been supposed, that some amount of
general infertility might arise in correlation with the different modes
of life of two varieties or incipient species. A considerable body of
facts already adduced renders it probable that this _is_ the mode in
which any widespread infertility would arise; and, if so, it has been
shown that, by the influence of natural selection and the known laws
which affect varieties, the infertility would be gradually increased.
But, if we suppose the infertility to arise sporadically within the two
forms, and to affect only a small proportion of the individuals in any
area, it will be difficult, if not impossible, to show that such
infertility would have any tendency to increase, or would produce any
but a prejudicial effect. If, for example, five per cent of each form
thus varied so as to be infertile with the other form, the result would
be hardly perceptible, because the individuals which formed cross-unions
and produced hybrids would constitute a very small portion of the whole
species; and the hybrid offspring, being at a disadvantage in the
struggle for existence and being themselves infertile, would soon die
out, while the much more numerous fertile portion of the two forms would
increase rapidly, and furnish a sufficient number of pure-bred offspring
of each form to take the place of the somewhat inferior hybrids between
them whenever the struggle for existence became severe. We must suppose
that the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility; but the latter
would necessarily lose half their proper increase by the sterility of
their hybrid offspring whenever they crossed with the other form, and
when they bred with their own form the tendency to sterility would die
out except in the very minute proportion of the five per cent
(one-twentieth) that chance would lead to pair together. Under these
circumstances the incipient sterility between the two forms would
rapidly be eliminated, and could never rise much above the numbers which
were produced by sporadic variation each year.

It was, probably, by a consideration of some such case as this that Mr.
Darwin came to the conclusion that infertility arising between incipient
species could not be increased by natural selection; and this is the
more likely, as he was always disposed to minimise both the frequency
and the amount even of structural variations.

We have yet to notice another mode of action of natural selection in
favouring and perpetuating any infertility that may arise between two
incipient species. If several distinct species are undergoing
modification at the same time and in the same area, to adapt them to
some new conditions that have arisen there, then any species in which
the structural or colour differences that have arisen between it and its
varieties or close allies were correlated with infertility of the
crosses between them, would have an advantage over the corresponding
varieties of other species in which there was no such physiological
peculiarity. Thus, incipient species which were infertile together would
have an advantage over other incipient species which were fertile, and,
whenever the struggle for existence became severe, would prevail over
them and take their place. Such infertility, being correlated with
constitutional or structural differences, would probably, as already
suggested, go on increasing as these differences increased; and thus, by
the time the new species became fully differentiated from its parent
form (or brother variety) the infertility might have become as well
marked as we usually find it to be between distinct species.

This discussion has led us to some conclusions of the greatest
importance as bearing on the difficult problem of the cause of the
sterility of the hybrids between distinct species. Accepting, as highly
probable, the fact of variations in fertility occurring in correlation
with variations in habits, colour, or structure, we see, that so long as
such variations occurred only sporadically, and affected but a small
proportion of the individuals in any area, the infertility could not be
increased by natural selection, but would tend to die out almost as fast
as it was produced. If, however, it was so closely correlated with
physical variations or diverse modes of life as to affect, even in a
small degree, a considerable proportion of the individuals of the two
forms in definite areas, it would be preserved by natural selection, and
the portion of the varying species thus affected would increase at the
expense of those portions which were more fertile when crossed. Each
further variation towards infertility between the two forms would be
again preserved, and thus the incipient infertility of the hybrid
offspring might be increased till it became so great as almost to amount
to sterility. Yet further, we have seen that if several competing
species in the same area were being simultaneously modified, those
between whose varieties infertility arose would have an advantage over
those whose varieties remained fertile _inter se_, and would ultimately
supplant them.

The preceding argument, it will be seen, depends entirely upon the
assumption that some amount of infertility characterises the distinct
varieties which are in process of differentiation into species; and it
may be objected that of such infertility there is no proof. This is
admitted; but it is urged that facts have been adduced which render such
infertility probable, at least in some cases, and this is all that is
required. It is by no means necessary that _all_ varieties should
exhibit incipient infertility, but only, some varieties; for we know
that, of the innumerable varieties that occur but few become developed
into distinct species, and it may be that the absence of infertility, to
obviate the effects of intercrossing, is one of the usual causes of
their failure. All I have attempted to show is, that _when_ incipient
infertility does occur in correlation with other varietal differences,
that infertility can be, and in fact must be, increased by natural
selection; and this, it appears to me, is a decided step in advance in
the solution of the problem.[62]


_Physiological Selection._

Another form of infertility has been suggested by Professor G.J. Romanes
as having aided in bringing about the characteristic infertility or
sterility of hybrids. It is founded on the fact, already noticed, that
certain individuals of some species possess what may be termed selective
sterility--that is, while fertile with some individuals of the species
they are sterile with others, and this altogether independently of any
differences of form, colour, or structure. The phenomenon, in the only
form in which it has been observed, is that of "infertility or absolute
sterility between two individuals, each of which is perfectly fertile
with all other individuals;" but Mr. Romanes thinks that "it would not
be nearly so remarkable, or physiologically improbable, that such
incompatibility should run through a whole race or strain."[63]
Admitting that this may be so, though we have at present no evidence
whatever in support of it, it remains to be considered whether such
physiological varieties could maintain themselves, or whether, as in the
cases of sporadic infertility already discussed, they would necessarily
die out unless correlated with useful characters. Mr. Romanes thinks
that they would persist, and urges that "whenever this one kind of
variation occurs _it cannot escape the preserving agency_ of
physiological selection. Hence, even if it be granted that the variation
which affects the reproductive system in this particular way is a
variation of comparatively rare occurrence, still, as _it must always be
preserved_ whenever it does occur, its influence in the manufacture of
specific types _must be cumulative_." The very positive statements which
I have italicised would lead most readers to believe that the alleged
fact had been demonstrated by a careful working out of the process in
some definite supposed cases. This, however, has nowhere been done in
Mr. Romanes' paper; and as it is _the_ vital theoretical point on which
any possible value of the new theory rests, and as it appears so opposed
to the self-destructive effects of simple infertility, which we have
already demonstrated when it occurs between the intermingled portion of
two varieties, it must be carefully examined. In doing so, I will
suppose that the required variation is not of "rare occurrence," but of
considerable amount, and that it appears afresh each year to about the
same extent, thus giving the theory every possible advantage.

Let us then suppose that a given species consists of 100,000 individuals
of each sex, with only the usual amount of fluctuating external
variability. Let a physiological variation arise, so that 10 per cent of
the whole number--10,000 individuals of each sex--while remaining
fertile _inter se_ become quite sterile with the remaining 90,000. This
peculiarity is not correlated with any external differences of form or
colour, or with inherent peculiarities of likes or dislikes leading to
any choice as to the pairing of the two sets of individuals. We have now
to inquire, What would be the result?

Taking, first, the 10,000 pairs of the physiological or abnormal
variety, we find that each male of these might pair with any one of the
whole 100,000 of the opposite sex. If, therefore, there was nothing to
limit their choice to particular individuals of either variety, the
probabilities are that 9000 of them would pair with the opposite
variety, and only 1000 with their own variety--that is, that 9000 would
form sterile unions, and only _one_ thousand would form fertile unions.

Taking, next, the 90,000 normal individuals of either sex, we find, that
each male of these has also a choice of 100,000 to pair with. The
probabilities are, therefore, that nine-tenths of them--that is,
81,000--would pair with their normal fellows, while 9000 would pair with
the opposite abnormal variety forming the above-mentioned sterile
unions.

Now, as the number of individuals forming a species remains constant,
generally speaking, from year to year, we shall have next year also
100,000 pairs, of which the two physiological varieties will be in the
proportion of eighty-one to one, or 98,780 pairs of the normal variety
to 1220[64] of the abnormal, that being the proportion of the fertile
unions of each. In this year we shall find, by the same rule of
probabilities, that only 15 males of the abnormal variety will pair with
their like and be fertile, the remaining 1205 forming sterile unions
with some of the normal variety. The following year the total 100,000
pairs will consist of 99,984 of the normal, and only 16 of the abnormal
variety; and the probabilities, of course, are, that the whole of these
latter will pair with some of the enormous preponderance of normal
individuals, and, their unions being sterile, the physiological variety
will become extinct in the third year.

If now in the second and each succeeding year a similar proportion as at
first (10 per cent) of the physiological variety is produced afresh from
the ranks of the normal variety, the same rate of diminution will go on,
and it will be found that, on the most favourable estimate, the
physiological variety can never exceed 12,000 to the 88,000 of the
normal form of the species, as shown by the following table:--


    1st Year. 10,000 of physiological variety to 90,000 of normal variety.
    2d   "     1,220 + 10,000 again produced.
    3d   "        16 +  1,220 + 10,000 do.               = 11,236
    4th  "         O +     16 +  1,220 + 10,000 do.      = 11,236
    5th  "                  O +     16 +  1,220 + 10,000 = 11,236
    and so on for any number of generations.


In the preceding discussion we have given the theory the advantage of
the large proportion of 10 per cent of this very exceptional variety
arising in its midst year by year, and we have seen that, even under
these favourable conditions, it is unable to increase its numbers much
above its starting-point, and that it remains wholly dependent on the
continued renewal of the variety for its existence beyond a few years.
It appears, then, that this form of inter-specific sterility cannot be
increased by natural or any other known form of selection, but that it
contains within itself its own principle of destruction. If it is
proposed to get over the difficulty by postulating a larger percentage
of the variety annually arising within the species, we shall not affect
the law of decrease until we approach equality in the numbers of the two
varieties. But with any such increase of the physiological variety the
species itself would inevitably suffer by the large proportion of
sterile unions in its midst, and would thus be at a great disadvantage
in competition with other species which were fertile throughout. Thus,
natural selection will always tend to weed out any species with too
great a tendency to sterility among its own members, and will therefore
prevent such sterility from becoming the general characteristic of
varying species, which this theory demands should be the case.

On the whole, then, it appears clear that no form of infertility or
sterility between the individuals of a species, can be increased by
natural selection unless correlated with some useful variation, while
all infertility not so correlated has a constant tendency to effect its
own elimination. But the opposite property, fertility, is of vital
importance to every species, and gives the offspring of the individuals
which possess it, in consequence of their superior numbers, a greater
chance of survival in the battle of life. It is, therefore, directly
under the control of natural selection, which acts both by the
self-preservation of fertile and the self-destruction of infertile
stocks--except always where correlated as above, when they become
useful, and therefore subject to be increased by natural selection.


_Summary and Concluding Remarks on Hybridity._

The facts which are of the greatest importance to a comprehension of
this very difficult subject are those which show the extreme
susceptibility of the reproductive system both in plants and animals. We
have seen how both these classes of organisms may be rendered infertile,
by a change of conditions which does not affect their general health, by
captivity, or by too close interbreeding. We have seen, also, that
infertility is frequently correlated with a difference of colour, or
with other characters; that it is not proportionate to divergence of
structure; that it varies in reciprocal crosses between pairs of the
same species; while in the cases of dimorphic and trimorphic plants the
different crosses between the same pair of individuals may be fertile or
sterile at the same time. It appears as if fertility depended on such a
delicate adjustment of the male and female elements to each other, that,
unless constantly kept up by the preservation of the most fertile
individuals, sterility is always liable to arise. This preservation
always occurs within the limits of each species, both because fertility
is of the highest importance to the continuance of the race, and also
because sterility (and to a less extent infertility) is self-destructive
as well as injurious to the species.

So long therefore as a species remains undivided, and in occupation of a
continuous area, its fertility is kept up by natural selection; but the
moment it becomes separated, either by geographical or selective
isolation, or by diversity of station or of habits, then, while each
portion must be kept fertile _inter se_, there is nothing to prevent
infertility arising between the two separated portions. As the two
portions will necessarily exist under somewhat different conditions of
life, and will usually have acquired some diversity of form and
colour--both which circumstances we know to be either the cause of
infertility or to be correlated with it,--the fact of some degree of
infertility usually appearing between closely allied but locally or
physiologically segregated species is exactly what we should expect.

The reason why varieties do not usually exhibit a similar amount of
infertility is not difficult to explain. The popular conclusions on this
matter have been drawn chiefly from what occurs among domestic animals,
and we have seen that the very first essential to their becoming
domesticated was that they should continue fertile under changed
conditions of life. During the slow process of the formation of new
varieties by conscious or unconscious selection, fertility has always
been an essential character, and has thus been invariably preserved or
increased; while there is some evidence to show that domestication
itself tends to increase fertility.

Among plants, wild species and varieties have been more frequently
experimented on than among animals, and we accordingly find numerous
cases in which distinct species of plants are perfectly fertile when
crossed, their hybrid offspring being also fertile _inter se_. We also
find some few examples of the converse fact--varieties of the same
species which when crossed are infertile or even sterile.

The idea that either infertility or geographical isolation is absolutely
essential to the formation of new species, in order to prevent the
swamping effects of intercrossing, has been shown to be unsound, because
the varieties or incipient species will, in most cases, be sufficiently
isolated by having adopted different habits or by frequenting different
stations; while selective association, which is known to be general
among distinct varieties or breeds of the same species, will produce an
effective isolation even when the two forms occupy the same area.

From the various considerations now adverted to, Mr. Darwin arrived at
the conclusion that the sterility or infertility of species with each
other, whether manifested in the difficulty of obtaining first crosses
between them or in the sterility of the hybrids thus obtained, is not a
constant or necessary result of specific difference, but is incidental
on unknown peculiarities of the reproductive system. These peculiarities
constantly tend to arise under changed conditions owing to the extreme
susceptibility of that system, and they are usually correlated with
variations of form or of colour. Hence, as fixed differences of form and
colour, slowly gained by natural selection in adaptation to changed
conditions, are what essentially characterise distinct species, some
amount of infertility between species is the usual result.

Here the problem was left by Mr. Darwin; but we have shown that its
solution may be carried a step further. If we accept the association of
some degree of infertility, however slight, as a not unfrequent
accompaniment of the external differences which always arise in a state
of nature between varieties and incipient species, it has been shown
that natural selection _has_ power to increase that infertility just as
it has power to increase other favourable variations. Such an increase
of infertility will be beneficial, whenever new species arise in the
same area with the parent form; and we thus see how, out of the
fluctuating and very unequal amounts of infertility correlated with
physical variations, there may have arisen that larger and more constant
amount which appears usually to characterise well-marked species.

The great body of facts of which a condensed account has been given in
the present chapter, although from an experimental point of view very
insufficient, all point to the general conclusion we have now reached,
and afford us a not unsatisfactory solution of the great problem of
hybridism in relation to the origin of species by means of natural
selection. Further experimental research is needed in order to complete
the elucidation of the subject; but until these additional facts are
forthcoming no new theory seems required for the explanation of the
phenomena.

FOOTNOTES:

[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.
ii. pp. 163-170.]

[Footnote 52: For a full account of these interesting facts and of the
various problems to which they give rise, the reader must consult
Darwin's volume on _The Different Forms of Flowers in Plants of the same
Species_, chaps, i.-iv.]

[Footnote 53: See _Nature_, vol. xxi. p. 207.]

[Footnote 54: Low's _Domesticated Animals of Great Britain_,
Introduction, p. lxiv.]

[Footnote 55: Low's _Domesticated Animals_, p. 28.]

[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.
379.]

[Footnote 57: _Origin of Species_, p. 239.]

[Footnote 58: _Origin of Species_, sixth edition, p. 9.]

[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.
(1870), Dr. Ogle has adduced some curious physiological facts bearing on
the presence or absence of white colours in the higher animals. He
states that a dark pigment in the olfactory region of the nostrils is
essential to perfect smell, and that this pigment is rarely deficient
except when the whole animal is pure white, and the creature is then
almost without smell or taste. He observes that there is no proof that,
in any of the cases given above, the black animals actually eat the
poisonous root or plant; and that the facts are readily understood if
the senses of smell and taste are dependent on a pigment which is absent
in the white animals, who therefore eat what those gifted with normal
senses avoid. This explanation however hardly seems to cover the facts.
We cannot suppose that almost all the sheep in the world (which are
mostly white) are without smell or taste. The cutaneous disease on the
white patches of hair on horses, the special liability of white terriers
to distemper, of white chickens to the gapes, and of silkworms which
produce yellow silk to the fungus, are not explained by it. The
analogous facts in plants also indicate a real constitutional relation
with colour, not an affection of the sense of smell and taste only.]

[Footnote 60: For all these facts, see _Animals and Plants under
Domestication_, vol. ii. pp. 335-338.]

[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.
102, 103.]

[Footnote 62: As this argument is a rather difficult one to follow,
while its theoretical importance is very great, I add here the following
briefer exposition of it, in a series of propositions; being, with a few
verbal alterations, a copy of what I wrote on the subject about twenty
years back. Some readers may find this easier to follow than the fuller
discussion in the text:--


    _Can Sterility of Hybrids have been Produced by Natural
    Selection?_

    1. Let there be a species which has varied into _two forms_ each
    adapted to certain existing conditions better than the parent
    form, which they soon supplant.

    2. If these _two forms_, which are supposed to coexist in the
    same district, do not intercross, natural selection will
    accumulate all favourable variations till they become well
    suited to their conditions of life, and form two slightly
    differing species.

    3. But if these _two forms_ freely intercross with each other,
    and produce hybrids, which are also quite fertile _inter se_,
    then the formation of the two distinct races or species will be
    retarded, or perhaps entirely prevented; for the offspring of
    the crossed unions will be _more vigorous_ owing to the cross,
    although _less adapted_ to their conditions of life than either
    of the pure breeds.

    4. Now, let a partial sterility of the hybrids of some
    considerable proportion of these two forms arise; and, as this
    would probably be due to some special conditions of life, we may
    fairly suppose it to arise in some definite portion of the area
    occupied by the two forms.

    5. The result will be that, in that area, the hybrids (although
    continually produced by first crosses almost as freely as
    before) will not themselves increase so rapidly as the two pure
    forms; and as the two pure forms are, by the terms of the
    problem, better suited to their several conditions of life than
    the hybrids, they will inevitably increase more rapidly, and
    will continually tend to supplant the hybrids altogether at
    every recurrent severe struggle for existence.

    6. We may fairly suppose, also, that as soon as any sterility
    appears some disinclination to _cross unions_ will appear, and
    this will further tend to the diminution of the production of
    hybrids.

    7. In the other part of the area, however, where hybridism
    occurs with perfect freedom, hybrids of various degrees may
    increase till they equal or even exceed in number the pure
    species--that is, the incipient species will be liable to be
    swamped by intercrossing.

    8. The first result, then, of a partial sterility of crosses
    appearing in one part of the area occupied by the two forms,
    will be--that the great majority of the individuals will there
    consist of the two pure forms only, while in the remaining part
    these will be in a minority,--which is the same as saying that
    the new _physiological variety_ of the two forms will be better
    suited to the conditions of existence than the remaining portion
    which has not varied physiologically.

    9. But when the struggle for existence becomes severe, that
    variety which is best adapted to the conditions of existence
    always supplants that which is imperfectly adapted; therefore,
    _by natural selection_ the _varieties_ which are _sterile_ when
    crossed will become established as the only ones.

    10. Now let variations in the _amount of sterility_ and in
    the _disinclination to crossed unions_ continue to occur--also
    in certain parts of the area: exactly the same result must
    recur, and the progeny of this new physiological variety will in
    time occupy the whole area.

    11. There is yet another consideration that would facilitate the
    process. It seems probable that the _sterility variations_
    would, to some extent, concur with, and perhaps depend upon, the
    _specific variations_; so that, just in proportion as the _two
    forms_ diverged and became better adapted to the conditions of
    existence, they would become more sterile when intercrossed. If
    this were the case, then natural selection would act with double
    strength; and those which were better adapted to survive both
    structurally and physiologically would certainly do so.]

[Footnote 63: Cases of this kind are referred to at p. 155. It must,
however, be noted, that such sterility in first crosses appears to be
equally rare between different species of the same genus as between
individuals of the same species. Mules and other hybrids are freely
produced between very distinct species, but are themselves infertile or
quite sterile; and it is this infertility or sterility of the hybrids
that is the characteristic--and was once thought to be the criterion--of
species, not the sterility of their first crosses. Hence we should not
expect to find any constant infertility in the first crosses between the
distinct strains or varieties that formed the starting-point of new
species, but only a slight amount of infertility in their mongrel
offspring. It follows, that Mr. Romanes' theory of _Physiological
Selection_--which assumes sterility or infertility between first crosses
as the fundamental fact in the origin of species--does not accord with
the general phenomena of hybridism in nature.]

[Footnote 64: The exact number is 1219.51, but the fractions are omitted
for clearness.]




CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS


    The Darwinian theory threw new light on organic colour--The
    problem to be solved--The constancy of animal colour indicates
    utility--Colour and environment--Arctic animals
    white--Exceptions prove the rule--Desert, forest, nocturnal, and
    oceanic animals--General theories of animal colour--Variable
    protective colouring--Mr. Poulton's experiments--Special or
    local colour adaptations--Imitation of particular objects--How
    they have been produced--Special protective colouring of
    butterflies--Protective resemblance among marine
    animals--Protection by terrifying enemies--Alluring
    coloration--The coloration of birds' eggs--Colour as a means of
    recognition--Summary of the preceding exposition--Influence of
    locality or of climate on colour--Concluding remarks.



Among the numerous applications of the Darwinian theory in the
interpretation of the complex phenomena presented by the organic world,
none have been more successful, or are more interesting, than those
which deal with the colours of animals and plants. To the older school
of naturalists colour was a trivial character, eminently unstable and
untrustworthy in the determination of species; and it appeared to have,
in most cases, no use or meaning to the objects which displayed it. The
bright and often gorgeous coloration of insect, bird, or flower, was
either looked upon as having been created for the enjoyment of mankind,
or as due to unknown and perhaps undiscoverable laws of nature.

But the researches of Mr. Darwin totally changed our point of view in
this matter. He showed, clearly, that some of the colours of animals are
useful, some hurtful to them; and he believed that many of the most
brilliant colours were developed by sexual choice; while his great
general principle, that all the fixed characters of organic beings have
been developed under the action of the law of utility, led to the
inevitable conclusion that so remarkable and conspicuous a character as
colour, which so often constitutes the most obvious distinction of
species from species, or group from group, must also have arisen from
survival of the fittest, and must, therefore, in most cases have some
relation to the wellbeing of its possessors. Continuous observation and
research, carried on by multitudes of observers during the last thirty
years, have shown this to be the case; but the problem is found to be
far more complex than was at first supposed. The modes in which colour
is of use to different classes of organisms is very varied, and have
probably not yet been all discovered; while the infinite variety and
marvellous beauty of some of its developments are such as to render it
hopeless to arrive at a complete and satisfactory explanation of every
individual case. So much, however, has been achieved, so many curious
facts have been explained, and so much light has been thrown on some of
the most obscure phenomena of nature, that the subject deserves a
prominent place in any account of the Darwinian theory.


_The Problem to be Solved._

Before dealing with the various modifications of colour in the animal
world it is necessary to say a few words on colour in general, on its
prevalence in nature, and how it is that the colours of animals and
plants require any special explanation. What we term colour is a
subjective phenomenon, due to the constitution of our mind and nervous
system; while, objectively, it consists of light-vibrations of different
wave-lengths emitted by, or reflected from, various objects. Every
visible object must be coloured, because to be visible it must send rays
of light to our eye. The kind of light it sends is modified by the
molecular constitution or the surface texture of the object. Pigments
absorb certain rays and reflect the remainder, and this reflected
portion has to our eyes a definite colour, according to the portion of
the rays constituting white light which are absorbed. Interference
colours are produced either by thin films or by very fine striae on the
surfaces of bodies, which cause rays of certain wave-lengths to
neutralise each other, leaving the remainder to produce the effects of
colour. Such are the colours of soap-bubbles, or of steel or glass on
which extremely fine lines have been ruled; and these colours often
produce the effect of metallic lustre, and are the cause of most of the
metallic hues of birds and insects.

As colour thus depends on molecular or chemical constitution or on the
minute surface texture of bodies, and, as the matter of which organic
beings are composed consists of chemical compounds of great complexity
and extreme instability, and is also subject to innumerable changes
during growth and development, we might naturally expect the phenomena
of colour to be more varied here than in less complex and more stable
compounds. Yet even in the inorganic world we find abundant and varied
colours; in the earth and in the water; in metals, gems, and minerals;
in the sky and in the ocean; in sunset clouds and in the many-tinted
rainbow. Here we can have no question of _use_ to the coloured object,
and almost as little perhaps in the vivid red of blood, in the brilliant
colours of red snow and other low algae and fungi, or even in the
universal mantle of green which clothes so large a portion of the
earth's surface. The presence of some colour, or even of many brilliant
colours, in animals and plants would require no other explanation than
does that of the sky or the ocean, of the ruby or the emerald--that is,
it would require a purely physical explanation only. It is the wonderful
individuality of the colours of animals and plants that attracts our
attention--the fact that the colours are localised in definite patterns,
sometimes in accordance with structural characters, sometimes altogether
independent of them; while often differing in the most striking and
fantastic manner in allied species. We are thus compelled to look upon
colour not merely as a physical but also as a biological characteristic,
which has been differentiated and specialised by natural selection, and
must, therefore, find its explanation in the principle of adaptation or
utility.


_The Constancy of Animal Colour indicates Utility._

That the colours and markings of animals have been acquired under the
fundamental law of utility is indicated by a general fact which has
received very little attention. As a rule, colour and marking are
constant in each species of wild animal, while, in almost every
domesticated animal, there arises great variability. We see this in our
horses and cattle, our dogs and cats, our pigeons and poultry. Now, the
essential difference between the conditions of life of domesticated and
wild animals is, that the former are protected by man, while the latter
have to protect themselves. The extreme variations in colour that
immediately arise under domestication indicate a tendency to vary in
this way, and the occasional occurrence of white or piebald or other
exceptionally coloured individuals of many species in a state of nature,
shows that this tendency exists there also; and, as these exceptionally
coloured individuals rarely or never increase, there must be some
constant power at work to keep it in check. This power can only be
natural selection or the survival of the fittest, which again implies
that some colours are useful, some injurious, in each particular case.
With this principle as our guide, let us see how far we can account both
for the general and special colours of the animal world.


_Colour and Environment._

The fact that first strikes us in our examination of the colours of
animals as a whole, is the close relation that exists between these
colours and the general environment. Thus, white prevails among arctic
animals; yellow or brown in desert species; while green is only a common
colour in tropical evergreen forests. If we consider these cases
somewhat carefully we shall find, that they afford us excellent
materials for forming a judgment on the various theories that have been
suggested to account for the colours of the animal world.

In the arctic regions there are a number of animals which are wholly
white all the year round, or which only turn white in winter. Among the
former are the polar bear and the American polar hare, the snowy owl and
the Greenland falcon; among the latter the arctic fox, the arctic hare,
the ermine, and the ptarmigan. Those which are permanently white remain
among the snow nearly all the year round, while those which change their
colour inhabit regions which are free from snow in summer. The obvious
explanation of this style of coloration is, that it is protective,
serving to conceal the herbivorous species from their enemies, and
enabling carnivorous animals to approach their prey unperceived. Two
other explanations have, however, been suggested. One is, that the
prevalent white of the arctic regions has a direct effect in producing
the white colour in animals, either by some photographic or chemical
action on the skin or by a reflex action through vision. The other is,
that the white colour is chiefly beneficial as a means of checking
radiation and so preserving animal heat during the severity of an arctic
winter. The first is part of the general theory that colour is the
effect of coloured light on the objects--a pure hypothesis which has, I
believe, no facts whatever to support it. The second suggestion is also
an hypothesis merely, since it has not been proved by experiment that a
white colour, _per se_, independently of the fur or feathers which is so
coloured, has any effect whatever in checking the radiation of low-grade
heat like that of the animal body. But both alike are sufficiently
disproved by the interesting exceptions to the rule of white coloration
in the arctic regions, which exceptions are, nevertheless, quite in
harmony with the theory of protection.

Whenever we find arctic animals which, from whatever cause, do not
require protection by the white colour, then neither the cold nor the
snow-glare has any effect upon their coloration. The sable retains its
rich brown fur throughout the Siberian winter; but it frequents trees at
that season and not only feeds partially on fruits or seeds, but is able
to catch birds among the branches of the fir-trees, with the bark of
which its colour assimilates. Then we have that thoroughly arctic
animal, the musk-sheep, which is brown and conspicuous; but this animal
is gregarious, and its safety depends on its association in small herds.
It is, therefore, of more importance for it to be able to recognise its
kind at a distance than to be concealed from its enemies, against which
it can well protect itself so long as it keeps together in a compact
body. But the most striking example is that of the common raven, which
is a true arctic bird, and is found even in mid-winter as far north as
any known bird or mammal. Yet it always retains its black coat, and the
reason, from our point of view, is obvious. The raven is a powerful bird
and fears no enemy, while, being a carrion-feeder, it has no need for
concealment in order to approach its prey. The colour of the raven and
of the musk-sheep are, therefore, both inconsistent with any other
theory than that the white colour of arctic animals has been acquired
for concealment, and to that theory both afford a strong support. Here
we have a striking example of the exception proving the rule.

In the desert regions of the earth we find an even more general
accordance of colour with surroundings. The lion, the camel, and all the
desert antelopes have more or less the colour of the sand or rock among
which they live. The Egyptian cat and the Pampas cat are sandy or earth
coloured. The Australian kangaroos are of similar tints, and the
original colour of the wild horse is supposed to have been sandy or clay
coloured. Birds are equally well protected by assimilative hues; the
larks, quails, goatsuckers, and grouse which abound in the North African
and Asiatic deserts are all tinted or mottled so as closely to resemble
the average colour of the soil in the districts they inhabit. Canon
Tristram, who knows these regions and their natural history so well,
says, in an often quoted passage: "In the desert, where neither trees,
brushwood, nor even undulations of the surface afford the slightest
protection to its foes, a modification of colour which shall be
assimilated to that of the surrounding country is absolutely necessary.
Hence, without exception, the upper plumage of every bird, whether lark,
chat, sylvain, or sand-grouse, and also the fur of all the smaller
mammals, and the skin of all the snakes and lizards, is of one uniform
isabelline or sand colour."

Passing on to the tropical regions, it is among their evergreen forests
alone that we find whole groups of birds whose ground colour is green.
Parrots are very generally green, and in the East we have an extensive
group of green fruit-eating pigeons; while the barbets, bee-eaters,
turacos, leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many
other groups, have so much green in their plumage as to tend greatly to
their concealment among the dense foliage. There can be no doubt that
these colours have been acquired as a protection, when we see that in
all the temperate regions, where the leaves are deciduous, the ground
colour of the great majority of birds, especially on the upper surface,
is a rusty brown of various shades, well corresponding with the bark,
withered leaves, ferns, and bare thickets among which they live in
autumn and winter, and especially in early spring when so many of them
build their nests.

Nocturnal animals supply another illustration of the same rule, in the
dusky colours of mice, rats, bats, and moles, and in the soft mottled
plumage of owls and goatsuckers which, while almost equally
inconspicuous in the twilight, are such as to favour their concealment
in the daytime.

An additional illustration of general assimilation of colour to the
surroundings of animals, is furnished by the inhabitants of the deep
oceans. Professor Moseley of the Challenger Expedition, in his British
Association lecture on this subject, says: "Most characteristic of
pelagic animals is the almost crystalline transparency of their bodies.
So perfect is this transparency that very many of them are rendered
almost entirely invisible when floating in the water, while some, even
when caught and held up in a glass globe, are hardly to be seen. The
skin, nerves, muscles, and other organs are absolutely hyaline and
transparent, but the liver and digestive tract often remain opaque and
of a yellow or brown colour, and exactly resemble when seen in the water
small pieces of floating seaweed." Such marine organisms, however, as
are of larger size, and either occasionally or habitually float on the
surface, are beautifully tinged with blue above, thus harmonising with
the colour of the sea as seen by hovering birds; while they are white
below, and are thus invisible against the wave-foam and clouds as seen
by enemies beneath the surface. Such are the tints of the beautiful
nudibranchiate mollusc, Glaucus atlanticus, and many others.


_General Theories of Animal Colour._

We are now in a position to test the general theories, or, to speak more
correctly, the popular notions, as to the origin of animal coloration,
before proceeding to apply the principle of utility to the explanation
of some among the many extraordinary manifestations of colour in the
animal world. The most generally received theory undoubtedly is, that
brilliancy and variety of colour are due to the direct action of light
and heat; a theory no doubt derived from the abundance of
bright-coloured birds, insects, and flowers which are brought from
tropical regions. There are, however, two strong arguments against this
theory. We have already seen how generally bright coloration is wanting
in desert animals, yet here heat and light are both at a maximum, and if
these alone were the agents in the production of colour, desert animals
should be the most brilliant. Again, all naturalists who have lived in
tropical regions know that the proportion of bright to dull coloured
species is little if any greater there than in the temperate zone, while
there are many tropical groups in which bright colours are almost
entirely unknown. No part of the world presents so many brilliant birds
as South America, yet there are extensive families, containing many
hundreds of species, which are as plainly coloured as our average
temperate birds. Such are the families of the bush-shrikes and
ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the
American creepers (Dendrocolaptidae), together with a large proportion
of the wood-warblers (Mniotiltidae), the finches, the wrens, and some
other groups. In the eastern hemisphere, also, we have the
babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the
honey-suckers (Meliphagidae), and several other smaller groups which are
certainly not coloured above the average standard of temperate birds.

Again, there are many families of birds which spread over the whole
world, temperate and tropical, and among these the tropical species
rarely present any exceptional brilliancy of colour. Such are the
thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named
group it is the temperate and arctic zones that afford the most
brilliant coloration.

The same general facts are found to prevail among insects. Although
tropical insects present some of the most gorgeous coloration in the
whole realm of nature, yet there are thousands and tens of thousands of
species which are as dull coloured as any in our cloudy land. The
extensive family of the carnivorous ground-beetles (Carabidae) attains
its greatest brilliancy in the temperate zone; while by far the larger
proportion of the great families of the longicorns and the weevils, are
of obscure colours even in the tropics. In butterflies, there is
undoubtedly a larger proportion of brilliant colour in the tropics; but
if we compare families which are almost equally developed over the
globe--as the Pieridae or whites and yellows, and the Satyridae or
ringlets--we shall find no great disproportion in colour between those
of temperate and tropical regions.

The various facts which have now briefly been noticed are sufficient to
indicate that the light and heat of the sun are not the direct causes of
the colours of animals, although they may favour the production of
colour when, as in tropical regions, the persistent high temperature
favours the development of the maximum of life. We will now consider the
next suggestion, that light reflected from surrounding coloured objects
tends to produce corresponding colours in the animal world.

This theory is founded on a number of very curious facts which prove,
that such a change does sometimes occur and is directly dependent on the
colours of surrounding objects; but these facts are comparatively rare
and exceptional in their nature, and the same theory will certainly not
apply to the infinitely varied colours of the higher animals, many of
which are exposed to a constantly varying amount of light and colour
during their active existence. A brief sketch of these dependent changes
of colour may, however, be advantageously given here.


_Variable Protective Colouring._

There are two distinct kinds of change of colour in animals due to the
colouring of the environment. In one case the change is caused by reflex
action set up by the animal _seeing_ the colour to be imitated, and the
change produced can be altered or repeated as the animal changes its
position. In the other case the change occurs but once, and is probably
not due to any conscious or sense action, but to some direct influence
on the surface tissues while the creature is undergoing a moult or
change to the pupa form.

The most striking example of the first class is that of the chameleon,
which changes to white, brown, yellowish, or green, according to the
colour of the object on which it rests. This change is brought about by
means of two layers of pigment cells, deeply seated in the skin, and of
bluish and yellowish colours. By suitable muscles these cells can be
forced upwards so as to modify the colour of the skin, which, when they
are not brought into action, is a dirty white. These animals are
excessively sluggish and defenceless, and the power of changing their
colour to that of their immediate surroundings is no doubt of great
service to them. Many of the flatfish are also capable of changing their
colour according to the colour of the bottom they rest on; and frogs
have a similar power to a limited extent. Some crustacea also change
colour, and the power is much developed in the Chameleon shrimp (Mysis
Chamaeleon) which is gray when on sand, but brown or green when among
brown or green seaweed. It has been proved by experiment that when this
animal is blinded the change does not occur. In all these cases,
therefore, we have some form of reflex or sense action by which the
change is produced, probably by means of pigment cells beneath the skin
as in the chameleon.

The second class consists of certain larvae, and pupae, which undergo
changes of colour when exposed to differently coloured surroundings.
This subject has been carefully investigated by Mr. E.B. Poulton, who
has communicated the results of his experiments to the Royal
Society.[65] It had been noticed that some species of larvae which fed
on several different plants had colours more or less corresponding to
the particular plant the individual fed on. Numerous cases are given in
Professor Meldola's article on "Variable Protective Colouring" (_Proc.
Zool. Soc._, 1873, p. 153), and while the general green coloration was
attributed to the presence of chlorophyll beneath the skin, the
particular change in correspondence to each food-plant was attributed to
a special function which had been developed by natural selection. Later
on, in a note to his translation of Weissmann's _Theory of Descent_,
Professor Meldola seemed disposed to think that the variations of colour
of some of the species might be phytophagic--that is, due to the direct
action of the differently coloured leaves on which the insect fed. Mr.
Poulton's experiments have thrown much light on this question, since he
has conclusively proved that, in the case of the sphinx caterpillar of
Smerinthus ocellatus, the change of colour is not due to the food but to
the coloured light reflected from the leaves.

This was shown by feeding two sets of larvae on the same plant but
exposed to differently coloured surroundings, obtained by sewing the
leaves together, so that in one case only the dark upper surface, in the
other the whitish under surface was exposed to view. The result in each
case was a corresponding change of colour in the larvae, confirming the
experiments on different individuals of the same batch of larvae which
had been supplied with different food-plants or exposed to a different
coloured light.

An even more interesting series of experiments was made on the colours
of pupae, which in many cases were known to be affected by the material
on which they underwent their transformations. The late Mr. T.W. Wood
proved, in 1867, that the pupae of the common cabbage butterflies
(Pieris brassicae and P. rapae) were either light, or dark, or green,
according to the coloured boxes they were kept in, or the colours of the
fences, walls, etc., against which they were suspended. Mrs. Barber in
South Africa found that the pupae of Papilio Nireus underwent a similar
change, being deep green when attached to orange leaves of the same
tint, pale yellowish-green when on a branch of the bottle-brush tree
whose half-dried leaves were of this colour, and yellowish when attached
to the wooden frame of a box. A few other observers noted similar
phenomena, but nothing more was done till Mr. Poulton's elaborate series
of experiments with the larvae of several of our common butterflies were
the means of clearing up several important points. He showed that the
action of the coloured light did not affect the pupa itself but the
larva, and that only for a limited period of time. After a caterpillar
has done feeding it wanders about seeking a suitable place to undergo
its transformation. When this is found it rests quietly for a day or
two, spinning the web from which it is to suspend itself; and it is
during this period of quiescence, and perhaps also the first hour or two
after its suspension, that the action of the surrounding coloured
surfaces determines, to a considerable extent, the colour of the pupa.
By the application of various surrounding colours during this period,
Mr. Poulton was able to modify the colour of the pupa of the common
tortoise-shell butterfly from nearly black to pale, or to a brilliant
golden; and that of Pieris rapae from dusky through pinkish to pale
green. It is interesting to note, that the colours produced were in all
cases such only as assimilated with the surroundings usually occupied by
the species, and also, that colours which did not occur in such
surroundings, as dark red or blue, only produced the same effects as
dusky or black.

Careful experiments were made to ascertain whether the effect was
produced through the sight of the caterpillar. The ocelli were covered
with black varnish, but neither this, nor cutting off the spines of the
tortoise-shell larva to ascertain whether they might be sense-organs,
produced any effect on the resulting colour. Mr. Poulton concludes,
therefore, that the colour-action probably occurs over the whole surface
of the body, setting up physiological processes which result in the
corresponding colour-change of the pupa. Such changes are, however, by
no means universal, or even common, in protectively coloured pupae,
since in Papilio machaon and some others which have been experimented
on, both in this country and abroad, no change can be produced on the
pupa by any amount of exposure to differently coloured surroundings. It
is a curious point that, with the small tortoise-shell larva, exposure
to light from gilded surfaces produced pupae with a brilliant golden
lustre; and the explanation is supposed to be that mica abounded in the
original habitat of the species, and that the pupae thus obtained
protection when suspended against micaceous rock. Looking, however, at
the wide range of the species and the comparatively limited area in
which micaceous rocks occur, this seems a rather improbable explanation,
and the occurrence of this metallic appearance is still a difficulty. It
does not, however, commonly occur in this country in a natural state.

The two classes of variable colouring here discussed are evidently
exceptional, and can have little if any relation to the colours of those
more active creatures which are continually changing their position with
regard to surrounding objects, and whose colours and markings are nearly
constant throughout the life of the individual, and (with the exception
of sexual differences) in all the individuals of the species. We will
now briefly pass in review the various characteristics and uses of the
colours which more generally prevail in nature; and having already
discussed those protective colours which serve to harmonise animals with
their general environment, we have to consider only those cases in which
the colour resemblance is more local or special in its character.


_Special or Local Colour Adaptations._

This form of colour adaptation is generally manifested by markings
rather than by colour alone, and is extremely prevalent both among
insects and vertebrates, so that we shall be able to notice only a few
illustrative cases. Among our native birds we have the snipe and
woodcock, whose markings and tints strikingly accord with the dead marsh
vegetation among which they live; the ptarmigan in its summer dress is
mottled and tinted exactly like the lichens which cover the stones of
the higher mountains; while young unfledged plovers are spotted so as
exactly to resemble the beach pebbles among which they crouch for
protection, as beautifully exhibited in one of the cases of British
birds in the Natural History Museum at South Kensington.

In mammalia, we notice the frequency of rounded spots on forest or tree
haunting animals of large size, as the forest deer and the forest cats;
while those that frequent reedy or grassy places are striped vertically,
as the marsh antelopes and the tiger. I had long been of opinion that
the brilliant yellow and black stripes of the tiger were adaptive, but
have only recently obtained proof that it is so. An experienced
tiger-hunter, Major Walford, states in a letter, that the haunts of the
tiger are invariably full of the long grass, dry and pale yellow for at
least nine months of the year, which covers the ground wherever there is
water in the rainy season, and he adds: "I once, while following up a
wounded tiger, failed for at least a minute to see him under a tree in
grass at a distance of about twenty yards--jungle open--but the natives
saw him, and I eventually made him out well enough to shoot him, but
even then I could not see at what part of him I was aiming. There can be
no doubt whatever that the colour of both the tiger and the panther
renders them almost invisible, especially in a strong blaze of light,
when among grass, and one does not seem to notice stripes or spots till
they are dead." It is the black shadows of the vegetation that
assimilate with the black stripes of the tiger; and, in like manner,
the spotty shadows of leaves in the forest so harmonise with the spots
of ocelots, jaguars, tiger-cats, and spotted deer as to afford them a
very perfect concealment.

In some cases the concealment is effected by colours and markings which
are so striking and peculiar that no one who had not seen the creature
in its native haunts would imagine them to be protective. An example of
this is afforded by the banded fruit pigeon of Timor, whose pure white
head and neck, black wings and back, yellow belly, and deeply-curved
black band across the breast, render it a very handsome and conspicuous
bird. Yet this is what Mr. H.O. Forbes says of it: "On the trees the
white-headed fruit pigeon (Ptilopus cinctus) sate motionless during the
heat of the day in numbers, on well-exposed branches; but it was with
the utmost difficulty that I or my sharp-eyed native servant could ever
detect them, even in trees where we knew they were sitting."[66] The
trees referred to are species of Eucalyptus which abound in Timor. They
have whitish or yellowish bark and very open foliage, and it is the
intense sunlight casting black curved shadows of one branch upon
another, with the white and yellow bark and deep blue sky seen through
openings of the foliage, that produces the peculiar combination of
colours and shadows to which the colours and markings of this bird have
become so closely assimilated.

Even such brilliant and gorgeously coloured birds as the sun-birds of
Africa are, according to an excellent observer, often protectively
coloured. Mrs. M.E. Barber remarks that "A casual observer would
scarcely imagine that the highly varnished and magnificently coloured
plumage of the various species of Noctarinea could be of service to
them, yet this is undoubtedly the case. The most unguarded moments of
the lives of these birds are those that are spent amongst the flowers,
and it is then that they are less wary than at any other time. The
different species of aloes, which blossom in succession, form the
principal sources of their winter supplies of food; and a legion of
other gay flowering plants in spring and summer, the aloe blossoms
especially, are all brilliantly coloured, and they harmonise admirably
with the gay plumage of the different species of sun-birds. Even the
keen eye of a hawk will fail to detect them, so closely do they resemble
the flowers they frequent. The sun-birds are fully aware of this fact,
for no sooner have they relinquished the flowers than they become
exceedingly wary and rapid in flight, darting arrow-like through the air
and seldom remaining in exposed situations. The black sun-bird
(Nectarinea amethystina) is never absent from that magnificent
forest-tree, the 'Kaffir Boom' (Erythrina caffra); all day long the
cheerful notes of these birds may be heard amongst its spreading
branches, yet the general aspect of the tree, which consists of a huge
mass of scarlet and purple-black blossoms without a single green leaf,
blends and harmonises with the colours of the black sun-bird to such an
extent that a dozen of them may be feeding amongst its blossoms without
being conspicuous, or even visible."[67]

Some other cases will still further illustrate how the colours of even
very conspicuous animals may be adapted to their peculiar haunts.

The late Mr. Swinhoe says of the Kerivoula picta, which he observed in
Formosa: "The body of this bat was of an orange colour, but the wings
were painted with orange-yellow and black. It was caught suspended, head
downwards, on a cluster of the fruit of the longan tree (Nephelium
longanum). Now this tree is an evergreen, and all the year round some
portion of its foliage is undergoing decay, the particular leaves being,
in such a stage, partially orange and black. This bat can, therefore, at
all seasons suspend from its branches and elude its enemies by its
resemblance to the leaves of the tree."[68]

Even more curious is the case of the sloths--defenceless animals which
feed upon leaves, and hang from the branches of trees with their back
downwards. Most of the species have a curious buff-coloured spot on the
back, rounded or oval in shape and often with a darker border, which
seems placed there on purpose to make them conspicuous; and this was a
great puzzle to naturalists, because the long coarse gray or greenish
hair was evidently like tree-moss and therefore protective. But an old
writer, Baron von Slack, in his _Voyage_ _to Surinam_ (1810), had
already explained the matter. He says: "The colour and even the shape of
the hair are much like withered moss, and serve to hide the animal in
the trees, but particularly when it has that orange-coloured spot
between the shoulders and lies close to the tree; it looks then exactly
like a piece of branch where the rest has been broken off, by which the
hunters are often deceived." Even such a huge animal as the giraffe is
said to be perfectly concealed by its colour and form when standing
among the dead and broken trees that so often occur on the outskirts of
the thickets where it feeds. The large blotch-like spots on the skin and
the strange shape of the head and horns, like broken branches, so tend
to its concealment that even the keen-eyed natives have been known to
mistake trees for giraffes or giraffes for trees.

Innumerable examples of this kind of protective colouring occur among
insects; beetles mottled like the bark of trees or resembling the sand
or rock or moss on which they live, with green caterpillars of the exact
general tints of the foliage they feed on; but there are also many cases
of detailed imitation of particular objects by insects that must be
briefly described.[69]


_Protective Imitation of Particular Objects._

The insects which present this kind of imitation most perfectly are the
Phasmidae, or stick and leaf insects. The well-known leaf-insects of
Ceylon and of Java, species of Phyllium, are so wonderfully coloured and
veined, with leafy expansions on the legs and thorax, that not one
person in ten can see them when resting on the food-plant close beneath
their eyes. Others resemble pieces of stick with all the minutiae of
knots and branches, formed by the insects' legs, which are stuck out
rigidly and unsymmetrically. I have often been unable to distinguish
between one of these insects and a real piece of stick, till I satisfied
myself by touching it and found it to be alive. One species, which was
brought me in Borneo, was covered with delicate semitransparent green
foliations, exactly resembling the hepaticae which cover pieces of
rotten stick in the damp forests. Others resemble dead leaves in all
their varieties of colour and form; and to show how perfect is the
protection obtained and how important it is to the possessors of it, the
following incident, observed by Mr. Belt in Nicaragua, is most
instructive. Describing the armies of foraging ants in the forest which
devour every insect they can catch, he says: "I was much surprised with
the behaviour of a green leaf-like locust. This insect stood immovably
among a host of ants, many of which ran over its legs without ever
discovering there was food within their reach. So fixed was its
instinctive knowledge that its safety depended on its immovability, that
it allowed me to pick it up and replace it among the ants without making
a single effort to escape. This species closely resembles a green
leaf."[70]

Caterpillars also exhibit a considerable amount of detailed resemblance
to the plants on which they live. Grass-feeders are striped
longitudinally, while those on ordinary leaves are always striped
obliquely. Some very beautiful protective resemblances are shown among
the caterpillars figured in Smith and Abbott's _Lepidopterous Insects of
Georgia_, a work published in the early part of the century, before any
theories of protection were started. The plates in this work are most
beautifully executed from drawings made by Mr. Abbott, representing the
insects, in every case, on the plants which they frequented, and no
reference is made in the descriptions to the remarkable protective
details which appear upon the plates. We have, first, the larva of
Sphinx fuciformis feeding on a plant with linear grass-like leaves and
small blue flowers; and we find the insect of the same green as the
leaves, striped longitudinally in accordance with the linear leaves, and
with the head blue corresponding both in size and colour with the
flowers. Another species (Sphinx tersa) is represented feeding on a
plant with small red flowers situated in the axils of the leaves; and
the larva has a row of seven red spots, unequal in size, and
corresponding very closely with the colour and size of the flowers. Two
other figures of sphinx larvae are very curious. That of Sphinx
pampinatrix feeds on a wild vine (Vitis indivisa), having green
tendrils, and in this species the curved horn on the tail is green, and
closely imitates in its curve the tip of the tendril. But in another
species (Sphinx cranta), which feeds on the fox-grape (Vitis vulpina),
the horn is very long and red, corresponding with the long red-tipped
tendrils of the plant. Both these larvae are green with oblique stripes,
to harmonise with the veined leaves of the vines; but a figure is also
given of the last-named species after it has done feeding, when it is of
a decided brown colour and has entirely lost its horn. This is because
it then descends to the ground to bury itself, and the green colour and
red horn would be conspicuous and dangerous; it therefore loses both at
the last moult. Such a change of colour occurs in many species of
caterpillars. Sometimes the change is seasonal; and, in those which
hibernate with us, the colour of some species, which is brownish in
autumn in adaptation to the fading foliage, becomes green in spring to
harmonise with the newly-opened leaves at that season.[71]

Some of the most curious examples of minute imitation are afforded by
the caterpillars of the geometer moths, which are always brown or
reddish, and resemble in form little twigs of the plant on which they
feed. They have the habit, when at rest, of standing out obliquely from
the branch, to which they hold on by their hind pair of prolegs or
claspers, and remain motionless for hours. Speaking of these protective
resemblances Mr. Jenner Weir says: "After being thirty years an
entomologist I was deceived myself, and took out my pruning scissors to
cut from a plum tree a spur which I thought I had overlooked. This
turned out to be the larva of a geometer two inches long. I showed it
to several members of my family, and defined a space of four inches in
which it was to be seen, but none of them could perceive that it was a
caterpillar."[72]

One more example of a protected caterpillar must be given. Mr. A.
Everett, writing from Sarawak, Borneo, says: "I had a caterpillar
brought me, which, being mixed by my boy with some other things, I took
to be a bit of moss with two exquisite pinky-white seed-capsules; but I
soon saw that it moved, and examining it more closely found out its real
character: it is covered with hair, with two little pink spots on the
upper surface, the general hue being more green. Its motions are very
slow, and when eating the head is withdrawn beneath a fleshy mobile
hood, so that the action of feeding does not produce any movement
externally. It was found in the limestone hills at Busan, the situation
of all others where mosses are most plentiful and delicate, and where
they partially clothe most of the protruding masses of rock."


_How these Imitations have been Produced._

To many persons it will seem impossible that such beautiful and detailed
resemblances as those now described--and these are only samples of
thousands that occur in all parts of the world--can have been brought
about by the preservation of accidental useful variations. But this will
not seem so surprising if we keep in mind the facts set forth in our
earlier chapters--the rapid multiplication, the severe struggle for
existence, and the constant variability of these and all other
organisms. And, further, we must remember that these delicate
adjustments are the result of a process which has been going on for
millions of years, and that we now see the small percentage of successes
among the myriads of failures. From the very first appearance of insects
and their various kinds of enemies the need of protection arose, and was
usually most easily met by modifications of colour. Hence, we may be
sure that the earliest leaf-eating insects acquired a green colour as
one of the necessities of their existence; and, as the species became
modified and specialised, those feeding on particular species of plants
would rapidly acquire the peculiar tints and markings best adapted to
conceal them upon those plants. Then, every little variation that, once
in a hundred years perhaps, led to the preservation of some larva which
was thereby rather better concealed than its fellows, would form the
starting-point of a further development, leading ultimately to that
perfection of imitation in details which now astonishes us. The
researches of Dr. Weismann illustrate this progressive adaptation. The
very young larvae of several species are green or yellowish without any
markings; they then, in subsequent moults, obtain certain markings, some
of which are often lost again before the larva is fully grown. The early
stages of those species which, like elephant hawk-moths (Chaerocampa),
have the anterior segments elongated and retractile, with large eye-like
spots to imitate the head of a vertebrate, are at first like those of
non-retractile species, the anterior segments being as large as the
rest. After the first moult they become smaller, comparatively; but it
is only after the second moult that the ocelli begin to appear, and
these are not fully defined till after the third moult. This progressive
development of the individual--the ontogeny--gives us a clue to the
ancestral development of the whole race--the phylogeny; and we are
enabled to picture to ourselves the very slow and gradual steps by which
the existing perfect adaptation has been brought about. In many larvae
great variability still exists, and in some there are two or more
distinctly-coloured forms--usually a dark and a light or a brown and a
green form. The larva of the humming-bird hawk-moth (Macroglossa
stellatarum) varies in this manner, and Dr. Weismann raised five
varieties from a batch of eggs from one moth. It feeds on species of
bedstraw (Galium verum and G. mollugo), and as the green forms are less
abundant than the brown, it has probably undergone some recent change of
food-plant or of habits which renders brown the more protective colour.


_Special Protective Colouring of Butterflies._

We will now consider a few cases of special protective colouring in the
perfect butterfly or moth. Mr. Mansel Weale states that in South Africa
there is a great prevalence of white and silvery foliage or bark,
sometimes of dazzling brilliancy, and that many insects and their larvae
have brilliant silvery tints which are protective, among them being
three species of butterflies whose undersides are silvery, and which are
thus effectually protected when at rest.[73] A common African butterfly
(Aterica meleagris) always settles on the ground with closed wings,
which so closely resemble the soil of the district that it can with
difficulty be seen, and the colour varies with the soil in different
localities. Thus specimens from Senegambia were dull brown, the soil
being reddish sand and iron-clay; those from Calabar and Cameroons were
light brown with numerous small white spots, the soil of those countries
being light brown clay with small quartz pebbles; while in other
localities where the colours of the soil were more varied the colours of
the butterfly varied also. Here we have variation in a single species
which has become specialised in certain areas to harmonise with the
colour of the soil.[74]

Many butterflies, in all parts of the world, resemble dead leaves on
their under side, but those in which this form of protection is carried
to the greatest perfection are the species of the Eastern genus Kallima.
In India K. inachis, and in the larger Malay islands K. paralekta, are
very common. They are rather large and showy butterflies, orange and
bluish on the upper side, with a very rapid flight, and frequenting dry
forests. Their habit is to settle always where there is some dead or
decaying foliage, and the shape and colour of the wings (on the under
surface), together with the attitude of the insect, is such as to
produce an absolutely perfect imitation of a dead leaf. This is effected
by the butterfly always settling on a twig, with the short tail of the
hind wings just touching it and forming the leaf-stalk. From this a dark
curved line runs across to the elongated tip of the upper wings,
imitating the midrib, on both sides of which are oblique lines, formed
partly by the nervures and partly by markings, which give the effect of
the usual veining of a leaf. The head and antennae fit exactly between
the closed upper wings so as not to interfere with the outline, which
has just that amount of irregular curvature that is seen in dry and
withered leaves. The colour is very remarkable for its extreme amount of
variability, from deep reddish-brown to olive or pale yellow, hardly two
specimens being exactly alike, but all coming within the range of colour
of leaves in various stages of decay. Still more curious is the fact
that the paler wings, which imitate leaves most decayed, are usually
covered with small black dots, often gathered into circular groups, and
so exactly resembling the minute fungi on decaying leaves that it is
hard at first to believe that the insects themselves are not attacked by
some such fungus. The concealment produced by this wonderful imitation
is most complete, and in Sumatra I have often seen one enter a bush and
then disappear like magic. Once I was so fortunate as to see the exact
spot on which the insect settled; but even then I lost sight of it for
some time, and only after a persistent search discovered that it was
close before my eyes.[75] Here we have a kind of imitation, which is
very common in a less developed form, carried to extreme perfection,
with the result that the species is very abundant over a considerable
area of country.


_Protective Resemblance among Marine Animals._

Among marine animals this form of protection is very common. Professor
Moseley tells us that all the inhabitants of the Gulf-weed are most
remarkably coloured, for purposes of protection and concealment, exactly
like the weed itself. "The shrimps and crabs which swarm in the weed are
of exactly the same shade of yellow as the weed, and have white markings
upon their bodies to represent the patches of Membranipora. The small
fish, Antennarius, is in the same way weed-colour with white spots. Even
a Planarian worm, which lives in the weed, is similarly yellow-coloured,
and also a mollusc, Scyllaea pelagica." The same writer tells us that "a
number of little crabs found clinging to the floats of the blue-shelled
mollusc, Ianthina, were all coloured of a corresponding blue for
concealment."[76]

Professor E.S. Morse of Salem, Mass., found that most of the New
England marine mollusca were protectively coloured; instancing among
others a little red chiton on rocks clothed with red calcareous algae,
and Crepidula plana, living within the apertures of the shells of larger
species of Gasteropods and of a pure white colour corresponding to its
habitat, while allied species living on seaweed or on the outside of
dark shells were dark brown.[77] A still more interesting case has been
recorded by Mr. George Brady. He says: "Amongst the Nullipore which
matted together the laminaria roots in the Firth of Clyde were living
numerous small starfishes (Ophiocoma bellis) which, except when their
writhing movements betrayed them, were quite undistinguishable from the
calcareous branches of the alga; their rigid angularly twisted rays had
all the appearance of the coralline, and exactly assimilated to its dark
purple colour, so that though I held in my hand a root in which were
half a dozen of the starfishes, I was really unable to detect them until
revealed by their movements."[78]

These few examples are sufficient to show that the principle of
protective coloration extends to the ocean as well as over the earth;
and if we consider how completely ignorant we are of the habits and
surroundings of most marine animals, it may well happen that many of the
colours of tropical fishes, which seem to us so strange and so
conspicuous, are really protective, owing to the number of equally
strange and brilliant forms of corals, sea-anemones, sponges, and
seaweeds among which they live.


_Protection by Terrifying Enemies._

A considerable number of quite defenceless insects obtain protection
from some of their enemies by having acquired a resemblance to dangerous
animals, or by some threatening or unusual appearance. This is obtained
either by a modification of shape, of habits, of colour, or of all
combined. The simplest form of this protection is the aggressive
attitude of the caterpillars of the Sphingidae, the forepart of the body
being erected so as to produce a rude resemblance to the figure of a
sphinx, hence the name of the family. The protection is carried further
by those species which retract the first three segments and have large
ocelli on each side of the fourth segment, thus giving to the
caterpillar, when the forepart of its body is elevated, the appearance
of a snake in a threatening attitude.

The blood-red forked tentacle, thrown out of the neck of the larvae of
the genus Papilio when alarmed, is, no doubt, a protection against the
attacks of ichneumons, and may, perhaps, also frighten small birds; and
the habit of turning up the tail possessed by the harmless rove-beetles
(Staphylinidae), giving the idea that they can sting, has, probably, a
similar use. Even an unusual angular form, like a crooked twig or
inorganic substance, may be protective; as Mr. Poulton thinks is the
case with the curious caterpillar of Notodonta ziczac, which, by means
of a few slight protuberances on its body, is able to assume an angular
and very unorganic-looking appearance. But perhaps the most perfect
example of this kind of protection is exhibited by the large caterpillar
of the Royal Persimmon moth (Bombyx regia), a native of the southern
states of North America, and known there as the "Hickory-horned devil."
It is a large green caterpillar, often six inches long, ornamented with
an immense crown of orange-red tubercles, which, if disturbed, it erects
and shakes from side to side in a very alarming manner. In its native
country the negroes believe it to be as deadly as a rattlesnake, whereas
it is perfectly innocuous. The green colour of the body suggests that
its ancestors were once protectively coloured; but, growing too large to
be effectually concealed, it acquired the habit of shaking its head
about in order to frighten away its enemies, and ultimately developed
the crown of tentacles as an addition to its terrifying powers. This
species is beautifully figured in Abbott and Smith's _Lepidopterous
Insects of Georgia_.


_Alluring Coloration._

Besides those numerous insects which obtain protection through their
resemblance to the natural objects among which they live, there are some
whose disguise is not used for concealment, but as a direct means of
securing their prey by attracting them within the enemy's reach. Only a
few cases of this kind of coloration have yet been observed, chiefly
among spiders and mantidae; but, no doubt, if attention were given to
the subject in tropical countries, many more would be discovered. Mr.
H.O. Forbes has described a most interesting example of this kind of
simulation in Java. While pursuing a large butterfly through the jungle,
he was stopped by a dense bush, on a leaf of which he observed one of
the skipper butterflies sitting on a bird's dropping. "I had often," he
says, "observed small Blues at rest on similar spots on the ground, and
have wondered what such a refined and beautiful family as the Lycaenidae
could find to enjoy, in food apparently so incongruous for a butterfly.
I approached with gentle steps, but ready net, to see if possible how
the present species was engaged. It permitted me to get quite close, and
even to seize it between my fingers; to my surprise, however, part of
the body remained behind, adhering as I thought to the excreta. I looked
closely, and finally touched with my finger the excreta to find if it
were glutinous. To my delighted astonishment I found that my eyes had
been most perfectly deceived, and that what seemed to be the excreta was
a most artfully coloured spider, lying on its back with its feet crossed
over and closely adpressed to the body." Mr. Forbes then goes on to
describe the exact appearance of such excreta, and how the various parts
of the spider are coloured to produce the imitation, even to the liquid
portion which usually runs a little down the leaf. This is exactly
imitated by a portion of the thin web which the spider first spins to
secure himself firmly to the leaf; thus producing, as Mr. Forbes
remarks, a living bait for butterflies and other insects so artfully
contrived as to deceive a pair of human eyes, even when intently
examining it.[79]

A native species of spider (Thomisus citreus) exhibits a somewhat
similar alluring protection by its close resemblance to buds of the
wayfaring tree, Viburnum lantana. It is pure creamy-white, the abdomen
exactly resembling in shape and colour the unopened buds of the flowers
among which it takes its station; and it has been seen to capture flies
which came to the flowers.

But the most curious and beautiful case of alluring protection is that
of a wingless Mantis in India, which is so formed and coloured as to
resemble a pink orchis or some other fantastic flower. The whole insect
is of a bright pink colour, the large and oval abdomen looking like the
labellum of an orchid. On each side, the two posterior legs have
immensely dilated and flattened thighs which represent the petals of a
flower, while the neck and forelegs imitate the upper sepal and column
of an orchid. The insect rests motionless, in this symmetrical attitude,
among bright green foliage, being of course very conspicuous, but so
exactly resembling a flower that butterflies and other insects settle
upon it and are instantly captured. It is a living trap, baited in the
most alluring manner to catch the unwary flower-haunting insects.[80]


_The Coloration of Birds' Eggs._

The colours of birds' eggs have long been a difficulty on the theory of
adaptive coloration, because, in so many cases it has not been easy to
see what can be the use of the particular colours, which are often so
bright and conspicuous that they seem intended to attract attention
rather than to be concealed. A more careful consideration of the subject
in all its bearings shows, however, that here too, in a great number of
cases, we have examples of protective coloration. When, therefore, we
cannot see the meaning of the colour, we may suppose that it has been
protective in some ancestral form, and, not being hurtful, has persisted
under changed conditions which rendered the protection needless.

We may divide all eggs, for our present purpose, into two great
divisions; those which are white or nearly so, and those which are
distinctly coloured or spotted. Egg-shells being composed mainly of
carbonate of lime, we may assume that the primitive colour of birds'
eggs was white, a colour that prevails now among the other egg-bearing
vertebrates--lizards, crocodiles, turtles, and snakes; and we might,
therefore, expect that this colour would continue where its presence had
no disadvantages. Now, as a matter of fact, we find that in all the
groups of birds which lay their eggs in concealed places, whether in
holes of trees or in the ground, or in domed or covered nests, the eggs
are either pure white or of very pale uniform coloration. Such is the
case with kingfishers, bee-eaters, penguins, and puffins, which nest in
holes in the ground; with the great parrot family, the woodpeckers, the
rollers, hoopoes, trogons, owls, and some others, which build in holes
in trees or other concealed places; while martins, wrens,
willow-warblers, and Australian finches, build domed or covered nests,
and usually have white eggs.

There are, however, many other birds which lay their white eggs in open
nests; and these afford some very interesting examples of the varied
modes by which concealment may be obtained. All the duck tribe, the
grebes, and the pheasants belong to this class; but these birds all have
the habit of covering their eggs with dead leaves or other material
whenever they leave the nest, so as effectually to conceal them. Other
birds, as the short-eared owl, the goatsucker, the partridge, and some
of the Australian ground pigeons, lay their white or pale eggs on the
bare soil; but in these cases the birds themselves are protectively
coloured, so that, when sitting, they are almost invisible; and they
have the habit of sitting close and almost continuously, thus
effectually concealing their eggs.

Pigeons and doves offer a very curious case of the protection of exposed
eggs. They usually build very slight and loose nests of sticks and
twigs, so open that light can be seen through them from below, while
they are generally well concealed by foliage above. Their eggs are white
and shining; yet it is a difficult matter to discover, from beneath,
whether there are eggs in the nest or not, while they are well hidden by
the thick foliage above. The Australian podargihuge goatsuckers--build
very similar nests, and their white eggs are protected in the same
manner. Some large and powerful birds, as the swans, herons, pelicans,
cormorants, and storks, lay white eggs in open nests; but they keep
careful watch over them, and are able to drive away intruders. On the
whole, then, we see that, while white eggs are conspicuous, and
therefore especially liable to attack by egg-eating animals, they are
concealed from observation in many and various ways. We may, therefore,
assume that, in cases where there seems to be no such concealment, we
are too ignorant of the whole of the conditions to form a correct
judgment.

We now come to the large class of coloured or richly spotted eggs, and
here we have a more difficult task, though many of them decidedly
exhibit protective tints or markings. There are two birds which nest on
sandy shores--the lesser tern and the ringed plover,--and both lay
sand-coloured eggs, the former spotted so as to harmonise with coarse
shingle, the latter minutely speckled like fine sand, which are the
kinds of ground the two birds choose respectively for their nests. "The
common sandpipers' eggs assimilate so closely with the tints around them
as to make their discovery a matter of no small difficulty, as every
oologist can testify who has searched for them. The pewits' eggs, dark
in ground colour and boldly marked, are in strict harmony with the sober
tints of moor and fallow, and on this circumstance alone their
concealment and safety depend. The divers' eggs furnish another example
of protective colour; they are generally laid close to the water's edge,
amongst drift and shingle, where their dark tints and black spots
conceal them by harmonising closely with surrounding objects. The snipes
and the great army of sandpipers furnish innumerable instances of
protectively coloured eggs. In all the instances given the sitting-bird
invariably leaves the eggs uncovered when it quits them, and
consequently their safety depends solely on the colours which adorn
them."[81] The wonderful range of colour and marking in the eggs of the
guillemot may be imputed to the inaccessible rocks on which it breeds,
giving it complete protection from enemies. Thus the pale or bluish
ground colour of the eggs of its allies, the auks and puffins, has
become intensified and blotched and spotted in the most marvellous
variety of patterns, owing to there being no selective agency to prevent
individual variation having full sway.

The common black coot (Fulica atra) has eggs which are coloured in a
specially protective manner. Dr. William Marshall writes, that it only
breeds in certain localities where a large water reed (Phragmites
arundinacea) abounds. The eggs of the coot are stained and spotted with
black on a yellowish-gray ground, and the dead leaves of the reed are of
the same colour, and are stained black by small parasitic fungi of the
Uredo family; and these leaves form the bed on which the eggs are laid.
The eggs and the leaves agree so closely in colour and markings that it
is a difficult thing to distinguish the eggs at any distance. It is to
be noted that the coot never covers up its eggs, as its ally the
moor-hen usually does.

The beautiful blue or greenish eggs of the hedge-sparrow, the
song-thrush, and sometimes those of the blackbird, seem at first sight
especially calculated to attract attention, but it is very doubtful
whether they are really so conspicuous when seen at a little distance
among their usual surroundings. For the nests of these birds are either
in evergreens, as holly or ivy, or surrounded by the delicate green
tints of our early spring vegetation, and may thus harmonise very well
with the colours around them. The great majority of the eggs of our
smaller birds are so spotted or streaked with brown or black on
variously tinted grounds that, when lying in the shadow of the nest and
surrounded by the many colours and tints of bark and moss, of purple
buds and tender green or yellow foliage, with all the complex glittering
lights and mottled shades produced among these by the spring sunshine
and by sparkling raindrops, they must have a quite different aspect from
that which they possess when we observe them torn from their natural
surroundings. We have here, probably, a similar case of general
protective harmony to that of the green caterpillars with beautiful
white or purple bands and spots, which, though gaudily conspicuous when
seen alone, become practically invisible among the complex lights and
shadows of the foliage they feed upon.

In the case of the cuckoo, which lays its eggs in the nests of a variety
of other birds, the eggs themselves are subject to considerable
variations of colour, the most common type, however, resembling those of
the pipits, wagtails, or warblers, in whose nests they are most
frequently laid. It also often lays in the nest of the hedge-sparrow,
whose bright blue eggs are usually not at all nearly matched, although
they are sometimes said to be so on the Continent. It is the opinion of
many ornithologists that each female cuckoo lays the same coloured eggs,
and that it usually chooses a nest the owners of which lay somewhat
similar eggs, though this is by no means universally the case. Although
birds which have cuckoos' eggs imposed upon them do not seem to neglect
them on account of any difference of colour, yet they probably do so
occasionally; and if, as seems probable, each bird's eggs are to some
extent protected by their harmony of colour with their surroundings, the
presence of a larger and very differently coloured egg in the nest might
be dangerous, and lead to the destruction of the whole set. Those
cuckoos, therefore, which most frequently placed their eggs among the
kinds which they resembled, would in the long run leave most progeny,
and thus the very frequent accord in colour might have been brought
about.

Some writers have suggested that the varied colours of birds' eggs are
primarily due to the effect of surrounding coloured objects on the
female bird during the period preceding incubation; and have expended
much ingenuity in suggesting the objects that may have caused the eggs
of one bird to be blue, another brown, and another pink.[82] But no
evidence has been presented to prove that any effects whatever are
produced by this cause, while there seems no difficulty in accounting
for the facts by individual variability and the action of natural
selection. The changes that occur in the conditions of existence of
birds must sometimes render the concealment less perfect than it may
once have been; and when any danger arises from this cause, it may be
met either by some change in the colour of the eggs, or in the
structure or position of the nest, or by the increased care which the
parents bestow upon the eggs. In this way the various divergences which
now so often puzzle us may have arisen.


_Colour as a Means of Recognition._

If we consider the habits and life-histories of those animals which are
more or less gregarious, comprising a large proportion of the herbivora,
some carnivora, and a considerable number of all orders of birds, we
shall see that a means of ready recognition of its own kind, at a
distance or during rapid motion, in the dusk of twilight or in partial
cover, must be of the greatest advantage and often lead to the
preservation of life. Animals of this kind will not usually receive a
stranger into their midst. While they keep together they are generally
safe from attack, but a solitary straggler becomes an easy prey to the
enemy; it is, therefore, of the highest importance that, in such a case,
the wanderer should have every facility for discovering its companions
with certainty at any distance within the range of vision.

Some means of easy recognition must be of vital importance to the young
and inexperienced of each flock, and it also enables the sexes to
recognise their kind and thus avoid the evils of infertile crosses; and
I am inclined to believe that its necessity has had a more widespread
influence in determining the diversities of animal coloration than any
other cause whatever. To it may probably be imputed the singular fact
that, whereas bilateral symmetry of coloration is very frequently lost
among domesticated animals, it almost universally prevails in a state of
nature; for if the two sides of an animal were unlike, and the diversity
of coloration among domestic animals occurred in a wild state, easy
recognition would be impossible among numerous closely allied forms.[83]
The wonderful diversity of colour and of marking that prevails,
especially in birds and insects, may be due to the fact that one of the
first needs of a new species would be, to keep separate from its nearest
allies, and this could be most readily done by some easily seen external
mark of difference. A few illustrations will serve to show how this
principle acts in nature.

My attention was first called to the subject by a remark of Mr. Darwin's
that, though, "the hare on her form is a familiar instance of
concealment through colour, yet the principle partly fails in a closely
allied species, the rabbit; for when running to its burrow it is made
conspicuous to the sportsman, and no doubt to all beasts of prey, by its
upturned white tail."[84] But a little consideration of the habits of
the animal will show that the white upturned tail is of the greatest
value, and is really, as it has been termed by a writer in _The Field_,
a "signal flag of danger." For the rabbit is usually a crepuscular
animal, feeding soon after sunset or on moonlight nights. When disturbed
or alarmed it makes for its burrow, and the white upturned tails of
those in front serve as guides and signals to those more remote from
home, to the young and the feeble; and thus each following the one or
two before it, all are able with the least possible delay to regain a
place of comparative safety. The apparent danger, therefore, becomes a
most important means of security.

The same general principle enables us to understand the singular, and
often conspicuous, markings on so many gregarious herbivora which are
yet, on the whole, protectively coloured. Thus, the American prong-buck
has a white patch behind and a black muzzle. The Tartarian antelope, the
Ovis poli of High Asia, the Java wild ox, several species of deer, and a
large number of antelopes have a similar conspicuous white patch behind,
which, in contrast to the dusky body, must enable them to be seen and
followed from a distance by their fellows. Where there are many species
of nearly the same general size and form inhabiting the same region--as
with the antelopes of Africa--we find many distinctive markings of a
similar kind. The gazelles have variously striped and banded faces,
besides white patches behind and on the flanks, as shown in the woodcut.
The spring-bok has a white patch on the face and one on the sides, with
a curiously distinctive white stripe above the tail, which is nearly
concealed when the animal is at rest by a fold of skin but comes into
full view when it is in motion, being thus quite analogous to the
upturned white tail of the rabbit. In the pallah the white rump-mark is
bordered with black, and the peculiar shape of the horns distinguishes
it when seen from the front. The sable-antelope, the gems-bok, the oryx,
the hart-beest, the bonte-bok, and the addax have each peculiar white
markings; and they are besides characterised by horns so remarkably
different in each species and so conspicuous, that it seems probable
that the peculiarities in length, twist, and curvature have been
differentiated for the purpose of recognition, rather than for any
speciality of defence in species whose general habits are so similar.

[Illustration: FIG. 18.--Gazella soemmerringi.]

It is interesting to note that these markings for recognition are very
slightly developed in the antelopes of the woods and marshes. Thus, the
grys-bok is nearly uniform in colour, except the long black-tipped ears;
and it frequents the wooded mountains. The duyker-bok and the rhoode-bok
are wary bush-haunters, and have no marks but the small white patch
behind. The wood-haunting bosch-bok goes in pairs, and has hardly any
distinctive marks on its dusky chestnut coat, but the male alone is
horned. The large and handsome koodoo frequents brushwood, and its
vertical white stripes are no doubt protective, while its magnificent
spiral horns afford easy recognition. The eland, which is an inhabitant
of the open country, is uniformly coloured, being sufficiently
recognisable by its large size and distinctive form; but the Derbyan
eland is a forest animal, and has a protectively striped coat. In like
manner, the fine Speke's antelope, which lives entirely in the swamps
and among reeds, has pale vertical stripes on the sides (protective),
with white markings on face and breast for recognition. An inspection of
the figures of antelopes and other animals in Wood's _Natural History_,
or in other illustrated works, will give a better idea of the
peculiarities of recognition markings than any amount of description.

Other examples of such coloration are to be seen in the dusky tints of
the musk-sheep and the reindeer, to whom recognition at a distance on
the snowy plains is of more importance than concealment from their few
enemies. The conspicuous stripes and bands of the zebra and the quagga
are probably due to the same cause, as may be the singular crests and
face-marks of several of the monkeys and lemurs.[85]

[Illustration: FIG. 19--Recognition marks of three African plovers.]

Among birds, these recognition marks are especially numerous and
suggestive. Species which inhabit open districts are usually
protectively coloured; but they generally possess some distinctive
markings for the purpose of being easily recognised by their kind, both
when at rest and during flight. Such are, the white bands or patches on
the breast or belly of many birds, but more especially the head and neck
markings in the form of white or black caps, collars, eye-marks or
frontal patches, examples of which are seen in the three species of
African plovers figured on page 221.

Recognition marks during flight are very important for all birds which
congregate in flocks or which migrate together; and it is essential
that, while being as conspicuous as possible, the marks shall not
interfere with the general protective tints of the species when at rest.
Hence they usually consist of well-contrasted markings on the wings and
tail, which are concealed during repose but become fully visible when
the bird takes flight. Such markings are well seen in our four British
species of shrikes, each having quite different white marks on the
expanded wings and on the tail feathers; and the same is the case with
our three species of Saxicola--the stone-chat, whin-chat, and
wheat-ear--which are thus easily recognisable on the wing, especially
when seen from above, as they would be by stragglers looking out for
their companions. The figures opposite, of the wings of two African
species of stone-curlew which are sometimes found in the same districts,
well illustrates these specific recognition marks. Though not very
greatly different to our eyes, they are no doubt amply so to the sharp
vision of the birds themselves.

Besides the white patches on the primaries here shown, the secondary
feathers are, in some cases, so coloured as to afford very distinctive
markings during flight, as seen in the central secondary quills of two
African coursers (Fig. 21).

[Illustration: FIG. 20.--Oedicnemus vermiculatus (above). Oe.
senegalensis (below).]

Most characteristic of all, however, are the varied markings of the
outer tail-feathers, whose purpose is so well shown by their being
almost always covered during repose by the two middle feathers, which
are themselves quite unmarked and protectively tinted like the rest of
the upper surface of the body. The figures of the expanded tails of two
species of East Asiatic snipe, whose geographical ranges overlap each
other, will serve to illustrate this difference; which is frequently
much greater and modified in an endless variety of ways (Fig. 22).

Numbers of species of pigeons, hawks, finches, warblers, ducks, and
innumerable other birds possess this class of markings; and they
correspond so exactly in general character with those of the mammalia,
already described, that we cannot doubt they serve a similar
purpose.[86]

[Illustration: FIG. 21.--Secondary quills.]

[Illustration: FIG. 22.--Scolopax megala (upper). S. stenura (lower).]

Those birds which are inhabitants of tropical forests, and which need
recognition marks that shall be at all times visible among the dense
foliage, and not solely or chiefly during flight, have usually small but
brilliant patches of colour on the head or neck, often not interfering
with the generally protective character of their plumage. Such are the
bright patches of blue, red, or yellow, by which the usually green
Eastern barbets are distinguished; and similar bright patches of colour
characterise the separate species of small green fruit-doves. To this
necessity for specialisation in colour, by which each bird may easily
recognise its kind, is probably due that marvellous variety in the
peculiar beauties of some groups of birds. The Duke of Argyll, speaking
of the humming birds, made the objection that "A crest of topaz is no
better in the struggle for existence than a crest of sapphire. A frill
ending in spangles of the emerald is no better in the battle of life
than a frill ending in spangles of the ruby. A tail is not affected for
the purposes of flight, whether its marginal or its central feathers are
decorated with white;" and he goes on to urge that mere beauty and
variety for their own sake are the only causes of these differences.
But, on the principles here suggested, the divergence itself is useful,
and must have been produced _pari passu_ with the structural differences
on which the differentiation of species depends; and thus we have
explained the curious fact that prominent differences of colour often
distinguish species otherwise very closely allied to each other.

Among insects, the principle of distinctive coloration for recognition
has probably been at work in the production of the wonderful diversity
of colour and marking we find everywhere, more especially among the
butterflies and moths; and here its chief function may have been to
secure the pairing together of individuals of the same species. In some
of the moths this has been secured by a peculiar odour, which attracts
the males to the females from a distance; but there is no evidence that
this is universal or even general, and among butterflies, especially,
the characteristic colour and marking, aided by size and form, afford
the most probable means of recognition. That this is so is shown by the
fact that "the common white butterfly often flies down to a bit of paper
on the ground, no doubt mistaking it for one of its own species;" while,
according to Mr. Collingwood, in the Malay Archipelago, "a dead
butterfly pinned upon a conspicuous twig will often arrest an insect of
the same species in its headlong flight, and bring it down within easy
reach of the net, especially if it be of the opposite sex."[87] In a
great number of insects, no doubt, form, motions, stridulating sounds,
or peculiar odours, serve to distinguish allied species from each other,
and this must be especially the case with nocturnal insects, or with
those whose colours are nearly uniform and are determined by the need of
protection; but by far the larger number of day-flying and active
insects exhibit varieties of colour and marking, forming the most
obvious distinction between allied species, and which have, therefore,
in all probability been acquired in the process of differentiation for
the purpose of checking the intercrossing of closely allied forms.[88]

Whether this principle extends to any of the less highly organised
animals is doubtful, though it may perhaps have affected the higher
mollusca. But in marine animals it seems probable that the colours,
however beautiful, varied, and brilliant they may often be, are in most
cases protective, assimilating them to the various bright-coloured
seaweeds, or to some other animals which it is advantageous for them to
imitate.[89]


_Summary of the Preceding Exposition._

Before proceeding to discuss some of the more recondite phenomena of
animal coloration, it will be well to consider for a moment the extent
of the ground we have already covered. Protective coloration, in some of
its varied forms, has not improbably modified the appearance of one-half
of the animals living on the globe. The white of arctic animals, the
yellowish tints of the desert forms, the dusky hues of crepuscular and
nocturnal species, the transparent or bluish tints of oceanic creatures,
represent a vast host in themselves; but we have an equally numerous
body whose tints are adapted to tropical foliage, to the bark of trees,
or to the soil or dead leaves on or among which they habitually live.
Then we have the innumerable special adaptations to the tints and forms
of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by
which such vast numbers of the insect tribes obtain protection; and we
have seen that these various forms of coloration are equally prevalent
in the waters of the seas and oceans, and are thus coextensive with the
domain of life upon the earth. The comparatively small numbers which
possess "terrifying" or "alluring" coloration may be classed under the
general head of the protectively coloured.

But under the next head--colour for recognition--we have a totally
distinct category, to some extent antagonistic or complementary to the
last, since its essential principle is visibility rather than
concealment. Yet it has been shown, I think, that this mode of
coloration is almost equally important, since it not only aids in the
preservation of existing species and in the perpetuation of pure races,
but was, perhaps, in its earlier stages, a not unimportant factor in
their development. To it we owe most of the variety and much of the
beauty in the colours of animals; it has caused at once bilateral
symmetry and general permanence of type; and its range of action has
been perhaps equally extensive with that of coloration for concealment.


_Influence of Locality or of Climate on Colour._

Certain relations between locality and coloration have long been
noticed. Mr. Gould observed that birds from inland or continental
localities were more brightly coloured than those living near the
sea-coast or on islands, and he supposed that the more brilliant
atmosphere of the inland stations was the explanation of the
phenomenon.[90] Many American naturalists have observed similar facts,
and they assert that the intensity of the colours of birds and mammals
increases from north to south, and also with the increase of humidity.
This change is imputed by Mr. J.A. Allen to the direct action of the
environment. He says: "In respect to the correlation of intensity of
colour in animals with the degree of humidity, it would perhaps be more
in accordance with cause and effect to express the law of correlation as
a _decrease_ of intensity of colour with a _decrease_ of humidity, the
paleness evidently resulting from exposure and the blanching effect of
intense sunlight, and a dry, often intensely heated atmosphere. With the
decrease of the aqueous precipitation the forest growth and the
protection afforded by arborescent vegetation gradually also decreases,
as of course does also the protection afforded by clouds, the
excessively humid regions being also regions of extreme cloudiness,
while the dry regions are comparatively cloudless districts."[91] Almost
identical changes occur in birds, and are imputed by Mr. Allen to
similar causes.

It will be seen that Mr. Gould and Mr. Allen impute opposite effects to
the same cause, brilliancy or intensity of colour being due to a
brilliant atmosphere according to the former, while paleness of colour
is imputed by the latter to a too brilliant sun. According to the
principles which have been established by the consideration of arctic,
desert, and forest animals respectively, we shall be led to conclude
that there has been no direct action in this case, but that the effects
observed are due to the greater or less need of protection. The pale
colour that is prevalent in arid districts is in harmony with the
general tints of the surface; while the brighter tints or more intense
coloration, both southward and in humid districts, are sufficiently
explained by the greater shelter due to a more luxuriant vegetation and
a shorter winter. The advocates of the theory that intensity of light
directly affects the colours of organisms, are led into perpetual
inconsistencies. At one time the brilliant colours of tropical birds and
insects are imputed to the intensity of a tropical sun, while the same
intensity of sunlight is now said to have a "bleaching" effect. The
comparatively dull and sober hues of our northern fauna were once
supposed to be the result of our cloudy skies; but now we are told that
cloudy skies and a humid atmosphere intensify colour.

In my _Tropical Nature_ (pp. 257-264) I have called attention to what is
perhaps the most curious and decided relation of colour to locality
which has yet been observed--the prevalence of white markings in the
butterflies and birds of islands.

So many cases are adduced from so many different islands, both in the
eastern and western hemisphere, that it is impossible to doubt the
existence of some common cause; and it seems probable to me now, after a
fuller consideration of the whole subject of colour, that here too we
have one of the almost innumerable results of the principle of
protective coloration. White is, as a rule, an uncommon colour in
animals, but probably only because it is so conspicuous. Whenever it
becomes protective, as in the case of arctic animals and aquatic birds,
it appears freely enough; while we know that white varieties of many
species occur occasionally in the wild state, and that, under
domestication, white or parti-coloured breeds are freely produced. Now
in all the islands in which exceptionally white-marked birds and
butterflies have been observed, we find two features which would tend to
render the conspicuous white markings less injurious--a luxuriant
tropical vegetation, and a decided scarcity of rapacious mammals and
birds. White colours, therefore, would not be eliminated by natural
selection; but variations in this direction would bear their part in
producing the recognition marks which are everywhere essential, and
which, in these islands, need not be so small or so inconspicuous as
elsewhere.


_Concluding Remarks._

On a review of the whole subject, then, we must conclude that there is
no evidence of the individual or prevalent colours of organisms being
directly determined by the amount of light, or heat, or moisture, to
which they are exposed; while, on the other hand, the two great
principles of the need of concealment from enemies or from their prey,
and of recognition by their own kind, are so wide-reaching in their
application that they appear at first sight to cover almost the whole
ground of animal coloration. But, although they are indeed wonderfully
general and have as yet been very imperfectly studied, we are acquainted
with other modes of coloration which have a different origin. These
chiefly appertain to the very singular class of warning colours, from
which arise the yet more extraordinary phenomena of mimicry; and they
open up so curious a field of inquiry and present so many interesting
problems, that a chapter must be devoted to them. Yet another chapter
will be required by the subject of sexual differentiation of colour and
ornament, as to the origin and meaning of which I have arrived at
different conclusions from Mr. Darwin. These various forms of coloration
having been discussed and illustrated, we shall be in a position to
attempt a brief sketch of the fundamental laws which have determined the
general coloration of the animal world.

FOOTNOTES:

[Footnote 65: _Proceedings of the Royal Society_, No. 243, 1886;
_Transactions of the Royal Society_, vol. clxxviii. B. pp. 311-441.]

[Footnote 66: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
460.]

[Footnote 67: _Trans. Phil. Soc._ (? _of S. Africa_), 1878, part iv, p.
27.]

[Footnote 68: _Proc. Zool. Soc._, 1862 p. 357.]

[Footnote 69: With reference to this general resemblance of insects to
their environment the following remarks by Mr. Poulton are very
instructive. He says: "Holding the larva of Sphinx ligustri in one hand
and a twig of its food-plant in the other, the wonder we feel is, not at
the resemblance but at the difference; we are surprised at the
difficulty experienced in detecting so conspicuous an object. And yet
the protection is very real, for the larvae will be passed over by those
who are not accustomed to their appearance, although the searcher may be
told of the presence of a large caterpillar. An experienced entomologist
may also fail to find the larvae till after a considerable search. This
is general protective resemblance, and it depends upon a general harmony
between the appearance of the organism and its whole environment. It is
impossible to understand the force of this protection for any larva,
without seeing it on its food-plant and in an entirely normal condition.
The artistic effect of green foliage is more complex than we often
imagine; numberless modifications are wrought by varied lights and
shadows upon colours which are in themselves far from uniform. In the
larva of Papilio machaon the protection is very real when the larva is
on the food-plant, and can hardly be appreciated at all when the two are
apart." Numerous other examples are given in the chapter on "Mimicry and
other Protective Resemblances among Animals," in my _Contributions to
the Theory of Natural Selection_.]

[Footnote 70: _The Naturalist in Nicaragua_, p. 19.]

[Footnote 71: R. Meldola, in _Proc. Zool. Soc._, 1873, p. 155.]

[Footnote 72: _Nature_, vol. iii. p. 166.]

[Footnote 73: _Trans. Ent. Soc. Lond._, 1878, p. 185.]

[Footnote 74: _Ibid._ (_Proceedings_, p. xlii.)]

[Footnote 75: Wallace's _Malay Archipelago_, vol. i. p. 204 (fifth
edition, p. 130), with figure.]

[Footnote 76: Moseley's _Notes by a Naturalist on the Challenger_.]

[Footnote 77: _Proceedings of the Boston Soc. of Nat. Hist._, vol. xiv.
1871.]

[Footnote 78: _Nature_, 1870, p. 376.]

[Footnote 79: _A Naturalist's Wanderings in the Eastern Archipelago_, p.
63.]

[Footnote 80: A beautiful drawing of this rare insect, Hymenopus
bicornis (in the nymph or active pupa state), was kindly sent me by Mr.
Wood-Mason, Curator of the Indian Museum at Calcutta. A species, very
similar to it, inhabits Java, where it is said to resemble a pink
orchid. Other Mantidae, of the genus Gongylus, have the anterior part of
the thorax dilated and coloured either white, pink, or purple; and they
so closely resemble flowers that, according to Mr. Wood-Mason, one of
them, having a bright violet-blue prothoracic shield, was found in Pegu
by a botanist, and was for a moment mistaken by him for a flower. See
_Proc. Ent. Soc. Lond._, 1878, p. liii.]

[Footnote 81: C. Dixon, in Seebohm's _History of British Birds_, vol.
ii. Introduction, p. xxvi. Many of the other examples here cited are
taken from the same valuable work.]

[Footnote 82: See A.H.S. Lucas, in _Proceedings of Royal Society of
Victoria_, 1887, p. 56.]

[Footnote 83: Professor Wm.H. Brewer of Yale College has shown that the
white marks or the spots of domesticated animals are rarely symmetrical,
but have a tendency to appear more frequently on the left side. This is
the case with horses, cattle, dogs, and swine. Among wild animals the
skunk varies considerably in the amount of white on the body, and this
too was found to be usually greatest on the left side. A close
examination of numerous striped or spotted species, as tigers, leopards,
jaguars, zebras, etc., showed that the bilateral symmetry was not exact,
although the general effect of the two sides was the same. This is
precisely what we should expect if the symmetry is not the result of a
general law of the organisation, but has been, in part at least,
produced and preserved for the useful purpose of recognition by the
animal's fellows of the same species, and especially by the sexes and
the young. See _Proc. of the Am. Ass. for Advancement of Science_, vol.
xxx. p. 246.]

[Footnote 84: _Descent of Man_, p. 542.]

[Footnote 85: It may be thought that such extremely conspicuous markings
as those of the zebra would be a great danger in a country abounding
with lions, leopards, and other beasts of prey; but it is not so. Zebras
usually go in bands, and are so swift and wary that they are in little
danger during the day. It is in the evening, or on moonlight nights,
when they go to drink, that they are chiefly exposed to attack; and Mr.
Francis Galton, who has studied these animals in their native haunts,
assures me, that in twilight they are not at all conspicuous, the
stripes of white and black so merging together into a gray tint that it
is very difficult to see them at a little distance. We have here an
admirable illustration of how a glaringly conspicuous style of marking
for recognition may be so arranged as to become also protective at the
time when protection is most needed; and we may also learn how
impossible it is for us to decide on the inutility of any kind of
coloration without a careful study of the habits of the species in its
native country.]

[Footnote 86: The principle of colouring for recognition was, I believe,
first stated in my article on "The Colours of Animals and Plants" in
Macmillan's _Magazine_, and more fully in my volume on _Tropical
Nature_. Subsequently Mrs. Barber gave a few examples under the head of
"Indicative or Banner Colours," but she applied it to the distinctive
colours of the males of birds, which I explain on another principle,
though this may assist.]

[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]

[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd
has an article on "Directive Coloration in Animals," in which he
recognises many of the cases here referred to, and suggests a few
others, though I think he includes many forms of coloration--as
"paleness of belly and inner side of legs"--which do not belong to this
class.]

[Footnote 89: For numerous examples of this protective colouring of
marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.
Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]

[Footnote 90: See _Origin of Species_, p. 107.]

[Footnote 91: The "Geographical Variation of North American Squirrels,"
_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter
Birds of Florida_, pp. 233-241.]




CHAPTER IX

WARNING COLORATION AND MIMICRY


    The skunk as an example of warning coloration--Warning colours
    among insects--Butterflies--Caterpillars--Mimicry--How mimicry
    has been produced--Heliconidae--Perfection of the
    imitation--Other cases of mimicry among Lepidoptera--Mimicry
    among protected groups--Its explanation--Extension of the
    principle--Mimicry in other orders of insects--Mimicry among the
    vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
    birds--Objections to the theory of mimicry--Concluding remarks
    on warning colours and mimicry.



We have now to deal with a class of colours which are the very opposite
of those we have hitherto considered, since, instead of serving to
conceal the animals that possess them or as recognition marks to their
associates, they are developed for the express purpose of rendering the
species conspicuous. The reason of this is that the animals in question
are either the possessors of some deadly weapons, as stings or poison
fangs, or they are uneatable, and are thus so disagreeable to the usual
enemies of their kind that they are never attacked when their peculiar
powers or properties are known. It is, therefore, important that they
should not be mistaken for defenceless or eatable species of the same
class or order, since in that case they might suffer injury, or even
death, before their enemies discovered the danger or the uselessness of
the attack. They require some signal or danger-flag which shall serve as
a warning to would-be enemies not to attack them, and they have usually
obtained this in the form of conspicuous or brilliant coloration, very
distinct from the protective tints of the defenceless animals allied to
them.

_The Skunk as illustrating Warning Coloration._

While staying a few days, in July 1887, at the Summit Hotel on the
Central Pacific Railway, I strolled out one evening after dinner, and on
the road, not fifty yards from the house, I saw a pretty little white
and black animal with a bushy tail coming towards me. As it came on at a
slow pace and without any fear, although it evidently saw me, I thought
at first that it must be some tame creature, when it suddenly occurred
to me that it was a skunk. It came on till within five or six yards of
me, then quietly climbed over a dwarf wall and disappeared under a small
outhouse, in search of chickens, as the landlord afterwards told me.
This animal possesses, as is well known, a most offensive secretion,
which it has the power of ejecting over its enemies, and which
effectually protects it from attack. The odour of this substance is so
penetrating that it taints, and renders useless, everything it touches,
or in its vicinity. Provisions near it become uneatable, and clothes
saturated with it will retain the smell for several weeks, even though
they are repeatedly washed and dried. A drop of the liquid in the eyes
will cause blindness, and Indians are said not unfrequently to lose
their sight from this cause. Owing to this remarkable power of offence
the skunk is rarely attacked by other animals, and its black and white
fur, and the bushy white tail carried erect when disturbed, form the
danger-signals by which it is easily distinguished in the twilight or
moonlight from unprotected animals. Its consciousness that it needs only
to be seen to be avoided gives it that slowness of motion and
fearlessness of aspect which are, as we shall see, characteristic of
most creatures so protected.


_Warning Colours among Insects._

It is among insects that warning colours are best developed, and most
abundant. We all know how well marked and conspicuous are the colours
and forms of the stinging wasps and bees, no one of which in any part of
the world is known to be protectively coloured like the majority of
defenceless insects. Most of the great tribe of Malacoderms among
beetles are distasteful to insect-eating animals. Our red and black
Telephoridae, commonly called "soldiers and sailors," were found, by Mr.
Jenner Weir, to be refused by small birds. These and the allied
Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by
Mr. Belt's tame monkey and by his fowls, though most other insects were
greedily eaten by them. The Coccinellidae or lady-birds are another
uneatable group, and their conspicuous and singularly spotted bodies
serve to distinguish them at a glance from all other beetles.

These uneatable insects are probably more numerous than is supposed,
although we already know immense numbers that are so protected. The most
remarkable are the