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Title: Darwinism (1889)
Author: Alfred Russel Wallace
Release Date: January 2, 2005 [EBook #14558]
Language: English
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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 versâ_. 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 Genève_, 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
Genève_, 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: 'Voilà
donc dans un même individu des loges et un style qui se rattachent
tantôt a un axe vertical, et tantôt a un gynobase; donc celui-ci n'est
qu'un axe veritable; mais cet axe est deprimé au lieu d'être 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 versâ_--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 versâ_. 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 three families of butterflies--Heliconidae, Danaidae,
and Acraeidae--comprising more than a thousand species, and
characteristic respectively of the three great tropical regions--South
America, Southern Asia, and Africa. All these butterflies have
peculiarities which serve to distinguish them from every other group in
their respective regions. They all have ample but rather weak wings, and
fly slowly; they are always very abundant; and they all have conspicuous
colours or markings, so distinct from those of other families that, in
conjunction with their peculiar outline and mode of flight, they can
usually be recognised at a glance. Other distinctive features are, that
their colours are always nearly the same on the under surface of their
wings as on the upper; they never try to conceal themselves, but rest on
the upper surfaces of leaves or flowers; and, lastly, they all have
juices which exhale a powerful scent, so that when one kills them by
pinching the body, the liquid that exudes stains the fingers yellow, and
leaves an odour that can only be removed by repeated washings.
Now, there is much direct evidence to show that this odour, though not
very offensive to us, is so to most insect-eating creatures. Mr. Bates
observed that, when set out to dry, specimens of Heliconidae were less
subject to the attacks of vermin; while both he and I noticed that they
were not attacked by insect-eating birds or dragonflies, and that their
wings were not found in the forest paths among the numerous wings of
other butterflies whose bodies had been devoured. Mr. Belt once observed
a pair of birds capturing insects for their young; and although the
Heliconidae swarmed in the vicinity, and from their slow flight could
have been easily caught, not one was ever pursued, although other
butterflies did not escape. His tame monkey also, which would greedily
munch up other butterflies, would never eat the Heliconidae. It would
sometimes smell them, but always rolled them up in its hand and then
dropped them.
We have also some corresponding evidence as to the distastefulness of
the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously
collected and took notes upon the Lepidoptera of Bombay, informed Mr.
Butler of the British Museum that the large and swift-flying butterfly
Charaxes psaphon, was continually persecuted by the bulbul, so that he
rarely caught a specimen of this species which had not a piece snipped
out of the hind wings. He offered one to a bulbul which he had in a
cage, and it was greedily devoured, whilst it was only by repeated
persecution that he succeeded in inducing the bird to touch a
Danais.[92]
Besides these three families of butterflies, there are certain groups of
the great genus Papilio--the true swallow-tailed butterflies--which have
all the characteristics of uneatable insects. They have a special
coloration, usually red and black (at least in the females), they fly
slowly, they are very abundant, and they possess a peculiar odour
somewhat like that of the Heliconidae. One of these groups is common in
tropical America, another in tropical Asia, and it is curious that,
although not very closely allied, they have each the same red and black
colours, and are very distinct from all the other butterflies of their
respective countries. There is reason to believe also that many of the
brilliantly coloured and weak-flying diurnal moths, like the fine
tropical Agaristidae and burnet-moths, are similarly protected, and that
their conspicuous colours serve as a warning of inedibility. The common
burnet-moth (Anthrocera filipendula) and the equally conspicuous
ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to
insect-eating creatures.
The most interesting and most conclusive example of warning coloration
is, however, furnished by caterpillars, because in this case the facts
have been carefully ascertained experimentally by competent observers.
In the year 1866, when Mr. Darwin was collecting evidence as to the
supposed effect of sexual selection in bringing about the brilliant
coloration of the higher animals, he was struck by the fact that many
caterpillars have brilliant and conspicuous colours, in the production
of which sexual selection could have no place. We have numbers of such
caterpillars in this country, and they are characterised not only by
their gay colours but by not concealing themselves. Such are the mullein
and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of
the buff-tip, and many others. Some of these caterpillars are
wonderfully conspicuous, as in the case of that noticed by Mr. Bates in
South America, which was four inches long, banded across with black and
yellow, and with bright red head, legs, and tail. Hence it caught the
eye of any one who passed by, even at the distance of many yards.
Mr. Darwin asked me to try and suggest some explanation of this
coloration; and, having been recently interested in the question of the
warning coloration of butterflies, I suggested that this was probably a
similar case,--that these conspicuous caterpillars were distasteful to
birds and other insect-eating creatures, and that their bright
non-protective colours and habit of exposing themselves to view, enabled
their enemies to distinguish them at a glance from the edible kinds and
thus learn not to touch them; for it must be remembered that the bodies
of caterpillars while growing are so delicate, that a wound from a
bird's beak would be perhaps as fatal as if they were devoured.[93] At
this time not a single experiment or observation had been made on the
subject, but after I had brought the matter before the Entomological
Society, two gentlemen, who kept birds and other tame animals, undertook
to make experiments with a variety of caterpillars.
Mr. Jenner Weir was the first to experiment with ten species of small
birds in his aviary, and he found that none of them would eat the
following smooth-skinned conspicuous caterpillars--Abraxas
grossulariata, Diloba caeruleocephala, Anthrocera filipendula, and
Cucullia verbasci. He also found that they would not touch any hairy or
spiny larvae, and he was satisfied that it was not the hairs or the
spines, but the unpleasant taste that caused them to be rejected,
because in one case a young smooth larva of a hairy species, and in
another case the pupa of a spiny larva, were equally rejected. On the
other hand, all green or brown caterpillars as well as those that
resemble twigs were greedily devoured.[94]
Mr. A.G. Butler also made experiments with some green lizards (Lacerta
viridis), which greedily ate all kinds of food, including flies of many
kinds, spiders, bees, butterflies, and green caterpillars; but they
would not touch the caterpillar of the gooseberry-moth (Abraxas
grossulariata), or the imago of the burnet-moth (Anthrocera
filipendula). The same thing happened with frogs. When the gooseberry
caterpillars were first given to them, "they sprang forward and licked
them eagerly into their mouths; no sooner, however, had they done so,
than they seemed to become aware of the mistake that they had made, and
sat with gaping mouths, rolling their tongues about, until they had got
quit of the nauseous morsels, which seemed perfectly uninjured, and
walked off as briskly as ever." Spiders seemed equally to dislike them.
This and another conspicuous caterpillar (Halia wavaria) were rejected
by two species--the geometrical garden spider (Epeira diadema) and a
hunting spider.[95]
Some further experiments with lizards were made by Professor Weismann,
quite confirming the previous observations; and in 1886 Mr. E.B. Poulton
of Oxford undertook a considerable series of experiments, with many
other species of larvae and fresh kinds of lizards and frogs. Mr.
Poulton then reviewed the whole subject, incorporating all recorded
facts, as well as some additional observations made by Mr. Jenner Weir
in 1886. More than a hundred species of larvae or of perfect insects of
various orders have now been made the subject of experiment, and the
results completely confirm my original suggestion. In almost every case
the protectively coloured larvae have been greedily eaten by all kinds
of insectivorous animals, while, in the immense majority of cases, the
conspicuous, hairy, or brightly coloured larvae have been rejected by
some or all of them. In some instances the inedibility of the larvae
extends to the perfect insect, but not in others. In the former cases
the perfect insect is usually adorned with conspicuous colours, as the
burnet and ragwort moths; but in the case of the buff-tip, the moth
resembles a broken piece of rotten stick, yet it is partly inedible,
being refused by lizards. It is, however, very doubtful whether these
are its chief enemies, and its protective form and colour may be needed
against insectivorous birds or mammals.
Mr. Samuel H. Scudder, who has largely bred North American butterflies,
has found so many of the eggs and larvae destroyed by hymenopterous and
dipterous parasites that he thinks at least nine-tenths, perhaps a
greater proportion, never reach maturity. Yet he has never found any
evidence that such parasites attack either the egg or the larva of the
inedible Danais archippus, so that in this case the insect is
distasteful to its most dangerous foes in all the stages of its
existence, a fact which serves to explain its great abundance and its
extension over almost the whole world.[96]
One case has been found of a protectively coloured larva,--one,
moreover, which in all its habits shows that it trusts to concealment to
escape its enemies--which was yet always rejected by lizards after they
had seized it, evidently under the impression that from its colour it
would be eatable. This is the caterpillar of the very common moth Mania
typica; and Mr. Poulton thinks that, in this case, the unpleasant taste
is an incidental result of some physiological processes in the organism,
and is itself a merely useless character. It is evident that the insect
would not conceal itself so carefully as it does if it had not some
enemies, and these are probably birds or small mammals, as its
food-plants are said to be dock and willow-herb, not suggestive of
places frequented by lizards; and it has been found by experiment that
lizards and birds have not always the same likes and dislikes. The case
is interesting, because it shows that nauseous fluids sometimes occur
sporadically, and may thus be intensified by natural selection when
required for the purpose of protection. Another exceptional case is
that of the very conspicuous caterpillar of the spurge hawk-moth
(Deilephila euphorbiae), which was at once eaten by a lizard, although,
as it exposes itself on its food-plant in the daytime and is very
abundant in some localities, it must almost certainly be disliked by
birds or by some animals who would otherwise devour it. If disturbed
while feeding it is said to turn round with fury and eject a quantity of
green liquid, of an acid and disagreeable smell similar to that of the
spurge milk, only worse.[97]
These facts, and Mr. Poulton's evidence that some larvae rejected by
lizards at first will be eaten if the lizards are very hungry, show that
there are differences in the amount of the distastefulness, and render
it probable that if other food were wanting many of these conspicuous
insects would be eaten. It is the abundance of the eatable kinds that
gives value to the inedibility of the smaller number; and this is
probably the reason why so many insects rely on protective colouring
rather than on the acquisition of any kind of defensive weapons. In the
long run the powers of attack and defence must balance each other. Hence
we see that even the powerful stings of bees and wasps only protect them
against some enemies, since a tribe of birds, the bee-eaters, have been
developed which feed upon them, and some frogs and lizards do so
occasionally.
The preceding outline will sufficiently explain the characteristics of
"warning coloration" and the end it serves in nature. There are many
other curious modifications of it, but these will be best appreciated
after we have discussed the remarkable phenomenon of "mimicry," which is
bound up with and altogether depends upon "warning colour," and is in
some cases the chief indication we have of the possession of some
offensive weapon to secure the safety of the species imitated.
_Mimicry._
This term has been given to a form of protective resemblance, in which
one species so closely resembles another in external form and colouring
as to be mistaken for it, although the two may not be really allied and
often belong to distinct families or orders. One creature seems
disguised in order to be made like another; hence the terms "mimic" and
mimicry, which imply no voluntary action on the part of the imitator. It
has long been known that such resemblances do occur, as, for example,
the clear-winged moths of the families Sesiidae and Aegeriidae, many of
which resemble bees, wasps, ichneumons, or saw-flies, and have received
names expressive of the resemblance; and the parasitic flies (Volucella)
which closely resemble bees, on whose larvae the larvae of the flies
feed.
The great bulk of such cases remained, however, unnoticed, and the
subject was looked upon as one of the inexplicable curiosities of
nature, till Mr. Bates studied the phenomenon among the butterflies of
the Amazon, and, on his return home, gave the first rational explanation
of it.[98] The facts are, briefly, these. Everywhere in that fertile
region for the entomologist the brilliantly coloured Heliconidae abound,
with all the characteristics which I have already referred to when
describing them as illustrative of "warning coloration." But along with
them other butterflies were occasionally captured, which, though often
mistaken for them, on account of their close resemblance in form,
colour, and mode of flight, were found on examination to belong to a
very distinct family, the Pieridae. Mr. Bates notices fifteen distinct
species of Pieridae, belonging to the genera Leptalis and Euterpe, each
of which closely imitates some one species of Heliconidae, inhabiting
the same region and frequenting the same localities. It must be
remembered that the two families are altogether distinct in structure.
The larvae of the Heliconidae are tubercled or spined, the pupae
suspended head downwards, and the imago has imperfect forelegs in the
male; while the larvae of the Pieridae are smooth, the pupae are
suspended with a brace to keep the head erect, and the forefeet are
fully developed in both sexes. These differences are as large and as
important as those between pigs and sheep, or between swallows and
sparrows; while English entomologists will best understand the case by
supposing that a species of Pieris in this country was coloured and
shaped like a small tortoise-shell, while another species on the
Continent was equally like a Camberwell beauty--so like in both cases
as to be mistaken when on the wing, and the difference only to be
detected by close examination. As an example of the resemblance,
woodcuts are given of one pair in which the colours are simple, being
olive, yellow, and black, while the very distinct neuration of the wings
and form of the head and body can be easily seen.
[Illustration: FIG. 23.--Methona psidii (Heliconidae). Leptalis orise
(Pieridae).]
Besides these Pieridae, Mr. Bates found four true Papilios, seven
Erycinidae, three Castnias (a genus of day-flying moths), and fourteen
species of diurnal Bombycidae, all imitating some species of Heliconidae
which inhabited the same district; and it is to be especially noted that
none of these insects were so abundant as the Heliconidae they
resembled, generally they were far less common, so that Mr. Bates
estimated the proportion in some cases as not one to a thousand. Before
giving an account of the numerous remarkable cases of mimicry in other
parts of the world, and between various groups of insects and of higher
animals, it will be well to explain briefly the use and purport of the
phenomenon, and also the mode by which it has been brought about.
_How Mimicry has been Produced._
The fact has been now established that the Heliconidae possess an
offensive odour and taste, which lead to their being almost entirely
free from attack by insectivorous creatures; they possess a peculiar
form and mode of flight, and do not seek concealment; while their
colours--although very varied, ranging from deep blue-black, with white,
yellow, or vivid red bands and spots, to the most delicate
semitransparent wings adorned with pale brown or yellow markings--are
yet always very distinctive, and unlike those of all the other families
of butterflies in the same country. It is, therefore, clear that if any
other butterflies in the same region, which are eatable and suffer great
persecution from insectivorous animals, should come to resemble any of
these uneatable species so closely as to be mistaken for them by their
enemies, they will obtain thereby immunity from persecution. This is the
obvious and sufficient reason why the imitation is useful, and therefore
why it occurs in nature. We have now to explain how it has probably been
brought about, and also why a still larger number of persecuted groups
have not availed themselves of this simple means of protection.
From the great abundance of the Heliconidae[99] all over tropical
America, the vast number of their genera and species, and their marked
distinctions from all other butterflies, it follows that they constitute
a group of high antiquity, which in the course of ages has become more
and more specialised, and owing to its peculiar advantages has now
become a dominant and aggressive race. But when they first arose from
some ancestral species or group which, owing to the food of the larvae
or some other cause, possessed disagreeable juices that caused them to
be disliked by the usual enemies of their kind, they were in all
probability not very different either in form or coloration from many
other butterflies. They would at that time be subject to repeated
attacks by insect-eaters, and, even if finally rejected, would often
receive a fatal injury. Hence arose the necessity for some
distinguishing mark, by which the devourers of butterflies in general
might learn that these particular butterflies were uneatable; and every
variation leading to such distinction, whether by form, colour, or mode
of flight, was preserved and accumulated by natural selection, till the
ancestral Heliconoids became well distinguished from eatable
butterflies, and thenceforth comparatively free from persecution. Then
they had a good time of it. They acquired lazy habits, and flew about
slowly. They increased abundantly and spread all over the country, their
larvae feeding on many plants and acquiring different habits; while the
butterflies themselves varied greatly, and colour being useful rather
than injurious to them, gradually diverged into the many coloured and
beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridae,
inhabiting the same district, happened to be sufficiently like some of
the Heliconidae to be occasionally mistaken for them. These, of course,
survived while their companions were devoured. Those among their
descendants that were still more like Heliconidae again survived, and at
length the imitation would become tolerably perfect. Thereafter, as the
protected group diverged into distinct species of many different
colours, the imitative group would occasionally be able to follow it
with similar variations,--a process that is going on now, for Mr. Bates
informs us that in each fresh district he visited he found closely
allied representative species or varieties of Heliconidae, and along
with them species of Leptalis (Pieridae), which had varied in the same
way so as still to be exact imitations. But this process of imitation
would be subject to check by the increasing acuteness of birds and other
animals which, whenever the eatable Leptalis became numerous, would
surely find them out, and would then probably attack both these and
their friends the Heliconidae in order to devour the former and reject
the latter. The Pieridae would, however, usually be less numerous,
because their larvae are often protectively coloured and therefore
edible, while the larvae of the Heliconidae are adorned with warning
colours, spines, or tubercles, and are uneatable. It seems probable that
the larvae and pupae of the Heliconidae were the first to acquire the
protective distastefulness, both because in this stage they are more
defenceless and more liable to fatal injury, and also because we now
find many instances in which the larvae are distasteful while the
perfect insects are eatable, but I believe none in which the reverse is
the case. The larvae of the Pieridae are now beginning to acquire
offensive juices, but have not yet obtained the corresponding
conspicuous colours; while the perfect insects remain eatable, except
perhaps in some Eastern groups, the under sides of whose wings are
brilliantly coloured although this is the part which is exposed when at
rest.
It is clear that if a large majority of the larvae of Lepidoptera, as
well as the perfect insects, acquired these distasteful properties, so
as seriously to diminish the food supply of insectivorous and nestling
birds, these latter would be forced by necessity to acquire
corresponding tastes, and to eat with pleasure what some of them now eat
only under pressure of hunger; and variation and natural selection would
soon bring about this change.
Many writers have denied the possibility of such wonderful resemblances
being produced by the accumulation of fortuitous variations, but if the
reader will call to mind the large amount of variability that has been
shown to exist in all organisms, the exceptional power of rapid increase
possessed by insects, and the tremendous struggle for existence always
going on, the difficulty will vanish, especially when we remember that
nature has the same fundamental groundwork to act upon in the two
groups, general similarity of forms, wings of similar texture and
outline, and probably some original similarity of colour and marking.
Yet there is evidently considerable difficulty in the process, or with
these great resources at her command nature would have produced more of
these mimicking forms than she has done. One reason of this deficiency
probably is, that the imitators, being always fewer in number, have not
been able to keep pace with the variations of the much more numerous
imitated form; another reason may be the ever-increasing acuteness of
the enemies, which have again and again detected the imposture and
exterminated the feeble race before it has had time to become further
modified. The result of this growing acuteness of enemies has been, that
those mimics that now survive exhibit, as Mr. Bates well remarks, "a
palpably intentional likeness that is perfectly staggering," and also
"that those features of the portrait are most attended to by nature
which produce the most effective deception when the insects are seen in
nature." No one, in fact, can understand the perfection of the imitation
who has not seen these species in their native wilds. So complete is it
in general effect that in almost every box of butterflies, brought from
tropical America by amateurs, are to be found some species of the
mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae,
placed together under the impression that they are the same species. Yet
more extraordinary, it sometimes deceives the very insects themselves.
Mr. Trimen states that the male Danais chrysippus is sometimes deceived
by the female Diadema bolina which mimics that species. Dr. Fritz
Müller, writing from Brazil to Professor Meldola, says, "One of the most
interesting of our mimicking butterflies is Leptalis melite. The female
alone of this species imitates one of our common white Pieridae, which
she copies so well that even her own male is often deceived; for I have
repeatedly seen the male pursuing the mimicked species, till, after
closely approaching and becoming aware of his error, he suddenly
returned."[100] This is evidently not a case of true mimicry, since the
species imitated is not protected; but it may be that the less abundant
Leptalis is able to mingle with the female Pieridae and thus obtain
partial immunity from attack. Mr. Kirby of the insect department of the
British Museum informs me that there are several species of South
American Pieridae which the female Leptalis melite very nearly
resembles. The case, however, is interesting as showing that the
butterflies are themselves deceived by a resemblance which is not so
great as that of some mimicking species.
_Other Examples of Mimicry among Lepidoptera._
In tropical Asia, and eastward to the Pacific Islands, the Danaidae take
the place of the Heliconidae of America, in their abundance, their
conspicuousness, their slow flight, and their being the subjects of
mimicry. They exist under three principal forms or genera. The genus
Euploea is the most abundant both in species and individuals, and
consists of fine broad-winged butterflies of a glossy or metallic
blue-black colour, adorned with pure white, or rich blue, or dusky
markings situated round the margins of the wings. Danais has generally
more lengthened wings, of a semitransparent greenish or a rich brown
colour, with radial or marginal pale spots; while the fine Hestias are
of enormous size, of a papery or semitransparent white colour, with
dusky or black spots and markings. Each of these groups is mimicked by
various species of the genus Papilio, usually with such accuracy that it
is impossible to distinguish them on the wing.[101] Several species of
Diadema, a genus of butterflies allied to our Vanessas, also mimic
species of Danais, but in this case the females only are affected, a
subject which will be discussed in another chapter.
Another protected group in the Eastern tropics is that of the beautiful
day-flying moths forming the family Agaristidae. These are usually
adorned with the most brilliant colours or conspicuous markings, they
fly slowly in forests among the butterflies and other diurnal insects,
and their great abundance sufficiently indicates their possession of
some distastefulness which saves them from attack. Under these
conditions we may expect to find other moths which are not so protected
imitating them, and this is the case. One of the common and wide-ranging
species (Opthalmis lincea), found in the islands from Amboyna to New
Ireland, is mimicked in a wonderful manner by one of the Liparidae (the
family to which our common "tussock" and "vapourer" moths belong). This
is a new species collected at Amboyna during the voyage of the
_Challenger_, and has been named Artaxa simulans. Both insects are
black, with the apex of the fore wings ochre coloured, and the outer
half of the hind wings bright orange. The accompanying woodcuts (for the
use of which I am indebted to Mr. John Murray of the _Challenger_
Office) well exhibit their striking resemblance to each other.
[Illustration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans
(Liparidae).]
In Africa exactly similar phenomena recur, species of Papilio and of
Diadema mimicking Danaidae or Acraeidae with the most curious accuracy.
Mr. Trimen, who studied this subject in South Africa, has recorded eight
species or varieties of Diadema, and eight of Papilio, which each mimic
some species of Danais; while eight species or varieties of Panopaea
(another genus of Nymphalidae), three of Melanitis (Eurytelidae), and
two of Papilio, resemble with equal accuracy some species of
Acraea.[102] He has also independently observed the main facts on which
the explanation of the phenomenon rests,--the unpleasant odour of the
Danais and Acraea, extending to their larvae and pupae; their great
abundance, slow flight, and disregard of concealment; and he states that
while lizards, mantidae, and dragonflies all hunt butterflies, and the
rejected wings are to be found abundantly at some of their
feeding-places, those of the two genera Danais and Acraea were never
among them.
The two groups of the great genus Papilio (the true swallow-tailed
butterflies) which have been already referred to as having the special
characteristics of uneatable insects, have also their imitators in other
groups; and thus, the belief in their inedibility--derived mainly from
their style of warning coloration and their peculiar habits--is
confirmed. In South America, several species of the "Aeneas" group of
these butterflies are mimicked by Pieridae and by day-flying moths of
the genera Castnia and Pericopis. In the East, Papilio hector, P.
diphilus, and P. liris, all belonging to the inedible group, are
mimicked by the females of other species of Papilio belonging to very
distinct groups; while in Northern India and China, many fine day-flying
moths (Epicopeia) have acquired the strange forms and peculiar colours
of some of the large inedible Papilios of the same regions.
In North America, the large and handsome Danais archippus, with rich
reddish-brown wings, is very common; and it is closely imitated by
Limenitis misippus, a butterfly allied to our "white admiral," but which
has acquired a colour quite distinct from that of the great bulk of its
allies. In the same country there is a still more interesting case. The
beautiful dark bronzy green butterfly, Papilio philenor, is inedible
both in larva and perfect insect, and it is mimicked by the equally dark
Limenitis ursula. There is also in the Southern and Western States a
dark female form of the yellow Papilio turnus, which in all probability
obtains protection from its general resemblance to P. philenor. Mr. W.H.
Edwards has found, by extensive experiment, that both the dark and
yellow females produce their own kinds, with very few exceptions; and he
thinks that the dark form has the advantage in the more open regions and
in the prairies, where insectivorous birds abound. But in open country
the dark form would be quite as conspicuous as the yellow form, if not
more so, so that the resemblance to an inedible species would be there
more needed.[103]
The only probable case of mimicry in this country is that of the moth,
Diaphora mendica, whose female only is white, while the larva is of
protective colours, and therefore almost certainly edible. A much more
abundant moth, of about the same size and appearing about the same time,
is Spilosoma menthrasti, also white, but in this case both it and its
larva have been proved to be inedible. The white colour of the female
Diaphora, although it must be very conspicuous at night, may, therefore,
have been acquired in order to resemble the uneatable Spilosoma, and
thus gain some protection.[104]
_Mimicry among Protected (Uneatable) Genera._
Before giving some account of the numerous other cases of warning
colours and of mimicry that occur in the animal kingdom, it will be well
to notice a curious phenomenon which long puzzled entomologists, but
which has at length received a satisfactory explanation.
We have hitherto considered, that mimicry could only occur when a
comparatively scarce and much persecuted species obtained protection by
its close external resemblance to a much more abundant uneatable species
inhabiting its own district; and this rule undoubtedly prevails among
the great majority of mimicking species all over the world. But Mr.
Bates also found a number of pairs of species of different genera of
Heliconidae, which resembled each other quite as closely as did the
other mimicking species he has described; and since all these insects
appear to be equally protected by their inedibility, and to be equally
free from persecution, it was not easy to see why this curious
resemblance existed, or how it had been brought about. That it is not
due to close affinity is shown by the fact that the resemblance occurs
most frequently between the two distinct sub-families into which (as Mr.
Bates first pointed out) the Heliconidae are naturally divided on
account of very important structural differences. One of these
sub-families (the true Heliconinae) consists of two genera only,
Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less
than sixteen genera; and, in the instances of mimicry we are now
discussing, one of the pairs or triplets that resemble each other is
usually a species of the large and handsome genus Heliconius, the others
being species of the genera Mechanitis, Melinaea, or Tithorea, though
several species of other Danaoid genera also imitate each other. The
following lists will give some idea of the number of these curious
imitative forms, and of their presence in every part of the Neotropical
area. The bracketed species are those that resemble each other so
closely that the difference is not perceptible when they are on the
wing.
In the Lower Amazon region are found--
{ Heliconius sylvana.
{ Melinaea egina.
{ Heliconius numata.
{ Melinaea mneme.
{ Tithorea harmonia.
{ Methona psidii.
{ Thyridia ino.
{ Ceratina ninonia.
{ Melinaea mnasias.
In Central America are found--
{ Heliconius zuleika.
Nicaragua { Melinaea hezia.
{ Mechanitis sp.
{ Heliconius formosus.
{ Tithorea penthias.
Guatemala { Heliconius telchina.
{ Melinaea imitata.
In the Upper Amazon region--
{ Heliconius pardalinus.
{ Melinaea pardalis.
{ Heliconius aurora.
{ Melinaea lucifer.
In New Grenada--
{ Heliconius ismenius.
{ Melinaea messatis.
{ Heliconius messene.
{ Melinaea mesenina.
{ (?) Mechanitis sp.
{ Heliconius hecalesia.
{ Tithorea hecalesina.
{ Heliconius hecuba.
{ Tithorea bonplandi.
In Eastern Peru and Bolivia--
{ Heliconius aristona.
{ Melinaea cydippe.
{ (?) Mechanitis mothone.
In Pernambuco--
{ Heliconius ethra.
{ Mechanitis nesaea.
In Rio Janeiro--
{ Helieonius eucrate.
{ Mechanitis lysimnia.
In South Brazil--
{ Thyridia megisto.
{ Ituna ilione.
{ Acraea thalia.
{ Eueides pavana.
Besides these, a number of species of Ithomia and Napeogenes, and of
Napeogenes and Mechanitis, resemble each other with equal accuracy, so
that they are liable to be mistaken for each other when on the wing; and
no doubt many other equally remarkable cases are yet unnoticed.
[Illustration: FIG. 25.--Wings of Ituna Ilione, female. Wings of
Thyridia megisto, female.]
The figures above of the fore and hind wings of two of these mimicking
species, from Dr. Fritz Müller's original paper in _Kosmos_, will serve
to show the considerable amount of difference, in the important
character of the neuration of the wings, between these butterflies,
which really belong to very distinct and not at all closely allied
genera. Other important characters are--(1) The existence of a small
basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)
the division of the cell between the veins 1_b_ and 2 of the hind wings
in the former genus, while it is undivided in the latter; and (3) the
existence in Thyridia of scent-producing tufts of hair on the upper edge
of the hind wing, while in Ituna these are wanting; but in place of them
are extensible processes at the end of the abdomen, also emitting a
powerful scent. These differences characterise two marked subdivisions
of the Danaoid Heliconinae, each containing several distinct genera; and
these subdivisions are further distinguished by very different forms of
larvae, that to which Ituna belongs having from two to four long
threadlike tentacles on the back, while in that containing Thyridia
these are always absent. The former usually feed on Asclepiadeae, the
latter on Solanaceae or Scrophulariaceae.
The two species figured, though belonging to such distinct and even
remote genera, have acquired almost identical tints and markings so as
to be deceptively alike. The surface of the wings is, in both,
transparent yellowish, with black transverse bands and white marginal
spots, while both have similar black-and white-marked bodies and long
yellow antennae. Dr. Müller states that they both show a preference for
the same flowers growing on the edges of the forest paths.[105]
We will now proceed to give the explanation of these curious
similarities, which have remained a complete puzzle for twenty years.
Mr. Bates, when first describing them, suggested that they might be due
to some form of parallel variation dependent on climatic influences; and
I myself adduced other cases of coincident local modifications of
colour, which did not appear to be explicable by any form of
mimicry.[106] But we neither of us hit upon the simple explanation given
by Dr. Fritz Müller in 1879.
His theory is founded on the assumed, but probable, fact, that
insect-eating birds only learn by experience to distinguish the edible
from the inedible butterflies, and in doing so necessarily sacrifice a
certain number of the latter. The quantity of insectivorous birds in
tropical America is enormous; and the number of young birds which every
year have to learn wisdom by experience, as regards the species of
butterflies to be caught or to be avoided, is so great that the
sacrifice of life of the inedible species must be considerable, and, to
a comparatively weak or scarce species, of vital importance. The number
thus sacrificed will be fixed by the quantity of young birds, and by the
number of experiences requisite to cause them to avoid the inedible
species for the future, and not at all by the numbers of individuals of
which each species consists. Hence, if two species are so much alike as
to be mistaken for one another, the fixed number annually sacrificed by
inexperienced birds will be divided between them, and both will benefit.
But if the two species are very unequal in numbers, the benefit will be
comparatively slight for the more abundant species, but very great for
the rare one. To the latter it may make all the difference between
safety and destruction.
To give a rough numerical example. Let us suppose that in a given
limited district there are two species of Heliconidae, one consisting of
only 1000, the other of 100,000 individuals, and that the quota required
annually in the same district for the instruction of young insectivorous
birds is 500. By the larger species this loss will be hardly felt; to
the smaller it will mean the most dreadful persecution resulting in a
loss of half the total population. But, let the two species become
superficially alike, so that the birds see no difference between them.
The quota of 500 will now be taken from a combined population of 101,000
butterflies, and if proportionate numbers of each suffer, then the weak
species will only lose five individuals instead of 500 as it did before.
Now we know that the different species of Heliconidae are not equally
abundant, some being quite rare; so that the benefit to be derived in
these latter cases would be very important. A slight inferiority in
rapidity of flight or in powers of eluding attack might also be a cause
of danger to an inedible species of scanty numbers, and in this case too
the being merged in another much more abundant species, by similarity
of external appearance, would be an advantage.
The question of fact remains. Do young birds pursue and capture these
distasteful butterflies till they have learned by bitter experience what
species to avoid? On this point Dr. Müller has fortunately been able to
obtain some direct evidence, by capturing several Acraeas and
Heliconidae which had evidently been seized by birds but had afterwards
escaped, as they had pieces torn out of the wing, sometimes
symmetrically out of both wings, showing that the insect had been seized
when at rest and with the two pairs of wings in contact. There is,
however, a general impression that this knowledge is hereditary, and
does not need to be acquired by young birds; in support of which view
Mr. Jenner Weir states that his birds always disregarded inedible
caterpillars. When, day by day, he threw into his aviary various larvae,
those which were edible were eaten immediately, those which were
inedible were no more noticed than if a pebble had been thrown before
the birds.
The cases, however, are not strictly comparable. The birds were not
young birds of the first year; and, what is more important, edible
larvae have a comparatively simple coloration, being always brown or
green and smooth. Uneatable larvae, on the other hand, comprise all that
are of conspicuous colours and are hairy or spiny. But with butterflies
there is no such simplicity of contrast. The eatable butterflies
comprise not only brown or white species, but hundreds of Nymphalidae,
Papilionidae, Lycaenidae, etc., which are gaily coloured and of an
immense variety of patterns. The colours and patterns of the inedible
kinds are also greatly varied, while they are often equally gay; and it
is quite impossible to suppose that any amount of instinct or inherited
habit (if such a thing exists) could enable young insectivorous birds to
distinguish all the species of one kind from all those of the other.
There is also some evidence to show that animals do learn by experience
what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J.
Bursch that very young chickens peck at insects which they afterwards
avoid. Lizards, too, often seized larvae which they were unable to eat
and ultimately rejected.
Although the Heliconidae present, on the whole, many varieties of
coloration and pattern, yet, in proportion to the number of distinct
species in each district, the types of coloration are few and very well
marked, and thus it becomes easier for a bird or other animal to learn
that all belonging to such types are uneatable. This must be a decided
advantage to the family in question, because, not only do fewer
individuals of each species need to be sacrificed in order that their
enemies may learn the lesson of their inedibility, but they are more
easily recognised at a distance, and thus escape even pursuit. There is
thus a kind of mimicry between closely allied species as well as between
species of distinct genera, all tending to the same beneficial end. This
may be seen in the four or five distinct species of the genus Heliconius
which all have the same peculiar type of coloration--a yellow band
across the upper wings and radiating red stripes on the lower,--and are
all found in the same forests of the Lower Amazon; in the numerous very
similar species of Ithomia with transparent wings, found in every
locality of the same region; and in the very numerous species of Papilio
of the "Aeneas" group, all having a similar style of marking, the
resemblance being especially close in the females. The very uniform type
of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of
the same character.[107] In all these cases the similarity of the allied
species is so great, that, when they are on the wing at some distance
off, it is difficult to distinguish one species from another. But this
close external resemblance is not always a sign of very near affinity;
for minute examination detects differences in the form and scalloping of
the wings, in the markings on the body, and in those on the under
surface of the wings, which do not usually characterise the closest
allies. It is to be further noted, that the presence of groups of very
similar species of the same genus, in one locality, is not at all a
common phenomenon among unprotected groups. Usually the species of a
genus found in one locality are each well marked and belong to somewhat
distinct types, while the closely allied forms--those that require
minute examination to discriminate them as distinct species--are most
generally found in separate areas, and are what are termed
representative forms.
The extension we have now given to the theory of mimicry is important,
since it enables us to explain a much wider range of colour phenomena
than those which were first imputed to mimicry. It is in the richest
butterfly region in the world--the Amazon valley--that we find the most
abundant evidence of the three distinct sets of facts, all depending on
the same general principle. The form of mimicry first elucidated by Mr.
Bates is characterised by the presence in each locality of certain
butterflies, or other insects, themselves edible and belonging to edible
groups, which derived protection from having acquired a deceptive
resemblance to some of the inedible butterflies in the same localities,
which latter were believed to be wholly free from the attacks of
insectivorous birds. Then came the extension of the principle, by Dr. F.
Müller, to the case of species of distinct genera of the inedible
butterflies resembling each other quite as closely as in the former
cases, and like them always found in the same localities. They derive
mutual benefit from becoming, in appearance, one species, from which a
certain toll is taken annually to teach the young insectivorous birds
that they are uneatable. Even when the two or more species are
approximately equal in numbers, they each derive a considerable benefit
from thus combining their forces; but when one of the species is scarce
or verging on extinction, the benefit becomes exceedingly great, being,
in fact, exactly apportioned to the need of the species.
The third extension of the same principle explains the grouping of
allied species of the same genera of inedible butterflies into sets,
each having a distinct type of coloration, and each consisting of a
number of species which can hardly be distinguished on the wing. This
must be useful exactly in the same way as in the last case, since it
divides the inevitable toll to insectivorous birds and other animals
among a number of species. It also explains the fact of the great
similarity of many species of inedible insects in the same locality--a
similarity which does not obtain to anything like the same extent among
the edible species. The explanation of the various phenomena of
resemblance and mimicry, presented by the distasteful butterflies, may
now be considered tolerably complete.
_Mimicry in other Orders of Insects._
A very brief sketch of these phenomena will be given, chiefly to show
that the same principle prevails throughout nature, and that, wherever a
rather extensive group is protected, either by distastefulness or
offensive weapons, there are usually some species of edible and
inoffensive groups that gain protection by imitating them. It has been
already stated that the Telephoridae, Lampyridae, and other families of
soft-winged beetles, are distasteful; and as they abound in all parts of
the world, and especially in the tropics, it is not surprising that
insects of many other groups should imitate them. This is especially the
case with the longicorn beetles, which are much persecuted by
insectivorous birds; and everywhere in tropical regions some of these
are to be found so completely disguised as to be mistaken for species of
the protected groups. Numbers of these imitations have been already
recorded by Mr. Bates and myself, but I will here refer to a few others.
In the recently published volumes on the Longicorn and Malacoderm
beetles of Central America[108] there are numbers of beautifully
coloured figures of the new species; and on looking over them we are
struck by the curious resemblance of some of the Longicorns to species
of the Malacoderm group. In some cases we discover perfect mimics, and
on turning to the descriptions we always find these pairs to come from
the same locality. Thus the Otheostethus melanurus, one of the
Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar
coloration, and size, and both are found at Chontales in Nicaragua, the
species mimicked having, however, as is usual, a wider range. The
curious and very rare little longicorn, Tethlimmena aliena, quite unlike
its nearest allies in the same country, is an exact copy on a somewhat
smaller scale of a malacoderm, Lygistopterus amabilis, both found at
Chontales. The pretty longicorn, Callia albicornis, closely resembles
two species of malacoderms (Silis chalybeipennis and Colyphus
signaticollis), all being small beetles with red head and thorax and
bright blue elytra, and all three have been found at Panama. Many other
species of Callia also resemble other malacoderms; and the longicorn
genus Lycidola has been named from its resemblance to various species of
the Lycidae, one of the species here figured (Lycidola belti) being a
good mimic of Calopteron corrugatum and of several other allied species,
all being of about the same size and found at Chontales. In these cases,
and in most others, the longicorn beetles have lost the general form and
aspect of their allies to take on the appearance of a distinct tribe.
Some other groups of beetles, as the Elateridae and Eucnemidae, also
deceptively mimic malacoderms.
Wasps and bees are often closely imitated by insects of other orders.
Many longicorn beetles in the tropics exactly mimic wasps, bees, or
ants. In Borneo a large black wasp, whose wings have a broad white patch
near the apex (Mygnimia aviculus), is closely imitated by a heteromerous
beetle (Coloborhombus fasciatipennis), which, contrary to the general
habit of beetles, keeps its wings expanded in order to show the white
patch on their apex, the wing-coverts being reduced to small oval
scales, as shown in the figure. This is a most remarkable instance of
mimicry, because the beetle has had to acquire so many characters which
are unknown among its allies (except in another species from Java)--the
expanded wings, the white band on them, and the oval scale-like
elytra.[109] Another remarkable case has been noted by Mr. Neville
Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly
imitated in colour, size, shape, attitude when at rest, and mode of
flight, by a beetle of the genus Laphria.[110]
The tiger-beetles (Cicindelidae) are also the subjects of mimicry by
more harmless insects. In the Malay Islands I found a heteromerous
beetle which exactly resembled a Therates, both being found running on
the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics
Collyris, another genus of the same family; while in the Philippine
Islands there is a cricket (Condylodeira tricondyloides), which so
closely resembles a tiger-beetle of the genus Tricondyla that the
experienced entomologist, Professor Westwood, at first placed it in his
cabinet among those beetles.
[Illustration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus
fasciatipennis (Beetle).]
[Illustration: FIG. 27.
a. Doliops sp. (Longicorn)
mimics Pachyrhynchus orbifae, (b) (a hard curculio).
c. Doliops curculionoides mimics (d) Pachyrhynchus sp.
e. Scepastus pachyrhynchoides (a grasshopper),
mimics (f) Apocyrtus sp. (a hard curculio).
g. Doliops sp. mimics (h) Pachyrhynchus sp.
i. Phoraspis (grasshopper) mimics (k) a Coccinella.
All the above are from the Philippines. The exact correspondence of the
colours of the insects themselves renders the mimicry much more complete
in nature than it appears in the above figures.]
One of the characters by which some beetles are protected is excessive
hardness of the elytra and integuments. Several genera of weevils
(Curculionidae) are thus saved from attack, and these are often mimicked
by species of softer and more eatable groups. In South America, the
genus Heilipus is one of these hard groups, and both Mr. Bates and M.
Roelofs, a Belgian entomologist, have noticed that species of other
genera exactly mimic them. So, in the Philippines, there is a group of
Curculionidae, forming the genus Pachyrhynchus, in which all the species
are adorned with the most brilliant metallic colours, banded and spotted
in a curious manner, and are very smooth and hard. Other genera of
Curculionidae (Desmidophorus, Alcides), which are usually very
differently coloured, have species in the Philippines which mimic the
Pachyrhynchi; and there are also several longicorn beetles (Aprophata,
Doliops, Acronia, and Agnia), which also mimic them. Besides these,
there are some longicorns and cetonias which reproduce the same colours
and markings; and there is even a cricket (Scepastus pachyrhynchoides),
which has taken on the form and peculiar coloration of these beetles in
order to escape from enemies, which then avoid them as uneatable.[111]
The figures on the opposite page exhibit several other examples of these
mimicking insects.
Innumerable other cases of mimicry occur among tropical insects; but we
must now pass on to consider a few of the very remarkable, but much
rarer instances, that are found among the higher animals.
_Mimicry among the Vertebrata._
Perhaps the most remarkable cases yet known are those of certain
harmless snakes which mimic poisonous species. The genus Elaps, in
tropical America, consists of poisonous snakes which do not belong to
the viper family (in which are included the rattlesnakes and most of
those which are poisonous), and which do not possess the broad
triangular head which characterises the latter. They have a peculiar
style of coloration, consisting of alternate rings of red and black, or
red, black, and yellow, of different widths and grouped in various ways
in the different species; and it is a style of coloration which does not
occur in any other group of snakes in the world. But in the same regions
are found three genera of harmless snakes, belonging to other families,
some few species of which mimic the poisonous Elaps, often so exactly
that it is with difficulty one can be distinguished from the other. Thus
Elaps fulvius in Guatemala is imitated by the harmless Pliocerus
equalis; Elaps corallinus in Mexico is mimicked by the harmless
Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by
Oxyrhopus trigeminus; while in other parts of South America similar
cases of mimicry occur, sometimes two harmless species imitating the
same poisonous snake.
A few other instances of mimicry in this group have been recorded. There
is in South Africa an egg-eating snake (Dasypeltis scaber), which has
neither fangs nor teeth, yet it is very like the Berg adder (Clothos
atropos), and when alarmed renders itself still more like by flattening
out its head and darting forward with a hiss as if to strike a foe.[112]
Dr. A.B. Meyer has also discovered that, while some species of the genus
Callophis (belonging to the same family as the American Elaps) have
large poison fangs, other species of the same genus have none; and that
one of the latter (C. gracilis) resembles a poisonous species (C.
intestinalis) so closely, that only an exact comparison will discover
the difference of colour and marking. A similar kind of resemblance is
said to exist between another harmless snake, Megaerophis flaviceps, and
the poisonous Callophis bivirgatus; and in both these cases the harmless
snake is less abundant than the poisonous one, as occurs in all examples
of true mimicry.[113]
In the genus Elaps, above referred to, the very peculiar style of colour
and marking is evidently a "warning colour" for the purpose of
indicating to snake-eating birds and mammals that these species are
poisonous; and this throws light on the long-disputed question of the
use of the rattle of the rattlesnake. This reptile is really both
sluggish and timid, and is very easily captured by those who know its
habits. If gently tapped on the head with a stick, it will coil itself
up and lie still, only raising its tail and rattling. It may then be
easily caught. This shows that the rattle is a warning to its enemies
that it is dangerous to proceed to extremities; and the creature has
probably acquired this structure and habit because it frequents open or
rocky districts where protective colour is needful to save it from being
pounced upon by buzzards or other snake-eaters. Quite parallel in
function is the expanded hood of the Indian cobra, a poisonous snake
which belongs also to the Elapidae. This is, no doubt, a warning to its
foes, not an attempt to terrify its prey; and the hood has been
acquired, as in the case of the rattlesnake, because, protective
coloration being on the whole useful, some mark was required to
distinguish it from other protectively coloured, but harmless, snakes.
Both these species feed on active creatures capable of escaping if their
enemy were visible at a moderate distance.
_Mimicry among Birds._
The varied forms and habits of birds do not favour the production among
them of the phenomena of warning colours or of mimicry; and the extreme
development of their instincts and reasoning powers, as well as their
activity and their power of flight, usually afford them other means of
evading their enemies. Yet there are a few imperfect, and one or two
very perfect cases of true mimicry to be found among them. The less
perfect examples are those presented by several species of cuckoos, an
exceedingly weak and defenceless group of birds. Our own cuckoo is, in
colour and markings, very like a sparrow-hawk. In the East, several of
the small black cuckoos closely resemble the aggressive drongo-shrikes
of the same country, and the small metallic cuckoos are like glossy
starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus)
resembles one of the fine pheasants (Euplocamus) of the same country,
both in form and in its rich metallic colours.
More perfect cases of mimicry occur between some of the dull-coloured
orioles in the Malay Archipelago and a genus of large honey-suckers--the
Tropidorhynchi or "Friar-birds." These latter are powerful and noisy
birds which go in small flocks. They have long, curved, and sharp beaks,
and powerful grasping claws; and they are quite able to defend
themselves, often driving away crows and hawks which venture to approach
them too nearly. The orioles, on the other hand, are weak and timid
birds, and trust chiefly to concealment and to their retiring habits to
escape persecution. In each of the great islands of the Austro-Malayan
region there is a distinct species of Tropidorhynchus, and there is
always along with it an oriole that exactly mimics it. All the
Tropidorhynchi have a patch of bare black skin round the eyes, and a
ruff of curious pale recurved feathers on the nape, whence their name of
Friar-birds, the ruff being supposed to resemble the cowl of a friar.
These peculiarities are imitated in the orioles by patches of feathers
of corresponding colours; while the different tints of the two species
in each island are exactly the same. Thus in Bouru both are earthy
brown; in Ceram they are both washed with yellow ochre; in Timor the
under surface is pale and the throat nearly white, and Mr. H.O. Forbes
has recently discovered another pair in the island of Timor Laut. The
close resemblance of these several pairs of birds, of widely different
families, is quite comparable with that of many of the insects already
described. It is so close that the preserved specimens have even
deceived naturalists; for, in the great French work, _Voyage de
l'Astrolabe_, the oriole of Bouru is actually described and figured as a
honey-sucker; and Mr. Forbes tells us that, when his birds were
submitted to Dr. Sclater for description, the oriole and the
honey-sucker were, previous to close examination, considered to be the
same species.
_Objections to the Theory of Mimicry._
To set forth adequately the varied and surprising facts of mimicry would
need a large and copiously illustrated volume; and no more interesting
subject could be taken up by a naturalist who has access to our great
collections and can devote the necessary time to search out the many
examples of mimicry that lie hidden in our museums. The brief sketch of
the subject that has been here given will, however, serve to indicate
its nature, and to show the weakness of the objections that were at
first made to it. It was urged that the action of "like conditions,"
with "accidental resemblances" and "reversion to ancestral types," would
account for the facts. If, however, we consider the actual phenomena as
here set forth, and the very constant conditions under which they occur,
we shall see how utterly inadequate are these causes, either singly or
combined. These constant conditions are--
1. That the imitative species occur in the same area and occupy
the very same station as the imitated.
2. That the imitators are always the more defenceless.
3. That the imitators are always less numerous in individuals.
4. That the imitators differ from the bulk of their allies.
5. That the imitation, however minute, is _external_ and
_visible_ only, never extending to internal characters or to
such as do not affect the external appearance.
These five characteristic features of mimicry show us that it is really
an exceptional form of protective resemblance. Different species in the
same group of organisms may obtain protection in different ways: some by
a general resemblance to their environment; some by more exactly
imitating the objects that surround them--bark, or leaf, or flower;
while others again gain an equal protection by resembling some species
which, from whatever cause, is almost as free from attack as if it were
a leaf or a flower. This immunity may depend on its being uneatable, or
dangerous, or merely strong; and it is the resemblance to such creatures
for the purpose of sharing in their safety that constitutes mimicry.
_Concluding Remarks on Warning Colours and Mimicry._
Colours which have been acquired for the purpose of serving as a warning
of inedibility, or of the possession of dangerous offensive weapons, are
probably more numerous than have been hitherto supposed; and, if so, we
shall be able to explain a considerable amount of colour in nature for
which no use has hitherto been conjectured. The brilliant and varied
colours of sea-anemones and of many coral animals will probably come
under this head, since we know that many of them possess the power of
ejecting stinging threads from various parts of their bodies which
render them quite uneatable to most animals. Mr. Gosse describes how, on
putting an Anthea into a tank containing a half-grown bullhead (Cottus
bubalis) which had not been fed for some time, the fish opened his mouth
and sucked in the morsel, but instantly shot it out again. He then
seized it a second time, and after rolling it about in his mouth for a
moment shot it out again, and then darted away to hide himself in a
hole. Some tropical fishes, however, of the genera Tetrodon,
Pseudoscarus, Astracion, and a few others, seem to have acquired the
power of feeding on corals and medusae; and the beautiful bands and
spots and bright colours with which they are frequently adorned, may be
either protective when feeding in the submarine coral groves, or may, in
some cases, be warning colours to show that they themselves are
poisonous and uneatable.
A remarkable illustration of the wide extension of warning colours, and
their very definite purpose in nature, is afforded by what may now be
termed "Mr. Belt's frog." Frogs in all parts of the world are, usually,
protectively coloured with greens or browns; and the little tree-frogs
are either green like the leaves they rest upon, or curiously mottled to
imitate bark or dead leaves. But there are a certain number of very
gaily coloured frogs, and these do not conceal themselves as frogs
usually do. Such was the small toad found by Darwin at Bahia Blanca,
which was intense black and bright vermilion, and crawled about in the
sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt
found a little frog gorgeously dressed in a livery of red and blue,
which did not attempt concealment and was very abundant, a combination
of characters which convinced him that it was uneatable. He, therefore,
took a few specimens home with him and gave them to his fowls and ducks,
but none would touch them. At last, by throwing down pieces of meat, for
which there was a great competition among the poultry, he managed to
entice a young duck into snatching up one of the little frogs. Instead
of swallowing it, however, the duck instantly threw it out of its mouth,
and went about jerking its head as if trying to get rid of some
unpleasant taste.[114]
The power of predicting what will happen in a given case is always
considered to be a crucial test of a true theory, and if so, the theory
of warning colours, and with it that of mimicry, must be held to be well
established. Among the creatures which probably have warning colours as
a sign of inedibility are, the brilliantly coloured nudibranchiate
molluscs, those curious annelids the Nereis and the Aphrodite or
sea-mouse, and many other marine animals. The brilliant colours of the
scallops (Pecten) and some other bivalve shells are perhaps an
indication of their hardness and consequent inedibility, as in the case
of the hard beetles; and it is not improbable that some of the
phosphorescent fishes and other marine organisms may, like the
glow-worm, hold out their lamp as a warning to enemies.[115] In
Queensland there is an exceedingly poisonous spider, whose bite will
kill a dog, and cause severe illness with excruciating pain in man. It
is black, with a bright vermilion patch on the middle of the body; and
it is so well recognised by this conspicuous coloration that even the
spider-hunting wasps avoid it.[116]
Locusts and grasshoppers are generally of green protective tints, but
there are many tropical species most gaudily decorated with red, blue,
and black colours. On the same general grounds as those by which Mr.
Belt predicted the inedibility of his conspicuous frog, we might safely
predict the same for these insects; but we have fortunately a proof that
they are so protected, since Mr. Charles Home states that one of the
bright coloured Indian locusts was invariably rejected when offered to
birds and lizards.[117]
* * * * *
The examples now given lead us to the conclusion that colours acquired
for the purpose of serving as a danger-signal to enemies are very
widespread in nature, and, with the corresponding colours of the species
which mimic them, furnish us with a rational explanation of a
considerable portion of the coloration of animals which is outside the
limits of those colours that have been acquired for either protection or
recognition. There remains, however, another set of colours, chiefly
among the higher animals, which, being connected with some of the most
interesting and most disputed questions in natural history, must be
discussed in a separate chapter.
FOOTNOTES:
[Footnote 92: _Nature_, vol. iii. p. 165. Professor Meldola observed
that specimens of Danais and Euplaea in collections were less subject to
the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was
corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.]
[Footnote 93: See Darwin's _Descent of Man_, p. 325.]
[Footnote 94: _Transactions of the Entomological Society of London_,
1869, p. 21.]
[Footnote 95: _Ibid._, p. 27.]
[Footnote 96: _Nature_, vol. iii. p. 147.]
[Footnote 97: Stainton's _Manual of Butterflies and Moths_, vol. i. p.
93; E.B. Poulton, _Proceedings of the Zool. Soc. of London_, 1887, pp.
191-274.]
[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp.
495-566, coloured plates.]
[Footnote 99: These butterflies are now divided into two sub-families,
one of which is placed with the Danaidae; but to avoid confusion I shall
always speak of the American genera under the old term Heliconidae.]
[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878,
p. 158.]
[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation
of Malayan Papilionidae; and, Wallace's _Contributions to Natural
Selection_ chaps. iii. and iv., where full details are given.]
[Footnote 102: See _Trans. Linn. Soc._, vol. xxvi., with two coloured
plates illustrating cases of mimicry.]
[Footnote 103: Edwards's _Butterflies of North America_, second series,
part vi.]
[Footnote 104: Professor Meldola informs me that he has recorded another
case of mimicry among British moths, in which Acidalia subsericata
imitates Asthena candidata. See _Ent. Mo. Mag._, vol. iv. p. 163.]
[Footnote 105: From Professor Meldola's translation of Dr. F. Müller's
paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]
[Footnote 106: _Island Life_, p. 255.]
[Footnote 107: This extension of the theory of mimicry was pointed out
by Professor Meldola in the paper already referred to; and he has
answered the objections to Dr. F. Müller's theory with great force in
the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]
[Footnote 108: Godman and Salvin's _Biologia Centrali-Americana,
Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]
[Footnote 109: _Trans. Ent. Soc._, 1885, p. 369.]
[Footnote 110: _Proc. Cambridge Phil. Soc._, vol. iii. part ii., 1877.]
[Footnote 111: _Compte-Rendu de la Société Entomologique de Belgaue_,
series ii., No. 59, 1878.]
[Footnote 112: _Nature_, vol. xxxiv. p. 547.]
[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]
[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]
[Footnote 115: Mr. Belt first suggested this use of the light of the
Lampyridae (fireflies and glow-worms)--_Naturalist in Nicaragua_, p.
320. Mr. Verrill and Professor Meldola made the same suggestion in the
case of medusae and other phosphorescent marine organisms (_Nature_,
vol. xxx. pp. 281, 289).]
[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]
[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]
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.
In the preceding chapters we have dealt chiefly with the coloration of
animals as distinctive of the several species; and we have seen that, in
an enormous number of cases, the colours can be shown to have a definite
purpose, and to be useful either as a means of protection or
concealment, of warning to enemies, or of recognition by their own kind.
We have now to consider a subordinate but very widespread
phenomenon---the differences of colour or of ornamental appendages in
the two sexes. These differences are found to have special relations
with the three classes of coloration above referred to, in many cases
confirming the explanation already given of their purport and use, and
furnishing us with important aid in formulating a general theory of
animal coloration.
In comparing the colours of the two sexes we find a perfect gradation,
from absolute identity of colour up to such extreme difference that it
is difficult to believe that the two forms can belong to the same
species; and this diversity in the colours of the sexes does not bear
any constant relation to affinity or systematic position. In both
insects and birds we find examples of complete identity and extreme
diversity of the sexes; and these differences occur sometimes in the
same tribe or family, and sometimes even in the same genus.
It is only among the higher and more active animals that sexual
differences of colour acquire any prominence. In the mollusca the two
sexes, when separated, are always alike in colour, and only very rarely
present slight differences in the form of the shell. In the extensive
group of crustacea the two sexes as a rule are identical in colour,
though there are often differences in the form of the prehensile organs;
but in a very few cases there are differences of colour also. Thus, in a
Brazilian species of shore-crab (Gelasimus) the female is grayish-brown,
while in the male the posterior part of the cephalo-thorax is pure
white, with the anterior part of a rich green. This colour is only
acquired by the males when they become mature, and is liable to rapid
change in a few minutes to dusky tints.[118] In some of the freshwater
fleas (Daphnoidae) the males are ornamented with red and blue spots,
while in others similar colours occur in both sexes. In spiders also,
though as a rule the two sexes are alike in colour, there are a few
exceptions, the males being ornamented with brilliant colours on the
abdomen, while the female is dull coloured.
_Sexual Coloration in Insects._
It is only when we come to the winged insects that we find any large
amount of peculiarity in sexual coloration, and even here it is only
developed in certain orders. Flies (Diptera), field-bugs (Hemiptera),
cicadas (Homoptera), and the grasshoppers, locusts, and crickets
(Orthoptera) present very few and unimportant sexual differences of
colour; but the last two groups have special musical organs very fully
developed in the males of some of the species, and these no doubt enable
the sexes to discover and recognise each other. In some cases, however,
when the female is protectively coloured, as in the well-known
leaf-insects already referred to (p. 207), the male is smaller and much
less protectively formed and coloured. In the bees and wasps
(Hymenoptera) it is also the rule that the sexes are alike in colour,
though there are several cases among solitary bees where they differ;
the female being black, and the male brown in Anthophora retusa, while
in Andraena fulva the female is more brightly coloured than the male. Of
the great order of beetles (Coleoptera) the same thing may be said.
Though often so rich and varied in their colours the sexes are usually
alike, and Mr. Darwin was only able to find about a dozen cases in which
there was any conspicuous difference between them.[119] They exhibit,
however, numerous sexual characters, in the length of the antennae, and
in horns, legs, or jaws remarkably enlarged or curiously modified in the
male sex.
It is in the family of dragonflies (order Neuroptera) that we first meet
with numerous cases of distinctive sexual coloration. In some of the
Agrionidae the males have the bodies rich blue and the wings black,
while the females have the bodies green and the wings transparent. In
the North American genus Hetaerina the males alone have a carmine spot
at the base of each wing; but in some other genera the sexes hardly
differ at all.
The great order of Lepidoptera, including the butterflies and moths,
affords us the most numerous and striking examples of diversity of
sexual colouring. Among the moths the difference is usually but slight,
being manifested in a greater intensity of the colour of the smaller
winged male; but in a few cases there is a decided difference, as in the
ghost-moth (Hepialus humuli), in which the male is pure white, while the
female is yellow with darker markings. This may be a recognition colour,
enabling the female more readily to discover her mate; and this view
receives some support from the fact that in the Shetland Islands the
male is almost as yellow as the female, since it has been suggested that
at midsummer, when this moth appears, there is in that high latitude
sufficient twilight all night to render any special coloration
unnecessary.[120]
Butterflies present us with a wonderful amount of sexual difference of
colour, in many cases so remarkable that the two sexes of the same
species remained for many years under different names and were thought
to be quite distinct species. We find, however, every gradation from
perfect identity to complete diversity, and in some cases we are able to
see a reason for this difference. Beginning with the most extraordinary
cases of diversity--as in Diadema misippus, where the male is black,
ornamented with a large white spot on each wing margined with rich
changeable blue, while the female is orange-brown with black spots and
stripes--we find the explanation in the fact that the female mimics an
uneatable Danais, and thus gains protection while laying its eggs on low
plants in company with that insect. In the allied species, Diadema
bolina, the females are also very different from the males, but are of
dusky brown tints, evidently protective and very variable, some
specimens having a general resemblance to the uneatable Euplaeas; so
that we see here some of the earlier stages of both forms of protection.
The remarkable differences in some South American Pieridae are similarly
explained. The males of Pieris pyrrha, P. lorena, and several others,
are white with a few black bands and marginal spots like so many of
their allies, while the females are gaily coloured with yellow and
brown, and exactly resemble some species of the uneatable Heliconidae of
the same district. Similarly, in the Malay Archipelago, the female of
Diadema anomala is glossy metallic blue, while the male is brown; the
reason for this reversal of the usual rule being, that the female
exactly mimics the brilliant colouring of the common and uneatable
Euplaea midamus, and thus secures protection. In the fine Adolias
dirtea, the male is black with a few specks of ochre-yellow and a broad
marginal band of rich metallic greenish-blue, while the female is
brownish-black entirely covered with rows of ochre-yellow spots. This
latter coloration does not appear to be protective when the insect is
seen in the cabinet, but it really is so. I have observed the female of
this butterfly in Sumatra, where it settles on the ground in the forest,
and its yellow spots so harmonise with the flickering gleams of sunlight
on the dead leaves that it can only be detected with the greatest
difficulty.
A hundred other cases might be quoted in which the female is either
more obscurely coloured than the male, or gains protection by imitating
some inedible species; and any one who has watched these female insects
flying slowly along in search of the plants on which to deposit their
eggs, will understand how important it must be to them not to attract
the attention of insect-eating birds by too conspicuous colours. The
number of birds which capture insects on the wing is much greater in
tropical regions than in Europe; and this is perhaps the reason why many
of our showy species are alike, or almost alike, in both sexes, while
they are protectively coloured on the under side which is exposed to
view when they are at rest. Such are our peacock, tortoise-shell, and
red admiral butterflies; while in the tropics we more commonly find that
the females are less conspicuous on the upper surface even when
protectively coloured beneath.
We may here remark, that the cases already quoted prove clearly that
either male or female may be modified in colour apart from the opposite
sex. In Pieris pyrrha and its allies the male retains the usual type of
coloration of the whole genus, while the female has acquired a distinct
and peculiar style of colouring. In Adolias dirtea, on the other hand,
the female appears to retain something like the primitive colour and
markings of the two sexes, modified perhaps for more perfect protection;
while the male has acquired more and more intense and brilliant colours,
only showing his original markings by the few small yellow spots that
remain near the base of the wings. In the more gaily coloured Pieridae,
of which our orange-tip butterfly may be taken as a type, we see in the
female the plain ancestral colours of the group, while the male has
acquired the brilliant orange tip to its wings, probably as a
recognition mark.
In those species in which the under surface is protectively coloured, we
often find the upper surface alike in both sexes, the tint of colour
being usually more intense in the male. But in some cases this leads to
the female being more conspicuous, as in some of the Lycaenidae, where
the female is bright blue and the male of a blue so much deeper and
soberer in tint as to appear the less brilliantly coloured of the two.
_Probable Causes of these Colours._
In the production of these varied results there have probably been
several causes at work. There seems to be a constant tendency in the
male of most animals--but especially of birds and insects--to develop
more and more intensity of colour, often culminating in brilliant
metallic blues or greens or the most splendid iridescent hues; while, at
the same time, natural selection is constantly at work, preventing the
female from acquiring these same tints, or modifying her colours in
various directions to secure protection by assimilating her to her
surroundings, or by producing mimicry of some protected form. At the
same time, the need for recognition must be satisfied; and this seems to
have led to diversities of colour in allied species, sometimes the
female, sometimes the male undergoing the greatest change according as
one or other could be modified with the greatest ease, and so as to
interfere least with the welfare of the race. Hence it is that sometimes
the males of allied species vary most, as in the different species of
Epicalia; sometimes the females, as in the magnificent green species of
Ornithoptera and the "Aeneas" group of Papilio.
The importance of the two principles--the need of protection and
recognition--in modifying the comparative coloration of the sexes among
butterflies, is beautifully illustrated in the case of the groups which
are protected by their distastefulness, and whose females do not,
therefore, need the protection afforded by sober colours.
In the great families, Heliconidae and Acraeidae, we find that the two
sexes are almost always alike; and, in the very few exceptions, that the
female, though differently, is not less gaily or less conspicuously
coloured. In the Danaidae the same general rule prevails, but the cases
in which the male exhibits greater intensity of colour than the female
are perhaps more numerous than in the other two families. There is,
however, a curious difference in this respect between the Oriental and
the American groups of distasteful Papilios with warning colours, both
of which are the subjects of mimicry. In the Eastern groups--of which P.
hector and P. coon may be taken as types--the two sexes are nearly
alike, the male being sometimes more intensely coloured and with fewer
pale markings; but in the American groups--represented by P. aeneas, P.
sesostris, and allies--there is a wonderful diversity, the males having
a rich green or bluish patch on the fore wings, while the females have a
band or spots of pure white, not always corresponding in position to the
green spot of the males. There are, however, transitional forms, by
which a complete series can be traced, from close similarity to great
diversity of colouring between the sexes; and this may perhaps be only
an extreme example of the intenser colour and more concentrated markings
which are a very prevalent characteristic of male butterflies.
There are, in fact, many indications of a regular succession of tints in
which colour development has occurred in the various groups of
butterflies, from an original grayish or brownish neutral tint. Thus in
the "Aeneas" group of Papilios we have the patch on the upper wings
yellowish in P. triopas, olivaceous in P. bolivar, bronzy-gray with a
white spot in P. erlaces, more greenish and buff in P. iphidamas,
gradually changing to the fine blue of P. brissonius, and the
magnificent green of P. sesostris. In like manner, the intense crimson
spots of the lower wings can be traced step by step from a yellow or
buff tint, which is one of the most widespread colours in the whole
order. The greater purity and intensity of colour seem to be usually
associated with more pointed wings, indicating greater vigour and more
rapid flight.
_Sexual Selection as a supposed Cause of Colour Development._
Mr. Darwin, as is well known, imputed most of the brilliant colours and
varied patterns of butterflies' wings to sexual selection--that is, to a
constant preference, by female butterflies, for the more brilliant
males; the colours thus produced being sometimes transmitted to the
males alone, sometimes to both sexes. This view has always seemed to me
to be unsupported by evidence, while it is also quite inadequate to
account for the facts. The only direct evidence, as set forth with his
usual fairness by Mr. Darwin himself, is opposed to his views. Several
entomologists assured him that, in moths, the females evince not the
least choice of their partners; and Dr. Wallace of Colchester, who has
largely bred the fine Bombyx cynthia, confirmed this statement. Among
butterflies, several males often pursue one female, and Mr. Darwin says,
that, unless the female exerts a choice the pairing must be left to
chance. But, surely, it may be the most vigorous or most persevering
male that is chosen, not necessarily one more brightly or differently
coloured, and this will be true "natural selection." Butterflies have
been noticed to prefer some coloured flowers to others; but that does
not prove, or even render probable, any preference for the colour
itself, but only for flowers of certain colours, on account of the more
agreeable or more abundant nectar obtained from them. Dr. Schulte called
Mr. Darwin's attention to the fact, that in the Diadema bolina the
brilliant blue colour surrounding the white spots is only visible when
we look towards the insect's head, and this is true of many of the
iridescent colours of butterflies, and probably depends upon the
direction of the striae on the scales. It is suggested, however, that
this display of colour will be seen by the female as the male is
approaching her, and that it has been developed by sexual
selection.[121] But in the majority of cases the males _follow_ the
female, hovering over her in a position which would render it almost
impossible for her to see the particular colours or patterns on his
upper surface; to do so the female should mount higher than the male,
and fly towards him--being the seeker instead of the sought, and this is
quite opposed to the actual facts. I cannot, therefore, think that this
suggestion adds anything whatever to the evidence for sexual selection
of colour by female butterflies. This question will, however, be again
touched upon after we have considered the phenomena of sexual colour
among the vertebrata.
_Sexual Coloration of Birds._
The general rule among vertebrates, as regards colour, is, for the two
sexes to be alike. This prevails, with only a few exceptions, in fishes,
reptiles, and mammalia; but in birds diversity of sexual colouring is
exceedingly frequent, and is, not improbably, present in a greater or
less degree in more than half of the known species. It is this class,
therefore, that will afford us the best materials for a discussion of
the problem, and that may perhaps lead us to a satisfactory explanation
of the causes to which sexual colour is due.
The most fundamental characteristic of birds, from our present point of
view, is a greater intensity of colour in the male. This is the case in
hawks and falcons; in many thrushes, warblers, and finches; in pigeons,
partridges, rails, plovers, and many others. When the plumage is highly
protective or of dull uniform tints, as in many of the thrushes and
warblers, the sexes are almost or quite identical in colour; but when
any rich markings or bright tints are acquired, they are almost always
wanting or much fainter in the female, as we see in the black-cap among
warblers, and the chaffinch among finches.
It is in tropical regions, where from a variety of causes colour has
been, developed to its fullest extent, that we find the most remarkable
examples of sexual divergence of colour. The most gorgeously coloured
birds known are the birds of paradise, the chatterers, the tanagers, the
humming-birds, and the pheasant-tribe, including the peacocks. In all
these the females are much less brilliant, and, in the great majority of
cases, exceptionally plain and dull coloured birds. Not only are the
remarkable plumes, crests, and gorgets of the birds of paradise entirely
wanting in the females, but these latter are usually without any bright
colour at all, and rank no higher than our thrushes in ornamental
plumage. Of the humming-birds the same may be said, except that the
females are often green, and sometimes slightly metallic, but from their
small size and uniform tints are never conspicuous. The glorious blues
and purples, the pure whites and intense crimsons of the male chatterers
are represented in the females by olive-greens or dull browns, as are
the infinitely varied tints of the male tanagers. And in pheasants, the
splendour of plumage which characterises the males is entirely absent in
the females, which, though often ornamental, have always comparatively
sober and protective tints. The same thing occurs with many other
groups. In the Eastern tropics are many brilliant birds belonging to the
families of the warblers, flycatchers, shrikes, etc., but the female is
always much less brilliant than the male and often quite dull coloured.
_Cause of Dull Colours of Female Birds._
The reason of this phenomenon is not difficult to find, if we consider
the essential conditions of a bird's existence, and the most important
function it has to fulfil. In order that the species may be continued,
young birds must be produced, and the female birds have to sit
assiduously on their eggs. While doing this they are exposed to
observation and attack by the numerous devourers of eggs and birds, and
it is of vital importance that they should be protectively coloured in
all those parts of the body which are exposed during incubation. To
secure this end all the bright colours and showy ornaments which
decorate the male have not been acquired by the female, who often
remains clothed in the sober hues which were probably once common to the
whole order to which she belongs. The different amounts of colour
acquired by the females have no doubt depended on peculiarities of
habits and of environment, and on the powers of defence or of
concealment possessed by the species. Mr. Darwin has taught us that
natural selection cannot produce absolute, but only relative perfection;
and as a protective colour is only one out of many means by which the
female birds are able to provide for the safety of their young, those
which are best endowed in other respects will have been allowed to
acquire more colour than those with whom the struggle for existence is
more severe.
_Relation of Sex Colour to Nesting Habits._
This principle is strikingly illustrated by the existence of
considerable numbers of birds in which both sexes are similarly and
brilliantly coloured,--in some cases as brilliantly as the males of many
of the groups above referred to. Such are the extensive families of the
kingfishers, the woodpeckers, the toucans, the parrots, the turacos, the
hangnests, the starlings, and many other smaller groups, all the species
of which are conspicuously or brilliantly coloured, while in all of them
the females are either coloured exactly like the males, or, when
differently coloured, are equally conspicuous. When searching for some
cause for this singular apparent exception to the rule of female
protective colouring, I came upon a fact which beautifully explains it;
for in all these cases, without exception, the species either nests in
holes in the ground or in trees, or builds a domed or covered nest, so
as completely to conceal the sitting-bird. We have here a case exactly
parallel to that of the butterflies protected by distastefulness, whose
females are either exactly like the males, or, if different, are equally
conspicuous. We can hardly believe that so exact a parallel should exist
between such remote classes of animals, except under the influence of a
general law; and, in the need of protection by all defenceless animals,
and especially by most female insects and birds, we have such a law,
which has been proved to have influenced the colours of a considerable
proportion of the animal kingdom.[122]
The general relation which exists between the mode of nesting and the
coloration of the sexes in those groups of birds which need protection
from enemies, may be thus expressed: When both sexes are brilliant or
conspicuous, the nest is such as to conceal the sitting-bird; but when
the male is brightly coloured and the female sits exposed on the nest,
she is always less brilliant and generally of quite sober and protective
hues.
It must be understood that the mode of nesting has influenced the
colour, not that the colour has determined the mode of nesting; and
this, I believe, has been generally, though not perhaps universally, the
case. For we know that colour varies more rapidly, and can be more
easily modified and fixed by selection, than any other character;
whereas habits, especially when connected with structure, and when they
pervade a whole group, are much more persistent and more difficult to
change, as shown by the habit of the dog turning round two or three
times before lying down, believed to be that of the wild ancestral form
which thus smoothed down the herbage so as to form a comfortable bed. We
see, too, that the general mode of nesting is characteristic of whole
families differing widely in size, form, and colours. Thus, all the
kingfishers and their allies in every part of the world nest in holes,
usually in banks, but sometimes in trees. The motmots and the puff-birds
(Bucconidae) build in similar places; while the toucans, barbets,
trogons, woodpeckers, and parrots all make their nests in hollow trees.
This habit, pervading all the members of extensive families, must
therefore be extremely ancient, more especially as it evidently depends
in some degree on the structure of the birds, the bills, and especially
the feet, of all these groups being unfitted for the construction of
woven arboreal nests.[123] But in all these families the colour varies
greatly from species to species, being constant only in the one
character of the similarity of the sexes, or, at all events, in their
being equally conspicuous even though differently coloured.
When I first put forward this view of the connection between the mode of
nesting and the coloration of female birds, I expressed the law in
somewhat different terms, which gave rise to some misunderstanding, and
led to numerous criticisms and objections. Several cases were brought
forward in which the females were far less brilliant than the males,
although the nest was covered. This is the case with the Maluridae, or
superb warblers of Australia, in which the males are very brilliant
during the pairing season and the females quite plain, yet they build
domed nests. Here, there can be little doubt, the covered nest is a
protection from rain or from some special enemies to the eggs; while the
birds themselves are protectively coloured in both sexes, except for a
short time during the breeding season when the male acquires brilliant
colours; and this is probably connected with the fact of their
inhabiting the open plains and thin scrub of Australia, where protective
colours are as generally advantageous as they are in our north-temperate
zones.
As I have now stated the law, I do not think there are any exceptions to
it, while there are an overwhelming number of cases which give it a
strong support. It has been objected that the domed nests of many birds
are as conspicuous as the birds themselves would be, and would,
therefore, be of no use as a protection to the birds and young. But, as
a matter of fact, they do protect from attack, for hawks or crows do not
pluck such nests to pieces, as in doing so they would be exposed to the
attack of the whole colony; whereas a hawk or falcon could carry off a
sitting-bird or the young at a swoop, and entirely avoid attack.
Moreover, each kind of covered nest is doubtless directed against the
attacks of the most dangerous enemies of the species, the purse-like
nests, often a yard long, suspended from the extremity of thin twigs,
being useful against the attacks of snakes, which, if they attempted to
enter them, would be easily made to lose their hold and fall to the
ground. Such birds as jays, crows, magpies, hawks, and other birds of
prey, have also been urged as an exception; but these are all aggressive
birds, able to protect themselves, and thus do not need any special
protection for their females during nidification. Some birds which build
in covered nests are comparatively dull coloured, like many of the
weaver birds, but in others the colours are more showy, and in all the
sexes are alike; so that none of these are in any way opposed to the
rule. The golden orioles have, however, been adduced as a decided
exception, since the females are showy and build in an open nest. But
even here the females are less brilliant than the males, and are
sometimes greenish or olivaceous on the upper surface; while they very
carefully conceal their nests among dense foliage, and the male is
sufficiently watchful and pugnacious to drive off most intruders.
On the other hand, how remarkable it is that the only small and brightly
coloured birds of our own country in which the male and female are
alike--the tits and starlings--either build in holes or construct
covered nests; while the beautiful hangnests (Icteridae) of South
America, which always build covered or purse-shaped nests, are equally
showy in both sexes, in striking contrast with the chatterers and
tanagers of the same country, whose females are invariably less
conspicuous than the males. On a rough estimate, there are about 1200
species of birds in the class of showy males and females, with concealed
nidification; while there are probably, from an equally rough estimate,
about the same number in the contrasted class of showy males and dull
females, with open nests. This will leave the great bulk of known birds
in the classes of those which are more or less protectively coloured in
both sexes; or which, from their organisation and habits, do not
require special protective coloration, such as many of the birds of
prey, the larger waders, and the oceanic birds.
There are a few very curious cases in which the female bird is actually
more brilliant than the male, and which yet have open nests. Such are
the dotterel (Eudromias morinellus), several species of phalarope, an
Australian creeper (Climacteris erythropus), and a few others; but in
every one of these cases the relation of the sexes in regard to
nidification is reversed, the male performing the duties of incubation,
while the female is the stronger and more pugnacious. This curious case,
therefore, quite accords with the general law of coloration.[124]
_Sexual Colours of other Vertebrates._
We may consider a few of the cases of sexual colouring of other classes
of vertebrates, as given by Mr. Darwin. In fishes, though the sexes are
usually alike, there are several species in which the males are more
brightly coloured, and have more elongated fins, spines, or other
appendages, and in some few cases the colours are decidedly different.
The males often fight together, and are altogether more vivacious and
excitable than the females during the breeding season; and with this we
may connect a greater intensity of coloration.
In frogs and toads the colours are usually alike, or a little more
intense in the males, and the same may be said of most snakes. It is in
lizards that we first meet with considerable sexual differences, many of
the species having gular pouches, frills, dorsal crests, or horns,
either confined to the males, or more developed in them than in the
females, and these ornaments are often brightly coloured. In most cases,
however, the tints of lizards are protective, the male being usually a
little more intense in coloration; and the difference in extreme cases
may be partly due to the need of protection for the female, which, when
laden with eggs, must be less active and less able to escape from
enemies than the male, and may, therefore, have retained more protective
colours, as so many insects and birds have certainly done.[125]
In mammalia there is often a somewhat greater intensity of colour in
the male, but rarely a decided difference. The female of the great red
kangaroo, however, is a delicate gray; while in the Lemur macaco of
Madagascar the male is jet-black and the female brown. In many monkeys
also there are some differences of colour, especially on the face. The
sexual weapons and ornaments of male mammalia, as horns, crests, manes,
and dewlaps, are well known, and are very numerous and remarkable.
Having thus briefly reviewed the facts, we will now consider the
theories to which they have given rise.
_Sexual Selection by the Struggles of Males._
Among the higher animals it is a very general fact that the males fight
together for the possession of the females. This leads, in polygamous
animals especially, to the stronger or better armed males becoming the
parents of the next generation, which inherits the peculiarities of the
parents; and thus vigour and offensive weapons are continually increased
in the males, resulting in the strength and horns of the bull, the tusks
of the boar, the antlers of the stag, and the spurs and fighting
instinct of the gamecock. But almost all male animals fight together,
though not specially armed; even hares, moles, squirrels, and beavers
fight to the death, and are often found to be scarred and wounded. The
same rule applies to almost all male birds; and these battles have been
observed in such different groups as humming-birds, finches,
goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of
the males are known to occur in the cases of crocodiles, lizards, and
tortoises; among fishes, in those of salmon and sticklebats. Even among
insects the same law prevails; and male spiders, beetles of many groups,
crickets, and butterflies often fight together.
From this very general phenomenon there necessarily results a form of
natural selection which increases the vigour and fighting power of the
male animal, since, in every case, the weaker are either killed,
wounded, or driven away. This selection would be more powerful if males
were always in excess of females, but after much research Mr. Darwin
could not obtain any satisfactory evidence that this was the case. The
same effect, however, is produced in some cases by constitution or
habits; thus male insects usually emerge first from the pupa, and among
migrating birds the males arrive first both in this country and in North
America. The struggle is thus intensified, and the most vigorous males
are the first to have offspring. This in all probability is a great
advantage, as the early breeders have the start in securing food, and
the young are strong enough to protect themselves while the later broods
are being produced.
It is to this form of male rivalry that Mr. Darwin first applied the
term "sexual selection." It is evidently a real power in nature; and to
it we must impute the development of the exceptional strength, size, and
activity of the male, together with the possession of special offensive
and defensive weapons, and of all other characters which arise from the
development of these or are correlated with them. But he has extended
the principle into a totally different field of action, which has none
of that character of constancy and of inevitable result that attaches to
natural selection, including male rivalry; for by far the larger portion
of the phenomena, which he endeavours to explain by the direct action of
sexual selection, can only be so explained on the hypothesis that the
immediate agency is female choice or preference. It is to this that he
imputes the origin of all secondary sexual characters other than weapons
of offence and defence, of all the ornamental crests and accessory
plumes of birds, the stridulating sounds of insects, the crests and
beards of monkeys and other mammals, and the brilliant colours and
patterns of male birds and butterflies. He even goes further, and
imputes to it a large portion of the brilliant colour that occurs in
both sexes, on the principle that variations occurring in one sex are
sometimes transmitted to the same sex only, sometimes to both, owing to
peculiarities in the laws of inheritance. In this extension of sexual
selection to include the action of female choice or preference, and in
the attempt to give to that choice such wide-reaching effects, I am
unable to follow him more than a very little way; and I will now state
some of the reasons why I think his views are unsound.
_Sexual Characters due to Natural Selection._
Besides the acquisition of weapons by the male for the purpose of
fighting with other males, there are some other sexual characters which
may have been produced by natural selection. Such are the various sounds
and odours which are peculiar to the male, and which serve as a call to
the female or as an indication of his presence. These are evidently a
valuable addition to the means of recognition of the two sexes, and are
a further indication that the pairing season has arrived; and the
production, intensification, and differentiation of these sounds and
odours are clearly within the power of natural selection. The same
remark will apply to the peculiar calls of birds, and even to the
singing of the males. These may well have originated merely as a means
of recognition between the two sexes of a species, and as an invitation
from the male to the female bird. When the individuals of a species are
widely scattered, such a call must be of great importance in enabling
pairing to take place as early as possible, and thus the clearness,
loudness, and individuality of the song becomes a useful character, and
therefore the subject of natural selection. Such is especially the case
with the cuckoo, and with all solitary birds, and it may have been
equally important at some period of the development of all birds. The
act of singing is evidently a pleasurable one; and it probably serves as
an outlet for superabundant nervous energy and excitement, just as
dancing, singing, and field sports do with us. It is suggestive of this
view that the exercise of the vocal power seems to be complementary to
the development of accessory plumes and ornaments, all our finest
singing birds being plainly coloured, and with no crests, neck or tail
plumes to display; while the gorgeously ornamented birds of the tropics
have no song, and those which expend much energy in display of plumage,
as the turkey, peacocks, birds of paradise, and humming-birds, have
comparatively an insignificant development of voice. Some birds have, in
the wings or tail, peculiarly developed feathers which produce special
sounds. In some of the little manakins of Brazil, two or three of the
wing-feathers are curiously shaped and stiffened in the male, so that
the bird is able to produce with them a peculiar snapping or cracking
sound; and the tail-feathers of several species of snipe are so narrowed
as to produce distinct drumming, whistling, or switching sounds when the
birds descend rapidly from a great height. All these are probably
recognition and call notes, useful to each species in relation to the
most important function of their lives, and thus capable of being
developed by the agency of natural selection.
_Decorative Plumage of Birds and its Display._
Mr. Darwin has devoted four chapters of his _Descent of Man_ to the
colours of birds, their decorative plumage, and its display at the
pairing season; and it is on this latter circumstance that he founds his
theory, that both the plumage and the colours have been developed by the
preference of the females, the more ornamented males becoming the
parents of each successive generation. Any one who reads these most
interesting chapters will admit, that the fact of the display is
demonstrated; and it may also be admitted, as highly probable, that the
female is pleased or excited by the display. But it by no means follows
that slight differences in the shape, pattern, or colours of the
ornamental plumes are what lead a female to give the preference to one
male over another; still less that all the females of a species, or the
great majority of them, over a wide area of country, and for many
successive generations, prefer exactly the same modification of the
colour or ornament.
The evidence on this matter is very scanty, and in most cases not at all
to the point. Some peahens preferred an old pied peacock; albino birds
in a state of nature have never been seen paired with other birds; a
Canada goose paired with a Bernicle gander; a male widgeon preferred a
pintail duck to its own species; a hen canary preferred a male
greenfinch to either linnet, goldfinch, siskin, or chaffinch. These
cases are evidently exceptional, and are not such as generally occur in
nature; and they only prove that the female does exert some choice
between very different males, and some observations on birds in a state
of nature prove the same thing; but there is no evidence that slight
variations in the colour or plumes, in the way of increased intensity or
complexity, are what determines the choice. On the other hand, Mr.
Darwin gives much evidence that it is _not_ so determined. He tells us
that Messrs. Hewitt, Tegetmeier, and Brent, three of the highest
authorities and best observers, "do not believe that the females prefer
certain males on account of the beauty of their plumage." Mr. Hewitt was
convinced "that the female almost invariably prefers the most vigorous,
defiant, and mettlesome male;" and Mr. Tegetmeier, "that a gamecock,
though disfigured by being dubbed, and with his hackles trimmed, would
be accepted as readily as a male retaining all his natural
ornaments."[126] Evidence is adduced that a female pigeon will sometimes
turn antipathy to a particular male without any assignable cause; or, in
other cases, will take a strong fancy to some one bird, and will desert
her own mate for him; but it is not stated that superiority or
inferiority of plumage has anything to do with these fancies. Two
instances are indeed given, of male birds being rejected, which had lost
their ornamental plumage; but in both cases (a widow-finch and a silver
pheasant) the long tail-plumes are the indication of sexual maturity.
Such cases do not support the idea that males with the tail-feathers a
trifle longer, or the colours a trifle brighter, are generally
preferred, and that those which are only a little inferior are as
generally rejected,--and this is what is absolutely needed to establish
the theory of the development of these plumes by means of the choice of
the female.
It will be seen, that female birds have unaccountable likes and dislikes
in the matter of their partners, just as we have ourselves, and this may
afford us an illustration. A young man, when courting, brushes or curls
his hair, and has his moustache, beard, or whiskers in perfect order,
and no doubt his sweetheart admires them; but this does not prove that
she marries him on account of these ornaments, still less that hair,
beard, whiskers, and moustache were developed by the continued
preferences of the female sex. So, a girl likes to see her lover well
and fashionably dressed, and he always dresses as well as he can when he
visits her; but we cannot conclude from this that the whole series of
male costumes, from the brilliantly coloured, puffed, and slashed
doublet and hose of the Elizabethan period, through the gorgeous coats,
long waistcoats, and pigtails of the early Georgian era, down to the
funereal dress-suit of the present day, are the direct result of female
preference. In like manner, female birds may be charmed or excited by
the fine display of plumage by the males; but there is no proof whatever
that slight differences in that display have any effect in determining
their choice of a partner.
_Display of Decorative Plumage._
The extraordinary manner in which most birds display their plumage at
the time of courtship, apparently with the full knowledge that it is
beautiful, constitutes one of Mr. Darwin's strongest arguments. It is,
no doubt, a very curious and interesting phenomenon, and indicates a
connection between the exertion of particular muscles and the
development of colour and ornament; but, for the reasons just given, it
does not prove that the ornament has been developed by female choice.
During excitement, and when the organism develops superabundant energy,
many animals find it pleasurable to exercise their various muscles,
often in fantastic ways, as seen in the gambols of kittens, lambs, and
other young animals. But at the time of pairing, male birds are in a
state of the most perfect development, and possess an enormous store of
vitality; and under the excitement of the sexual passion they perform
strange antics or rapid flights, as much probably from an internal
impulse to motion and exertion as with any desire to please their mates.
Such are the rapid descent of the snipe, the soaring and singing of the
lark, and the dances of the cock-of-the-rock and of many other birds.
It is very suggestive that similar strange movements are performed by
many birds which have no ornamental plumage to display. Goatsuckers,
geese, carrion vultures, and many other birds of plain plumage have been
observed to dance, spread their wings or tails, and perform strange
love-antics. The courtship of the great albatross, a most unwieldy and
dull coloured bird, has been thus described by Professor Moseley: "The
male, standing by the female on the nest, raises his wings, spreads his
tail and elevates it, throws up his head with the bill in the air, or
stretches it straight out, or forwards, as far as he can, and then
utters a curious cry."[127] Mr. Jenner Weir informs me that "the male
blackbird is full of action, spreads out his glossy wing and tail, turns
his rich golden beak towards the female, and chuckles with delight,"
while he has never seen the more plain coloured thrush demonstrative to
the female. The linnet distends his rosy breast, and slightly expands
his brown wings and tail; while the various gay coloured Australian
finches adopt such attitudes and postures as, in every case, to show off
their variously coloured plumage to the best advantage.[128]
_A Theory of Animal Coloration._
Having rejected Mr. Darwin's theory of female choice as incompetent to
account for the brilliant colours and markings of the higher animals,
the preponderance of these colours and markings in the male sex, and
their display during periods of activity or excitement, I may be asked
what explanation I have to offer as a preferable substitute. In my
_Tropical Nature_ I have already indicated such a theory, which I will
now briefly explain, supporting it by some additional facts and
arguments, which appear to me to have great weight, and for which I am
mainly indebted to a most interesting and suggestive posthumous work by
Mr. Alfred Tylor.[129]
The fundamental or ground colours of animals ar has been shown in
preceding chapters, very largely protective, and it is not improbable
that the primitive colours of all animals were so. During the long
course of animal development other modes of protection than concealment
by harmony of colour arose, and thenceforth the normal development of
colour due to the complex chemical and structural changes ever going on
in the organism, had full play; and the colours thus produced were again
and again modified by natural selection for purposes of warning,
recognition, mimicry, or special protection, as has been already fully
explained in the preceding chapters.
Mr. Taylor has, however, called attention to an important principle
which underlies the various patterns or ornamental markings of
animals--namely, that diversified coloration follows the chief lines of
structure, and changes at points, such as the joints, where function
changes. He says, "If we take highly decorated species--that is, animals
marked by alternate dark or light bands or spots, such as the zebra,
some deer, or the carnivora, we find, first, that the region of the
spinal column is marked by a dark stripe; secondly, that the regions of
the appendages, or limbs, are differently marked; thirdly, that the
flanks are striped or spotted, along or between the regions of the lines
of the ribs; fourthly, that the shoulder and hip regions are marked by
curved lines; fifthly, that the pattern changes, and the direction of
the lines, or spots, at the head, neck, and every joint of the limbs;
and lastly, that the tips of the ears, nose, tail, and feet, and the eye
are emphasised in colour. In spotted animals the greatest length of the
spot is generally in the direction of the largest development of the
skeleton."
This structural decoration is well seen in many insects. In
caterpillars, similar spots and markings are repeated in each segment,
except where modified for some form of protection. In butterflies, the
spots and bands usually have reference to the form of the wing and the
arrangement of the nervures; and there is much evidence to show that the
primitive markings are always spots in the cells, or between the
nervures, or at the junctions of nervures, the extension and coalescence
of these spots forming borders, bands, or blotches, which have become
modified in infinitely varied ways for protection, warning, or
recognition. Even in birds, the distribution of colours and markings
follows generally the same law. The crown of the head, the throat, the
ear-coverts, and the eyes have usually distinct tints in all highly
coloured birds; the region of the furcula has often a distinct patch of
colour, as have the pectoral muscles, the uropygium or root of the tail,
and the under tail-coverts.[130]
Mr. Tylor was of opinion the primitive form of ornamentation consisted
of spots, the confluence of these in certain directions forming lines or
bands; and, these again, sometimes coalescing into blotches, or into
more or less uniform tints covering a large portion of the surface of
the body. The young lion and tiger are both spotted; and in the Java hog
(Sus vittatus) very young animals are banded, but have spots over the
shoulders and thighs. These spots run into stripes as the animal grows
older; then the stripes expand, and at last, meeting together, the adult
animal becomes of a uniform dark brown colour. So many of the species of
deer are spotted when young, that Darwin concludes the ancestral form,
from which all deer are derived, must have been spotted. Pigs and tapirs
are banded or spotted when young; an imported young specimen of Tapirus
Bairdi was covered with white spots in longitudinal rows, here and there
forming short stripes.[131] Even the horse, which Darwin supposes to be
descended from a striped animal, is often spotted, as in dappled horses;
and great numbers show a tendency to spottiness, especially on the
haunches.
Ocelli may also be developed from spots, or from bars, as pointed out by
Mr. Darwin. Spots are an ordinary form of marking in disease, and these
spots sometimes run together, forming blotches. There is evidence that
colour markings are in some way dependent on nerve distribution. In the
disease known as frontal herpes, an eruption occurs which corresponds
exactly to the distribution of the ophthalmic division of the fifth
cranial nerve, mapping out all its little branches even to the one which
goes to the tip of the nose. In a Hindoo suffering from herpes the
pigment was destroyed in the arm along the course of the ulnar nerve,
with its branches along both sides of one finger and the half of
another. In the leg the sciatic and scaphenous nerves were partly mapped
out, giving to the patient the appearance of an anatomical diagram.[132]
These facts are very interesting, because they help to explain the
general dependence of marking on structure which has been already
pointed out. For, as the nerves everywhere follow the muscles, and these
are attached to the various bones, we see how it happens, that the
tracts in which distinct developments of colour appear, should so often
be marked out by the chief divisions of the bony structure in
vertebrates, and by the segments in the annulosa. There is, however,
another correspondence of even greater interest and importance.
Brilliant colours usually appear just in proportion to the development
of tegumentary appendages. Among birds the most brilliant colours are
possessed by those which have developed frills, crests, and elongated
tails like the humming-birds; immense tail-coverts like the peacock;
enormously expanded wing-feathers, as in the argus-pheasant; or
magnificent plumes from the region of the coracoids in many of the birds
of paradise. It is to be noted, also, that all these accessory plumes
spring from parts of the body which, in other species, are distinguished
by patches of colour; so that we may probably impute the development of
colour and of accessory plumage to the same fundamental cause.
Among insects, the most brilliant and varied coloration occurs in the
butterflies and moths, groups in which the wing-membranes have received
their greatest expansion, and whose specialisation has been carried
furthest in the marvellous scaly covering which is the seat of the
colour. It is suggestive, that the only other group in which functional
wings are much coloured is that of the dragonflies, where the membrane
is exceedingly expanded. In like manner, the colours of beetles, though
greatly inferior to those of the lepidoptera, occur in a group in which
the anterior pair of wings has been thickened and modified in order to
protect the vital parts, and in which these wing-covers (elytra), in the
course of development in the different groups, must have undergone great
changes, and have been the seat of very active growth.
_The Origin of Accessory Plumes._
Mr. Darwin supposes, that these have in almost every case been developed
by the preference of female birds for such males as possessed them in a
higher degree than others; but this theory does not account for the fact
that these plumes usually appear in a few definite parts of the body. We
require some cause to initiate the development in one part rather than
in another. Now, the view that colour has arisen over surfaces where
muscular and nervous development is considerable, and the fact that it
appears especially upon the accessory or highly developed plumes, leads
us to inquire whether the same cause has not primarily determined the
development of these plumes. The immense tuft of golden plumage in the
best known birds of paradise (Paradisea apoda and P. minor) springs
from a very small area on the side of the breast. Mr. Frank E. Beddard,
who has kindly examined a specimen for me, says that "this area lies
upon the pectoral muscles, and near to the point where the fibres of the
muscle converge towards their attachment to the humerus. The plumes
arise, therefore, close to the most powerful muscle of the body, and
near to where the activities of that muscle would be at a maximum.
Furthermore, the area of attachment of the plumes is just above the
point where the arteries and nerves for the supply of the pectoral
muscles, and neighbouring regions, leave the interior of the body. The
area of attachment of the plume is, also, as you say in your letter,
just above the junction of the coracoid and sternum." Ornamental plumes
of considerable size rise from the same part in many other species of
paradise birds, sometimes extending laterally in front, so as to form
breast shields. They also occur in many humming-birds, and in some
sun-birds and honey-suckers; and in all these cases there is a wonderful
amount of activity and rapid movement, indicating a surplus of vitality,
which is able to manifest itself in the development of these accessory
plumes.[133]
In a quite distinct set of birds, the gallinaceae, we find the
ornamental plumage usually arising from very different parts, in the
form of elongated tail-feathers or tail-coverts, and of ruffs or hackles
from the neck. Here the wings are comparatively little used, the most
constant activities depending on the legs, since the gallinaceae are
pre-eminently walking, running, and scratching birds. Now the
magnificent train of the peacock--the grandest development of accessory
plumes in this order--springs from an oval or circular area, about three
inches in diameter, just above the base of the tail, and, therefore,
situated over the lower part of the spinal column near the insertion of
the powerful muscles which move the hind limbs and elevate the tail. The
very frequent presence of neck-ruffs or breast-shields in the males of
birds with accessory plumes may be partly due to selection, because they
must serve as a protection in their mutual combats, just as does the
lion's or the horse's mane. The enormously lengthened plumes of the bird
of paradise and of the peacock can, however, have no such use, but must
be rather injurious than beneficial in the bird's ordinary life. The
fact that they have been developed to so great an extent in a few
species is an indication of such perfect adaptation to the conditions of
existence, such complete success in the battle for life, that there is,
in the adult male at all events, a surplus of strength, vitality, and
growth-power which is able to expend itself in this way without injury.
That such is the case is shown by the great abundance of most of the
species which possess these wonderful superfluities of plumage. Birds of
paradise are among the commonest birds in New Guinea, and their loud
voices can be often heard when the birds themselves are invisible in the
depths of the forest; while Indian sportsmen have described the peafowl
as being so abundant, that from twelve to fifteen hundred have been seen
within an hour at one spot; and they range over the whole country from
the Himalayas to Ceylon. Why, in allied species, the development of
accessory plumes has taken different forms, we are unable to say, except
that it may be due to that individual variability which has served as
the starting-point for so much of what seems to us strange in form, or
fantastic in colour, both in the animal and vegetable world.
_Development of Accessory Plumes and their Display._
If we have found a _vera causa_ for the origin of ornamental appendages
of birds and other animals in a surplus of vital energy, leading to
abnormal growths in those parts of the integument where muscular and
nervous action are greatest, the continuous development of these
appendages will result from the ordinary action of natural selection in
preserving the most healthy and vigorous individuals, and the still
further selective agency of sexual struggle in giving to the very
strongest and most energetic the parentage of the next generation. And,
as all the evidence goes to show that, so far as female birds exercise
any choice, it is of "the most vigorous, defiant, and mettlesome male,"
this form of sexual selection will act in the same direction, and help
to carry on the process of plume development to its culmination. That
culmination will be reached when the excessive length or abundance of
the plumes begins to be injurious to the bearer of them; and it may be
this check to the further lengthening of the peacock's train that has
led to the broadening of the feathers at the ends, and the consequent
production of the magnificent eye-spots which now form its crowning
ornament.
The display of these plumes will result from the same causes which led
to their production. Just in proportion as the feathers themselves
increased in length and abundance, the skin-muscles which serve to
elevate them would increase also; and the nervous development as well as
the supply of blood to these parts being at a maximum, the erection of
the plumes would become a habit at all periods of nervous or sexual
excitement. The display of the plumes, like the existence of the plumes
themselves, would be the chief external indication of the maturity and
vigour of the male, and would, therefore, be necessarily attractive to
the female. We have, thus, no reason for imputing to her any of those
aesthetic emotions which are excited in us, by the beauty of form,
colour, and pattern of these plumes; or the still more improbable
aesthetic tastes, which would cause her to choose her mate on account of
minute differences in their forms, colours, or patterns.
As co-operating causes in the production of accessory ornamental plumes,
I have elsewhere suggested[134] that crests and other erectile feathers
may have been useful in making the bird more formidable in appearance,
and thus serving to frighten away enemies; while long tail or wing
feathers might serve to distract the aim of a bird of prey. But though
this might be of some use in the earlier stages of their development, it
is probably of little importance compared with the vigour and pugnacity
of which the plumes are the indication, and which enable most of their
possessors to defend themselves against the enemies which are dangerous
to weaker and more timid birds. Even the tiny humming-birds are said to
attack birds of prey that approach too near to their nests.
_The Effect of Female Preference will be Neutralised by Natural
Selection._
The various facts and arguments now briefly set forth, afford an
explanation of the phenomena of male ornament, as being due to the
general laws of growth and development, and make it unnecessary to call
to our aid so hypothetical a cause as the cumulative action of female
preference. There remains, however, a general argument, arising from the
action of natural selection itself, which renders it almost
inconceivable that female preference could have been effective in the
way suggested; while the same argument strongly supports the view here
set forth. Natural selection, as we have seen in our earlier chapters,
acts perpetually and on an enormous scale in weeding out the "unfit" at
every stage of existence, and preserving only those which are in all
respects the very best. Each year, only a small percentage of young
birds survive to take the place of the old birds which die; and the
survivors will be those which are best able to maintain existence from
the egg onwards, an important factor being that their parents should be
well able to feed and protect them, while they themselves must in turn
be equally able to feed and protect their own offspring. Now this
extremely rigid action of natural selection must render any attempt to
select mere ornament utterly nugatory, unless the most ornamented always
coincide with "the fittest" in every other respect; while, if they do so
coincide, then any selection of ornament is altogether superfluous. If
the most brightly coloured and fullest plumaged males are _not_ the most
healthy and vigorous, have _not_ the best instincts for the proper
construction and concealment of the nest, and for the care and
protection of the young, they are certainly not the fittest, and will
not survive, or be the parents of survivors. If, on the other hand,
there _is_ generally this correlation--if, as has been here argued,
ornament is the natural product and direct outcome of superabundant
health and vigour, then no other mode of selection is needed to account
for the presence of such ornament. The action of natural selection does
not indeed disprove the existence of female selection of ornament as
ornament, but it renders it entirely ineffective; and as the direct
evidence for any such female selection is almost _nil_, while the
objections to it are certainly weighty, there can be no longer any
reason for upholding a theory which was provisionally useful in calling
attention to a most curious and suggestive body of facts, but which is
now no longer tenable. The term "sexual selection" must, therefore, be
restricted to the direct results of male struggle and combat. This is
really a form of natural selection, and is a matter of direct
observation; while its results are as clearly deducible as those of any
of the other modes in which selection acts. And if this restriction of
the term is needful in the case of the higher animals it is much more so
with the lower. In butterflies the weeding out by natural selection
takes place to an enormous extent in the egg, larva, and pupa states;
and perhaps not more than one in a hundred of the eggs laid produces a
perfect insect which lives to breed. Here, then, the impotence of female
selection, if it exist, must be complete; for, unless the most
brilliantly coloured males are those which produce the best protected
eggs, larvae, and pupae, and unless the particular eggs, larvae, and
pupae, which are able to survive, are those which produce the most
brilliantly coloured butterflies, any choice the female might make must
be completely swamped. If, on the other hand, there _is_ this
correlation between colour development and perfect adaptation to
conditions in all stages, then this development will necessarily proceed
by the agency of natural selection and the general laws which determine
the production of colour and of ornamental appendages.[135]
_General Laws of Animal Coloration._
The condensed account which has now been given of the phenomena of
colour in the animal world will sufficiently show the wonderful
complexity and extreme interest of the subject; while it affords an
admirable illustration of the importance of the great principle of
utility, and of the effect of the theories of natural selection and
development in giving a new interest to the most familiar facts of
nature. Much yet remains to be done, both in the observation of new
facts as to the relations between the colours of animals and their
habits or economy, and, more especially, in the elucidation of the laws
of growth which determine changes of colour in the various groups; but
so much is already known that we are able, with some confidence, to
formulate the general principles which have brought about all the beauty
and variety of colour which everywhere delight us in our contemplation
of animated nature. A brief statement of these principles will fitly
conclude our exposition of the subject.
1. Colour may be looked upon as a necessary result of the highly complex
chemical constitution of animal tissues and fluids. The blood, the bile,
the bones, the fat, and other tissues have characteristic, and often
brilliant colours, which we cannot suppose to have been determined for
any special purpose, as colours, since they are usually concealed. The
external organs, with their various appendages and integuments, would,
by the same general laws, naturally give rise to a greater variety of
colour.
2. We find it to be the fact that colour increases in variety and
intensity as external structures and dermal appendages become more
differentiated and developed. It is on scales, hair, and especially on
the more highly specialised feathers, that colour is most varied and
beautiful; while among insects colour is most fully developed in those
whose wing membranes are most expanded, and, as in the lepidoptera, are
clothed with highly specialised scales. Here, too, we find an additional
mode of colour production in transparent lamellae or in fine surface
striae which, by the laws of interference, produce the wonderful
metallic hues of so many birds and insects.
3. There are indications of a progressive change of colour, perhaps in
some definite order, accompanying the development of tissues or
appendages. Thus spots spread and fuse into bands, and when a lateral or
centrifugal expansion has occurred--as in the termination of the
peacocks' train feathers, the outer web of the secondary quills of the
Argus pheasant, or the broad and rounded wings of many butterflies--into
variously shaded or coloured ocelli. The fact that we find gradations of
colour in many of the more extensive groups, from comparatively dull or
simple to brilliant and varied hues, is an indication of some such law
of development, due probably to progressive local segregation in the
tissues of identical chemical or organic molecules, and dependent on
laws of growth yet to be investigated.
4. The colours thus produced, and subject to much individual variation,
have been modified in innumerable ways for the benefit of each species.
The most general modification has been in such directions as to favour
concealment when at rest in the usual surroundings of the species,
sometimes carried on by successive steps till it has resulted in the
most minute imitation of some inanimate object or exact mimicry of some
other animal. In other cases bright colours or striking contrasts have
been preserved, to serve as a warning of inedibility or of dangerous
powers of attack. Most frequent of all has been the specialisation of
each distinct form by some tint or marking for purposes of easy
recognition, especially in the case of gregarious animals whose safety
largely depends upon association and mutual defence.
5. As a general rule the colours of the two sexes are alike; but in the
higher animals there appears a tendency to deeper or more intense
colouring in the male, due probably to his greater vigour and
excitability. In many groups in which this superabundant vitality is at
a maximum, the development of dermal appendages and brilliant colours
has gone on increasing till it has resulted in a great diversity between
the sexes; and in most of these cases there is evidence to show that
natural selection has caused the female to retain the primitive and more
sober colours of the group for purposes of protection.
_Concluding Remarks._
The general principles of colour development now sketched out enable us
to give some rational explanation of the wonderful amount of brilliant
colour which occurs among tropical animals. Looking on colour as a
normal product of organisation, which has either been allowed free play,
or has been checked and modified for the benefit of the species, we can
see at once that the luxuriant and perennial vegetation of the tropics,
by affording much more constant means of concealment, has rendered
brilliant colour less hurtful there than in the temperate and colder
regions. Again, this perennial vegetation supplies abundance of both
vegetable and insect food throughout the year, and thus a greater
abundance and greater variety of the forms of life are rendered
possible, than where recurrent seasons of cold and scarcity reduce the
possibilities of life to a minimum. Geology furnishes us with another
reason, in the fact, that throughout the tertiary period tropical
conditions prevailed far into the temperate regions, so that the
possibilities of colour development were still greater than they are at
the present time. The tropics, therefore, present to us the results of
animal development in a much larger area and under more favourable
conditions than prevail to-day. We see in them samples of the
productions of an earlier and a better world, from an animal point of
view; and this probably gives a greater variety and a finer display of
colour than would have been produced, had conditions always been what
they are now. The temperate zones, on the other hand, have recently
suffered the effects of a glacial period of extreme severity, with the
result that almost the only gay coloured birds they now possess are
summer visitors from tropical or sub-tropical lands. It is to the
unbroken and almost unchecked course of development from remote
geological times that has prevailed in the tropics, favoured by abundant
food and perennial shelter, that we owe such superb developments as the
frills and crests and jewelled shields of the humming-birds, the golden
plumes of the birds of paradise, and the resplendent train of the
peacock. This last exhibits to us the culmination of that marvel and
mystery of animal colour which is so well expressed by a poet-artist in
the following lines. The marvel will ever remain to the sympathetic
student of nature, but I venture to hope that in the preceding chapters
I have succeeded in lifting--if only by one of its corners--the veil of
mystery which has for long shrouded this department of nature.
_On a Peacock's Feather._
In Nature's workshop but a shaving,
Of her poem but a word,
But a tint brushed from her palette,
This feather of a bird!
Yet set it in the sun glance,
Display it in the shine,
Take graver's lens, explore it,
Note filament and line,
Mark amethyst to sapphire,
And sapphire to gold,
And gold to emerald changing
The archetype unfold!
Tone, tint, thread, tissue, texture,
Through every atom scan,
Conforming still, developing,
Obedient to plan.
This but to form a pattern
On the garment of a bird!
What then must be the poem,
This but its lightest word!
Sit before it; ponder o'er it,
'Twill thy mind advantage more,
Than a treatise, than a sermon,
Than a library of lore.
FOOTNOTES:
[Footnote 118: Darwin's _Descent of Man_, p. 271.]
[Footnote 119: Darwin's _Descent of Man_, p. 294, and footnote.]
[Footnote 120: _Nature_, 1871, p. 489.]
[Footnote 121: Darwin in _Nature_, 1880, p. 237.]
[Footnote 122: See the author's _Contributions to Natural Selection_,
chap. vii. in which these facts were first brought forward.]
[Footnote 123: On this point see the author's _Contributions to Natural
Selection_, chap. v. i.]
[Footnote 124: Seebohm's _History of British Birds_, vol. ii.,
introduction, p. xiii.]
[Footnote 125: For details see Darwin's _Descent of Man_, chap. xii.]
[Footnote 126: _Descent of Man_, pp. 417, 418, 420.]
[Footnote 127: _Notes of a Naturalist on the Challenger._]
[Footnote 128: _Descent of Man_, pp. 401, 402.]
[Footnote 129: _Coloration in Animals and Plants_, London, 1886.]
[Footnote 130: _Coloration of Animals_, Pl. X, p. 90; and Pls. II, III,
and IV, pp. 30, 40, 42.]
[Footnote 131: See coloured Fig. in _Proc. Zool. Soc._, 1871, p. 626.]
[Footnote 132: A. Tylor's _Coloration_, p. 40; and Photograph in
Hutchinson's _Illustrations of Clinical Surgery_, quoted by Tylor.]
[Footnote 133: For activity and pugnacity of humming-birds, see
_Tropical Nature_, pp. 130, 213.]
[Footnote 134: _Tropical Nature_, p. 209. In Chapter V of this work the
views here advocated were first set forth, and the reader is referred
there for further details.]
[Footnote 135: The Rev. O. Pickard-Cambridge, who has devoted himself to
the study of spiders, has kindly sent me the following extract from a
letter, written in 1869, in which he states his views on this
question:--
"I myself doubt that particular application of the Darwinian
theory which attributes male peculiarities of form, structure,
colour, and ornament to female appetency or predilection. There
is, it seems to me, undoubtedly something in the male
organisation of a special, and sexual nature, which, of its own
vital force, develops the remarkable male peculiarities so
commonly seen, and of no imaginable use to that sex. In as far
as these peculiarities show a great vital power, they point out
to us the finest and strongest individuals of the sex, and show
us which of them would most certainly appropriate to themselves
the best and greatest number of females, and leave behind them
the strongest and greatest number of progeny. And here would
come in, as it appears to me, the proper application of Darwin's
theory of Natural Selection; for the possessors of greatest
vital power being those most frequently produced and reproduced,
the external signs of it would go on developing in an
ever-increasing exaggeration, only to be checked where it became
really detrimental in some respect or other to the individual."
This passage, giving the independent views of a close observer--one,
moreover, who has studied the species of an extensive group of animals
both in the field and in the laboratory--very nearly accords with my own
conclusions above given; and, so far as the matured opinions of a
competent naturalist have any weight, afford them an important support.]
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.
The colours of plants are both less definite and less complex than are
those of animals, and their interpretation on the principle of utility
is, on the whole, more direct and more easy. Yet here, too, we find that
in our investigation of the uses of the various colours of fruits and
flowers, we are introduced to some of the most obscure recesses of
nature's workshop, and are confronted with problems of the deepest
interest and of the utmost complexity.
So much has been written on this interesting subject since Mr. Darwin
first called attention to it, and its main facts have become so
generally known by means of lectures, articles, and popular books, that
I shall give here a mere outline sketch, for the purpose of leading up
to a discussion of some of the more fundamental problems which arise out
of the facts, and which have hitherto received less attention than they
deserve.
_The General Colour Relations of Plants._
The green colour of the foliage of leafy plants is due to the existence
of a substance called chlorophyll, which is almost universally developed
in the leaves under the action of light. It is subject to definite
chemical changes during the processes of growth and of decay, and it is
owing to these changes that we have the delicate tints of spring
foliage, and the more varied, intense, and gorgeous hues of autumn. But
these all belong to the class of intrinsic or normal colours, due to the
chemical constitution of the organism; as colours they are unadaptive,
and appear to have no more relation to the wellbeing of the plants
themselves than have the colours of gems and minerals. We may also
include in the same category those algae and fungi which have bright
colours--the "red snow" of the arctic regions, the red, green, or purple
seaweeds, the brilliant scarlet, yellow, white, or black agarics, and
other fungi. All these colours are probably the direct results of
chemical composition or molecular structure, and, being thus normal
products of the vegetable organism, need no special explanation from our
present point of view; and the same remark will apply to the varied
tints of the bark of trunks, branches, and twigs, which are often of
various shades of brown and green, or even vivid reds or yellows.
There are, however, a few cases in which the need of protection, which
we have found to be so important an agency in modifying the colours of
animals, has also determined those of some of the smaller members of the
vegetable kingdom. Dr. Burchell found a mesembryanthomum in South Africa
like a curiously shaped pebble, closely resembling the stones among
which it grew;[136] and Mr. J.P. Mansel Weale states that in the same
country one of the Asclepiadeae has tubers growing above ground among
stones which they exactly resemble, and that, when not in leaf, they are
for this reason quite invisible.[137] It is clear that such resemblances
must be highly useful to these plants, inhabiting an arid country
abounding in herbivorous mammalia, which, in times of drought or
scarcity, will devour everything in the shape of a fleshy stem or tuber.
True mimicry is very rare in plants, though adaptation to like
conditions often produces in foliage and habit a similarity that is
deceiving. Euphorbias growing in deserts often closely resemble cacti.
Seaside plants and high alpine plants of different orders are often much
alike; and innumerable resemblances of this kind are recorded in the
names of plants, as Veronica epacridea (the veronica like an epacris),
Limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the
resembling species in each case belonging to totally distinct families.
But in these cases, and in most others that have been observed, the
essential features of true mimicry are absent, inasmuch as the one plant
cannot be supposed to derive any benefit from its close resemblance to
the other, and this is still more certain from the fact that the two
species usually inhabit different localities. A few cases exist,
however, in which there does seem to be the necessary accordance and
utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the
only species of the genus Ajuga in South Africa, which is strikingly
like an orchid of the same country; while a balsam (Impatiens capensis),
also a solitary species of the genus in that country, is equally like an
orchid, growing in the same locality and visited by the same insects. As
both these genera of plants are specialised for insect fertilisation,
and both of the plants in question are isolated species of their
respective genera, we may suppose that, when they first reached South
Africa they were neglected by the insects of the country; but, being
both remotely like orchids in form of flower, those varieties that
approached nearest to the familiar species of the country were visited
by insects and cross-fertilised, and thus a closer resemblance would at
length be brought about. Another case of close general resemblance, is
that of our common white dead-nettle (Lamium album) to the
stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this
is a case of true mimicry, the dead-nettle being benefited by being
mistaken by grazing animals for the stinging-nettle.[138]
_Colours of Fruits._
It is when we come to the essential parts of plants on which their
perpetuation and distribution depends, that we find colour largely
utilised for a distinct purpose in flowers and fruits. In the former we
find attractive colours and guiding marks to secure cross-fertilisation
by insects; in the latter attractive or protective coloration, the first
to attract birds or other animals when the fruits are intended to be
eaten, the second to enable them to escape being eaten when it would be
injurious to the species. The colour phenomena of fruits being much the
most simple will be considered first.
The perpetuation and therefore the very existence of each species of
flowering plant depend upon its seeds being preserved from destruction
and more or less effectually dispersed over a considerable area. The
dispersal is effected either mechanically or by the agency of animals.
Mechanical dispersal is chiefly by means of air-currents, and large
numbers of seeds are specially adapted to be so carried, either by being
clothed with down or pappus, as in the well-known thistle and dandelion
seeds; by having wings or other appendages, as in the sycamore, birch,
and many other trees; by being thrown to a considerable distance by the
splitting of the seed-vessel, and by many other curious devices.[139]
Very large numbers of seeds, however, are so small and light that they
can be carried enormous distances by gales of wind, more especially as
most of this kind are flattened or curved, so as to expose a large
surface in proportion to their weight. Those which are carried by
animals have their surfaces, or that of the seed-vessel, armed with
minute hooks, or some prickly covering which attaches itself to the hair
of mammalia or the feathers of birds, as in the burdock, cleavers, and
many other species. Others again are sticky, as in Plumbago europaea,
mistletoe, and many foreign plants.
All the seeds or seed-vessels which are adapted to be dispersed in any
of these ways are of dull protective tints, so that when they fall on
the ground they are almost indistinguishable; besides which, they are
usually small, hard, and altogether unattractive, never having any
soft, juicy pulp; while the edible seeds often bear such a small
proportion to the hard, dry envelopes or appendages, that few animals
would care to eat them.
_The Meaning of Nuts._
There is, however, another class of fruits or seeds, usually termed
nuts, in which there is a large amount of edible matter, often very
agreeable to the taste, and especially attractive and nourishing to a
large number of animals. But when eaten, the seed is destroyed and the
existence of the species endangered. It is evident, therefore, that it
is by a kind of accident that these nuts are eatable; and that they are
not intended to be eaten is shown by the special care nature seems to
have taken to conceal or to protect them. We see that all our common
nuts are green when on the tree, so as not easily to be distinguished
from the leaves; but when ripe they turn brown, so that when they fall
on to the ground they are equally indistinguishable among the dead
leaves and twigs, or on the brown earth. Then they are almost always
protected by hard coverings, as in hazel-nuts, which are concealed by
the enlarged leafy involucre, and in the large tropical brazil-nuts and
cocoa-nuts by such a hard and tough case as to be safe from almost every
animal. Others have an external bitter rind, as in the walnut; while in
the chestnuts and beech-nuts two or three fruits are enclosed in a
prickly involucre.
Notwithstanding all these precautions, nuts are largely devoured by
mammalia and birds; but as they are chiefly the product of trees or
shrubs of considerable longevity, and are generally produced in great
profusion, the perpetuation of the species is not endangered. In some
cases the devourers of nuts may aid in their dispersal, as they probably
now and then swallow the seed whole, or not sufficiently crushed to
prevent germination; while squirrels have been observed to bury nuts,
many of which are forgotten and afterwards grow in places they could not
have otherwise reached.[140] Nuts, especially the larger kinds which are
so well protected by their hard, nearly globular cases, have their
dispersal facilitated by rolling down hill, and more especially by
floating in rivers and lakes, and thus reaching other localities. During
the elevation of land areas this method would be very effective, as the
new land would always be at a lower level than that already covered with
vegetation, and therefore in the best position for being stocked with
plants from it.
The other modes of dispersal of seeds are so clearly adapted to their
special wants, that we feel sure they must have been acquired by the
process of variation and natural selection. The hooked and sticky seeds
are always those of such herbaceous plants as are likely, from their
size, to come in contact with the wool of sheep or the hair of cattle;
while seeds of this kind never occur on forest trees, on aquatic plants,
or even on very dwarf creepers or trailers. The winged seed-vessels or
seeds, on the other hand, mostly belong to trees and to tall shrubs or
climbers. We have, therefore, a very exact adaptation to conditions in
these different modes of dispersal; while, when we come to consider
individual cases, we find innumerable other adaptations, some of which
the reader will find described in the little work by Sir John Lubbock
already referred to.
_Edible or Attractive Fruits._
It is, however, when we come to true fruits (in a popular sense) that we
find varied colours evidently intended to attract animals, in order that
the fruits may be eaten, while the seeds pass through the body
undigested and are then in the fittest state for germination. This end
has been gained in a great variety of ways, and with so many
corresponding adaptations as to leave no doubt as to the value of the
result. Fruits are pulpy or juicy, and usually sweet, and form the
favourite food of innumerable birds and some mammals. They are always
coloured so as to contrast with the foliage or surroundings, red being
the most common as it is certainly the most conspicuous colour, but
yellow, purple, black, or white being not uncommon. The edible portion
of fruits is developed from different parts of the floral envelopes, or
of the ovary, in the various orders and genera. Sometimes the calyx
becomes enlarged and fleshy, as in the apple and pear tribe; more often
the integuments of the ovary itself are enlarged, as in the plum, peach,
grape, etc.; the receptacle is enlarged and forms the fruit of the
strawberry; while the mulberry, pineapple, and fig are examples of
compound fruits formed in various ways from a dense mass of flowers.
In all cases the seeds themselves are protected from injury by various
devices. They are small and hard in the strawberry, raspberry, currant,
etc., and are readily swallowed among the copious pulp. In the grape
they are hard and bitter; in the rose (hip) disagreeably hairy; in the
orange tribe very bitter; and all these have a smooth, glutinous
exterior which facilitates their being swallowed. When the seeds are
larger and are eatable, they are enclosed in an excessively hard and
thick covering, as in the various kinds of "stone" fruit (plums,
peaches, etc.), or in a very tough core, as in the apple. In the nutmeg
of the Eastern Archipelago we have a curious adaptation to a single
group of birds. The fruit is yellow, somewhat like an oval peach, but
firm and hardly eatable. This splits open and shows the glossy black
covering of the seed or nutmeg, over which spreads the bright scarlet
arillus or "mace," an adventitious growth of no use to the plant except
to attract attention. Large fruit pigeons pluck out this seed and
swallow it entire for the sake of the mace, while the large nutmeg
passes through their bodies and germinates; and this has led to the wide
distribution of wild nutmegs over New Guinea and the surrounding
islands.
In the restriction of bright colour to those edible fruits the eating of
which is beneficial to the plant, we see the undoubted result of natural
selection; and this is the more evident when we find that the colour
never appears till the fruit is ripe--that is, till the seeds within it
are fully matured and in the best state for germination. Some
brilliantly coloured fruits are poisonous, as in our bitter-sweet
(Solanum dulcamara), cuckoo-pint (Arum) and the West Indian manchineel.
Many of these are, no doubt, eaten by animals to whom they are harmless;
and it has been suggested that even if some animals are poisoned by them
the plant is benefited, since it not only gets dispersed, but finds, in
the decaying body of its victim, a rich manure heap.[141] The particular
colours of fruits are not, so far as we know, of any use to them other
than as regards conspicuousness, hence a tendency to _any_ decided
colour has been preserved and accumulated as serving to render the fruit
easily visible among its surroundings of leaves or herbage. Out of 134
fruit-bearing plants in Mongredien's _Trees and Shrubs_, and Hooker's
_British Flora_, the fruits of no less than sixty-eight, or rather more
than half, are red, forty-five are black, fourteen yellow, and seven
white. The great prevalence of red fruits is almost certainly due to
their greater conspicuousness having favoured their dispersal, though it
may also have arisen in part from the chemical changes of chlorophyll
during ripening and decay producing red tints as in many fading leaves.
Yet the comparative scarcity of yellow in fruits, while it is the most
common tint of fading leaves, is against this supposition.
There are, however, a few instances of coloured fruits which do not seem
to be intended to be eaten; such are the colocynth plant (Cucumis
colocynthus), which has a beautiful fruit the size and colour of an
orange, but nauseous beyond description to the taste. It has a hard
rind, and may perhaps be dispersed by being blown along the ground, the
colour being an adventitious product; but it is quite possible,
notwithstanding its repulsiveness to us, that it may be eaten by some
animals. With regard to the fruit of another plant, Calotropis procera,
there is less doubt, as it is dry and full of thin, flat-winged seeds,
with fine silky filaments, eminently adapted for wind-dispersal; yet it
is of a bright yellow colour, as large as an apple, and therefore very
conspicuous. Here, therefore, we seem to have colour which is a mere
byproduct of the organism and of no use to it; but such cases are
exceedingly rare, and this rarity, when compared with the great
abundance of cases in which there is an obvious purpose in the colour,
adds weight to the evidence in favour of the theory of the attractive
coloration of edible fruits in order that birds and other animals may
assist in their dispersal. Both the above-named plants are natives of
Palestine and the adjacent arid countries.[142]
_The Colours of Flowers._
Flowers are much more varied in their colours than fruits, as they are
more complex and more varied in form and structure; yet there is some
parallelism between them in both respects. Flowers are frequently
adapted to attract insects as fruits are to attract birds, the object
being in the former to secure cross-fertilisation, in the latter
dispersal; while just as colour is an index of the edibility of fruits
which supply pulp or juice to birds, so are the colours of flowers an
indication of the presence of nectar or of pollen which are devoured by
insects.
The main facts and many of the details, as to the relation of insects to
flowers, were discovered by Sprengel in 1793. He noticed the curious
adaptation of the structure of many flowers to the particular insects
which visit them; he proved that insects do cross-fertilise flowers, and
he believed that this was the object of the adaptations, while the
presence of nectar and pollen ensured the continuance of their visits;
yet he missed discovering the _use_ of this cross-fertilisation. Several
writers at a later period obtained evidence that cross-fertilisation of
plants was a benefit to them; but the wide generality of this fact and
its intimate connection with the numerous and curious adaptations
discovered by Sprengel, was first shown by Mr. Darwin, and has since
been demonstrated by a vast mass of observations, foremost among which
are his own researches on orchids, primulas, and other plants.[143]
By an elaborate series of experiments carried on for many years Mr.
Darwin demonstrated the great value of cross-fertilisation in increasing
the rapidity of growth, the strength and vigour of the plant, and in
adding to its fertility. This effect is produced immediately, not as he
expected would be the case, after several generations of crosses. He
planted seeds from cross-fertilised and self-fertilised plants on two
sides of the same pot exposed to exactly similar conditions, and in most
cases the difference in size and vigour was amazing, while the plants
from cross-fertilised parents also produced more and finer seeds. These
experiments entirely confirmed the experience of breeders of animals
already referred to (p. 160), and led him to enunciate his famous
aphorism, "Nature abhors perpetual self-fertilisation".[144] In this
principle we appear to have a sufficient reason for the various
contrivances by which so many flowers secure cross-fertilisation, either
constantly or occasionally. These contrivances are so numerous, so
varied, and often so highly complex and extraordinary, that they have
formed the subject of many elaborate treatises, and have also been amply
popularised in lectures and handbooks. It will be unnecessary,
therefore, to give details here, but the main facts will be summarised
in order to call attention to some difficulties of the theory which seem
to require further elucidation.
_Modes of securing Cross-Fertilisation._
When we examine the various modes in which the cross-fertilisation of
flowers is brought about, we find that some are comparatively simple in
their operation and needful adjustments, others highly complex. The
simple methods belong to four principal classes:--(1) By dichogamy--that
is, by the anthers and the stigma becoming mature or in a fit state for
fertilisation at slightly different times on the same plant. The result
of this is that, as plants in different stations, on different soils, or
exposed to different aspects flower earlier or later, the mature pollen
of one plant can only fertilise some plant exposed to somewhat different
conditions or of different constitution, whose stigma will be mature at
the same time; and this difference has been shown by Darwin to be that
which is adapted to secure the fullest benefit of cross-fertilisation.
This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and
many others. (2) By the flower being self-sterile with its own pollen,
as in the crimson flax. This absolutely prevents self-fertilisation. (3)
By the stamens and anthers being so placed that the pollen cannot fall
upon the stigma, while it does fall upon a visiting insect which carries
it to the stigma of another flower. This effect is produced in a variety
of very simple ways, and is often aided by the motion of the stamens
which bend down out of the way of the stigmas before the pollen is ripe,
as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers
being on different plants, forming the class Dioecia of Linnaeus. In
these cases the pollen may be carried to the stigmas either by the wind
or by the agency of insects.
[Illustration: FIG. 28.
Malva sylvestris, adapted for insect-fertilisation.
Malva rotundifolia, adapted for self-fertilisation.]
Now these four methods are all apparently very simple, and easily
produced by variation and selection. They are applicable to flowers of
any shape, requiring only such size and colour as to attract insects,
and some secretion of nectar to ensure their repeated visits, characters
common to the great majority of flowers. All these methods are common,
except perhaps the second; but there are many flowers in which the
pollen from another plant is prepotent over the pollen from
fertilisation, the same flower, and this has nearly the same effect as
self-sterility if the flowers are frequently crossed by insects. We
cannot help asking, therefore, why have other and much more elaborate
methods been needed? And how have the more complex arrangements of so
many flowers been brought about? Before attempting to answer these
questions, and in order that the reader may appreciate the difficulty of
the problem and the nature of the facts to be explained, it will be
necessary to give a summary of the more elaborate modes of securing
cross-fertilisation.
(1) We first have dimorphism and heteromorphism, the phenomena of which
have been already sketched in our seventh chapter.
Here we have both a mechanical and a physiological modification, the
stamens and pistil being variously modified in length and position,
while the different stamens in the same flower have widely different
degrees of fertility when applied to the same stigma,--a phenomenon
which, if it were not so well established, would have appeared in the
highest degree improbable. The most remarkable case is that of the three
different forms of the loosestrife (Lythrum salicaria) here figured
(Fig. 29 on next page).
(2) Some flowers have irritable stamens which, when their bases are
touched by an insect, spring up and dust it with pollen. This occurs in
our common berberry.
[Illustration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]
(3) In others there are levers or processes by which the anthers are
mechanically brought down on to the head or back of an insect entering
the flower, in such a position as to be carried to the stigma of the
next flower it visits. This may be well seen in many species of Salvia
and Erica.
(4) In some there is a sticky secretion which, getting on to the
proboscis of an insect, carries away the pollen, and applies it to the
stigma of another flower. This occurs in our common milkwort (Polygala
vulgaris).
(5) In papilionaceous plants there are many complex adjustments, such as
the squeezing out of pollen from a receptacle on to an insect, as in
Lotus corniculatus, or the sudden springing out and exploding of the
anthers so as thoroughly to dust the insect, as in Medicago falcata,
this occurring after the stigma has touched the insect and taken off
some pollen from the last flower.
(6) Some flowers or spathes form closed boxes in which insects find
themselves entrapped, and when they have fertilised the flower, the
fringe of hairs opens and allows them to escape. This occurs in many
species of Arum and Aristolochia.
(7) Still more remarkable are the traps in the flower of Asclepias which
catch flies, butterflies, and wasps by the legs, and the wonderfully
complex arrangements of the orchids. One of these, our common Orchis
pyramidalis, may be briefly described to show how varied and beautiful
are the arrangements to secure cross-fertilisation. The broad trifid lip
of the flower offers a support to the moth which is attracted by its
sweet odour, and two ridges at the base guide the proboscis with
certainty to the narrow entrance of the nectary. When the proboscis has
reached the end of the spur, its basal portion depresses the little
hinged rostellum that covers the saddle-shaped sticky glands to which
the pollen masses (pollinia) are attached. On the proboscis being
withdrawn, the two pollinia stand erect and parallel, firmly attached to
the proboscis. In this position, however, they would be useless, as they
would miss the stigmatic surface of the next flower visited by the moth.
But as soon as the proboscis is withdrawn, the two pollen masses begin
to diverge till they are exactly as far apart as are the stigmas of the
flower; and then commences a second movement which brings them down
till they project straight forward nearly at right angles to their first
position, so as exactly to hit against the stigmatic surfaces of the
next flower visited on which they leave a portion of their pollen. The
whole of these motions take about half a minute, and in that time the
moth will usually have flown to another plant, and thus effect the most
beneficial kind of cross-fertilisation.[145] This description will be
better understood by referring to the illustration opposite, from
Darwin's _Fertilisation of Orchids_(Fig. 30).
[Illustration: FIG. 30.--Orchis pyramidalis.]
_The Interpretation of these Facts._
Having thus briefly indicated the general character of the more complex
adaptations for cross-fertilisation, the details of which are to be
found in any of the numerous works on the subject,[146] we find
ourselves confronted with the very puzzling question--Why were these
innumerable highly complex adaptations produced, when the very same
result may be effected--and often is effected--by extremely simple
means? Supposing, as we must do, that all flowers were once of simple
and regular forms, like a buttercup or a rose, how did such irregular
and often complicated flowers as the papilionaceous or pea family, the
labiates or sage family, and the infinitely varied and fantastic orchids
ever come into existence? No cause has yet been suggested but the need
of attracting insects to cross-fertilise them; yet the attractiveness of
regular flowers with bright colours and an ample supply of nectar is
equally great, and cross-fertilisation can be quite as effectively
secured in these by any of the four simple methods already described.
Before attempting to suggest a possible solution of this difficult
problem, we have yet to pass in review a large body of curious
adaptations connected with insect fertilisation, and will first call
attention to that portion of the phenomena which throw some light upon
the special colours of flowers in their relation to the various kinds of
insects which visit them. For these facts we are largely indebted to
the exact and long-continued researches of Professor Hermann Müller.
_Summary of Additional Facts bearing on Insect Fertilisation._
1. That the size and colour of a flower are important factors in
determining the visits of insects, is shown by the general fact of more
insects visiting conspicuous than inconspicuous flowers. As a single
instance, the handsome Geranium palustre was observed by Professor
Müller to be visited by sixteen different species of insects, the
equally showy G. pratense by thirteen species, while the smaller and
much less conspicuous G. molle was visited by eight species, and G.
pusillum by only one. In many cases, however, a flower may be very
attractive to only a few species of insects; and Professor Müller
states, as the result of many years' assiduous observation, that "a
species of flower is the more visited by insects the more conspicuous it
is."
2. Sweet odour is usually supplementary to the attraction of colour.
Thus it is rarely present in the largest and most gaudily coloured
flowers which inhabit open places, such as poppies, paeonies,
sunflowers, and many others; while it is often the accompaniment of
inconspicuous flowers, as the mignonette; of such as grow in shady
places, as the violet and primrose; and especially of white or yellowish
flowers, as the white jasmine, clematis, stephanotis, etc.
3. White flowers are often fertilised by moths, and very frequently give
out their scent only by night, as in our butterfly-orchis (Habenaria
chlorantha); and they sometimes open only at night, as do many of the
evening primroses and other flowers. These flowers are often long tubed
in accordance with the length of the moths' probosces, as in the genus
Pancratium, our butterfly orchis, white jasmine, and a host of others.
4. Bright red flowers are very attractive to butterflies, and are
sometimes specially adapted to be fertilised by them, as in many pinks
(Dianthus deltoides, D. superbus, D. atrorubens), the corn-cockle
(Lychnis Githago), and many others. Blue flowers are especially
attractive to bees and other hymenoptera (though they frequent flowers
of all colours), no less than sixty-seven species of this order having
been observed to visit the common "sheep's-bit" (Jasione montana). Dull
yellow or brownish flowers, some of which smell like carrion, are
attractive to flies, as the Arum and Aristolochia; while the dull
purplish flowers of the Scrophularia are specially attractive to wasps.
5. Some flowers have neither scent nor nectar, and yet attract insects
by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary
glistens as if moist, and flies alight on it and carry away pollen to
another flower; while in grass of parnassus (Parnassia palustris) there
are a number of small stalked yellow balls near the base of the flower,
which look like drops of honey but are really dry. In this case there is
a little nectar lower down, but the special attraction is a sham; and as
there are fresh broods of insects every year, it takes time for them to
learn by experience, and thus enough are always deceived to effect
cross-fertilisation.[147] This is analogous to the case of the young
birds, which have to learn by experience the insects that are inedible,
as explained at page 253.
6. Many flowers change their colour as soon as fertilised; and this is
beneficial, as it enables bees to avoid wasting time in visiting those
blossoms which have been already fertilised and their nectar exhausted.
The common lungwort (Pulmonaria officinalis), is at first red, but later
turns blue; and H. Müller observed bees visiting many red flowers in
succession, but neglecting the blue. In South Brazil there is a species
of Lantana, whose flowers are yellow the first day, orange the second,
and purple the third; and Dr. Fritz Müller observed that many
butterflies visited the yellow flowers only, some both the yellow and
the orange flowers, but none the purple.
7. Many flowers have markings which serve as guides to insects; in some
cases a bright central eye, as in the borage and forget-me-not; or lines
or spots converging to the centre, as in geraniums, pinks, and many
others. This enables insects to go quickly and directly to the opening
of the flower, and is equally important in aiding them to obtain a
better supply of food, and to fertilise a larger number of flowers.
8. Flowers have been specially adapted to the kinds of insects that
most abound where they grow. Thus the gentians of the lowlands are
adapted to bees, those of the high alps to butterflies only; and while
most species of Rhinanthus (a genus to which our common "yellow rattle"
belongs) are bee-flowers, one high alpine species (R. alpinus) has been
also adapted for fertilisation by butterflies only. The reason of this
is, that in the high alps butterflies are immensely more plentiful than
bees, and flowers adapted to be fertilised by bees can often have their
nectar extracted by butterflies without effecting cross-fertilisation.
It is, therefore, important to have a modification of structure which
shall make butterflies the fertilisers, and this in many cases has been
done.[148]
9. Economy of time is very important both to the insects and the
flowers, because the fine working days are comparatively few, and if no
time is wasted the bees will get more honey, and in doing so will
fertilise more flowers. Now, it has been ascertained by several
observers that many insects, bees especially, keep to one kind of flower
at a time, visiting hundreds of blossoms in succession, and passing over
other species that may be mixed with them. They thus acquire quickness
in going at once to the nectar, and the change of colour in the flower,
or incipient withering when fertilised, enables them to avoid those
flowers that have already had their honey exhausted. It is probably to
assist the insects in keeping to one flower at a time, which is of vital
importance to the perpetuation of the species, that the flowers which
bloom intermingled at the same season are usually very distinct both in
form and colour. In the sandy districts of Surrey, in the early spring,
the copses are gay with three flowers--the primrose, the wood-anemone,
and the lesser celandine, forming a beautiful contrast, while at the
same time the purple and the white dead-nettles abound on hedge banks. A
little later, in the same copses, we have the blue wild hyacinth (Scilla
nutans), the red campion (Lychnis dioica), the pure white great starwort
(Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon),
all distinct and well-contrasted flowers. In damp meadows in summer we
have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.
maculata), and the yellow rattle (Rhinanthus Crista-galli); while in
drier meadows we have cowslips, ox-eye daisies, and buttercups, all very
distinct both in form and colour. So in cornfields we have the scarlet
poppies, the purple corn-cockle, the yellow corn-marygold, and the blue
cornflower; while on our moors the purple heath and the dwarf gorse make
a gorgeous contrast. Thus the difference of colour which enables the
insect to visit with rapidity and unerring aim a number of flowers of
the same kind in succession, serves to adorn our meadows, banks, woods,
and heaths with a charming variety of floral colour and form at each
season of the year.[149]
_Fertilisation of Flowers by Birds._
In the temperate regions of the Northern Hemisphere, insects are the
chief agents in cross-fertilisation when this is not effected by the
wind; but in warmer regions, and in the Southern hemisphere, birds are
found to take a considerable part in the operation, and have in many
cases led to modifications in the form and colour of flowers. Each part
of the globe has special groups of birds which are flower-haunters.
America has the humming-birds (Trochilidae), and the smaller group of
the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
(Nectarineidae) take the place of the humming-birds, and another small
group, the flower-peckers (Dicaeidae), assist them. In the Australian
region there are also two flower-feeding groups, the Meliphagidae, or
honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent
researches by American naturalists have shown that many flowers are
fertilised by humming-birds, such as passion-flowers, trumpet-flowers,
fuchsias, and lobelias; while some, as the Salvia splendens of Mexico,
are specially adapted to their visits. We may thus perhaps explain the
number of very large tubular flowers in the tropics, such as the huge
brugmansias and bignonias; while in the Andes and in Chile, where
humming-birds are especially plentiful, we find great numbers of red
tubular flowers, often of large size and apparently adapted to these
little creatures. Such are the beautiful Lapageria and Philesia, the
grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium,
Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the
most extraordinary modifications of flower structure adapted to bird
fertilisation are the species of Marcgravia, in which the pedicels and
bracts of the terminal portion of a pendent bunch of flowers have been
modified into pitchers which secrete nectar and attract insects, while
birds feeding on the nectar, or insects, have the pollen of the
overhanging flowers dusted on their backs, and, carrying it to other
flowers, thus cross-fertilise them (see Illustration).
[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
nepenthoides.]
In Australia and New Zealand the fine "glory peas" (Clianthus), the
Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
of the New Zealand flax (Phormium tenax), are cross-fertilised by
birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
fertilised by Nectarineas.
The great extent to which insect and bird agency is necessary to flowers
is well shown by the case of New Zealand. The entire country is
comparatively poor in species of insects, especially in bees and
butterflies which are the chief flower fertilisers; yet according to the
researches of local botanists no less than one-fourth of all the
flowering plants are incapable of self-fertilisation, and, therefore,
wholly dependent on insect or bird agency for the continuance of the
species.
The facts as to the cross-fertilisation of flowers which have now been
very briefly summarised, taken in connection with Darwin's experiments
proving the increased vigour and fertility given by cross-fertilisation,
seem amply to justify his aphorism that "Nature abhors
self-fertilisation," and his more precise statement, that, "No plant is
perpetually self-fertilised;" and this view has been upheld by
Hildebrand, Delpino, and other botanists.[150]
_Self-Fertilisation of Flowers._
But all this time we have been only looking at one side of the question,
for there exists an abundance of facts which seem to imply, just as
surely, the utter uselessness of cross-fertilisation. Let us, then, see
what these facts are before proceeding further.
1. An immense variety of plants are habitually self-fertilised, and
their numbers probably far exceed those which are habitually
cross-fertilised by insects. Almost all the very small or obscure
flowered plants with hermaphrodite flowers are of this kind. Most of
these, however, may be insect fertilised occasionally, and may,
therefore, come under the rule that no species are perpetually
self-fertilised.
2. There are many plants, however, in which special arrangements exist
to secure self-fertilisation. Sometimes the corolla closes and brings
the anthers and stigma into contact; in others the anthers cluster round
the stigmas, both maturing together, as in many buttercups, stitchwort
(Stellaria media), sandwort (Spergula), and some willow-herbs
(Epilobium); or they arch over the pistil, as in Galium aparine and
Alisma Plantago. The style is also modified to bring it into contact
with the anthers, as in the dandelion, groundsel, and many other
plants.[151] All these, however, may be occasionally cross-fertilised.
3. In other cases precautions are taken to prevent cross-fertilisation,
as in the numerous cleistogamous or closed flowers. These occur in no
less than fifty-five different genera, belonging to twenty-four natural
orders, and in thirty-two of these genera the normal flowers are
irregular, and have therefore been specially modified for insect
fertilisation.[152] These flowers appear to be degradations of the
normal flowers, and are closed up by various modifications of the petals
or other parts, so that it is impossible for insects to reach the
interior, yet they produce seed in abundance, and are often the chief
means by which the species is continued. Thus, in our common dog-violet
the perfect flowers rarely produce seed, while the rudimentary
cleistogamic flowers do so in abundance. The sweet violet also produces
abundance of seed from its cleistogamic flowers, and few from its
perfect flowers; but in Liguria it produces only perfect flowers which
seed abundantly. No case appears to be known of a plant which has
cleistogamic flowers only, but a small rush (Juncus bufonius) is in this
condition in some parts of Russia, while in other parts perfect flowers
are also produced.[153] Our common henbit dead-nettle (Lamium
amplexicaule) produces cleistogamic flowers, as do also some orchids.
The advantage gained by the plant is great economy of specialised
material, since with very small flowers and very little expenditure of
pollen an abundance of seed is produced.
4. A considerable number of plants which have evidently been specially
modified for insect fertilisation have, by further modification, become
quite self-fertile. This is the case with the garden-pea, and also with
our beautiful bee-orchis, in which the pollen-masses constantly fall on
to the stigmas, and the flower, being thus self-fertilised, produces
abundance of capsules and of seed. Yet in many of its close allies
insect agency is absolutely required; but in one of these, the
fly-orchis, comparatively very little seed is produced, and
self-fertilisation would therefore be advantageous to it. When
garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed
to do them no good, as the seeds from these crosses produced less
vigorous plants than seed from those which were self-fertilised; a fact
directly opposed to what usually occurs in cross-fertilised plants.
5. As opposed to the theory that there is any absolute need for
cross-fertilisation, it has been urged by Mr. Henslow and others that
many self-fertilised plants are exceptionally vigorous, such as
groundsel, chickweed, sow-thistle, buttercups, and other common weeds;
while most plants of world-wide distribution are self-fertilised, and
these have proved themselves to be best fitted to survive in the battle
of life. More than fifty species of common British plants are very
widely distributed, and all are habitually self-fertilised.[154] That
self-fertilisation has some great advantage is shown by the fact that it
is usually the species which have the smallest and least conspicuous
flowers which have spread widely, while the large and showy flowered
species of the same genera or families, which require insects to
cross-fertilise them, have a much more limited distribution.
6. It is now believed by some botanists that many inconspicuous and
imperfect flowers, including those that are wind-fertilised, such as
plantains, nettles, sedges, and grasses, do not represent primitive or
undeveloped forms, but are degradations from more perfect flowers which
were once adapted to insect fertilisation. In almost every order we find
some plants which have become thus reduced or degraded for wind or
self-fertilisation, as Poterium and Sanguisorba among the Rosaceae;
while this has certainly been the case in the cleistogamic flowers. In
most of the above-mentioned plants there are distinct rudiments of
petals or other floral organs, and as the chief use of these is to
attract insects, they could hardly have existed in primitive
flowers.[155] We know, moreover, that when the petals cease to be
required for the attraction of insects, they rapidly diminish in size,
lose their bright colour or almost wholly disappear.[156]
_Difficulties and Contradictions._
The very bare summary that has now been given of the main facts relating
to the fertilisation of flowers, will have served to show the vast
extent and complexity of the inquiry, and the extraordinary
contradictions and difficulties which it presents. We have direct proof
of the beneficial results of intercrossing in a great number of cases;
we have an overwhelming mass of facts as to the varied and complex
structure of flowers evidently adapted to secure this intercrossing by
insect agency; yet we see many of the most vigorous plants which spread
widely over the globe, with none of these adaptations, and evidently
depending on self-fertilisation for their continued existence and
success in the battle of life. Yet more extraordinary is it to find
numerous cases in which the special arrangements for cross-fertilisation
appear to have been a failure, since they have either been supplemented
by special means for self-fertilisation, or have reverted back in
various degrees to simpler forms in which self-fertilisation becomes the
rule. There is also a further difficulty in the highly complex modes by
which cross-fertilisation is often brought about; for we have seen that
there are several very effective yet very simple modes of securing
intercrossing, involving a minimum of change in the form and structure
of the flower; and when we consider that the result attained with so
much cost of structural modification is by no means an unmixed good, and
is far less certain in securing the perpetuation of the species than is
self-fertilisation, it is most puzzling to find such complex methods
resorted to, sometimes to the extent of special precautions against the
possibility of self-fertilisation ever taking place. Let us now see
whether any light can be thrown on these various anomalies and
contradictions.
_Intercrossing not necessarily Advantageous._
No one was more fully impressed than Mr. Darwin with the beneficial
effects of intercrossing on the vigour and fertility of the species or
race, yet he clearly saw that it was not always and necessarily
advantageous. He says: "The most important conclusion at which I have
arrived is, that the mere act of intercrossing by itself does no good.
The good depends on the individuals which are crossed differing slightly
in constitution, owing to their progenitors having been subjected during
several generations to slightly different conditions. This conclusion,
as we shall hereafter see, is closely connected with various important
physiological problems, such as the benefit derived from slight changes
in the conditions of life."[157] Mr. Darwin has also adduced much direct
evidence proving that slight changes in the conditions of life are
beneficial to both animals and plants, maintaining or restoring their
vigour and fertility in the same way as a favourable cross seems to
restore it.[158] It is, I believe, by a careful consideration of these
two classes of facts that we shall find the clue to the labyrinth in
which this subject has appeared to involve us.
_Supposed Evil Results of Close Interbreeding._
Just as we have seen that intercrossing is not necessarily good, we
shall be forced to admit that close interbreeding is not necessarily
bad. Our finest breeds of domestic animals have been thus produced, and
by a careful statistical inquiry Mr. George Darwin has shown that the
most constant and long-continued intermarriages among the British
aristocracy have produced no prejudicial results. The rabbits on Porto
Santo are all the produce of a single female; they have lived on the
same small island for 470 years, and they still abound there and appear
to be vigorous and healthy (see p. 161).
We have, however, on the other hand, overwhelming evidence that in many
cases, among our domestic animals and cultivated plants, close
interbreeding does produce bad results, and the apparent contradiction
may perhaps be explained on the same general principles, and under
similar limitations, as were found to be necessary in defining the value
of intercrossing. It appears probable, then, that it is not
interbreeding in itself that is hurtful, but interbreeding without
rigid selection or some change of conditions. Under nature, as in the
case of the Porto Santo rabbits, the rapid increase of these animals
would in a very few years stock the island with a full population, and
thereafter natural selection would act powerfully in the preservation
only of the healthiest and the most fertile, and under these conditions
no deterioration would occur. Among the aristocracy there has been a
constant selection of beauty, which is generally synonymous with health,
while any constitutional infertility has led to the extinction of the
family. With domestic animals the selection practised is usually neither
severe enough nor of the right kind. There is no natural struggle for
existence, but certain points of form and colour characteristic of the
breed are considered essential, and thus the most vigorous or the most
fertile are not always those which are selected to continue the stock.
In nature, too, the species always extends over a larger area and
consists of much greater numbers, and thus a difference of constitution
soon arises in different parts of the area, which is wanting in the
limited numbers of pure bred domestic animals. From a consideration of
these varied facts we conclude that an occasional disturbance of the
organic equilibrium is what is essential to keep up the vigour and
fertility of any organism, and that this disturbance may be equally well
produced either by a cross between individuals of somewhat different
constitutions, or by occasional slight changes in the conditions of
life. Now plants which have great powers of dispersal enjoy a constant
change of conditions, and can, therefore, exist permanently, or at all
events, for very long periods, without intercrossing; while those which
have limited powers of dispersal, and are restricted to a comparatively
small and uniform area, need an occasional cross to keep up their
fertility and general vigour. We should, therefore, expect that those
groups of plants which are adapted both for cross-and
self-fertilisation, which have showy flowers and possess great powers of
seed-dispersal, would be the most abundant and most widely distributed;
and this we find to be the case, the Compositae possessing all these
characteristics in the highest degree, and being the most generally
abundant group of plants with conspicuous flowers in all parts of the
world.
_How the Struggle for Existence Acts among Flowers._
Let us now consider what will be the action of the struggle for
existence under the conditions we have seen to exist.
Everywhere and at all times some species of plants will be dominant and
aggressive; while others will be diminishing in numbers, reduced to
occupy a smaller area, and generally having a hard struggle to maintain
themselves. Whenever a self-fertilising plant is thus reduced in numbers
it will be in danger of extinction, because, being limited to a small
area, it will suffer from the effects of too uniform conditions which
will produce weakness and infertility. But while this change is in
progress, any crosses between individuals of slightly different
constitution will be beneficial, and all variations favouring either
insect agency on the one hand, or wind-dispersal of pollen on the other,
will lead to the production of a somewhat stronger and more fertile
stock. Increased size or greater brilliancy of the flower, more abundant
nectar, sweeter odour, or adaptations for more effectual
cross-fertilisation would all be preserved, and thus would be initiated
some form of specialisation for insect agency in cross-fertilisation;
and in every different species so circumstanced the result would be
different, depending as it would on many and complex combinations of
variation of parts of the flower, and of the insect species which most
abounded in the district.
Species thus favourably modified might begin a new era of development,
and, while spreading over a somewhat wider area, give rise to new
varieties or species, all adapted in various degrees and modes to secure
cross-fertilisation by insect agency. But in course of ages some change
of conditions might prove adverse. Either the insects required might
diminish in numbers or be attracted by other competing flowers, or a
change of climate might give the advantage to other more vigorous
plants. Then self-fertilisation with greater means of dispersal might be
more advantageous; the flowers might become smaller and more numerous;
the seeds smaller and lighter so as to be more easily dispersed by the
wind, while some of the special adaptations for insect fertilisation
being useless would, by the absence of selection and by the law of
economy of growth, be reduced to a rudimentary form. With these
modifications the species might extend its range into new districts,
thereby obtaining increased vigour by the change of conditions, as
appears to have been the case with so many of the small flowered
self-fertilised plants. Thus it might continue to exist for a long
series of ages, till under other changes--geographical or biological--it
might again suffer from competition or from other adverse circumstances,
and be at length again confined to a limited area, or reduced to very
scanty numbers.
But when this cycle of change had taken place, the species would be very
different from the original form. The flower would have been at one time
modified to favour the visits of insects and to secure
cross-fertilisation by their aid, and when the need for this passed
away, some portions of these structures would remain, though in a
reduced or rudimentary condition. But when insect agency became of
importance a second time, the new modifications would start from a
different or more advanced basis, and thus a more complex result might
be produced. Owing to the unequal rates at which the reduction of the
various parts might occur, some amount of irregularity in the flower
might arise, and on a second development towards insect
cross-fertilisation this irregularity, if useful, might be increased by
variation and selection.
The rapidity and comparative certainty with which such changes as are
here supposed do really take place, are well shown by the great
differences in floral structure, as regards the mode of fertilisation,
in allied genera and species, and even in some cases in varieties of the
same species. Thus in the Ranunculaceae we find the conspicuous part of
the flower to be the petals in Ranunculus, the sepals in Helleborus,
Anemone, etc., and the stamens in most species of Thalictrum. In all
these we have a simple regular flower, but in Aquilegia it is made
complex by the spurred petals, and in Delphinium and Aconitum it becomes
quite irregular. In the more simple class self-fertilisation occurs
freely, but it is prevented in the more complex flowers by the stamens
maturing before the pistil. In the Caprifoliaceae we have small and
regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
regular open flowers without honey, as in the elder (Sambucus); and
tubular flowers increasing in length and irregularity, till in some,
like our common honeysuckle, they are adapted for fertilisation by moths
only, with abundant honey and delicious perfume to attract them. In the
Scrophulariaceae we find open, almost regular flowers, as Veronica and
Verbascum, fertilised by flies and bees, but also self-fertilised;
Scrophularia adapted in form and colour to be fertilised by wasps; and
the more complex and irregular flowers of Linaria, Rhinanthus,
Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
In the genera Geranium, Polygonum, Veronica, and several others there is
a gradation of forms from large and bright to small and obscure coloured
flowers, and in every case the former are adapted for insect
fertilisation, often exclusively, while in the latter self-fertilisation
constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
are two forms (which have been named _major_ and _minor_), the larger
and more conspicuous adapted to insect fertilisation only, the smaller
capable of self-fertilisation; and two similar forms exist in the
eyebright (Euphrasia officinalis). In both these cases there are special
modifications in the length and curvature of the style as well as in the
size and shape of the corolla; and the two forms are evidently becoming
each adapted to special conditions, since in some districts the one, in
other districts the other is most abundant.[159]
These examples show us that the kind of change suggested above is
actually going on, and has presumably always been going on in nature
throughout the long geological epochs during which the development of
flowers has been progressing. The two great modes of gaining increased
vigour and fertility--intercrossing and dispersal over wider areas--have
been resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-changing
conditions. During all the modifications that ensued, useless parts were
reduced or suppressed, owing to the absence of selection and the
principle of economy of growth; and thus at each fresh adaptation some
rudiments of old structures were re-developed, but not unfrequently in
a different form and for a distinct purpose.
The chief types of flowering plants have existed during the millions of
ages of the whole tertiary period, and during this enormous lapse of
time many of them may have been modified in the direction of insect
fertilisation, and again into that of self-fertilisation, not once or
twice only, but perhaps scores or even hundreds of times; and at each
such modification a difference in the environment may have led to a
distinct line of development. At one epoch the highest specialisation of
structure in adaptation to a single species or group of insects may have
saved a plant from extinction; while, at other times, the simplest mode
of self-fertilisation, combined with greater powers of dispersal and a
constitution capable of supporting diverse physical conditions, may have
led to a similar result. With some groups the tendency seems to have
been almost continuously to greater and greater specialisation, while
with others a tendency to simplification and degradation has resulted in
such plants as the grasses and sedges.
We are now enabled dimly to perceive how the curious anomaly of very
simple and very complex methods of securing cross-fertilisation--both
equally effective--may have been brought about. The simple modes may be
the result of a comparatively direct modification from the more
primitive types of flowers, which were occasionally, and, as it were,
accidentally visited and fertilised by insects; while the more complex
modes, existing for the most part in the highly irregular flowers, may
result from those cases in which adaptation to insect-fertilisation, and
partial or complete degradation to self-fertilisation or to
wind-fertilisation, have again and again recurred, each time producing
some additional complexity, arising from the working up of old rudiments
for new purposes, till there have been reached the marvellous flower
structures of the papilionaceous tribes, of the asclepiads, or of the
orchids.
We thus see that the existing diversity of colour and of structure in
flowers is probably the ultimate result of the ever-recurring struggle
for existence, combined with the ever-changing relations between the
vegetable and animal kingdoms during countless ages. The constant
variability of every part and organ, with the enormous powers of
increase possessed by plants, have enabled them to become again and
again readjusted to each change of condition as it occurred, resulting
in that endless variety, that marvellous complexity, and that exquisite
colouring which excite our admiration in the realm of flowers, and
constitute them the perennial charm and crowning glory of nature.
_Flowers the Product of Insect Agency._
In his _Origin of Species_, Mr. Darwin first stated that flowers had
been rendered conspicuous and beautiful in order to attract insects,
adding: "Hence we may conclude that, if insects had not been developed
on the earth, our plants would not have been decked with beautiful
flowers, but would have produced only such poor flowers as we see on our
fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
all fertilised through the agency of the wind." The argument in favour
of this view is now much stronger than when he wrote; for not only have
we reason to believe that most of these wind-fertilised flowers are
degraded forms of flowers which have once been insect fertilised, but we
have abundant evidence that whenever insect agency becomes comparatively
ineffective, the colours of the flowers become less bright, their size
and beauty diminish, till they are reduced to such small, greenish,
inconspicuous flowers as those of the rupture-wort (Herniaria glabra),
the knotgrass (Polygonum aviculare), or the cleistogamic flowers of the
violet. There is good reason to believe, therefore, not only that
flowers have been developed in order to attract insects to aid in their
fertilisation, but that, having been once produced, in however great
profusion, if the insect races were all to become extinct, flowers (in
the temperate zones at all events) would soon dwindle away, and that
ultimately all floral beauty would vanish from the earth.
We cannot, therefore, deny the vast change which insects have produced
upon the earth's surface, and which has been thus forcibly and
beautifully delineated by Mr. Grant Allen: "While man has only tilled a
few level plains, a few great river valleys, a few peninsular mountain
slopes, leaving the vast mass of earth untouched by his hand, the insect
has spread himself over every land in a thousand shapes, and has made
the whole flowering creation subservient to his daily wants. His
buttercup, his dandelion, and his meadow-sweet grow thick in every
English field. His thyme clothes the hillside; his heather purples the
bleak gray moorland. High up among the alpine heights his gentian
spreads its lakes of blue; amid the snows of the Himalayas his
rhododendrons gleam with crimson light. Even the wayside pond yields him
the white crowfoot and the arrowhead, while the broad expanses of
Brazilian streams are beautified by his gorgeous water-lilies. The
insect has thus turned the whole surface of the earth into a boundless
flower-garden, which supplies him from year to year with pollen or
honey, and itself in turn gains perpetuation by the baits that it offers
for his allurement."[160]
_Concluding Remarks on Colour in Nature._
In the last four chapters I have endeavoured to give a general and
systematic, though necessarily condensed view of the part which is
played by colour in the organic world. We have seen in what infinitely
varied ways the need of concealment has led to the modification of
animal colours, whether among polar snows or sandy deserts, in tropical
forests or in the abysses of the ocean. We next find these general
adaptations giving way to more specialised types of coloration, by which
each species has become more and more harmonised with its immediate
surroundings, till we reach the most curiously minute resemblances to
natural objects in the leaf and stick insects, and those which are so
like flowers or moss or birds' droppings that they deceive the acutest
eye. We have learnt, further, that these varied forms of protective
colouring are far more numerous than has been usually suspected,
because, what appear to be very conspicuous colours or markings when the
species is observed in a museum or in a menagerie, are often highly
protective when the creature is seen under the natural conditions of its
existence. From these varied classes of facts it seems not improbable
that fully one-half of the species in the animal kingdom possess colours
which have been more or less adapted to secure for them concealment or
protection.
Passing onward we find the explanation of a distinct type of colour or
marking, often superimposed upon protective tints, in the importance of
easy recognition by many animals of their fellows, their parents, or
their mates. By this need we have been able to account for markings that
seem calculated to make the animal conspicuous, when the general tints
and well-known habits of the whole group demonstrate the need of
concealment. Thus also we are able to explain the constant symmetry in
the markings of wild animals, as well as the numerous cases in which the
conspicuous colours are concealed when at rest and only become visible
during rapid motion. In striking contrast to ordinary protective
coloration we have "warning colours," usually very conspicuous and often
brilliant or gaudy, which serve to indicate that their possessors are
either dangerous or uneatable to the usual enemies of their tribe. This
kind of coloration is probably more prevalent than has been hitherto
supposed, because in the case of many tropical animals we are quite
unacquainted with their special and most dangerous enemies, and are also
unable to determine whether they are or are not distasteful to those
enemies. As a kind of corollary to the "warning colours," we find the
extraordinary phenomena of "mimicry," in which defenceless species
obtain protection by being mistaken for those which, from any cause,
possess immunity from attack. Although a large number of instances of
warning colour and of mimicry are now recorded, it is probably still an
almost unworked field of research, more especially in tropical regions
and among the inhabitants of the ocean.
The phenomena of sexual diversities of coloration next engaged our
attention, and the reasons why Mr. Darwin's theory of "sexual
selection," as regards colour and ornament, could not be accepted were
stated at some length, together with the theory of animal coloration and
ornament we propose to substitute for it. This theory is held to be in
harmony with the general facts of animal coloration, while it entirely
dispenses with the very hypothetical and inadequate agency of female
choice in producing the detailed colours, patterns, and ornaments, which
in so many cases distinguish the male sex.
If my arguments on this point are sound, they will dispose also of Mr.
Grant Allen's view of the direct action of the colour sense on the
animal integuments.[161] He argues that the colours of insects and birds
reproduce generally the colours of the flowers they frequent or the
fruits they eat, and he adduces numerous cases in which flower-haunting
insects and fruit-eating birds are gaily coloured. This he supposes to
be due to the colour-taste, developed by the constant presence of bright
flowers and fruits, being applied to the selection of each variation
towards brilliancy in their mates; thus in time producing the gorgeous
and varied hues they now possess. Mr. Allen maintains that "insects are
bright where bright flowers exist in numbers, and dull where flowers are
rare or inconspicuous;" and he urges that "we can hardly explain this
wide coincidence otherwise than by supposing that a taste for colour is
produced through the constant search for food among entomophilous
blossoms, and that this taste has reacted upon its possessors through
the action of unconscious sexual selection."
The examples Mr. Allen quotes of bright insects being associated with
bright flowers seem very forcible, but are really deceptive or
erroneous; and quite as many cases could be quoted which prove the very
opposite. For example, in the dense equatorial forests flowers are
exceedingly scarce, and there is no comparison with the amount of floral
colour to be met with in our temperate meadows, woods, and hillsides.
The forests about Para in the lower Amazon are typical in this respect,
yet they abound with the most gorgeously coloured butterflies, almost
all of which frequent the forest depths, keeping near the ground, where
there is the greatest deficiency of brilliant flowers. In contrast with
this let us take the Cape of Good Hope--the most flowery region probably
that exists upon the globe,--where the country is a complete
flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite
iridaceous and other bulbs, and numerous flowering shrubs and trees; yet
the Cape butterflies are hardly equal, either in number or variety, to
those of any country in South Europe, and are utterly insignificant when
compared with those of the comparatively flowerless forest-depths of the
Amazon or of New Guinea. Neither is there any relation between the
colours of other insects and their haunts. Few are more gorgeous than
some of the tiger-beetles and the carabi, yet these are all carnivorous;
while many of the most brilliant metallic buprestidae and longicorns are
always found on the bark of fallen trees. So with the humming-birds;
their brilliant metallic tints can only be compared with metals or gems,
and are totally unlike the delicate pinks and purples, yellows and reds
of the majority of flowers. Again, the Australian honey-suckers
(Meliphagidae) are genuine flower-haunters, and the Australian flora is
more brilliant in colour display than that of most tropical regions, yet
these birds are, as a rule, of dull colours, not superior on the average
to our grain-eating finches. Then, again, we have the grand pheasant
family, including the gold and the silver pheasants, the gorgeous
fire-backed and ocellated pheasants, and the resplendent peacock, all
feeding on the ground on grain or seeds or insects, yet adorned with the
most gorgeous colours.
There is, therefore, no adequate basis of facts for this theory to rest
upon, even if there were the slightest reason to believe that not only
birds, but butterflies and beetles, take any delight in colour for its
own sake, apart from the food-supply of which it indicates the presence.
All that has been proved or that appears to be probable is, that they
are able to perceive differences of colour, and to associate each colour
with the particular flowers or fruits which best satisfy their wants.
Colour being in its nature diverse, it has been beneficial for them to
be able to distinguish all its chief varieties, as manifested more
particularly in the vegetable kingdom, and among the different species
of their own group; and the fact that certain species of insects show
some preference for a particular colour may be explained by their having
found flowers of that colour to yield them a more abundant supply of
nectar or of pollen. In those cases in which butterflies frequent
flowers of their own colour, the habit may well have been acquired from
the protection it affords them.
It appears to me that, in imputing to insects and birds the same love of
colour for its own sake and the same aesthetic tastes as we ourselves
possess, we may be as far from the truth as were those writers who held
that the bee was a good mathematician, and that the honeycomb was
constructed throughout to satisfy its refined mathematical instincts;
whereas it is now generally admitted to be the result of the simple
principle of economy of material applied to a primitive cylindrical
cell.[162]
In studying the phenomena of colour in the organic world we have been
led to realise the wonderful complexity of the adaptations which bring
each species into harmonious relation with all those which surround it,
and which thus link together the whole of nature in a network of
relations of marvellous intricacy. Yet all this is but, as it were, the
outward show and garment of nature, behind which lies the inner
structure--the framework, the vessels, the cells, the circulating
fluids, and the digestive and reproductive processes,--and behind these
again those mysterious chemical, electrical, and vital forces which
constitute what we term Life. These forces appear to be fundamentally
the same for all organisms, as is the material of which all are
constructed; and we thus find behind the outer diversities an inner
relationship which binds together the myriad forms of life.
Each species of animal or plant thus forms part of one harmonious whole,
carrying in all the details of its complex structure the record of the
long story of organic development; and it was with a truly inspired
insight that our great philosophical poet apostrophised the humble
weed--
Flower in the crannied wall,
I pluck you out of the crannies,
I hold you here, root and all, in my hand,
Little flower--but _if_ I could understand
What you are, root and all, and all in all,
I should know what God and man is.
FOOTNOTES:
[Footnote 136: Burchell's _Travels_, vol. i. p. 10.]
[Footnote 137: _Nature_, vol. iii. p. 507.]
[Footnote 138: _Flowers, Fruits, and Leaves_, p. 128 (Fig. 79).]
[Footnote 139: For a popular sketch of these, see Sir J. Lubbock's
_Flowers, Fruits, and Leaves_, or any general botanical work.]
[Footnote 140: _Nature_, vol. xv. p, 117.]
[Footnote 141: Grant Allen's _Colour Sense_, p. 113.]
[Footnote 142: Canon Tristram's _Natural History of the Bible_, pp. 483,
484.]
[Footnote 143: For a complete historical account of this subject with
full references to all the works upon it, see the Introduction to
Hermann Müller's _Fertilisation of Flowers_, translated by D'Arcy W.
Thompson.]
[Footnote 144: For the full detail of his experiments, see _Cross-and
Self-Fertilisation of Plants_, 1876.]
[Footnote 145: See Darwin's _Fertilisation of Orchids_ for the many
extraordinary and complex arrangements in these plants.]
[Footnote 146: The English reader may consult Sir John Lubbock's
_British Wild Flowers in Relation to Insects_, and H. Müller's great and
original work, _The Fertilisation of Flowers_.]
[Footnote 147: Müller's _Fertilisation of Flowers_, p. 248.]
[Footnote 148: "Alpenblumen," by D.H. Müller. See _Nature_, vol. xxiii.
p. 333.]
[Footnote 149: This peculiarity of local distribution of colour in
flowers may be compared, as regards its purpose, with the recognition
colours of animals. Just as these latter colours enable the sexes to
recognise each other, and thus avoid sterile unions of distinct species,
so the distinctive form and colour of each species of flower, as
compared with those that usually grow around it, enables the fertilising
insects to avoid carrying the pollen of one flower to the stigma of a
distinct species.]
[Footnote 150: See H. Müller's _Fertilisation of Flowers_, p. 18.]
[Footnote 151: The above examples are taken from Rev. G. Henslow's paper
on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,
_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown
that the same thing occurs among tropical orchids, in his paper "On the
Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
_Journ. Linn. Soc._, xxi. p. 538.]
[Footnote 152: These are the numbers given by Darwin, but I am informed
by Mr. Hemsley that many additions have been since made to the list, and
that cleistogamic flowers probably occur in nearly all the natural
orders.]
[Footnote 153: For a full account of cleistogamic flowers, see Darwin's
_Forms of Flowers_, chap. viii.]
[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._
Second series, _Botany_, vol. i. p. 391.]
[Footnote 155: The Rev. George Henslow, in his _Origin of Floral
Structures_, says: "There is little doubt but that all wind-fertilised
angiosperms are degradations from insect-fertilised flowers....
_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_
presumably was so formerly, but has reacquired an entomophilous habit;
the whole tribe Poterieae being, in fact, a degraded group which has
descended from Potentilleae. Plantains retain their corolla but in a
degraded form. Junceae are degraded Lilies; while Cyperaceae and
Gramineae among monocotyledons may be ranked with Amentiferae among
dicotyledons, as representing orders which have retrograded very far
from the entomophilous forms from which they were possibly and probably
descended" (p. 266).
"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well
illustrate the change from an entomophilous to the anemophilous state.
_P. lanceolata_ has polymorphic flowers, and is visited by
pollen-seeking insects, so that it can be fertilised either by insects
or the wind. _P. media_ illustrates transitions in point of structure,
as the filaments are pink, the anthers motionless, and the pollen grains
aggregated, and it is regularly visited by _Bombus terrestris_. On the
other hand, the slender filaments, versatile anthers, powdery pollen,
and elongated protogynous style are features of other species indicating
anemophily; while the presence of a degraded corolla shows its ancestors
to have been entomophilous. _P. media_, therefore, illustrates, not a
primitive entomophilous condition, but a return to it; just as is the
case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show
no capacity of restoring the corolla, the attractive features having to
be borne by the calyx, which is purplish in Sanguisorba, by the pink
filaments of Plantago, and by the yellow anthers in the Sallow willow"
(p. 271).
"The interpretation, then, I would offer of inconspicuousness and all
kinds of degradations is the exact opposite to that of conspicuousness
and great differentiations; namely, that species with minute flowers,
rarely or never visited by insects, and habitually self-fertilised, have
primarily arisen through the neglect of insects, and have in consequence
assumed their present floral structures" (p. 282).
In a letter just received from Mr. Henslow, he gives a few additional
illustrations of his views, of which the following are the most
important: "Passing to Incompletae, the orders known collectively as
'Cyclospermeae' are related to Caryophylleae; and to my mind are
degradations from it, of which Orache is anemophilous. Cupuliferae have
an inferior ovary and rudimentary calyx-limb on the top. These, as far
as I know, cannot be interpreted except as degradations. The whole of
Monocotyledons appear to me (from anatomical reasons especially) to be
degradations from Dicotyledons, and primarily through the agency of
growth in water. Many subsequently became terrestrial, but retained the
effects of their primitive habitat through heredity. The 3-merous [sic]
perianth of grasses, the parts of the flower being in whorls, point to a
degradation from a sub-liliaceous condition."
Mr. Henslow informs me that he has long held these views, but, as far as
he knows, alone. Mr. Grant Allen, however, set forth a similar theory in
his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of
Flowers_ (chap. v.), where he develops it fully and uses similar
arguments to those of Mr. Henslow.]
[Footnote 156: H. Müller gives ample proof of this in his _Fertilisation
of Flowers_.]
[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]
[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]
[Footnote 159: Müller's _Fertilisation of Flowers_, pp. 448, 455. Other
cases of recent degradation and readaptation to insect-fertilisation are
given by Professor Henslow (see footnote, p. 324).]
[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
[Footnote 161: _The Colour Sense_, chap. ix.]
[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]
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.
The theory which we may now take as established--that all the existing
forms of life have been derived from other forms by a natural process of
descent with modification, and that this same process has been in action
during past geological time--should enable us to give a rational account
not only of the peculiarities of form and structure presented by animals
and plants, but also of their grouping together in certain areas, and
their general distribution over the earth's surface.
In the absence of any exact knowledge of the facts of distribution, a
student of the theory of evolution might naturally anticipate that all
groups of allied organisms would be found in the same region, and that,
as he travelled farther and farther from any given centre, the forms of
life would differ more and more from those which prevailed at the
starting-point, till, in the remotest regions to which he could
penetrate, he would find an entirely new assemblage of animals and
plants, altogether unlike those with which he was familiar. He would
also anticipate that diversities of climate would always be associated
with a corresponding diversity in the forms of life.
Now these anticipations are to a considerable extent justified.
Remoteness on the earth's surface is usually an indication of diversity
in the fauna and flora, while strongly contrasted climates are always
accompanied by a considerable contrast in the forms of life. But this
correspondence is by no means exact or proportionate, and the converse
propositions are often quite untrue. Countries which are near to each
other often differ radically in their animal and vegetable productions;
while similarity of climate, together with moderate geographical
proximity, are often accompanied by marked diversities in the prevailing
forms of life. Again, while many groups of animals--genera, families,
and sometimes even orders--are confined to limited regions, most of the
families, many genera, and even some species are found in every part of
the earth. An enumeration of a few of these anomalies will better
illustrate the nature of the problem we have to solve.
As examples of extreme diversity, notwithstanding geographical
proximity, we may adduce Madagascar and Africa, whose animal and
vegetable productions are far less alike than are those of Great Britain
and Japan at the remotest extremities of the great northern continent;
while an equal, or perhaps even a still greater, diversity exists
between Australia and New Zealand. On the other hand, Northern Africa
and South Europe, though separated by the Mediterranean Sea, have faunas
and floras which do not differ from each other more than do the various
countries of Europe. As a proof that similarity of climate and general
adaptability have had but a small part in determining the forms of life
in each country, we have the fact of the enormous increase of rabbits
and pigs in Australia and New Zealand, of horses and cattle in South
America, and of the common sparrow in North America, though in none of
these cases are the animals natives of the countries in which they
thrive so well. And lastly, in illustration of the fact that allied
forms are not always found in adjacent regions, we have the tapirs,
which are found only on opposite sides of the globe, in tropical America
and the Malayan Islands; the camels of the Asiatic deserts, whose
nearest allies are the llamas and alpacas of the Andes; and the
marsupials, only found in Australia and on the opposite side of the
globe, in America. Yet, again, although mammalia may be said to be
universally distributed over the globe, being found abundantly on all
the continents and on a great many of the larger islands, yet they are
entirely wanting in New Zealand, and in a considerable number of other
islands which are, nevertheless, perfectly able to support them when
introduced.
Now most of these difficulties can be solved by means of well-known
geographical and geological facts. When the productions of remote
countries resemble each other, there is almost always continuity of land
with similarity of climate between them. When adjacent countries differ
greatly in their productions, we find them separated by a sea or strait
whose great depth is an indication of its antiquity or permanence. When
a group of animals inhabits two countries or regions separated by wide
oceans, it is found that in past geological times the same group was
much more widely distributed, and may have reached the countries it
inhabits from an intermediate region in which it is now extinct. We
know, also, that countries now united by land were divided by arms of
the sea at a not very remote epoch; while there is good reason to
believe that others now entirely isolated by a broad expanse of sea were
formerly united and formed a single land area. There is also another
important factor to be taken account of in considering how animals and
plants have acquired their present peculiarities of
distribution,--changes of climate. We know that quite recently a glacial
epoch extended over much of what are now the temperate regions of the
northern hemisphere, and that consequently the organisms which inhabit
those parts must be, comparatively speaking, recent immigrants from more
southern lands. But it is a yet more important fact that, down to middle
Tertiary times at all events, an equable temperate climate, with a
luxuriant vegetation, extended to far within the arctic circle, over
what are now barren wastes, covered for ten months of the year with snow
and ice. The arctic zone has, therefore, been in past times capable of
supporting almost all the forms of life of our temperate regions; and we
must take account of this condition of things whenever we have to
speculate on the possible migrations of organisms between the old and
new continents.
_The Conditions which have determined Distribution._
When we endeavour to explain in detail the facts of the existing
distribution of organic beings, we are confronted by several preliminary
questions, upon the solution of which will depend our treatment of the
phenomena presented to us. Upon the theory of descent which we have
adopted, all the different species of a genus, as well as all the genera
which compose a family or higher group, have descended from some common
ancestor, and must therefore, at some remote epoch, have occupied the
same area, from which their descendants have spread to the regions they
now inhabit. In the numerous cases in which the same group now occupies
countries separated by oceans or seas, by lofty mountain-chains, by wide
deserts, or by inhospitable climates, we have to consider how the
migration which must certainly have taken place has been effected. It is
possible that during some portion of the time which has elapsed since
the origin of the group the interposing barriers have not been in
existence; or, on the other hand, the particular organisms we are
dealing with may have the power of overpassing the barriers, and thus
reaching their present remote dwelling-places. As this is really the
fundamental question of distribution on which the solution of all its
more difficult problems depends, we have to inquire, in the first place,
what is the nature of, and what are the limits to, the changes of the
earth's surface, especially during the Tertiary and latter part of the
Secondary periods, as it was during those periods that most of the
existing types of the higher animals and plants came into existence;
and, in the next place, what are the extreme limits of the powers of
dispersal possessed by the chief groups of animals and plants. We will
first consider the question of barriers, more especially those formed by
seas and oceans.
_The Permanence of Oceans._
It was formerly a very general belief, even amongst geologists, that the
great features of the earth's surface, no less than the smaller ones,
were subject to continual mutations, and that during the course of
known geological time the continents and great oceans had again and
again changed places with each other. Sir Charles Lyell, in the last
edition of his _Principles of Geology_ (1872), said: "Continents,
therefore, although permanent for whole geological epochs, shift their
positions entirely in the course of ages;" and this may be said to have
been the orthodox opinion down to the very recent period when, by means
of deep-sea soundings, the nature of the ocean bottom was made known.
The first person to throw doubt on this view appears to have been the
veteran American geologist, Professor Dana. In 1849, in the Report of
Wilke's Exploring Expedition, he adduced the argument against a former
continent in the Pacific during the Tertiary period, from the absence of
all native quadrupeds. In 1856, in articles in the _American Journal_,
he discussed the development of the American continent, and argued for
its general permanence; and in his _Manual of Geology_ in 1863 and later
editions, the same views were more fully enforced and were latterly
applied to all continents. Darwin, in his _Journal of Researches_,
published in 1845, called attention to the fact that all the small
islands far from land in the Pacific, Indian, and Atlantic Oceans are
either of coralline or volcanic formation. He excepted, however, the
Seychelles and St. Paul's rocks; but the former have since been shown to
be no exception, as they consist entirely of coral rock; and although
Darwin himself spent a few hours on St. Paul's rocks on his outward
voyage in the _Beagle_, and believed he had found some portions of them
to be of a "cherty," and others of a "felspathic" nature, this also has
been shown to be erroneous, and the careful examination of the rocks by
the Abbé Renard clearly proves them to be wholly of volcanic
origin.[163] We have, therefore, at the present time, absolutely no
exception whatever to the remarkable fact that all the oceanic islands
of the globe are either of volcanic or coral formation; and there is,
further, good reason to believe that those of the latter class in every
case rest upon a volcanic foundation.
In his _Origin of Species_, Darwin further showed that no true oceanic
island had any native mammals or batrachia when first discovered, this
fact constituting the test of the class to which an island belongs;
whence he argued that none of them had ever been connected with
continents, but all had originated in mid-ocean. These considerations
alone render it almost certain that the areas now occupied by the great
oceans have never, during known geological time, been occupied by
continents, since it is in the highest degree improbable that every
fragment of those continents should have completely disappeared, and
have been replaced by volcanic islands rising out of profound oceanic
abysses; but recent research into the depth of the oceans and the nature
of the deposits now forming on their floors, adds greatly to the
evidence in this direction, and renders it almost a certainty that they
represent very ancient if not primaeval features of the earth's surface.
A very brief outline of the nature of this evidence will be now given.
The researches of the _Challenger_ expedition into the nature of the
sea-bottom show, that the whole of the land debris brought down by
rivers to the ocean (with the exception of pumice and other floating
matter), is deposited comparatively near to the shores, and that the
fineness of the material is an indication of the distance to which it
has been carried. Everything in the nature of gravel and sand is laid
down within a very few miles of land, only the finer muddy sediments
being carried out for 20 or 50 miles, and the very finest of all, under
the most favourable conditions, rarely extending beyond 150, or at the
utmost, 300 miles from land into the deep ocean.[164] Beyond these
distances, and covering the entire ocean floor, are various oozes formed
wholly from the debris of marine organisms; while intermingled with
these are found various volcanic products which have been either carried
through the air or floated on the surface, and a small but perfectly
recognisable quantity of meteoric matter. Ice-borne rocks are also found
abundantly scattered over the ocean bottom within a definite distance of
the arctic and antarctic circles, clearly marking out the limit of
floating icebergs in recent geological times.
Now the whole series of marine stratified rocks, from the earliest
Palaeozoic to the most recent Tertiary beds, consist of materials
closely corresponding to the land debris now being deposited within a
narrow belt round the shores of all continents; while no rocks have been
found which can be identified with the various oozes now forming in the
deep abysses of the ocean. It follows, therefore, that all the
geological formations have been formed in comparatively shallow water,
and always adjacent to the continental land of the period. The great
thickness of some of the formations is no indication of a deep sea, but
only of slow subsidence during the time that the deposition was in
progress. This view is now adopted by many of the most experienced
geologists, especially by Dr. Archibald Geikie, Director of the
Geological Survey of Great Britain, who, in his lecture on "Geographical
Evolution," says: "From all this evidence we may legitimately conclude
that the present land of the globe, though consisting in great measure
of marine formations, has never lain under the deep sea; but that its
site must always have been near land. Even its thick marine limestones
are the deposits of comparatively shallow water."[165]
But besides these geological and physical considerations, there is a
mechanical difficulty in the way of repeated change of position of
oceans and continents which has not yet received the attention it
deserves. According to the recent careful estimate by Mr. John Murray,
the land area of the globe is to the water area as ·28 to ·72. The mean
height of the land above sea-level is 2250 feet, while the mean depth of
the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic
miles, and that of the waters of the ocean 323,800,000 cubic miles; and
it follows that if the whole of the solid matter of the earth's surface
were reduced to one level, it would be everywhere covered by an ocean
about two miles deep. The accompanying diagram will serve to render
these figures more intelligible. The length of the sections of land and
ocean are in the proportion of their respective areas, while the mean
height of the land and the mean depth of the ocean are exhibited on a
greatly increased vertical scale. If we considered the continents and
their adjacent oceans separately they would differ a little, but not
very materially, from this diagram; in some cases the proportion of land
to ocean would be a little greater, in others a little less.
[Illustration: FIG. 32.]
Now, if we try to imagine a process of elevation and depression by which
the sea and land shall completely change places, we shall be met by
insuperable difficulties. We must, in the first place, assume a general
equality between elevation and subsidence during any given period,
because if the elevation over any extensive continental area were not
balanced by some subsidence of approximately equal amount, an
unsupported hollow would be left under the earth's crust. Let us now
suppose a continental area to sink, and an adjacent oceanic area to
rise, it will be seen that the greater part of the land will disappear
long before the new land has approached the surface of the ocean. This
difficulty will not be removed by supposing a portion of a continent to
subside, and the immediately adjacent portion of the ocean on the other
side of the continent to rise, because in almost every case we find that
within a comparatively short distance from the shores of all existing
continents, the ocean floor sinks rapidly to a depth of from 2000 to
3000 fathoms, and maintains a similar depth, generally speaking, over a
large portion of the oceanic areas. In order, therefore, that any area
of continental extent be upraised from the great oceans, there must be a
subsidence of a land area five or six times as great, unless it can be
shown that an extensive elevation of the ocean floor up to and far
above the surface could occur without an equivalent depression
elsewhere. The fact that the waters of the ocean are sufficient to cover
the whole globe to a depth of two miles, is alone sufficient to indicate
that the great ocean basins are permanent features of the earth's
surface, since any process of alternation of these with the land areas
would have been almost certain to result again and again in the total
disappearance of large portions, if not of all, of the dry land of the
globe. But the continuity of terrestrial life since the Devonian and
Carboniferous periods, and the existence of very similar forms in the
corresponding deposits of every continent--as well as the occurrence of
sedimentary rocks, indicating the proximity of land at the time of their
deposit, over a large portion of the surface of all the continents, and
in every geological period--assure us that no such disappearance has
ever occurred.
_Oceanic and Continental Areas._
When we speak of the permanence of oceanic and continental areas as one
of the established facts of modern research, we do not mean that
existing continents and oceans have always maintained the exact areas
and outlines that they now present, but merely, that while all of them
have been undergoing changes in outline and extent from age to age, they
have yet maintained substantially the same positions, and have never
actually changed places with each other. There are, moreover, certain
physical and biological facts which enable us to mark out these areas
with some confidence.
We have seen that there are a large number of islands which may be
classed as oceanic, because they have never formed parts of continents,
but have originated in mid-ocean, and have derived their forms of life
by migration across the sea. Their peculiarities are seen to be very
marked in comparison with those islands which there is good reason to
believe are really fragments of more extensive land areas, and are hence
termed "continental." These continental islands consist in every case of
a variety of stratified rocks of various ages, thus corresponding
closely with the usual structure of continents; although many of the
islands are small like Jersey or the Shetland Islands, or far from
continental land like the Falkland Islands or New Zealand. They all
contain indigenous mammalia or batrachia, and generally a much greater
variety of birds, reptiles, insects, and plants, than do the oceanic
islands. From these various characteristics we conclude that they have
all once formed parts of continents, or at all events of much larger
land areas, and have become isolated, either by subsidence of the
intervening land or by the effects of long-continued marine denudation.
Now, if we trace the thousand-fathom line around all our existing
continents we find that, with only two exceptions, every island which
can be classed as "continental" falls within this line, while all that
lie beyond it have the undoubted characteristics of "oceanic" islands.
We, therefore, conclude that the thousand-fathom line marks out,
approximately, the "continental area,"--that is, the limits within which
continental development and change throughout known geological time have
gone on. There may, of course, have been some extensions of land beyond
this limit, while some areas within it may always have been ocean; but
so far as we have any direct evidence, this line may be taken to mark
out, approximately, the most probable boundary between the "continental
area," which has always consisted of land and shallow sea in varying
proportions, and the great oceanic basins, within the limits of which
volcanic activity has been building up numerous islands, but whose
profound depths have apparently undergone little change.
_Madagascar and New Zealand._
The two exceptions just referred to are Madagascar and New Zealand, and
all the evidence goes to show that in these cases the land connection
with the nearest continental area was very remote in time. The
extraordinary isolation of the productions of Madagascar--almost all the
most characteristic forms of mammalia, birds, and reptiles of Africa
being absent from it--renders it certain that it must have been
separated from that continent very early in the Tertiary, if not as far
back as the latter part of the Secondary period; and this extreme
antiquity is indicated by a depth of considerably more than a thousand
fathoms in the Mozambique Channel, though this deep portion is less than
a hundred miles wide between the Comoro Islands and the mainland.[166]
Madagascar is the only island on the globe with a fairly rich mammalian
fauna which is separated from a continent by a depth greater than a
thousand fathoms; and no other island presents so many peculiarities in
these animals, or has preserved so many lowly organised and archaic
forms. The exceptional character of its productions agrees exactly with
its exceptional isolation by means of a very deep arm of the sea.
New Zealand possesses no known mammals and only a single species of
batrachian; but its geological structure is perfectly continental. There
is also much evidence that it does possess one mammal, although no
specimens have been yet obtained.[167] Its reptiles and birds are highly
peculiar and more numerous than in any truly oceanic island. Now the sea
which directly separates New Zealand from Australia is more than 2000
fathoms deep, but in a north-west direction there is an extensive bank
under 1000 fathoms, extending to and including Lord Howe's Island, while
north of this are other banks of the same depth, approaching towards a
submarine extension of Queensland on the one hand, and New Caledonia on
the other, and altogether suggestive of a land union with Australia at
some very remote period. Now the peculiar relations of the New Zealand
fauna and flora with those of Australia and of the tropical Pacific
Islands to the northward indicate such a connection, probably during the
Cretaceous period; and here, again, we have the exceptional depth of the
dividing sea and the form of the ocean bottom according well with the
altogether exceptional isolation of New Zealand, an isolation which has
been held by some naturalists to be great enough to justify its claim to
be one of the primary Zoological Regions.
_The Teachings of the Thousand-Fathom Line._
If now we accept the annexed map as showing us approximately how far
beyond their present limits our continents may have extended during any
portion of the Tertiary and Secondary periods, we shall obtain a
foundation of inestimable value for our inquiries into those migrations
of animals and plants during past ages which have resulted in their
present peculiarities of distribution. We see, for instance, that the
South American and African continents have always been separated by
nearly as wide an ocean as at present, and that whatever similarities
there may be in their productions must be due to the similar forms
having been derived from a common origin in one of the great northern
continents. The radical difference between the higher forms of life of
the two continents accords perfectly with their permanent separation. If
there had been any direct connection between them during Tertiary times,
we should hardly have found the deep-seated differences between the
Quadrumana of the two regions--no family even being common to both; nor
the peculiar Insectivora of the one continent, and the equally peculiar
Edentata of the other. The very numerous families of birds quite
peculiar to one or other of these continents, many of which, by their
structural isolation and varied development of generic and specific
forms, indicate a high antiquity, equally suggest that there has been no
near approach to a land connection during the same epoch.
Looking to the two great northern continents, we see indications of a
possible connection between them both in the North Atlantic and the
North Pacific oceans; and when we remember that from middle Tertiary
times backward--so far as we know continuously to the earliest
Palaeozoic epoch--a temperate and equable climate, with abundant woody
vegetation, prevailed up to and within the arctic circle, we see what
facilities may have been afforded for migration from one continent to
the other, sometimes between America and Europe, sometimes between
America and Asia. Admitting these highly probable connections, no
bridging of the Atlantic in more southern latitudes (of which there is
not a particle of evidence) will have been necessary to account for all
the intermigration that has occurred between the two continents. If, on
the other hand, we remember how long must have been the route, and how
diverse must always have been the conditions between the more northern
and the more southern portions of the American and Euro-Asiatic
continents, we shall not be surprised that many widespread forms in
either continent have not crossed into the other; and that while the
skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys
(Meleagris) are confined to America, the pigs and the hedgehogs, the
true flycatchers and the pheasants are found only in the Euro-Asiatic
continent. But, just as there have been periods which facilitated
intermigration between America and the Old World, there have almost
certainly been periods, perhaps of long duration even geologically, when
these continents have been separated by seas as wide as, or even wider
than, those of the present day; and thus may be explained such curious
anomalies as the origination of the camel-tribe in America, and its
entrance into Asia in comparatively recent Tertiary times, while the
introduction of oxen and bears into America from the Euro-Asiatic
continent appears to have been equally recent.[168]
We shall find on examination that this view of the general permanence of
the oceanic and continental areas, with constant minor fluctuations of
land and sea over the whole extent of the latter, enables us to
understand, and offer a rational explanation of, most of the difficult
problems of geographical distribution; and further, that our power of
doing this is in direct proportion to our acquaintance with the
distribution of fossil forms of life during the Tertiary period. We
must, also, take due note of many other facts of almost equal importance
for a due appreciation of the problems presented for solution, the most
essential being, the various powers of dispersal possessed by the
different groups of animals and plants, the geological antiquity of the
species and genera, and the width and depth of the seas which separate
the countries they, inhabit. A few illustrations will now be given of
the way in which these branches of knowledge enable us to deal with the
difficulties and anomalies that present themselves.
_The Distribution of Marsupials._
This singular and lowly organised type of mammals constitutes almost the
sole representative of the class in Australia and New Guinea, while it
is entirely unknown in Asia, Africa, or Europe. It reappears in America,
where several species of opossums are found; and it was long thought
necessary to postulate a direct southern connection of these distant
countries, in order to account for this curious fact of distribution.
When, however, we look to what is known of the geological history of the
marsupials the difficulty vanishes. In the Upper Eocene deposits of
Western Europe the remains of several animals closely allied to the
American opossums have been found; and as, at this period, a very mild
climate prevailed far up into the arctic regions, there is no difficulty
in supposing that the ancestors of the group entered America from Europe
or Northern Asia during early Tertiary times.
But we must go much further back for the origin of the Australian
marsupials. All the chief types of the higher mammalia were in existence
in the Eocene, if not in the preceding Cretaceous period, and as we find
none of these in Australia, that country must have been finally
separated from the Asiatic continent during the Secondary or Mesozoic
period. Now during that period, in the Upper and the Lower Oolite and in
the still older Trias, the jaw-bones of numerous small mammalia have
been found, forming eight distinct genera, which are believed to have
been either marsupials or some allied lowly forms. In North America
also, in beds of the Jurassic and Triassic formations, the remains of an
equally great variety of these small mammalia have been discovered; and
from the examination of more than sixty specimens, belonging to at least
six distinct genera, Professor Marsh is of opinion that they represent a
generalised type, from which the more specialised marsupials and
insectivora were developed.
From the fact that very similar mammals occur both in Europe and America
at corresponding periods, and in beds which represent a long succession
of geological time, and that during the whole of this time no fragments
of any higher forms have been discovered, it seems probable that both
the northern continents (or the larger portion of their area) were then
inhabited by no other mammalia than these, with perhaps other equally
low types. It was, probably, not later than the Jurassic age when some
of these primitive marsupials were able to enter Australia, where they
have since remained almost completely isolated; and, being free from
the competition of higher forms, they have developed into the great
variety of types we now behold there. These occupy the place, and have
to some extent acquired the form and structure of distinct orders of the
higher mammals--the rodents, the insectivora, and the carnivora,--while
still preserving the essential characteristics and lowly organisation of
the marsupials. At a much later period--probably in late Tertiary
times--the ancestors of the various species of rats and mice which now
abound in Australia, and which, with the aerial bats, constitute its
only forms of placental mammals, entered the country from some of the
adjacent islands. For this purpose a land connection was not necessary,
as these small creatures might easily be conveyed among the branches or
in the crevices of trees uprooted by floods and carried down to the sea,
and then floated to a shore many miles distant. That no actual land
connection with, or very close approximation to, an Asiatic island has
occurred in recent times, is sufficiently proved by the fact that no
squirrel, pig, civet, or other widespread mammal of the Eastern
hemisphere has been able to reach the Australian continent.
_The Distribution of Tapirs._
These curious animals form one of the puzzles of geographical
distribution, being now confined to two very remote regions of the
globe--the Malay Peninsula and adjacent islands of Sumatra and Borneo,
inhabited by one species, and tropical America, where there are three or
four species, ranging from Brazil to Ecuador and Guatemala. If we
considered these living forms only, we should be obliged to speculate on
enormous changes of land and sea in order that these tropical animals
might have passed from one country to the other. But geological
discoveries have rendered all such hypothetical changes unnecessary.
During Miocene and Pliocene times tapirs abounded over the whole of
Europe and Asia, their remains having been found in the tertiary
deposits of France, India, Burmah, and China. In both North and South
America fossil remains of tapirs occur only in caves and deposits of
Post-Pliocene age, showing that they are comparatively recent immigrants
into that continent. They perhaps entered by the route of Kamchatka and
Alaska, where the climate, even now so much milder and more equable than
on the north-east of America, might have been warm enough in late
Pliocene times to have allowed the migration of these animals. In Asia
they were driven southwards by the competition of numerous higher and
more powerful forms, but have found a last resting-place in the swampy
forests of the Malay region.
_What these Facts Prove._
Now these two cases, of the marsupials and the tapirs, are in the
highest degree instructive, because they show us that, without any
hypothetical bridging of deep oceans, and with only such changes of sea
and land as are indicated by the extent of the comparatively shallow
seas surrounding and connecting the existing continents, we are able to
account for the anomaly of allied forms occurring only in remote and
widely separated areas. These examples really constitute crucial tests,
because, of all classes of animals, mammalia are least able to surmount
physical barriers. They are obviously unable to pass over wide arms of
the sea, while the necessity for constant supplies of food and water
renders sandy deserts or snow-clad plains equally impassable. Then,
again, the peculiar kinds of food on which alone many of them can
subsist, and their liability to the attacks of other animals, put a
further check upon their migrations. In these respects almost all other
organisms have great advantages over mammals. Birds can often fly long
distances, and can thus cross arms of the sea, deserts, or mountain
ranges; insects not only fly, but are frequently carried great distances
by gales of wind, as shown by the numerous cases of their visits to
ships hundreds of miles from land. Reptiles, though slow of movement,
have advantages in their greater capacity for enduring hunger or thirst,
their power of resisting cold or drought in a state of torpidity, and
they have also some facilities for migration across the sea by means of
their eggs, which may be conveyed in crevices of timber or among masses
of floating vegetable matter. And when we come to the vegetable kingdom,
the means of transport are at their maximum, numbers of seeds having
special adaptations for being carried by mammalia or birds, and for
floating in the water, or through the air, while many are so small and
so light that there is practically no limit to the distances they may be
carried by gales and hurricanes.
We may, therefore, feel quite certain that the means of distribution
that have enabled the larger mammalia to reach the most remote regions
from a common starting-point, will be at least as efficacious, and
usually far more efficacious, with all other land animals and plants;
and if in every case the existing distribution of this class can be
explained on the theory of oceanic and continental permanence, with the
limited changes of sea and land already referred to, no valid objections
can be taken against this theory founded on anomalies of distribution in
other orders. Yet nothing is more common than for students of this or
that group to assort that the theory of oceanic permanence is quite
inconsistent with the distribution of its various species and genera.
Because a few Indian genera and closely allied species of birds are
found in Madagascar, a land termed "Lemuria" has been supposed to have
united the two countries during a comparatively recent geological epoch;
while the similarity of fossil plants and reptiles, from the Permian and
Miocene formations of India and South Africa, has been adduced as
further evidence of this connection. But there are also genera of
snakes, of insects, and of plants, common to Madagascar and South
America only, which have been held to necessitate a direct land
connection between these countries. These views evidently refute
themselves, because any such land connections must have led to a far
greater similarity in the productions of the several countries than
actually exists, and would besides render altogether inexplicable the
absence of all the chief types of African and Indian mammalia from
Madagascar, and its marvellous individuality in every department of the
organic world.[169]
_Powers of Dispersal as illustrated by Insular Organisms._
Having arrived at the conclusion that our existing oceans have remained
practically unaltered throughout the Tertiary and Secondary periods of
geology, and that the distribution of the mammalia is such as might
have been brought about by their known powers of dispersal, and by such
changes of land and sea as have probably or certainly occurred, we are,
of course, restricted to similar causes to explain the much wider and
sometimes more eccentric distribution of other classes of animals and of
plants. In doing so, we have to rely partly on direct evidence of
dispersal, afforded by the land organisms that have been observed far
out at sea, or which have taken refuge on ships, as well as by the
periodical visitants to remote islands; but very largely on indirect
evidence, afforded by the frequent presence of certain groups on remote
oceanic islands, which some ancestral forms must, therefore, have
reached by transmission across the ocean from distant lands.
_Birds._
These vary much in their powers of flight, and their capability of
traversing wide seas and oceans. Many swimming and wading birds can
continue long on the wing, fly swiftly, and have, besides, the power of
resting safely on the surface of the water. These would hardly be
limited by any width of ocean, except for the need of food; and many of
them, as the gulls, petrels, and divers, find abundance of food on the
surface of the sea itself. These groups have a wide distribution
_across_ the oceans; while waders--especially plovers, sandpipers,
snipes, and herons--are equally cosmopolitan, travelling _along_ the
coasts of all the continents, and across the narrow seas which separate
them. Many of these birds seem unaffected by climate, and as the
organisms on which they feed are equally abundant on arctic, temperate,
and tropical shores, there is hardly any limit to the range even of some
of the species.
Land-birds are much more restricted in their range, owing to their
usually limited powers of flight, their inability to rest on the surface
of the sea or to obtain food from it, and their greater specialisation,
which renders them less able to maintain themselves in the new countries
they may occasionally reach. Many of them are adapted to live only in
woods, or in marshes, or in deserts; they need particular kinds of food
or a limited range of temperature; and they are adapted to cope only
with the special enemies or the particular group of competitors among
which they have been developed. Such birds as these may pass again and
again to a new country, but are never able to establish themselves in
it; and it is this organic barrier, as it is termed, rather than any
physical barrier, which, in many cases, determines the presence of a
species in one area and its absence from another. We must always
remember, therefore, that, although the presence of a species in a
remote oceanic island clearly proves that its ancestors must at one time
have found their way there, the absence of a species does not prove the
contrary, since it also may have reached the island, but have been
unable to maintain itself, owing to the inorganic or organic conditions
not being suitable to it. This general principle applies to all classes
of organisms, and there are many striking illustrations of it. In the
Azores there are eighteen species of land-birds which are permanent
residents, but there are also several others which reach the islands
almost every year after great storms, but have never been able to
establish themselves. In Bermuda the facts are still more striking,
since there are only ten species of resident birds, while no less than
twenty other species of land-birds and more than a hundred species of
waders and aquatics are frequent visitors, often in great numbers, but
are never able to establish themselves. On the same principle we account
for the fact that, of the many continental insects and birds that have
been let loose, or have escaped from confinement, in this country,
hardly one has been able to maintain itself, and the same phenomenon is
still more striking in the case of plants. Of the thousands of hardy
plants which grow easily in our gardens, very few have ever run wild,
and when the experiment is purposely tried it invariably fails. Thus A.
de Candolle informs us that several botanists of Paris, Geneva, and
especially of Montpellier, have sown the seeds of many hundreds of
species of exotic hardy plants, in what appeared to be the most
favourable situations, but that in hardly a single case has any one of
them become naturalised.[170] Still more, then, in plants than in
animals the absence of a species does not prove that it has never
reached the locality, but merely that it has not been able to maintain
itself in competition with the native productions. In other cases, as
we have seen, facts of an exactly opposite nature occur. The rat, the
pig, and the rabbit, the water-cress, the clover, and many other plants,
when introduced into New Zealand, nourish exceedingly, and even
exterminate their native competitors; so that in these cases we may feel
sure that the species in question did not exist in New Zealand simply
because they had been unable to reach that country by their natural
means of dispersal. I will now give a few cases, in addition to those
recorded in my previous works, of birds and insects which have been
observed far from any land.
_Birds and Insects at Sea._
Captain D. Fullarton of the ship _Timaru_ recorded in his log the
occurrence of a great number of small land-birds about the ship on 15th
March 1886, when in Lat. 48° 31' N., Long. 8° 16' W. He says: "A great
many small land-birds about us; put about sixty into a coop, evidently
tired out." And two days later, 17th March, "Over fifty of the birds
cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two
small birds, name unknown, one kind like a linnet, and a large bird like
a starling. In all there have been on board over seventy birds, besides
some that hovered about us for some time and then fell into the sea
exhausted." Easterly winds and severe weather were experienced at the
time.[171] The spot where this remarkable flight of birds was met with
is about 160 miles due west of Brest, and this is the least distance the
birds must have been carried. It is interesting to note that the
position of the ship is nearly in the line from the English and French
coasts to the Azores, where, after great storms, so many bird stragglers
arrive annually. These birds were probably blown out to sea during their
spring migration along the south coast of England to Wales and Ireland.
During the autumnal migration, however, great flocks of
birds--especially starlings, thrushes, and fieldfares--have been
observed every year flying out to sea from the west coast of Ireland,
almost the whole of which must perish. At the Nash Lighthouse, in the
Bristol Channel on the coast of Glamorganshire, an enormous number of
small birds were observed on 3d September, including nightjars,
buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins,
wheatears, and blackbirds. These had probably crossed from
Somersetshire, and had they been caught by a storm the larger portion of
them must have been blown out to sea.[172]
These facts enable us to account sufficiently well for the birds of
oceanic islands, the number and variety of which are seen to be
proportionate to their facilities for reaching the island and
maintaining themselves in it. Thus, though more birds yearly reach
Bermuda than the Azores, the number of residents in the latter islands
is much larger, due to the greater extent of the islands, their number,
and their more varied surface. In the Galapagos the land-birds are still
more numerous, due in part to their larger area and greater proximity to
the continent, but chiefly to the absence of storms, so that the birds
which originally reached the islands have remained long isolated and
have developed into many closely allied species adapted to the special
conditions. All the species of the Galapagos but one are peculiar to the
islands, while the Azores possess only one peculiar species, and Bermuda
none--a fact which is clearly due to the continual immigration of fresh
individuals keeping up the purity of the breed by intercrossing. In the
Sandwich Islands, which are extremely isolated, being more than 2000
miles from any continent or large island, we have a condition of things
similar to what prevails in the Galapagos, the land-birds, eighteen in
number, being all peculiar, and belonging, except one, to peculiar
genera. These birds have probably all descended from three or four
original types which reached the islands at some remote period, probably
by means of intervening islets that have since disappeared. In St.
Helena we have a degree of permanent isolation which has prevented any
land-birds from reaching the island; for although its distance from the
continent, 1100 miles, is not so great as in the case of the Sandwich
Islands, it is situated in an ocean almost entirely destitute of small
islands, while its position within the tropics renders it free from
violent storms. Neither is there, on the nearest part of the coast of
Africa, a perpetual stream of migrating birds like that which supplies
the innumerable stragglers which every year reach Bermuda and the
Azores.
_Insects._
Winged insects have been mainly dispersed in the same way as birds, by
their power of flight, aided by violent or long-continued winds. Being
so small, and of such low specific gravity, they are occasionally
carried to still greater distances; and thus no islands, however remote,
are altogether without them. The eggs of insects, being often deposited
in borings or in crevices of timber, may have been conveyed long
distances by floating trees, as may the larvae of those species which
feed on wood. Several cases have been published of insects coming on
board ships at great distances from land; and Darwin records having
caught a large grasshopper when the ship was 370 miles from the coast of
Africa, whence the insect had probably come.
In the _Entomologists' Monthly Magazine_ for June 1885, Mr. MacLachlan
has recorded the occurrence of a swarm of moths in the Atlantic ocean,
from the log of the ship _Pleione_. The vessel was homeward bound from
New Zealand, and in Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths
appeared about the ship, settling in numbers on the spars and rigging.
The wind for four days previously had been very light from north,
north-west, or north-east, and sometimes calm. The north-east trade wind
occasionally extends to the ship's position at that time of year. The
captain adds that "frequently, in that part of the ocean, he has had
moths and butterflies come on board." The position is 960 miles
south-west of the Cape Verde Islands, and about 440 north-east of the
South American coast. The specimen preserved is Deiopeia pulchella, a
very common species in dry localities in the Eastern tropics, and rarely
found in Britain, but, Mr. MacLachlan thinks, not found in South
America. They must have come, therefore, from the Cape Verde Islands, or
from some parts of the African coast, and must have traversed about a
thousand miles of ocean with the assistance, no doubt, of a strong
north-east trade wind for a great part of the distance. In the British
Museum collection there is a specimen of the same moth caught at sea
during the voyage of the _Rattlesnake_, in Lat. 6° N., Long. 22-1/2°
W., being between the former position and Sierra Leone, thus rendering
it probable that the moths came from that part of the African coast, in
which case the swarm encountered by the _Pleione_ must have travelled
more than 1200 miles.
A similar case was recorded by Mr. F.A. Lucas in the American periodical
_Science_ of 8th April 1887. He states that in 1870 he met with numerous
moths of many species while at sea in the South Atlantic (Lat. 25° S.,
Long. 24° W.), about 1000 miles from the coast of Brazil. As this
position is just beyond the south-east trades, the insects may have been
brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
8920) is the record of a small longicorn beetle which flew on board a
ship 500 miles off the west coast of Africa. Numerous other cases are
recorded of insects at less distances from land, and, taken in
connection with those already given, they are sufficient to show that
great numbers must be continually carried out to sea, and that
occasionally they are able to reach enormous distances. But the
reproductive powers of insects are so great that all we require, in
order to stock a remote island, is that some few specimens shall reach
it even once in a century, or once in a thousand years.
_Insects at great Altitudes._
Equally important is the proof we possess that insects are often carried
to great altitudes by upward currents of air. Humboldt noticed them up
to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
Müller has collected many interesting cases of the same character in
Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
height of 11,000 feet, while numerous flies and beetles, some of
considerable size, have been caught on the glaciers and snow-fields of
various parts of the Alps. Upward currents of air, whirlwinds and
tornadoes, occur in all parts of the world, and large numbers of insects
are thus carried up into the higher regions of the atmosphere, where
they are liable to be caught by strong winds, and thus conveyed enormous
distances over seas or continents. With such powerful means of
dispersal the distribution of insects over the entire globe, and their
presence in the most remote oceanic islands, offer no difficulties.
_The Dispersal of Plants._
The dispersal of seeds is effected in a greater variety of ways than are
available in the case of any animals. Some fruits or seed-vessels, and
some seeds, will float for many weeks, and after immersion in salt water
for that period the seeds will often germinate. Extreme cases are the
double cocoa-nut of the Seychelles, which has been found on the coast of
Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria
(soap-berry), which has been brought to Bermuda by the Gulf Stream from
the West Indies, and has grown after a journey in the sea of about 1500
miles; and the West Indian bean, Entada scandens, which reached the
Azores from the West Indies, a distance of full 3000 miles, and
afterwards germinated at Kew. By these means we can account for the
similarity in the shore flora of the Malay Archipelago and most of the
islands of the Pacific; and from an examination of the fruits and seeds,
collected among drift during the voyage of the _Challenger_, Mr. Hemsley
has compiled a list of 121 species which are probably widely dispersed
by this means.
A still larger number of species owe their dispersal to birds in several
distinct ways. An immense number of fruits in all parts of the world are
devoured by birds, and have been attractively coloured (as we have
seen), in order to be so devoured, because the seeds pass through the
birds' bodies and germinate where they fall. We have seen how frequently
birds are forced by gales of wind across a wide expanse of ocean, and
thus seeds must be occasionally carried. It is a very suggestive fact,
that all the trees and shrubs in the Azores bear berries or small fruits
which are eaten by birds; while all those which bear larger fruits, or
are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs,
etc.--are entirely wanting. Game-birds and waders often have portions of
mud attached to their feet, and Mr. Darwin has proved by experiment that
such mud frequently contains seeds. One partridge had such a quantity of
mud attached to its foot as to contain seeds from which eighty-two
plants germinated; this proves that a very small portion of mud may
serve to convey seeds, and such an occurrence repeated even at long
intervals may greatly aid in stocking remote islands with vegetation.
Many seeds also adhere to the feathers of birds, and thus, again, may be
conveyed as far as birds are ever carried. Dr. Guppy found a small hard
seed in the gizzard of a Cape Petrel, taken about 550 miles east of
Tristan da Cunha.
_Dispersal of Seeds by the Wind._
In the preceding cases we have been able to obtain direct evidence of
transportal; but although we know that many seeds are specially adapted
to be dispersed by the wind, we cannot obtain direct proof that they are
so carried for hundreds or thousands of miles across the sea, owing to
the difficulty of detecting single objects which are so small and
inconspicuous. It is probable, however, that the wind as an agent of
dispersal is really more effective than any of those we have hitherto
considered, because a very large number of plants have seeds which are
very small and light, and are often of such a form as to facilitate
aerial carriage for enormous distances. It is evident that such seeds
are especially liable to be transported by violent winds, because they
become ripe in autumn at the time when storms are most prevalent, while
they either lie upon the surface of the ground, or are disposed in dry
capsules on the plant ready to be blown away. If inorganic particles
comparable in weight, size, or form with such seeds are carried for
great distances, we may be sure that seeds will also be occasionally
carried in the same way. It will, therefore, be necessary to give a few
examples of wind-carriage of small objects.
On 27th July 1875 a remarkable shower of small pieces of hay occurred at
Monkstown, near Dublin. They appeared floating slowly down from a great
height, as if falling from a dark cloud which hung overhead. The pieces
picked up were wet, and varied from single blades of grass to tufts
weighing one or two ounces. A similar shower occurred a few days earlier
in Denbighshire, and was observed to travel in a direction contrary to
that of the wind in the lower atmosphere.[174] There is no evidence of
the distance from which the hay was brought, but as it had been carried
to a great height, it was in a position to be conveyed to almost any
distance by a violent wind, had such occurred at the time.
_Mineral Matter carried by the Wind._
The numerous cases of sand and volcanic dust being carried enormous
distances through the atmosphere sufficiently prove the importance of
wind as a carrier of solid matter, but unfortunately the matter
collected has not been hitherto examined with a view to determine the
maximum size and weight of the particles. A few facts, however, have
been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
at Genoa on 15th October 1885, and was believed to have been brought
from the African desert, consisted of quartz, hornblende, and other
minerals, and contained particles having a diameter of 1/500 inch, each
weighing 1/200,000 grain. This dust had probably travelled over 600
miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
distant, during the great eruption, there are many solid particles even
larger than those mentioned above. Some of this dust was given me by
Professor Judd, and I found in it several ovoid particles of a much
larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
the same eruption, which fell on board the ship _Arabella_, 970 miles
from the volcano, also contained solid particles 1/500 inch diameter.
Mr. John Murray of the _Challenger_ Expedition writes to me that he
finds in the deep sea deposits 500 and even 700 miles west of the coast
of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
and similar particles are found at equally great distances from the
south-west coasts of Australia; and he considers these to be atmospheric
dust carried to that distance by the wind. Taking the sp. gr. of quartz
at 2.6, these particles would weigh about 1/25,000 grain each. These
interesting facts can, however, by no means be taken as indicating the
extreme limits of the power of wind in carrying solid particles. During
the Krakatoa eruption no gale of special violence occurred, and the
region is one of comparative calms. The grains of quartz found by Mr.
Murray more nearly indicate the limit, but the very small portions of
matter brought up by the dredge, as compared with the enormous areas of
sea-bottom, over which the atmospheric dust must have been scattered,
render it in the highest degree improbable that the maximum limit either
of size of particles, or of distance from land has been reached.
Let us, however, assume that the quartz grains, found by Mr. Murray in
the deep-sea ooze 700 miles from land, give us the extreme limit of the
power of the atmosphere as a carrier of solid particles, and let us
compare with these the weights of some seeds. From a small collection of
the seeds of thirty species of herbaceous plants sent me from Kew, those
in the above table were selected, and small portions of eight of them
carefully weighed in a chemical balance.[175] By counting these portions
I was able to estimate the number of seeds weighing one grain. The three
very minute species, whose numbers are marked with an asterisk (*), were
estimated by the comparison of their sizes with those of the smaller
weighed seeds.
No| Species. |Approximate | Approximate | Remarks.
| |No. of Seeds| Dimensions. |
| |In one Grain| |
| | | in. in. in. |
1|Draba verna | 1,800 |1/60 x 1/90 x 1/160|Oval, flat.
2|Hypericum perforatum | 520 | 1/30 x 1/80 |Cylindrical.
3|Astilbe rivularis | 4,500 | 1/50 x 1/100 |Elongate, flat, tailed,
| | | | wavy.
4|Saxifraga coriophylla| 750 | 1/40 x 1/75 |Surface rough, adhere
| | | | to the dry capsules.
5|Oenothera rosea | 640 | 1/40 x 1/80 |Ovate.
6|Hypericum hirsutum | 700 | 1/30 x 1/100 |Cylindrical, rough.
7|Mimulus luteus | 2,900 | 1/60 x 1/100 |Oval, minute.
8|Penthorum sedoides | 8,000* | 1/70 x 1/150 |Flattened, very minute.
9|Sagina procumbens | 12,000* | 1/120 |Sub-triangular, flat.
10|Orchis maculata | 15,000* | --- |Margined, flat,
| | | | very minute.
11|Gentiana purpurea | 35 | 1/25 |Wavy, rough, with this
| | | | coriaceous margins.
12|Silene alpina | --- | 1/30 |Flat, with fringed
| | | | margins.
13|Adenophora communis | --- | 1/20 x 1/40 |Very thin, wavy, light.
|Quartz grains | 25,000 | 1/250 |Deep sea ... 700 miles.
|Do. |200,000 | 1/500 |Genoa ... 600 miles.
If now we compare the seeds with the quartz grains, we find that
several are from twice to three times the weight of the grains found by
Mr. Murray, and others five times, eight times, and fifteen times as
heavy; but they are proportionately very much larger, and, being usually
irregular in shape or compressed, they expose a very much larger surface
to the air. The surface is often rough, and several have dilated margins
or tailed appendages, increasing friction and rendering the uniform rate
of falling through still air immensely less than in the case of the
smooth, rounded, solid quartz grains. With these advantages it is a
moderate estimate that seeds ten times the weight of the quartz grains
could be carried quite as far through the air by a violent gale and
under the most favourable conditions. These limits will include five of
the seeds here given, as well as hundreds of others which do not exceed
them in weight; and to these we may add some larger seeds which have
other favourable characteristics, as is the case with numbers 11-13,
which, though very much larger than the rest, are so formed as in all
probability to be still more easily carried great distances by a gale of
wind. It appears, therefore, to be absolutely certain that every
autumnal gale capable of conveying solid mineral particles to great
distances, must also carry numbers of small seeds at least as far; and
if this is so, the wind alone will form one of the most effective agents
in the dispersal of plants.
Hitherto this mode of conveyance, as applying to the transmission of
seeds for great distances across the ocean, has been rejected by
botanists, for two reasons. In the first place, there is said to be no
direct evidence of such conveyance; and, secondly, the peculiar plants
of remote oceanic islands do not appear to have seeds specially adapted
for aerial transmission. I will consider briefly each of these
objections.
_Objection to the Theory of Wind-Dispersal._
To obtain direct evidence of the transmission of such minute and
perishable objects, which do not exist in great quantities, and are
probably carried to the greatest distances but rarely and as single
specimens, is extremely difficult. A bird or insect can be seen if it
comes on board ship, but who would ever detect the seeds of Mimulus or
Orchis even if a score of them fell on a ship's deck? Yet if but one
such seed per century were carried to an oceanic island, that island
might become rapidly overrun by the plant, if the conditions were
favourable to its growth and reproduction. It is further objected that
search has been made for such seeds, and they have not been found.
Professor Kerner of Innsbruck examined the snow on the surface of
glaciers, and assiduously collected all the seeds he could find, and
these were all of plants which grew in the adjacent mountains or in the
same district. In like manner, the plants growing on moraines were found
to be those of the adjacent mountains, plateaux, or lowlands. Hence he
concluded that the prevalent opinion that seeds may be carried through
the air for very great distances "is not supported by fact."[176] The
opinion is certainly not supported by Kerner's facts, but neither is it
opposed by them. It is obvious that the seeds that would be carried by
the wind to moraines or to the surface of glaciers would be, first and
in the greatest abundance, those of the immediately surrounding
district; then, very much more rarely, those from more remote mountains;
and lastly, in extreme rarity, those from distant countries or
altogether distinct mountain ranges. Let us suppose the first to be so
abundant that a single seed could be found by industrious search on each
square yard of the surface of the glacier; the second so scarce that
only one could possibly be found in a hundred yards square; while to
find one of the third class it would be necessary exhaustively to
examine a square mile of surface. Should we expect that _one_ ever to be
found, and should the fact that it could not be found be taken as a
proof that it was not there? Besides, a glacier is altogether in a bad
position to receive such remote wanderers, since it is generally
surrounded by lofty mountains, often range behind range, which would
intercept the few air-borne seeds that might have been carried from a
distant land. The conditions in an oceanic island, on the other hand,
are the most favourable, since the land, especially if high, will
intercept objects carried by the wind, and will thus cause more of the
solid matter to fall on it than on an equal area of ocean. We know that
winds at sea often blow violently for days together, and the rate of
motion is indicated by the fact that 72 miles an hour was the average
velocity of the wind observed during twelve hours at the Ben Nevis
observatory, while the velocity sometimes rises to 120 miles an hour. A
twelve hours' gale might, therefore, carry light seeds a thousand miles
as easily and certainly as it could carry quartz-grains of much greater
specific gravity, rotundity, and smoothness, 500 or even 100 miles; and
it is difficult even to imagine a sufficient reason why they should not
be so carried--perhaps very rarely and under exceptionally favourable
conditions,--but this is all that is required.
As regards the second objection, it has been observed that orchideae,
which have often exceedingly small and light seeds, are remarkably
absent from oceanic islands. This, however, may be very largely due to
their extreme specialisation and dependence on insect agency for their
fertilisation; while the fact that they do occur in such very remote
islands as the Azores, Tahiti, and the Sandwich Islands, proves that
they must have once reached these localities either by the agency of
birds or by transmission through the air; and the facts I have given
above render the latter mode at least as probable as the former. Sir
Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
yet having no pappus, while other species of the genus possess it. This
is certainly remarkable, and proves that the plant must have, or once
have had, some other means of dispersal across wide oceans.[177] One of
the most widely dispersed species in the whole world (Sonchus oleraceus)
possesses pappus, as do four out of five of the species which are common
to Europe and New Zealand, all of which have a very wide distribution.
The same author remarks on the limited area occupied by most species of
Compositae, notwithstanding their facilities for dispersal by means of
their feathered seeds; but it has been already shown that limitations
of area are almost always due to the competition of allied forms,
facilities for dispersal being only one of many factors in determining
the wide range of species. It is, however, a specially important factor
in the case of the inhabitants of remote oceanic islands, since, whether
they are peculiar species or not, they or their remote ancestors must at
some time or other have reached their present position by natural means.
I have already shown elsewhere, that the flora of the Azores strikingly
supports the view of the species having been introduced by aerial
transmission only, that is, by the agency of birds and the wind, because
all plants that could not possibly have been carried by these means are
absent.[178] In the same way we may account for the extreme rarity of
Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on
Insular Floras, says that they "are wanting in a large number of oceanic
islands where there is no true littoral flora," as St. Helena, Juan
Fernandez, and all the islands of the South Atlantic and South Indian
Oceans. Even in the tropical islands, such as Mauritius and Bourbon,
there are no endemic species, and very few in the Galapagos and the
remoter Pacific Islands. All these facts are quite in accordance with
the absence of facilities for transmission through the air, either by
birds or the wind, owing to the comparatively large size and weight of
the seeds; and an additional proof is thus afforded of the extreme
rarity of the successful floating of seeds for great distances across
the ocean.[179]
_Explanation of North Temperate Plants in the Southern Hemisphere._
If we now admit that many seeds which are either minute in size, of thin
texture or wavy form, or so fringed or margined as to afford a good hold
to the air, are capable of being carried for many hundreds of miles by
exceptionally violent and long-continued gales of wind, we shall not
only be better able to account for the floras of some of the remotest
oceanic islands, but shall also find in the fact a sufficient
explanation of the wide diffusion of many genera, and even species, of
arctic and north temperate plants in the southern hemisphere or on the
summits of tropical mountains. Nearly fifty of the flowering plants of
Tierra-del-Fuego are found also in North America or Europe, but in no
intermediate country; while fifty-eight species are common to New
Zealand and Northern Europe; thirty-eight to Australia, Northern Europe,
and Asia; and no less than seventy-seven common to New Zealand,
Australia, and South America.[180] On lofty mountains far removed from
each other, identical or closely allied plants often occur. Thus the
fine Primula imperialis of a single mountain peak in Java has been found
(or a closely allied species) in the Himalayas; and many other plants of
the high mountains of Java, Ceylon, and North India are either identical
or closely allied forms. So, in Africa, some species, found on the
summits of the Cameroons and Fernando Po in West Africa, are closely
allied to species in the Abyssinian highlands and in Temperate Europe;
while other Abyssinian and Cameroons species have recently been found on
the mountains of Madagascar. Some peculiar Australian forms have been
found represented on the summit of Kini Balu in Borneo. Again, on the
summit of the Organ mountains in Brazil there are species allied to
those of the Andes, but not found in the intervening lowlands.
_No Proof of Recent Lower Temperature in the Tropics._
Now all these facts, and numerous others of like character, were
supposed by Mr. Darwin to be due to a lowering of temperature during
glacial epochs, which allowed these temperate forms to migrate across
the intervening tropical lowlands. But any such change within the epoch
of existing species is almost inconceivable. In the first place, it
would necessitate the extinction of much of the tropical flora (and with
it of the insect life), because without such extinction alpine
herbaceous plants could certainly never spread over tropical forest
lowlands; and, in the next place, there is not a particle of direct
evidence that any such lowering of temperature in inter-tropical
lowlands ever took place. The only alleged evidence of the kind is that
adduced by the late Professor Agassiz and Mr. Hartt; but I am informed
by my friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
who succeeded Mr. Hartt, and spent several years completing the
geological survey of Brazil, that the supposed moraines and glaciated
granite rocks near Rio Janeiro and elsewhere, as well as the so-called
boulder-clay of the same region, are entirely explicable as the results
of sub-aerial denudation and weathering, and that there is no proof
whatever of glaciation in any part of Brazil.
_Lower Temperature not needed to Explain the Facts._
But any such vast physical change as that suggested by Darwin, involving
as it does such tremendous issues as regards its effects on the tropical
fauna and flora of the whole world, is really quite uncalled for,
because the facts to be explained are of the same essential nature as
those presented by remote oceanic islands, between which and the nearest
continents no temperate land connection is postulated. In proportion to
their limited area and extreme isolation, the Azores, St. Helena, the
Galapagos, and the Sandwich Islands, each possess a fairly rich--the
last a very rich--indigenous flora; and the means which sufficed to
stock them with a great variety of plants would probably suffice to
transmit others from mountain-top to mountain-top in various parts of
the globe. In the case of the Azores, we have large numbers of species
identical with those of Europe, and others closely allied, forming an
exactly parallel case to the species found on the various mountain
summits which have been referred to. The distances from Madagascar to
the South African mountains and to Kilimandjaro, and from the latter to
Abyssinia, are no greater than from Spain to the Azores, while there are
other equatorial mountains forming stepping-stones at about an equal
distance to the Cameroons. Between Java and the Himalayas we have the
lofty mountains of Sumatra and of North-western Burma, forming steps at
about the same distance apart; while between Kini Balu and the
Australian Alps we have the unexplored snow mountains of New Guinea,
the Bellenden Ker mountains in Queensland, and the New England and Blue
Mountains of New South Wales. Between Brazil and Bolivia the distances
are no greater; while the unbroken range of mountains from Arctic
America to Tierra-del-Fuego offers the greatest facilities for
transmission, the partial gap between the lofty peak of Chiriqui and the
high Andes of New Grenada being far less than from Spain to the Azores.
Thus, whatever means have sufficed for stocking oceanic islands must
have been to some extent effective in transmitting northern forms from
mountain to mountain, across the equator, to the southern hemisphere;
while for this latter form of dispersal there are special facilities, in
the abundance of fresh and unoccupied surfaces always occurring in
mountain regions, owing to avalanches, torrents, mountain-slides, and
rock-falls, thus affording stations on which air-borne seeds may
germinate and find a temporary home till driven out by the inroads of
the indigenous vegetation. These temporary stations may be at much lower
altitudes than the original habitat of the species, if other conditions
are favourable. Alpine plants often descend into the valleys on glacial
moraines, while some arctic species grow equally well on mountain
summits and on the seashore. The distances above referred to between the
loftier mountains may thus be greatly reduced by the occurrence of
suitable conditions at lower altitudes, and the facilities for
transmission by means of aerial currents proportionally increased.[181]
_Facts Explained by the Wind-Carriage of Seeds._
But if we altogether reject aerial transmission of seeds for great
distances, except by the agency of birds, it will be difficult, if not
impossible, to account for the presence of so many identical species of
plants on remote mountain summits, or for that "continuous current of
vegetation" described by Sir Joseph Hooker as having apparently long
existed from the northern to the southern hemisphere. It may be admitted
that we can, possibly, account for the greater portion of the floras of
remote oceanic islands by the agency of birds alone; because, when blown
out to sea land-birds must reach some island or perish, and all which
come within sight of an island will struggle to reach it as their only
refuge. But, with mountain summits the case is altogether different,
because, being surrounded by land instead of by sea, no bird would need
to fly, or to be carried by the wind, for several hundred miles at a
stretch to another mountain summit, but would find a refuge in the
surrounding uplands, ridges, valleys, or plains. As a rule the birds
that frequent lofty mountain tops are peculiar species, allied to those
of the surrounding district; and there is no indication whatever of the
passage of birds from one remote mountain to another in any way
comparable with the flights of birds which are known to reach the Azores
annually, or even with the few regular migrants from Australia to New
Zealand. It is almost impossible to conceive that the seeds of the
Himalayan primula should have been thus carried to Java; but, by means
of gales of wind, and intermediate stations from fifty to a few hundred
miles apart, where the seeds might vegetate for a year or two and
produce fresh seed to be again carried on in the same manner, the
transmission might, after many failures, be at last effected.
A very important consideration is the vastly larger scale on which
wind-carriage of seeds must act, as compared with bird-carriage. It can
only be a few birds which carry seeds attached to their feathers or
feet. A very small proportion of these would carry the seeds of Alpine
plants; while an almost infinitesimal fraction of these latter would
convey the few seeds attached to them safely to an oceanic island or
remote mountain. But winds, in the form of whirlwinds or tornadoes,
gales or hurricanes, are perpetually at work over large areas of land
and sea. Insects and light particles of matter are often carried up to
the tops of high mountains; and, from the very nature and origin of
winds, they usually consist of ascending or descending currents, the
former capable of suspending such small and light objects as are many
seeds long enough for them to be carried enormous distances. For each
single seed carried away by external attachment to the feet or feathers
of a bird, countless millions are probably carried away by violent
winds; and the chance of conveyance to a great distance and in a
definite direction must be many times greater by the latter mode than
by the former.[182] We have seen that inorganic particles of much
greater specific gravity than seeds, and nearly as heavy as the smallest
kinds, are carried to great distances through the air, and we can
therefore hardly doubt that some seeds are carried as far. The direct
agency of the wind, as a supplement to bird-transport, will help to
explain the presence in oceanic islands of plants growing in dry or
rocky places whose small seeds are not likely to become attached to
birds; while it seems to be the only effective agency possible in the
dispersal of those species of alpine or sub-alpine plants found on the
summits of distant mountains, or still more widely separated in the
temperate zones of the northern and southern hemispheres.
_Concluding Remarks._
On the general principles that have been now laid down, it will be found
that all the chief facts of the geographical distribution of animals and
plants can be sufficiently understood. There will, of course, be many
cases of difficulty and some seeming anomalies, but these can usually be
seen to depend on our ignorance of some of the essential factors of the
problem. Either we do not know the distribution of the group in recent
geological times, or we are still ignorant of the special methods by
which the organisms are able to cross the sea. The latter difficulty
applies especially to the lizard tribe, which are found in almost all
the tropical oceanic islands; but the particular mode in which they are
able to traverse a wide expanse of ocean, which is a perfect barrier to
batrachia and almost so to snakes, has not yet been discovered. Lizards
are found in all the larger Pacific Islands as far as Tahiti, while
snakes do not extend beyond the Fiji Islands; and the latter are also
absent from Mauritius and Bourbon, where lizards of seven or eight
species abound. Naturalists resident in the Pacific Islands would make a
valuable contribution to our science by studying the life-history of the
native lizards, and endeavouring to ascertain the special facilities
they possess for crossing over wide spaces of ocean.
FOOTNOTES:
[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_ (Cambridge,
Mass., 1888), vol. i. p. 127, footnote.]
[Footnote 164: Even the extremely fine Mississippi mud is nowhere found
beyond a hundred miles from the mouths of the river in the Gulf of
Mexico (A. Agassiz, _Three Cruises of the Blake_, vol. i. p. 128).]
[Footnote 165: I have given a full summary of the evidence for the
permanence of oceanic and continental areas in my _Island Life_, chap.
vi.]
[Footnote 166: For a full account of the peculiarities of the Madagascar
fauna, see my _Island Life_, chap. xix.]
[Footnote 167: See _Island Life_, p. 446, and the whole of chaps. xxi.
xxii. More recent soundings have shown that the Map at p. 443, as well
as that of the Madagascar group at p. 387, are erroneous, the ocean
around Norfolk Island and in the Straits of Mozambique being more than
1000 fathoms deep. The general argument is, however, unaffected.]
[Footnote 168: For some details of these migrations, see the author's
_Geographical Distribution of Animals_, vol. i. p. 140; also Heilprin's
_Geographical and Geological Distribution of Animals_.]
[Footnote 169: For a full discussion of this question, see _Island
Life_, pp. 390-420.]
[Footnote 170: _Géographie Botanique_, p. 798.]
[Footnote 171: _Nature_, 1st April 1886.]
[Footnote 172: Report of the Brit. Assoc. Committee on Migration of
Birds during 1886.]
[Footnote 173: _Trans. Ent. Soc._, 1871, p. 184.]
[Footnote 174: _Nature_ (1875), vol. xii. pp. 279, 298.]
[Footnote 175: I am indebted to Professor R. Meldola of the Finsbury
Technical Institute, and Rev. T.D. Titmas of Charterhouse for furnishing
me with the weights required.]
[Footnote 176: See _Nature_, vol. vi. p. 164, for a summary of Kerner's
paper.]
[Footnote 177: It seems quite possible that the absence of pappus in
this case is a recent adaptation, and that it has been brought about by
causes similar to those which have reduced or aborted the wings of
insects in oceanic islands. For when a plant has once reached one of the
storm-swept islands of the southern ocean, the pappus will be injurious
for the same reason that the wings of insects are injurious, since it
will lead to the seeds being blown out to sea and destroyed. The seeds
which are heaviest and have least pappus will have the best chance of
falling on the ground and remaining there to germinate, and this process
of selection might rapidly lead to the entire disappearance of the
pappus.]
[Footnote 178: See _Island Life_, p. 251.]
[Footnote 179: Mr. Hemsley suggests that it is not so much the
difficulty of transmission by floating, as the bad conditions the seeds
are usually exposed to when they reach land. Many, even if they
germinate, are destroyed by the waves, as Burchell noticed at St.
Helena; while even a flat and sheltered shore would be an unsuitable
position for many inland plants. Air-borne seeds, on the other hand, may
be carried far inland, and so scattered that some of them are likely to
reach suitable stations.]
[Footnote 180: For fuller particulars, see Sir J. Hooker's _Introduction
to Floras of New Zealand and Australia_, and a summary in my _Island
Life_, chaps. xxii. xxiii.]
[Footnote 181: For a fuller discussion of this subject, see my _Island
Life_, chap. xxiii.]
[Footnote 182: A very remarkable case of wind conveyance of seeds on a
large scale is described in a letter from Mr. Thomas Hanbury to his
brother, the late Daniel Hanbury, which has been kindly communicated to
me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856,"
and the passage referred to is as follows:--
"For the past three days we have had very warm weather for this time of
year, in fact almost as warm as the middle of summer. Last evening the
wind suddenly changed round to the north and blew all night with
considerable violence, making a great change in the atmosphere.
"This morning, myriads of small white particles are floating about in
the air; there is not a single cloud and no mist, yet the sun is quite
obscured by this substance, and it looks like a white fog in England. I
enclose thee a sample, thinking it may interest. It is evidently a
vegetable production; I think, apparently, some kind of seed."
Mr. Hemsley adds, that this substance proves to be the plumose seeds of
a poplar or willow. In order to produce the effects described--_quite
obscuring the sun like a white fog_,--the seeds must have filled the air
to a very great height; and they must have been brought from some
district where there were extensive tracts covered with the tree which
produced them.]
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.
The theory of evolution in the organic world necessarily implies that
the forms of animals and plants have, broadly speaking, progressed from
a more generalised to a more specialised structure, and from simpler to
more complex forms. We know, however, that this progression has been by
no means regular, but has been accompanied by repeated degradation and
degeneration; while extinction on an enormous scale has again and again
stopped all progress in certain directions, and has often compelled a
fresh start in development from some comparatively low and imperfect
type.
The enormous extension of geological research in recent times has made
us acquainted with a vast number of extinct organisms, so vast that in
some important groups--such as the mollusca--the fossil are more
numerous than the living species; while in the mammalia they are not
much less numerous, the preponderance of living species being chiefly in
the smaller and in the arboreal forms which have not been so well
preserved as the members of the larger groups. With such a wealth of
material to illustrate the successive stages through which animals have
passed, it will naturally be expected that we should find important
evidence of evolution. We should hope to learn the steps by which some
isolated forms have been connected with their nearest allies, and in
many cases to have the gaps filled up which now separate genus from
genus, or species from species. In some cases these expectations are
fulfilled, but in many other cases we seek in vain for evidence of the
kind we desire; and this absence of evidence with such an apparent
wealth of material is held by many persons to throw doubt on the theory
of evolution itself. They urge, with much appearance of reason, that all
the arguments we have hitherto adduced fall short of demonstration, and
that the crucial test consists in being able to show, in a great number
of cases, those connecting links which we say must have existed. Many of
the gaps that still remain are so vast that it seems incredible to these
writers that they could ever have been filled up by a close succession
of species, since these must have spread over so many ages, and have
existed in such numbers, that it seems impossible to account for their
total absence from deposits in which great numbers of species belonging
to other groups are preserved and have been discovered. In order to
appreciate the force, or weakness, of these objections, we must inquire
into the character and completeness of that record of the past life of
the earth which geology has unfolded, and ascertain the nature and
amount of the evidence which, under actual conditions, we may expect to
find.
_The Number of known Species of Extinct Animals._
When we state that the known fossil mollusca are considerably more
numerous than those which now live on the earth, it appears at first
sight that our knowledge is very complete, but this is far from being
the case. The species have been continually changing throughout
geological time, and at each period have probably been as numerous as
they are now. If we divide the fossiliferous strata into twelve great
divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
find not only that each has a very distinct and characteristic molluscan
fauna, but that the different subdivisions often present a widely
different series of species; so that although a certain number of
species are common to two or more of the great divisions, the totality
of the species that have lived upon the earth must be very much more
than twelve times--perhaps even thirty or forty times--the number now
living. In like manner, although the species of fossil mammals now
recognised by more or less fragmentary fossil remains may not be much
less numerous than the living species, yet the duration of existence of
these was comparatively so short that they were almost completely
changed, perhaps six or seven times, during the Tertiary period; and
this is certainly only a fragment of the geological time during which
mammalia existed on the globe.
There is also reason to believe that the higher animals were much more
abundant in species during past geological epochs than now, owing to the
greater equability of the climate which rendered even the arctic regions
as habitable as the temperate zones are in our time.
The same equable climate would probably cause a more uniform
distribution of moisture, and render what are now desert regions capable
of supporting abundance of animal life. This is indicated by the number
and variety of the species of large animals that have been found fossil
in very limited areas which they evidently inhabited at one period. M.
Albert Gaudry found, in the deposits of a mountain stream at Pikermi in
Greece, an abundance of large mammalia such as are nowhere to be found
living together at the present time. Among them were two species of
Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and
a giraffe larger than those now living, several monkeys, carnivora
ranging from martens and civets to lions and hyaenas of the largest
size, numerous antelopes of at least five distinct genera, and besides
these many forms altogether extinct. Such were the great herds of
Hipparion, an ancestral form of horse; the Helladotherium, a huge animal
bigger than the giraffe; the Ancylotherium, one of the Edentata; the
huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the
monstrous Chalicotherium, allied to the swine and ruminants, but as
large as a rhinoceros; and to prey upon these, the great Machairodus or
sabre-toothed tiger. And all these remains were found in a space 300
paces long by 60 paces broad, many of the species existing in enormous
quantities.
The Pikermi fossils belong to the Upper Miocene formation, but an
equally rich deposit of Upper Eocene age has been discovered in
South-Western France at Quercy, where M. Filhol has determined the
presence of no less than forty-two species of beasts of prey alone.
Equally remarkable are the various discoveries of mammalian fossils in
North America, especially in the old lake bottoms now forming what are
called the "bad lands" of Dakota and Nebraska, belonging to the Miocene
period. Here are found an enormous assemblage of remains, often perfect
skeletons, of herbivora and carnivora, as varied and interesting as
those from the localities already referred to in Europe; but altogether
distinct, and far exceeding, in number and variety of species of the
larger animals, the whole existing fauna of North America. Very similar
phenomena occur in South America and in Australia, leading us to the
conclusion that the earth at the present time is impoverished as regards
the larger animals, and that at each successive period of Tertiary time,
at all events, it contained a far greater number of species than now
inhabit it. The very richness and abundance of the remains which we find
in limited areas, serve to convince us how imperfect and fragmentary
must be our knowledge of the earth's fauna at any one past epoch; since
we cannot believe that all, or nearly all, of the animals which
inhabited any district were entombed in a single lake, or overwhelmed by
the floods of a single river.
But the spots where such rich deposits occur are exceedingly few and far
between when compared with the vast areas of continental land, and we
have every reason to believe that in past ages, as now, numbers of
curious species were rare or local, the commoner and more abundant
species giving a very imperfect idea of the existing series of animal
forms. Yet more important, as showing the imperfection of our knowledge,
is the enormous lapse of time between the several formations in which we
find organic remains in any abundance, so vast that in many cases we
find ourselves almost in a new world, all the species and most of the
genera of the higher animals having undergone a complete change.
_Causes of the Imperfection of the Geological Record._
These facts are quite in accordance with the conclusions of geologists
as to the necessary imperfection of the geological record, since it
requires the concurrence of a number of favourable conditions to
preserve any adequate representation of the life of a given epoch. In
the first place, the animals to be preserved must not die a natural
death by disease, or old age, or by being the prey of other animals, but
must be destroyed by some accident which shall lead to their being
embedded in the soil. They must be either carried away by floods, sink
into bogs or quicksands, or be enveloped in the mud or ashes of a
volcanic eruption; and when thus embedded they must remain undisturbed
amid all the future changes of the earth's surface.
But the chances against this are enormous, because denudation is always
going on, and the rocks we now find at the earth's surface are only a
small fragment of those which were originally laid down. The
alternations of marine and freshwater deposits, and the frequent
unconformability of strata with those which overlie them, tell us
plainly of repeated elevations and depressions of the surface, and of
denudation on an enormous scale. Almost every mountain range, with its
peaks, ridges, and valleys, is but the remnant of some vast plateau
eaten away by sub-aerial agencies; every range of sea-cliffs tell us of
long slopes of land destroyed by the waves; while almost all the older
rocks which now form the surface of the earth have been once covered
with newer deposits which have long since disappeared. Nowhere are the
evidences of this denudation more apparent than in North and South
America, where granitic or metamorphic rocks cover an area hardly less
than that of all Europe. The same rocks are largely developed in Central
Africa and Eastern Asia; while, besides those portions that appear
exposed on the surface, areas of unknown extent are buried under strata
which rest on them uncomformably, and could not, therefore, constitute
the original capping under which the whole of these rocks must once have
been deeply buried; because granite can only be formed, and metamorphism
can only go on, deep down in the crust of the earth. What an
overwhelming idea does this give us of the destruction of whole piles
of rock, miles in thickness and covering areas comparable with those of
continents; and how great must have been the loss of the innumerable
fossil forms which those rocks contained! In view of such destruction we
are forced to conclude that our palaeontological collections, rich
though they may appear, are really but small and random samples, giving
no adequate idea of the mighty series of organism which have lived upon
the earth.[183]
Admitting, however, the extreme imperfection of the geological record as
a whole, it may be urged that certain limited portions of it are fairly
complete--as, for example, the various Miocene deposits of India,
Europe, and North America,--and that in these we ought to find many
examples of species and genera linked together by intermediate forms. It
may be replied that in several cases this really occurs; and the reason
why it does not occur more often is, that the theory of evolution
requires that distinct genera should be linked together, not by a direct
passage, but by the descent of both from a common ancestor, which may
have lived in some much earlier age the record of which is either
wanting or very incomplete. An illustration given by Mr. Darwin will
make this more clear to those who have not studied the subject. The
fantail and pouter pigeons are two very distinct and unlike breeds,
which we yet know to have been both derived from the common wild
rock-pigeon. Now, if we had every variety of living pigeon before us, or
even all those which have lived during the present century, we should
find no intermediate types between these two--none combining in any
degree the characters of the pouter with that of the fantail. Neither
should we ever find such an intermediate form, even had there been
preserved a specimen of every breed of pigeon since the ancestral
rock-pigeon was first tamed by man--a period of probably several
thousand years. We thus see that a complete passage from one very
distinct species to another could not be expected even had we a complete
record of the life of any one period. What we require is a complete
record of all the species that have existed since the two forms began
to diverge from their common ancestor, and this the known imperfection
of the record renders it almost impossible that we should ever attain.
All that we have a right to expect is, that, as we multiply the fossil
forms in any group, the gaps that at first existed in that group shall
become less wide and less numerous; and also that, in some cases, a
tolerably direct series shall be found, by which the more specialised
forms of the present day shall be connected with more generalised
ancestral types. We might also expect that when a country is now
characterised by special groups of animals, the fossil forms that
immediately preceded them shall, for the most part, belong to the same
groups; and further, that, comparing the more ancient with the more
modern types, we should find indications of progression, the earlier
forms being, on the whole, lower in organisation, and less specialised
in structure than the later. Now evidence of evolution of these varied
kinds is what we do find, and almost every fresh discovery adds to their
number and cogency. In order, therefore, to show that the testimony
given by geology is entirely in favour of the theory of descent with
modification, some of the more striking of the facts will now be given.
_Geological Evidences of Evolution._
In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls
attention to some recent discoveries in the Hungarian plains, of fossil
lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of
the Austrian Geological Survey. The beds in which they occur have
accumulated to the thickness of 2000 feet, containing throughout
abundance of fossils, and divisible into eight zones, each of which
exhibits a well-marked and characteristic fauna. Professor Judd then
describes the bearing of these discoveries as follows--
"The group of shells which affords the most interesting evidence
of the origin of new forms through descent with modification is
that of the genus Vivipara or Paludina, which occurs in
prodigious abundance throughout the whole series of freshwater
strata. We shall not, of course, attempt in this place to enter
into any details concerning the forty distinct _forms_ of this
genus (Dr. Neumayr very properly hesitates to call them all
_species_), which are named and described in this monograph,
and between which, as the authors show, so many connecting
links, clearly illustrating the derivation of the newer from the
older types, have been detected. On the minds of those who
carefully examine the admirably engraved figures given in the
plates accompanying this valuable memoir, or still better, the
very large series of specimens from among which the subjects of
these figures are selected, and which are now in the museum of
the Reichsanstalt of Vienna, but little doubt will, we suspect,
remain that the authors have fully made out their case, and have
demonstrated that, beyond all controversy, the series with
highly complicated ornamentation were variously derived by
descent--the lines of which are in most cases perfectly clear
and obvious--from the simple and unornamented Vivipara
achatinoides of the Congerien-Schichten (the lower division of
the series of strata). It is interesting to notice that a large
portion of these unquestionably derived forms depart so widely
from the type of the genus Vivipara, that they have been
separated on so high an authority as that of Sandberger, as a
new genus, under the name of Tulotoma. And hence we are led to
the conclusion that a vast number of forms, certainly exhibiting
specific distinctions, and according to some naturalists,
differences even entitled to be regarded of generic value, have
all a common ancestry."
It is, as Professor Judd remarks, owing to the exceptionally favourable
circumstances of a long-continued and unbroken series of deposits being
formed under physical conditions either identical or very slowly
changing, that we owe so complete a record of the process of organic
change. Usually, some disturbing elements, such as a sudden change of
physical conditions, or the immigration of new sets of forms from other
areas and the consequent retreat or partial extinction of the older
fauna, interferes with the continuity of organic development, and
produces those puzzling discordances so generally met with in geological
formations of marine origin. While a case of the kind now described
affords evidence of the origin of species complete and conclusive,
though on a necessarily very limited scale, the very rarity of the
conditions which are essential to such completeness serves to explain
why it is that in most cases the direct evidence of evolution is not to
be obtained.
Another illustration of the filling up of gaps between existing groups
is afforded by Professor Huxley's researches on fossil crocodiles. The
gap between the existing crocodiles and the lizards is very wide, but as
we go back in geological time we meet with fossil forms which are to
some extent intermediate and form a connected series. The three living
genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
formation, and allied forms of another genus, Holops, in the Chalk. From
the Chalk backward to the Lias another group of genera occurs, having
anatomical characteristics intermediate between the living crocodiles
and the most ancient forms. These, forming two genera Belodon and
Stagonolepis, are found in a still older formation, the Trias. They have
characters resembling some lizards, especially the remarkable Hatteria
of New Zealand, and have also some resemblances to the
Dinosaurians--reptiles which in some respects approach birds.
Considering how comparatively few are the remains of this group of
animals, the evidence which it affords of progressive development is
remarkably clear.[184]
Among the higher animals the rhinoceros, the horse, and the deer afford
good evidence of advance in organisation and of the filling up of the
gaps which separate the living forms from their nearest allies. The
earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
of the United States, and were to some extent intermediate between the
rhinoceros and tapir families, having like the latter four toes to the
front feet, and three to those behind. These are followed in the Upper
Eocene by the genus Amynodon, in which the skull assumes more distinctly
the rhinocerotic type. Following this in the Lower Miocene we have the
Aceratherium, like the last in its feet, but still more decidedly a
rhinoceros in its general structure. From this there are two diverging
lines--one in the Old World, the other in the New. In the former, to
which the Aceratherium is supposed to have migrated in early Miocene
times, when a mild climate and luxuriant vegetation prevailed far within
the arctic circle, it gave rise to the Ceratorhinus and the various
horned rhinoceroses of late Tertiary times and of those now living. In
America a number of large hornless rhinoceroses were developed--they
are found in the Upper Miocene, Pliocene, and Post-Pliocene
formations--and then became extinct. The true rhinoceroses have three
toes on all the feet.[185]
_The Pedigree of the Horse Tribe._
Yet more remarkable is the evidence afforded by the ancestral forms of
the horse tribe which have been discovered in the American tertiaries.
The family Equidae, comprising the living horse, asses, and zebras,
differ widely from all other mammals in the peculiar structure of the
feet, all of which terminate in a single large toe forming the hoof.
They have forty teeth, the molars being formed of hard and soft material
in crescentic folds, so as to be a powerful agent in grinding up hard
grasses and other vegetable food. The former peculiarities depend upon
modifications of the skeleton, which have been thus described by
Professor Huxley:--
"Let us turn in the first place to the fore-limb. In most
quadrupeds, as in ourselves, the fore-arm contains distinct
bones, called the radius and the ulna. The corresponding region
in the horse seems at first to possess but one bone. Careful
observation, however, enables us to distinguish in this bone a
part which clearly answers to the upper end of the ulna. This is
closely united with the chief mass of the bone which represents
the radius, and runs out into a slender shaft, which may be
traced for some distance downwards upon the back of the radius,
and then in most cases thins out and vanishes. It takes still
more trouble to make sure of what is nevertheless the fact, that
a small part of the lower end of the bone of a horse's fore-arm,
which is only distinct in a very young foal, is really the lower
extremity of the ulna.
"What is commonly called the knee of a horse is its wrist. The
'cannon bone' answers to the middle bone of the five metacarpal
bones which support the palm of the hand in ourselves. The
pastern, coronary, and coffin bones of veterinarians answer to
the joints of our middle fingers, while the hoof is simply a
greatly enlarged and thickened nail. But if what lies below the
horse's 'knee' thus corresponds to the middle finger in
ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two
slender splintlike bones, about two-thirds as long as the cannon
bone, which gradually taper to their lower ends and bear no
finger joints, or, as they are termed, phalanges. Sometimes,
small bony or gristly nodules are to be found at the bases of
these two metacarpal splints, and it is probable that these
represent rudiments of the first and fifth toes. Thus, the part
of the horse's skeleton which corresponds with that of the human
hand, contains one overgrown middle digit, and at least two
imperfect lateral digits; and these answer, respectively, to the
third, the second, and the fourth fingers in man.
"Corresponding modifications are found in the hind limb. In
ourselves, and in most quadrupeds, the leg contains two distinct
bones, a large bone, the tibia, and a smaller and more slender
bone, the fibula. But, in the horse, the fibula seems, at first,
to be reduced to its upper end; a short slender bone united with
the tibia, and ending in a point below, occupying its place.
Examination of the lower end of a young foal's shin-bone,
however, shows a distinct portion of osseous matter which is the
lower end of the fibula; so that the, apparently single, lower
end of the shin-bone is really made up of the coalesced ends of
the tibia and fibula, just as the, apparently single, lower end
of the fore-arm bone is composed of the coalesced radius and
ulna.
"The heel of the horse is the part commonly known as the hock.
The hinder cannon bone answers to the middle metatarsal bone of
the human foot, the pastern, coronary, and coffin bones, to the
middle toe bones; the hind hoof to the nail; as in the forefoot.
And, as in the forefoot, there are merely two splints to
represent the second and the fourth toes. Sometimes a rudiment
of a fifth toe appears to be traceable.
"The teeth of a horse are not less peculiar than its limbs. The
living engine, like all others, must be well stoked if it is to
do its work; and the horse, if it is to make good its wear and
tear, and to exert the enormous amount of force required for its
propulsion, must be well and rapidly fed. To this end, good
cutting instruments and powerful and lasting crushers are
needful. Accordingly, the twelve cutting teeth of a horse are
close-set and concentrated in the forepart of its mouth, like so
many adzes or chisels. The grinders or molars are large, and
have an extremely complicated structure, being composed of a
number of different substances of unequal hardness. The
consequence of this is that they wear away at different rates;
and, hence, the surface of each grinder is always as uneven as
that of a good millstone."[186]
We thus see that the Equidae differ very widely in structure from most
other mammals. Assuming the truth of the theory of evolution, we should
expect to find traces among extinct animals of the steps by which this
great modification has been effected; and we do really find traces of
these steps, imperfectly among European fossils, but far more completely
among those of America.
It is a singular fact that, although no horse inhabited America when
discovered by Europeans, yet abundance of remains of extinct horses have
been found both in North and South America in Post-Tertiary and Upper
Pliocene deposits; and from these an almost continuous series of
modified forms can be traced in the Tertiary formation, till we reach,
at the very base of the series, a primitive form so unlike our perfected
animal, that, had we not the intermediate links, few persons would
believe that the one was the ancestor of the other. The tracing out of
this marvellous history we owe chiefly to Professor Marsh of Yale
College, who has himself discovered no less than thirty species of
fossil Equidae; and we will allow him to tell the story of the
development of the horse from a humble progenitor in his own words.
"The oldest representative of the horse at present known is the
diminutive Eohippus from the Lower Eocene. Several species have
been found, all about the size of a fox. Like most of the early
mammals, these ungulates had forty-four teeth, the molars with
short crowns and quite distinct in form from the premolars. The
ulna and fibula were entire and distinct, and there were four
well-developed toes and a rudiment of another on the forefeet,
and three toes behind. In the structure of the feet and teeth,
the Eohippus unmistakably indicates that the direct ancestral
line to the modern horse has already separated from the other
perissodactyles, or odd-toed ungulates.
"In the next higher division of the Eocene another genus,
Orohippus, makes its appearance, replacing Eohippus, and showing
a greater, though still distant, resemblance to the equine type.
The rudimentary first digit of the forefoot has disappeared, and
the last premolar has gone over to the molar series. Orohippus
was but little larger than Eohippus, and in most other respects
very similar. Several species have been found, but none occur
later than the Upper Eocene.
"Near the base of the Miocene, we find a third closely allied
genus, Mesohippus, which is about as large as a sheep, and one
stage nearer the horse. There are only three toes and a
rudimentary splint on the forefeet, and three toes behind. Two
of the premolar teeth are quite like the molars. The ulna is no
longer distinct or the fibula entire, and other characters show
clearly that the transition is advancing.
"In the Upper Miocene Mesohippus is not found, but in its place
a fourth form, Miohippus, continues the line. This genus is near
the Anchitherium of Europe, but presents several important
differences. The three toes in each foot are more nearly of a
size, and a rudiment of the fifth metacarpal bone is retained.
All the known species of this genus are larger than those of
Mesohippus, and none of them pass above the Miocene formation.
"The genus Protohippus of the Lower Pliocene is yet more equine,
and some of its species equalled the ass in size. There are
still three toes on each foot, but only the middle one,
corresponding to the single toe of the horse, comes to the
ground. This genus resembles most nearly the Hipparion of
Europe.
"In the Pliocene we have the last stage of the series before
reaching the horse, in the genus Pliohippus, which has lost the
small hooflets, and in other respects is very equine. Only in
the Upper Pliocene does the true Equus appear and complete the
genealogy of the horse, which in the Post-Tertiary roamed over
the whole of North and South America, and soon after became
extinct. This occurred long before the discovery of the
continent by Europeans, and no satisfactory reason for the
extinction has yet been given. Besides the characters I have
mentioned, there are many others in the skeleton, skull, teeth,
and brain of the forty or more intermediate species, which show
that the transition from the Eocene Eohippus to the modern Equus
has taken place in the order indicated"[187] (see Fig. 33).
[Illustration: FIG. 33.--Geological development of the horse tribe
(Eohippus since discovered).]
Well may Professor Huxley say that this is demonstrative evidence of
evolution; the doctrine resting upon exactly as secure a foundation as
did the Copernican theory of the motions of the heavenly bodies at the
time of its promulgation. Both have the same basis--the coincidence of
the observed facts with the theoretical requirements.
_Development of Deer's Horns._
Another clear and unmistakable proof of evolution is afforded by one of
the highest and latest developed tribes of mammals--the true deer. These
differ from all other ruminants in possessing solid deciduous horns
which are always more or less branched. They first appear in the Middle
Miocene formation, and continue down to our time; and their development
has been carefully traced by Professor Boyd Dawkins, who thus summarises
his results:--
"In the middle stage of the Miocene the cervine antler consists
merely of a simple forked crown (as in Cervus dicroceros), which
increases in size in the Upper Miocene, although it still
remains small and erect, like that of the roe. In Cervus
Matheroni it measures 11·4 inches, and throws off not more than
four tines, all small. The deer living in Auvergne in the
succeeding or Pliocene age, present us with another stage in the
history of antler development. There, for the first time, we see
antlers of the Axis and Rusa type, larger and longer, and more
branching than any antlers were before, and possessing three or
more well-developed tines. Deer of this type abounded in
Pliocene Europe. They belong to the Oriental division of the
Cervidae, and their presence in Europe confirms the evidence of
the flora, brought forward by the Comte de Saporta, that the
Pliocene climate was warm. They have probably disappeared from
Europe in consequence of the lowering of the temperature in the
Pleistocene age, while their descendants have found a congenial
home in the warmer regions of Eastern Asia.
"In the latest stage of the Pliocene--the Upper Pliocene of the
Val d'Arno--the Cervus dicranios of Nesti presents us with
antlers much smaller than those of the Irish elk, but very
complicated in their branching. This animal survived into the
succeeding age, and is found in the pre-glacial forest bed of
Norfolk, being described by Dr. Falconer under the name of
Sedgwick's deer. The Irish elk, moose, stag, reindeer, and
fallow deer appear in Europe in the Pleistocene age, all with
highly complicated antlers in the adult, and the first
possessing the largest antlers yet known. Of these the Irish elk
disappeared in the Prehistoric age, after having lived in
countless herds in Ireland, while the rest have lived on into
our own times in Euro-Asia, and, with the exception of the last,
also in North America.
"From this survey it is obvious that the cervine antlers have
increased in size and complexity from the Mid-Miocene to the
Pleistocene age, and that their successive changes are analogous
to those which are observed in the development of antlers in the
living deer, which begin with a simple point, and increase in
number of tines till their limit of growth be reached. In other
words, the development of antlers indicated at successive and
widely-separated pages of the geological record is the same as
that observed in the history of a single living species. It is
also obvious that the progressive diminution of size and
complexity in the antlers, from the present time back into the
early Tertiary age, shows that we are approaching the zero of
antler development in the Mid-Miocene. No trace of any
antler-bearing ruminant has been met with in the lower Miocenes,
either of Europe or the United States."[188]
_Progressive Brain-Development._
The three illustrations now given sufficiently prove that, whenever the
geological record approaches to completeness, we have evidence of the
progressive change of species in definite directions, and from less
developed to more developed types--exactly such a change as we may
expect to find if the evolution theory be the true one. Many other
illustrations of a similar change could be given, but the animal groups
in which they occur being less familiar, the details would be less
interesting, and perhaps hardly intelligible. There is, however, one
very remarkable proof of development that must be briefly noticed--that
afforded by the steady increase in the size of the brain. This may be
best stated in the words of Professor Marsh:--
"The real progress of mammalian life in America, from the
beginning of the Tertiary to the present, is well illustrated by
the brain-growth, in which we have the key to many other
changes. The earliest known Tertiary mammals all had very small
brains, and in some forms this organ was proportionally less
than in certain reptiles. There was a gradual increase in the
size of the brain during this period, and it is interesting to
find that this growth was mainly confined to the cerebral
hemispheres, or higher portion of the brain. In most groups of
mammals the brain has gradually become more convoluted, and thus
increased in quality as well as quantity. In some also the
cerebellum and olfactory lobes, the lower parts of the brain,
have even diminished in size. In the long struggle for existence
during Tertiary time the big brains won, then as now; and the
increasing power thus gained rendered useless many structures
inherited from primitive ancestors, but no longer adapted to new
conditions."
This remarkable proof of development in the organ of the mental
faculties, forms a fitting climax to the evidence already adduced of the
progressive evolution of the general structure of the body, as
illustrated by the bony skeleton. We now pass on to another class of
facts equally suggestive of evolution.
_The Local Relations of Fossil and Living Animals._
If all existing animals have been produced from ancestral forms--mostly
extinct--under the law of variation and natural selection, we may expect
to find in most cases a close relation between the living forms of each
country and those which inhabited it in the immediately preceding epoch.
But if species have originated in some quite different way, either by
any kind of special creation, or by sudden advances of organisation in
the offspring of preceding types, such close relationship would not be
found; and facts of this kind become, therefore, to some extent a test
of evolution under natural selection or some other law of gradual
change. Of course the relationship will not appear when extensive
migration has occurred, by which the inhabitants of one region have been
able to take possession of another region, and destroy or drive out its
original inhabitants, as has sometimes happened. But such cases are
comparatively rare, except where great changes of climate are known to
have occurred; and we usually do find a remarkable continuity between
the existing fauna and flora of a country and those of the immediately
preceding age. A few of the more remarkable of these cases will now be
briefly noticed.
The mammalian fauna of Australia consists, as is well known, wholly of
the lowest forms--the Marsupials and Monotremata--except only a few
species of mice. This is accounted for by the complete isolation of the
country from the Asiatic continent during the whole period of the
development of the higher animals. At some earlier epoch the ancestral
marsupials, which abounded both in Europe and North America in the
middle of the Secondary period, entered the country, and have since
remained there, free from the competition of higher forms, and have
undergone a special development in accordance with the peculiar
conditions of a limited area. While in the large continents higher forms
of mammalia have been developed, which have almost or wholly
exterminated the less perfect marsupials, in Australia these latter have
become modified into such varied forms as the leaping kangaroos, the
burrowing wombats, the arboreal phalangers, the insectivorous
bandicoots, and the carnivorous Dasyuridae or native cats, culminating
in the Thylacinus or "tiger-wolf" of Tasmania--animals as unlike each
other as our sheep, rabbits, squirrels, and dogs, but all retaining the
characteristic features of the marsupial type.
Now in the caves and late Tertiary or Post-Tertiary deposits of
Australia the remains of many extinct mammalia have been found, but all
are marsupials. There are many kangaroos, some larger than any living
species, and others more allied to the tree-kangaroos of New Guinea; a
large wombat as large as a tapir; the Diprotodon, a thick-limbed
kangaroo the size of a rhinoceros or small elephant; and a quite
different animal, the Nototherium, nearly as large. The carnivorous
Thylacinus of Tasmania is also found fossil; and a huge phalanger,
Thylacoleo, the size of a lion, believed by Professor Owen and by
Professor Oscar Schmidt to have been equally carnivorous and
destructive.[189] Besides these, there are many other species more
resembling the living forms both in size and structure, of which they
may be, in some cases, the direct ancestors. Two species of extinct
Echidna, belonging to the very low Monotremata, have also been found in
New South Wales.
Next to Australia, South America possesses the most remarkable
assemblage of peculiar mammals, in its numerous Edentata--the sloths,
ant-eaters, and armadillos; its rodents, such as the cavies and
chinchillas; its marsupial opossums, and its quadrumana of the family
Cebidae. Remains of extinct species of all these have been found in the
caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
deposits of the pampas many distinct genera of these groups have been
found, some of gigantic size and extraordinary form. There are
armadillos of many types, some being as large as elephants; gigantic
sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
others; rodents belonging to the American families Cavidae and
Chinchillidae; and ungulates allied to the llama; besides many other
extinct forms of intermediate types or of uncertain affinities.[190] The
extinct Moas of New Zealand--huge wingless birds allied to the living
Apteryx--illustrate the same general law.
The examples now quoted, besides illustrating and enforcing the general
fact of evolution, throw some light on the usual character of the
modification and progression of animal forms. In the cases where the
geological record is tolerably complete, we find a continuous
development of some kind--either in complexity of ornamentation, as in
the fossil Paludinas of the Hungarian lake-basins; in size and in the
specialisation of the feet and teeth, as in the American fossil horses;
or in the increased development of the branching horns, as in the true
deer. In each of these cases specialisation and adaptation to the
conditions of the environment appear to have reached their limits, and
any change of these conditions, especially if it be at all rapid or
accompanied by the competition of less developed but more adaptable
forms, is liable to cause the extinction of the most highly developed
groups. Such we know was the case with the horse tribe in America, which
totally disappeared in that continent at an epoch so recent that we
cannot be sure that the disappearance was not witnessed, perhaps caused,
by man; while even in the Eastern hemisphere it is the smaller
species--the asses and the zebras--that have persisted, while the larger
and more highly developed true horses have almost, if not quite,
disappeared in a state of nature. So we find, both in Australia and
South America, that in a quite recent period many of the largest and
most specialised forms have become extinct, while only the smaller types
have survived to our day; and a similar fact is to be observed in many
of the earlier geological epochs, a group progressing and reaching a
maximum of size or complexity and then dying out, or leaving at most but
few and pigmy representatives.
_Cause of Extinction of Large Animals._
Now there are several reasons for the repeated extinction of large
rather than of small animals. In the first place, animals of great bulk
require a proportionate supply of food, and any adverse change of
conditions would affect them more seriously than it would smaller
animals. In the next place, the extreme specialisation of many of these
large animals would render it less easy for them to be modified in any
new direction suited to changed conditions. Still more important,
perhaps, is the fact that very large animals always increase slowly as
compared with small ones--the elephant producing a single young one
every three years, while a rabbit may have a litter of seven or eight
young two or three times a year. Now the probability of favourable
variations will be in direct proportion to the population of the
species, and as the smaller animals are not only many hundred times more
numerous than the largest, but also increase perhaps a hundred times as
rapidly, they are able to become quickly modified by variation and
natural selection in harmony with changed conditions, while the large
and bulky species, being unable to vary quickly enough, are obliged to
succumb in the struggle for existence. As Professor Marsh well observes:
"In every vigorous primitive type which was destined to survive many
geological changes, there seems to have been a tendency to throw off
lateral branches, which became highly specialised and soon died out,
because they were unable to adapt themselves to new conditions." And he
goes on to show how the whole narrow path of the persistent Suilline
type, throughout the entire series of the American tertiaries, is
strewed with the remains of such ambitious offshoots, many of them
attaining the size of a rhinoceros; "while the typical pig, with an
obstinacy never lost, has held on in spite of catastrophes and
evolution, and still lives in America to-day."
_Indications of General Progression in Plants and Animals._
One of the most powerful arguments formerly adduced against evolution
was, that geology afforded no evidence of the gradual development of
organic forms, but that whole tribes and classes appeared suddenly at
definite epochs, and often in great variety and exhibiting a very
perfect organisation. The mammalia, for example, were long thought to
have first appeared in Tertiary times, where they are represented in
some of the earlier deposits by all the great divisions of the class
fully developed--carnivora, rodents, insectivora, marsupials, and even
the perissodactyle and artiodactyle divisions of the ungulata--as
clearly defined as at the present day. The discovery in 1818 of a single
lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
the generalisation, since either its mammalian character was denied, or
the geological position of the strata, in which it was found, was held
to have been erroneously determined. But since then, at intervals of
many years, other remains of mammalia have been discovered in the
Secondary strata, ranging from the Upper Oolite to the Upper Trias both
in Europe and the United States, and one even (Tritylodon) in the Trias
of South Africa. All these are either marsupials, or of some still lower
type of mammalia; but they consist of many distinct forms classed in
about twenty genera. Nevertheless, a great gap still exists between
these mammals and those of the Tertiary strata, since no mammal of any
kind has been found in any part of the Cretaceous formation, although in
several of its subdivisions abundance of land plants, freshwater shells,
and air-breathing reptiles have been discovered. So with fishes. In the
last century none had been obtained lower than the Carboniferous
formation; thirty years later they were found to be very abundant in the
Devonian rocks, and later still they were discovered in the Upper Ludlow
and Lower Ludlow beds of the Silurian formation.
We thus see that such sudden appearances are deceptive, and are, in
fact, only what we ought to expect from the known imperfection of the
geological record. The conditions favourable to the fossilisation of any
group of animals occur comparatively rarely, and only in very limited
areas; while the conditions essential for their permanent preservation
in the rocks, amid all the destruction caused by denudation or
metamorphism, are still more exceptional. And when they are thus
preserved to our day, the particular part of the rocks in which they lie
hidden may not be on the surface but buried down deep under other
strata, and may thus, except in the case of mineral-bearing deposits, be
altogether out of our reach. Then, again, how large a proportion of the
earth consists of wild and uncivilised regions in which no exploration
of the rocks has been yet made, so that whether we shall find the
fossilised remains of any particular group of animals which lived during
a limited period of the earth's history, and in a limited area, depends
upon at least a fivefold combination of chances. Now, if we take each of
these chances separately as only ten to one against us (and some are
certainly more than this), then the actual chance against our finding
the fossil remains, say of any one order of mammalia, or of land plants,
at any particular geological horizon, will be about a hundred thousand
to one.
It may be said, if the chances are so great, how is it that we find such
immense numbers of fossil species exceeding in number, in some groups,
all those that are now living? But this is exactly what we should
expect, because the number of species of organisms that have ever lived
upon the earth, since the earliest geological times, will probably be
many hundred times greater than those now existing of which we have any
knowledge; and hence the enormous gaps and chasms in the geological
record of extinct forms is not to be wondered at. Yet, notwithstanding
these chasms in our knowledge, if evolution is true, there ought to have
been, on the whole, progression in all the chief types of life. The
higher and more specialised forms should have come into existence later
than the lower and more generalised forms; and however fragmentary the
portions we possess of the whole tree of life upon the earth, they ought
to show us broadly that such a progressive evolution has taken place. We
have seen that in some special groups, already referred to, such a
progression is clearly visible, and we will now cast a hasty glance over
the entire series of fossil forms, in order to see if a similar
progression is manifested by them as a whole.
_The Progressive Development of Plants._
Ever since fossil plants have been collected and studied, the broad fact
has been apparent that the early plants--those of the Coal
formation--were mainly cryptogamous, while in the Tertiary deposits the
higher flowering plants prevailed. In the intermediate secondary epoch
the gymnosperms--cycads and coniferae--formed a prominent part of the
vegetation, and as these have usually been held to be a kind of
transition form between the flowerless and flowering plants, the
geological succession has always, broadly speaking, been in accordance
with the theory of evolution. Beyond this, however, the facts were very
puzzling. The highest cryptogams--ferns, lycopods, and
equisetaceae--appeared suddenly, and in immense profusion in the Coal
formation, at which period they attained a development they have never
since surpassed or even equalled; while the highest plants--the
dicotyledonous and monocotyledonous angiosperms--which now form the bulk
of the vegetation of the world, and exhibit the most wonderful
modifications of form and structure, were almost unknown till the
Tertiary period, when they suddenly appeared in full development, and,
for the most part, under the same generic forms as now exist.
During the latter half of the present century, however, great additions
have been made to our knowledge of fossil plants; and although there
are still indications of vast gaps in our knowledge, due, no doubt, to
the very exceptional conditions required for the preservation of plant
remains, we now possess evidence of a more continuous development of the
various types of vegetation. According to Mr. Lester F. Ward, between
8000 and 9000 species of fossil plants have been described or indicated;
and, owing to the careful study of the nervation of leaves, a large
number of these are referable to their proper orders or genera, and
therefore give us some notion--which, though very imperfect, is probably
accurate in its main outlines--of the progressive development of
vegetation on the earth.[191] The following is a summary of the facts as
given by Mr. Ward:--
The lowest forms of vegetable life--the cellular plants--have been found
in Lower Silurian deposits in the form of three species of marine algae;
and in the whole Silurian formation fifty species have been recognised.
We cannot for a moment suppose, however, that this indicates the first
appearance of vegetable life upon the earth, for in these same Lower
Silurian beds the more highly organised vascular cryptogams appear in
the form of rhizocarps--plants allied to Marsilea and Azolla,--and a
very little higher, ferns, lycopods, and even conifers appear. We have
indications, however, of a still more ancient vegetation, in the
carbonaceous shales and thick beds of graphite far down in the Middle
Laurentian, since there is no other known agency than the vegetable cell
by means of which carbon can be extracted from the atmosphere and fixed
in the solid state. These great beds of graphite, therefore, imply the
existence of abundance of vegetable life at the very commencement of the
era of which we have any geological record.[192]
Ferns, as already stated, begin in the Middle Silurian formation with
the Eopteris Morrieri. In the Devonian, we have 79 species, in the
Carboniferous 627, and in the Permian 186 species; after which fossil
ferns diminish greatly, though they are found in every formation; and
the fact that fully 3000 living species are known, while the richest
portion of the Tertiary in fossil plants--the Miocene--- has only
produced 87 species, will serve to indicate the extreme imperfection of
the geological record.
The Equisetaceae (horsetails) which also first appear in the Silurian and
reach their maximum development in the Coal formation, are, in all
succeeding formations, far less numerous than ferns, and only thirty
living species are known. Lycopodiaceae, though still more abundant in
the Coal formation, are very rarely found in any succeeding deposit,
though the living species are tolerably numerous, about 500 having been
described. As we cannot suppose them to have really diminished and then
increased again in this extraordinary manner, we have another indication
of the exceptional nature of plant preservation and the extreme and
erratic character of the imperfection of the record.
Passing now to the next higher division of plants--the gymnosperms--we
find Coniferae appearing in the Upper Silurian, becoming tolerably
abundant in the Devonian, and reaching a maximum in the Carboniferous,
from which formation more than 300 species are known, equal to the
number recorded as now living. They occur in all succeeding formations,
being abundant in the Oolite, and excessively so in the Miocene, from
which 250 species have been described. The allied family of gymnosperms,
the Cycadaceae, first appear in the Carboniferous era, but very
scantily; are most abundant in the Oolite, from which formation 116
species are known, and then steadily diminish to the Tertiary, although
there are seventy-five living species.
We now come to the true flowering plants, and we first meet with
monocotyledons in the Carboniferous and Permian formations. The
character of these fossils was long disputed, but is now believed to be
well established; and the sub-class continues to be present in small
numbers in all succeeding deposits, becoming rather plentiful in the
Upper Cretaceous, and very abundant in the Eocene and Miocene. In the
latter formation 272 species have been discovered; but the 116 species
in the Eocene form a larger proportion of the total vegetation of the
period.
True dicotyledons appear very much later, in the Cretaceous period, and
only in its upper division, if we except a single species from the
Urgonian beds of Greenland. The remarkable thing is that we here find
the sub-class fully developed and in great luxuriance of types, all the
three divisions--Apetalae, Polypetalae, and Gamopetalae--being
represented, with a total of no less than 770 species. Among them are
such familiar forms as the poplar, the birch, the beech, the sycamore,
and the oak; as well as the fig, the true laurel, the sassafras, the
persimmon, the maple, the walnut, the magnolia, and even the apple and
the plum tribes. Passing on to the Tertiary period the numbers increase,
till they reach their maximum in the Miocene, where more than 2000
species of dicotyledons have been discovered. Among these the
proportionate number of the higher gamopetalae has slightly increased,
but is considerably less than at the present day.
_Possible Cause of sudden late Appearance of Exogens._
The sudden appearance of fully developed exogenous flowering plants in
the Cretaceous period is very analogous to the equally sudden appearance
of all the chief types of placental mammalia in the Eocene; and in both
cases we must feel sure that this suddenness is only apparent, due to
unknown conditions which have prevented their preservation (or their
discovery) in earlier formations. The case of the dicotyledonous plants
is in some respects the most extraordinary, because in the earlier
Mesozoic formations we appear to have a fair representation of the flora
of the period, including such varied forms as ferns, equisetums, cycads,
conifers, and monocotyledons. The only hint at an explanation of this
anomaly has been given by Mr. Ball, who supposes that all these groups
inhabited the lowlands, where there was not only excessive heat and
moisture, but also a superabundance of carbonic acid in the
atmosphere--conditions under which these groups had been developed, but
which were prejudicial to the dicotyledons. These latter are supposed to
have originated on the high table-lands and mountain ranges, in a rarer
and drier atmosphere in which the quantity of carbonic acid gas was much
less; and any deposits formed in lake beds at high altitudes and at such
a remote epoch have been destroyed by denudation, and hence we have no
record of their existence.[193]
During a few weeks spent recently in the Rocky Mountains, I was struck
by the great scarcity of monocotyledons and ferns in comparison with
dicotyledons--a scarcity due apparently to the dryness and rarity of the
atmosphere favouring the higher groups. If we compare Coulter's _Rocky
Mountain Botany_ with Gray's _Botany of the Northern (East) United
States_, we have two areas which differ chiefly in the points of
altitude and atmospheric moisture. Unfortunately, in neither of these
works are the species consecutively numbered; but by taking the pages
occupied by the two divisions of dicotyledons on the one hand,
monocotyledons and ferns on the other, we can obtain a good
approximation. In this way we find that in the flora of the
North-Eastern States the monocotyledons and ferns are to the
dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they
are in the proportion of only 34 to 100; while if we take an exclusively
Alpine flora, as given by Mr. Ball, there are not one-fifth as many
monocotyledons as dicotyledons. These facts show that even at the
present day elevated plateaux and mountains are more favourable to
dicotyledons than to monocotyledons, and we may, therefore, well suppose
that the former originated within such elevated areas, and were for long
ages confined to them. It is interesting to note that their richest
early remains have been found in the central regions of the North
American continent, where they now, proportionally, most abound, and
where the conditions of altitude and a dry atmosphere were probably
present at a very early period.
[Illustration: FIG. 34.--Diagram illustrating the Geological
Distribution of Plants.]
The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
will illustrate our present knowledge of the development of the
vegetable kingdom in geological time. The shaded vertical bands exhibit
the proportions of the fossil forms actually discovered, while the
outline extensions are intended to show what we may fairly presume to
have been the approximate periods of origin, and progressive increase of
the number of species, of the chief divisions of the vegetable kingdom.
These seem to accord fairly well with their respective grades of
development, and thus offer no obstacle to the acceptance of the belief
in their progressive evolution.
_Geological Distribution of Insects._
The marvellous development of insects into such an endless variety of
forms, their extreme specialisation, and their adaptation to almost
every possible condition of life, would almost necessarily imply an
extreme antiquity. Owing, however, to their small size, their lightness,
and their usually aerial habits, no class of animals has been so
scantily preserved in the rocks; and it is only recently that the whole
of the scattered material relating to fossil insects and their allies
have been brought together by Mr. Samuel H. Scudder of Boston, and we
have thus learned their bearing on the theory of evolution.[194]
The most striking fact which presents itself on a glance at the
distribution of fossil insects, is the completeness of the
representation of all the chief types far back in the Secondary period,
at which time many of the existing families appear to have been
perfectly differentiated. Thus in the Lias we find dragonflies
"apparently as highly specialised as to-day, no less than four tribes
being present." Of beetles we have undoubted Curculionidae from the Lias
and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
Lias. All these forms are well represented, but there are many other
families doubtfully identified in equally ancient rocks. Diptera of the
families Empidae, Asilidae, and Tipulidae have been found as far back as
the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
the Oolite; while ants, representing the highly specialised Hymenoptera,
have occurred in the Purbeck and Lias.
This remarkable identity of the families of very ancient with those of
existing insects is quite comparable with the apparently sudden
appearance of existing genera of trees in the Cretaceous epoch. In both
cases we feel certain that we must go very much farther back in order to
find the ancestral forms from which they were developed, and that at any
moment some fresh discovery may revolutionise our ideas as to the
antiquity of certain groups. Such a discovery was made while Mr.
Scudder's work was passing through the press. Up to that date all the
existing orders of true insects appeared to have originated in the
Trias, the alleged moth and beetle of the Coal formation having been
incorrectly determined. But now, undoubted remains of beetles have been
found in the Coal measures of Silesia, thus supporting the
interpretation of the borings in carboniferous trees as having been made
by insects of this order, and carrying back this highly specialised form
of insect life well into Palaeozoic times. Such a discovery renders all
speculation as to the origin of true insects premature, because we may
feel sure that all the other orders of insects, except perhaps
hymenoptera and lepidoptera, were contemporaneous with the highly
specialised beetles.
The less highly organised terrestrial arthropoda--the Arachnida and
Myriapoda--are, as might be expected, much more ancient. A fossil spider
has been found in the Carboniferous, and scorpions in the Upper Silurian
rocks of Scotland, Sweden, and the United States. Myriapoda have been
found abundantly in the Carboniferous and Devonian formations; but all
are of extinct orders, exhibiting a more generalised structure than
living forms.
Much more extraordinary, however, is the presence in the Palaeozoic
formations of ancestral forms of true insects, termed by Mr. Scudder
Palaeodictyoptera. They consist of generalised cockroaches and
walking-stick insects (Orthopteroidea); ancient mayflies and allied
forms, of which there are six families and more than thirty genera
(Neuropteroidea); three genera of Hemipteroidea resembling various
Homoptera and Hemiptera, mostly from the Carboniferous formation, a few
from the Devonian, and one ancestral cockroach (Palaeoblattina) from
the Middle Silurian sandstone of France. If this occurrence of a true
hexapod insect from the Middle Silurian be really established, taken in
connection with the well-defined Coleoptera from the Carboniferous, the
origin of the entire group of terrestrial arthropoda is necessarily
thrown back into the Cambrian epoch, if not earlier. And this cannot be
considered improbable in view of the highly differentiated land
plants--ferns, equisetums, and lycopods--in the Middle or Lower
Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian;
while the beds of graphite in the Laurentian were probably formed from
terrestrial vegetation.
On the whole, then, we may affirm that, although the geological record
of the insect life of the earth is exceptionally imperfect, it yet
decidedly supports the evolution hypothesis. The most specialised order,
Lepidoptera, is the most recent, only dating back to the Oolite; the
Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
Orthoptera and Neuroptera extend to the Trias. The recent discovery of
Coleoptera in the Carboniferous shows, however, that the preceding
limits are not absolute, and will probably soon be overpassed. Only the
more generalised ancestral forms of winged insects have been traced back
to Silurian time, and along with them the less highly organised
scorpions; facts which serve to show us the extreme imperfection of our
knowledge, and indicate possibilities of a world of terrestrial life in
the remotest Palaeozoic times.
_Geological Succession of Vertebrata._
The lowest forms of vertebrates are the fishes, and these appear first
in the geological record in the Upper Silurian formation. The most
ancient known fish is a Pteraspis, one of the bucklered ganoids or
plated fishes--by no means a very low type--allied to the sturgeon
(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
nearly extinct. Almost equally ancient are the sharks, which under
various forms still abound in our seas. We cannot suppose these to be
nearly the earliest fishes, especially as the two lowest orders, now
represented by the Amphioxus or lancelet and the lampreys, have not yet
been found fossil. The ganoids were greatly developed in the Devonian
era, and continued till the Cretaceous, when they gave way to the true
osseous fishes, which had first appeared in the Jurassic period, and
have continued to increase till the present day. This much later
appearance of the higher osseous fishes is quite in accordance with
evolution, although some of the very lowest forms, the lancelet and the
lampreys, together with the archaic ceratodus, have survived to our
time.
The Amphibia, represented by the extinct labyrinthodons, appear first in
the Carboniferous rocks, and these peculiar forms became extinct early
in the Secondary period. The labyrinthodons were, however, highly
specialised, and do not at all indicate the origin of the class, which
may be as ancient as the lower forms of fishes. Hardly any recognisable
remains of our existing groups--the frogs, toads, and salamanders--are
found before the Tertiary period, a fact which indicates the extreme
imperfection of the record as regards this class of animals.
True reptiles have not been found till we reach the Permian where
Prohatteria and Proterosaurus occur, the former closely allied to the
lizard-like Sphenodon of New Zealand, the latter having its nearest
allies in the same group of reptiles--Rhyncocephala, other forms of
which occur in the Trias. In this last-named formation the earliest
crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
Fossil serpents have been first found in the Cretaceous formation, but
the conditions for the preservation of these forms have evidently been
unfavourable, and the record is correspondingly incomplete. The marine
Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
land animal that has ever lived upon the earth[196]--all belong to
special developments of the reptilian type which flourished during the
Secondary epoch, and then became extinct.
Birds are among the rarest of fossils, due, no doubt, to their aerial
habits removing them from the ordinary dangers of flood, bog, or ice
which overwhelm mammals and reptiles, and also to their small specific
gravity which keeps them floating on the surface of water till devoured.
Their remains were long confined to Tertiary deposits, where many living
genera and a few extinct forms have been found. The only birds yet known
from the older rocks are the toothed birds (Odontornithes) of the
Cretaceous beds of the United States, belonging to two distinct families
and many genera; a penguin-like form (Enaliornis) from the Upper
Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
from the Upper Oolite of Bavaria. The record is thus imperfect and
fragmentary in the extreme; but it yet shows us, in the few birds
discovered in the older rocks, more primitive and generalised types,
while the Tertiary birds had already become specialised like those
living, and had lost both the teeth and the long vertebral tail, which
indicate reptilian affinities in the earlier Mammalia have been found,
as already stated, as far back as the Trias formation, in Europe in the
United States and in South Africa, all being very small, and belonging
either to the Marsupial order, or to some still lower and more
generalised type, out of which both Marsupials and Insectivora were
developed. Other allied forms have been found in the Lower and Upper
Oolite both of Europe and the United States. But there is then a great
gap in the whole Cretaceous formation, from which no mammal has been
obtained, although both in the Wealden and the Upper Chalk in Europe,
and in the Upper Cretaceous deposits of the United States an abundant
and well-preserved terrestrial flora has been discovered. Why no mammals
have left their remains here it is impossible to say. We can only
suppose that the limited areas in which land plants have been so
abundantly preserved, did not present the conditions which are needed
for the fossilisation and preservation of mammalian remains.
When we come to the Tertiary formation, we find mammals in abundance;
but a wonderful change has taken place. The obscure early types have
disappeared, and we discover in their place a whole series of forms
belonging to existing orders, and even sometimes to existing families.
Thus, in the Eocene we have remains of the opossum family; bats
apparently belonging to living genera; rodents allied to the South
American cavies and to dormice and squirrels; hoofed animals belonging
to the odd-toed and even-toed groups; and ancestral forms of cats,
civets, dogs, with a number of more generalised forms of carnivora.
Besides these there are whales, lemurs, and many strange ancestral forms
of proboscidea.[197]
The great diversity of forms and structures at so remote an epoch would
require for their development an amount of time, which, judging by the
changes that have occurred in other groups, would carry us back far into
the Mesozoic period. In order to understand why we have no record of
these changes in any part of the world, we must fall back upon some such
supposition as we made in the case of the dicotyledonous plants.
Perhaps, indeed, the two cases are really connected, and the upland
regions of the primeval world, which saw the development of our higher
vegetation, may have also afforded the theatre for the gradual
development of the varied mammalian types which surprise us by their
sudden appearance in Tertiary times.
[Illustration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]
Notwithstanding these irregularities and gaps in the record, the
accompanying table, summarising our actual knowledge of the geological
distribution of the five classes of vertebrata, exhibits a steady
progression from lower to higher types, excepting only the deficiency in
the bird record which is easily explained. The comparative perfection of
type in which each of these classes first appears, renders it certain
that the origin of each and all of them must be sought much farther back
than any records which have yet been discovered. The researches of
palaeontologists and embryologists indicate a reptilian origin for birds
and mammals, while reptiles and amphibia arose, perhaps independently,
from fishes.
_Concluding Remarks._
The brief review we have now taken of the more suggestive facts
presented by the geological succession of organic forms, is sufficient
to show that most, if not all, of the supposed difficulties which it
presents in the way of evolution, are due either to imperfections in the
geological record itself, or to our still very incomplete knowledge of
what is really recorded in the earth's crust. We learn, however, that
just as discovery progresses, gaps are filled up and difficulties
disappear; while, in the case of many individual groups, we have already
obtained all the evidence of progressive development that can reasonably
be expected. We conclude, therefore, that the geological difficulty has
now disappeared; and that this noble science, when properly understood,
affords clear and weighty evidence of evolution.
FOOTNOTES:
[Footnote 183: The reader who desires to understand this subject more
fully, should study chap. x. of the _Origin of Species_, and chap. xiv.
of Sir Charles Lyell's _Principles of Geology_.]
[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the
Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in
_Nature_, vol. xii. p. 38.]
[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin
and Development of the Rhinoceros Group," read before the British
Association in 1883.]
[Footnote 186: American Addresses, pp. 73-76.]
[Footnote 187: Lecture on the Introduction and Succession of Vertebrate
Life in America, _Nature_, vol. xvi. p. 471.]
[Footnote 188: _Nature_, vol. xxv. p. 84.]
[Footnote 189: See _The Mammalia in their Relation to Primeval Times_,
p. 102.]
[Footnote 190: For a brief enumeration and description of these fossils,
see the author's _Geographical Distribution of Animals_, vol. i. p.
146.]
[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.
Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William
Dawson, speaking of the value of leaves for the determination of fossil
plants, says: "In my own experience I have often found determinations of
the leaves of trees confirmed by the discovery of their fruits or of the
structure of their stems. Thus, in the rich cretaceous plant-beds of the
Dunvegan series, we have beech-nuts associated in the same bed with
leaves referred to _Fagus_. In the Laramie beds I determined many years
ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux
found in beds in the United States leaves which he referred to the same
genus. Later, I found in collections made on the Red Deer River of
Canada my fruits and Lesquereux's leaves on the same slab. The presence
of trees of the genera _Carya_ and _Juglans_ in the same formation was
inferred from their leaves, and specimens have since been obtained of
silicified wood with the microscopic structure of the modern butternut.
Still we are willing to admit that determinations from leaves alone are
liable to doubt."--_The Geological History of Plants_, p. 196.]
[Footnote 192: Sir J. William Dawson's _Geological History of Plants_,
p. 18.]
[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.
of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]
[Footnote 194: Systematic Review of our Present Knowledge of Fossil
Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.
Survey_, No. 31, Washington, 1886).]
[Footnote 195: For the facts as to the early appearance of the above
named groups of reptiles I am indebted to Mr. E. Lydekker of the
Geological Department of the Natural History Museum.]
[Footnote 196: According to Professor Marsh this creature was 50 or 60
feet long, and when erect, at least 30 feet in height. It fed upon the
foliage of the mountain forests of the Cretaceous epoch, the remains of
which are preserved with it.]
[Footnote 197: For fuller details, see the author's _Geographical
Distribution of Animals_, and Heilprin's _Geographical and Geological
Distribution of Animals_.]
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.
Having now set forth and illustrated at some length the most important
of the applications of the development hypothesis in the explanation of
the broader and more generally interesting phenomena presented by the
organic world, we propose to discuss some of the more fundamental
problems and difficulties which have recently been adduced by eminent
naturalists. It is the more necessary to do this, because there is now a
tendency to minimise the action of natural selection in the production
of organic forms, and to set up in its place certain fundamental
principles of variation or laws of growth, which it is urged are the
real originators of the several lines of development, and of most of the
variety of form and structure in the vegetable and animal kingdoms.
These views have, moreover, been seized upon by popular writers to throw
doubt and discredit on the whole theory of evolution, and especially on
Darwin's presentation of that theory, to the bewilderment of the general
public, who are quite unable to decide how far the new views, even if
well established, tend to subvert the Darwinian theory, or whether they
are really more than subsidiary parts of it, and quite powerless without
it to produce any effect whatever.
The writers whose special views we now propose to consider are: (1) Mr.
Herbert Spencer, on modification of structures arising from modification
of functions, as set forth in his _Factors of Organic Evolution_. (2)
Dr. E.D. Cope, who advocates similar views in detail, in his work
entitled _The Origin of the Fittest_, and may be considered the head of
a school of American naturalists who minimise the agency of natural
selection. (3) Dr. Karl Semper, who has especially studied the direct
influence of the environment in the whole animal kingdom, and has set
forth his views in a volume on _The Natural Conditions of Existence as
they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that
fundamental laws of growth, and the antagonism of vegetative and
reproductive forces, account for much that has been imputed to natural
selection.
We will now endeavour to ascertain what are the more important facts and
arguments adduced by each of the above writers, and how far they offer a
substitute for the action of natural selection; having done which, a
brief account will be given of the views of Dr. Aug. Weismann, whose
theory of heredity will, if established, strike at the very root of the
arguments of the first three of the writers above referred to.
_Mr. Herbert Spencer's Factors of Organic Evolution._
Mr. Spencer, while fully recognising the importance and wide range of
the principle of natural selection, thinks that sufficient weight has
not been given to the effects of use and disuse as a factor in
evolution, or to the direct action of the environment in determining or
modifying organic structures. As examples of the former class of
actions, he adduces the decreased size of the jaws in the civilised
races of mankind, the inheritance of nervous disease produced by
overwork, the great and inherited development of the udders in cows and
goats, and the shortened legs, jaws, and snout in improved races of
pigs--the two latter examples being quoted from Mr. Darwin,--and other
cases of like nature. As examples of the latter, Mr. Darwin is again
quoted as admitting that there are many cases in which the action of
similar conditions appears to have produced corresponding changes in
different species; and we have a very elaborate discussion of the direct
action of the medium in modifying the protoplasm of simple organisms, so
as to bring about the difference between the outer surface and the inner
part that characterises the cells or other units of which they are
formed.
Now, although this essay did little more than bring together facts which
had been already adduced by Mr. Darwin or by Mr. Spencer himself, and
lay stress upon their importance, its publication in a popular review
was immediately seized upon as "an avowed and definite declaration
against some of the leading ideas on which the Mechanical Philosophy
depends," and as being "fatal to the adequacy of the Mechanical
Philosophy as any explanation of organic evolution,"[198]--an expression
of opinion which would be repudiated by every Darwinian. For, even
admitting the interpretation which Mr. Spencer puts on the facts he
adduces, they are all included in the causes which Darwin himself
recognised as having acted in bringing about the infinitude of forms in
the organic world. In the concluding chapter of the _Origin of Species_
he says: "I have now recapitulated the facts and considerations which
have thoroughly convinced me that species have been modified during a
long course of descent. This has been effected chiefly through the
natural selection of numerous successive, slight, favourable variations;
aided in an important manner by the inherited effects of the use and
disuse of parts; and in an unimportant manner--that is, in relation to
adaptive structures whether past or present, by the direct action of
external conditions, and by variations which seem to us, in our
ignorance, to arise spontaneously." This passage, summarising Darwin's
whole inquiry, and explaining his final point of view, shows how very
inaccurate may be the popular notion, as expressed by the Duke of
Argyll, of any supposed additions to the causes of change of species as
recognised by Darwin.
But, as we shall see presently, there is now much reason to believe
that the supposed inheritance of acquired modifications--that is, of the
effects of use and disuse, or of the direct influence of the
environment--is not a fact; and if so, the very foundation is taken away
from the whole class of objections on which so much stress is now laid.
It therefore becomes important to inquire whether the facts adduced by
Darwin, Spencer, and others, do really necessitate such inheritance, or
whether any other interpretation of them is possible. I believe there is
such an interpretation; and we will first consider the cases of disuse
on which Mr. Spencer lays most stress.
The cases Mr. Spencer adduces as demonstrating the effects of disuse in
diminishing the size and strength of organs are, the diminished size of
the jaws in the races of civilised men, and the diminution of the
muscles used in closing the jaws in the case of pet-dogs fed for
generations on soft food. He argues that the minute reduction in any one
generation could not possibly have been useful, and, therefore, not the
subject of natural selection; and against the theory of correlation of
the diminished jaw with increased brain in man, he urges that there are
cases of large brain development, accompanied by jaws above the average
size. Against the theory of economy of nutrition in the case of the
pet-dogs, he places the abundant food of these animals which would
render such economy needless.
But neither he nor Mr. Darwin has considered the effects of the
withdrawal of the action of natural selection in keeping up the parts in
question to their full dimensions, which, of itself, seems to me quite
adequate to produce the results observed. Recurring to the evidence,
adduced in Chapter III, of the constant variation occurring in all parts
of the organism, while selection is constantly acting on these
variations in eliminating all that fall below the best working standard,
and preserving only those that are fully up to it; and, remembering
further, that, of the whole number of the increase produced annually,
only a small percentage of the best adapted can be preserved, we shall
see that every useful organ will be kept up nearly to its higher limit
of size and efficiency. Now Mr. Galton has proved experimentally that,
when any part has thus been increased (or diminished) by selection,
there is in the offspring a strong tendency to revert to a mean or
average size, which tends to check further increase. And this mean
appears to be, not the mean of the actual existing individuals but a
lower mean, or that from which they had been recently raised by
selection.[199] He calls this the law of "Regression towards
Mediocrity," and it has been proved by experiments with vegetables and
by observations on mankind. This regression, in every generation, takes
place even when both parents have been selected for their high
development of the organ in question; but when there is no such
selection, and crosses are allowed among individuals of every grade of
development, the deterioration will be very rapid; and after a time not
only will the average size of the part be greatly reduced, but the
instances of full development will become very rare. Thus what Weismann
terms "panmixia," or free intercrossing, will co-operate with Galton's
law of "regression towards mediocrity," and the result will be that,
whenever selection ceases to act on any part or organ which has
heretofore been kept up to a maximum of size and efficiency, the organ
in question will rapidly decrease till it reaches a mean value
considerably below the mean of the progeny that has usually been
produced each year, and very greatly below the mean of that portion
which has survived annually; and this will take place by the general law
of heredity, and quite irrespective of any _use_ or _disuse_ of the part
in question. Now, no observations have been adduced by Mr. Spencer or
others, showing that the average amount of change supposed to be due to
_disuse_ is greater than that due to the law of regression towards
mediocrity; while even if it were somewhat greater, we can see many
possible contributory causes to its production. In the case of civilised
man's diminished jaw, there may well be some correlation between the jaw
and the brain, seeing that increased mental activity would lead to the
withdrawal of blood and of nervous energy from adjacent parts, and might
thus lead to diminished growth of those parts in the individual. And in
the case of pet-dogs, the selection of small or short-headed individuals
would imply the unconscious selection of those with less massive
temporal muscles, and thus lead to the concomitant reduction of those
muscles. The amount of reduction observed by Darwin in the wing-bones of
domestic ducks and poultry, and in the hind legs of tame rabbits, is
very small, and is certainly no greater than the above causes will well
account for; while so many of the external characters of all our
domestic animals have been subject to long-continued artificial
selection, and we are so ignorant of the possible correlations of
different parts, that the phenomena presented by them seem sufficiently
explained without recurrence to the assumption that any changes in the
individual, due to disuse, are inherited by the offspring.
_Supposed Effects of Disuse among Wild Animals._
It may be urged, however, that among wild animals we have many undoubted
results of disuse much more pronounced than those among domestic kinds,
results which cannot be explained by the causes already adduced. Such
are the reduced size of the wings of many birds on oceanic islands; the
abortion of the eyes in many cave animals, and in some which live
underground; and the loss of the hind limbs in whales and in some
lizards. These cases differ greatly in the amount of the reduction of
parts which has taken place, and may be due to different causes. It is
remarkable that in some of the birds of oceanic islands the reduction is
little if at all greater than in domestic birds, as in the water-hen of
Tristan d'Acunha. Now if the reduction of wing were due to the
hereditary effects of disuse, we should expect a very much greater
effect in a bird inhabiting an oceanic island than in a domestic bird,
where the disuse has been in action for an indefinitely shorter period.
In the case of many other birds, however--as some of the New Zealand
rails and the extinct dodo of Mauritius--the wings have been reduced to
a much more rudimentary condition, though it is still obvious that they
were once organs of flight; and in these cases we certainly require some
other causes than those which have reduced the wings of our domestic
fowls. One such cause may have been of the same nature as that which has
been so efficient in reducing the wings of the insects of oceanic
islands--the destruction of those which, during the occasional use of
their wings, were carried out to sea. This form of natural selection may
well have acted in the case of birds whose powers of flight were
already somewhat reduced, and to whom, there being no enemies to escape
from, their use was only a source of danger. We may thus, perhaps,
account for the fact that many of these birds retain small but useless
wings with which they never fly; for, the wings having been reduced to
this functionless condition, no power could reduce them further except
correlation of growth or economy of nutrition, causes which only rarely
come into play.
The complete loss of eyes in some cave animals may, perhaps, be
explained in a somewhat similar way. Whenever, owing to the total
darkness, they became useless, they might also become injurious, on
account of their delicacy of organisation and liability to accidents and
disease; in which case natural selection would begin to act to reduce,
and finally abort them; and this explains why, in some cases, the
rudimentary eye remains, although completely covered by a protective
outer skin. Whales, like moas and cassowaries, carry us back to a remote
past, of whose conditions we know too little for safe speculation. We
are quite ignorant of the ancestral forms of either of these groups, and
are therefore without the materials needful for determining the steps by
which the change took place, or the causes which brought it about.[200]
On a review of the various examples that have been given by Mr. Darwin
and others of organs that have been reduced or aborted, there seems too
much diversity in the results for all to be due to so direct and uniform
a cause as the individual effects of disuse accumulated by heredity. For
if that were the only or chief efficient cause, and a cause capable of
producing a decided effect during the comparatively short period of the
existence of animals in a state of domestication, we should expect to
find that, in wild species, all unused parts or organs had been reduced
to the smallest rudiments, or had wholly disappeared. Instead of this we
find various grades of reduction, indicating the probable result of
several distinct causes, sometimes acting separately, sometimes in
combination, such as those we have already pointed out.
And if we find no positive evidence of _disuse_, acting by its direct
effect on the individual, being transmitted to the offspring, still less
can we find such evidence in the case of the _use_ of organs. For here
the very fact of _use_, in a wild state, implies _utility_, and utility
is the constant subject for the action of natural selection; while among
domestic animals those parts which are exceptionally used are so used in
the service of man, and have thus become the subjects of artificial
selection. Thus "the great and inherited development of the udders in
cows and goats," quoted by Spencer from Darwin, really affords no proof
of inheritance of the increase due to use, because, from the earliest
period of the domestication of these animals, abundant milk-production
has been highly esteemed, and has thus been the subject of selection;
while there are no cases among wild animals that may not be better
explained by variation and natural selection.
_Difficulty as to Co-adaptation of Parts by Variation and Selection._
Mr. Spencer again brings forward this difficulty, as he did in his
_Principles of Biology_ twenty-five years ago, and urges that all the
adjustments of bones, muscles, blood-vessels, and nerves which would be
required during, for example, the development of the neck and fore-limbs
of the giraffe, could not have been effected by "simultaneous fortunate
spontaneous variations." But this difficulty is fully disposed of by the
facts of simultaneous variation adduced in our third chapter, and has
also been specially considered in Chapter VI, p. 127. The best answer to
this objection may, perhaps, be found in the fact that the very thing
said to be impossible by variation and natural selection has been again
and again effected by variation and artificial selection. During the
process of formation of such breeds as the greyhound or the bulldog, of
the race-horse and carthorse, of the fantail pigeon or the otter-sheep,
many co-ordinate adjustments have been produced; and no difficulty has
occurred, whether the change has been effected by a single variation--as
in the last case named--or by slow steps, as in all the others. It seems
to be forgotten that most animals have such a surplus of vitality and
strength for all the ordinary occasions of life that any slight
superiority in one part can be at once utilised; while the moment any
want of balance occurs, variations in the insufficiently developed parts
will be selected to bring back the harmony of the whole organisation.
The fact that, in all domestic animals, variations do occur, rendering
them swifter or stronger, larger or smaller, stouter or slenderer, and
that such variations can be separately selected and accumulated for
man's purposes, is sufficient to render it certain that similar or even
greater changes may be effected by natural selection, which, as Darwin
well remarks, "acts on every internal organ, on every shade of
constitututional difference, on the whole machinery of life." The
difficulty as to co-adaptation of parts by variation and natural
selection appears to me, therefore, to be a wholly imaginary difficulty
which has no place whatever in the operations of nature.
_Direct Action of the Environment._
Mr. Spencer's last objection to the wide scope given by Darwinians to
the agency of natural selection is, that organisms are acted upon by the
environment, which produces in them definite changes, and that these
changes in the individual are transmitted by inheritance, and thus
become increased in successive generations. That such changes are
produced in the individual there is ample evidence, but that they are
inherited independently of any form of selection or of reversion is
exceedingly doubtful, and Darwin nowhere expresses himself as satisfied
with the evidence. The two very strongest cases he mentions are the
twenty-nine species of American trees which all differed in a
corresponding way from their nearest European allies; and the American
maize which became changed after three generations in Europe. But in the
case of the trees the differences alleged may be partly due to
correlation with constitutional peculiarities dependent on climate,
especially as regards the deeper tint of the fading leaves and the
smaller size of the buds and seeds in America than in Europe; while the
less deeply toothed or serrated leaves in the American species are, in
our present complete ignorance of the causes and uses of serration,
quite as likely to be due to some form of adaptation as to any direct
action of the climate. Again, we are not told how many of the allied
species do not vary in this particular manner, and this is certainly an
important factor in any conclusion we may form on the question.
In the case of the maize it appears that one of the more remarkable and
highly selected American varieties was cultivated in Germany, and in
three years nearly all resemblance to the original parent was lost; and
in the sixth year it closely resembled a common European variety, but
was of somewhat more vigorous growth. In this case no selection appears
to have been practised, and the effects may have been due to that
"reversion to mediocrity" which invariably occurs, and is more
especially marked in the case of varieties which have been rapidly
produced by artificial selection. It may be considered as a partial
reversion to the wild or unimproved stock; and the same thing would
probably have occurred, though perhaps less rapidly, in America itself.
As this is stated by Darwin to be the most remarkable case known to him
"of the direct and prompt action of climate on a plant," we must
conclude that such direct effects have not been proved to be accumulated
by inheritance, independently of reversion or selection.
The remaining part of Mr. Spencer's essay is devoted to a consideration
of the hypothetical action of the environment on the lower organisms
which consist of simple cells or formless masses of protoplasm; and he
shows with great elaboration that the outer and inner parts of these
are necessarily subject to different conditions; and that the outer
actions of air or water lead to the formation of integuments, and
sometimes to other definite modifications of the surface, whence arise
permanent differences of structure. Although in these cases also it is
very difficult to determine how much is due to direct modification by
external agencies transmitted and accumulated by inheritance, and how
much to spontaneous variations accumulated by natural selection, the
probabilities in favour of the former mode of action are here greater,
because there is no differentiation of nutritive and reproductive cells
in these simple organisms; and it can be readily seen that any change
produced in the latter will almost certainly affect the next
generation.[201] We are thus carried back almost to the origin of life,
and can only vaguely speculate on what took place under conditions of
which we know so little.
_The American School of Evolutionists._
The tentative views of Mr. Spencer which we have just discussed, are
carried much further, and attempts have been made to work them out in
great detail, by many American naturalists, whose best representative is
Dr. E.D. Cope of Philadelphia.[202] This school endeavours to explain
all the chief modifications of form in the animal kingdom by fundamental
laws of growth and the inherited effects of use and effort, returning,
in fact, to the teachings of Lamarck as being at least equally important
with those of Darwin.
The following extract will serve to show the high position claimed by
this school as original discoverers, and as having made important
additions to the theory of evolution:
"Wallace and Darwin have propounded as the cause of modification in
descent their law of natural selection. This law has been epitomised by
Spencer as the 'survival of the fittest.' This neat expression no doubt
covers the case, but it leaves the origin of the fittest entirely
untouched. Darwin assumes a 'tendency to variation' in nature, and it is
plainly necessary to do this, in order that materials for the exercise
of a selection should exist. Darwin and Wallace's law is then only
restrictive, directive, conservative, or destructive of something
already created. I propose, then, to seek for the originative laws by
which these subjects are furnished; in other words, for the causes of
the origin of the fittest."[203]
Mr. Cope lays great stress on the existence of a special developmental
force termed "bathmism" or growth-force, which acts by means of
retardation and acceleration "without any reference to fitness at all;"
that "instead of being controlled by fitness it is the controller of
fitness." He argues that "all the characteristics of generalised groups
from genera up (excepting, perhaps, families) have been evolved under
the law of acceleration and retardation," combined with some
intervention of natural selection; and that specific characters, or
species, have been evolved by natural selection with some assistance
from the higher law. He, therefore, makes species and genera two
absolutely distinct things, the latter not developed out of the former;
generic characters and specific characters are, in his opinion,
fundamentally different, and have had different origins, and whole
groups of species have been simultaneously modified, so as to belong to
another genus; whence he thinks it "highly probable that the same
specific form has existed through a succession of genera, and perhaps in
different epochs of geologic time."
Useful characters, he concludes, have been produced by the special
location of growth-force by use; useless ones have been produced by
location of growth-force without the influence of use. Another element
which determines the direction of growth-force, and which precedes use,
is effort; and "it is thought that effort becomes incorporated into the
metaphysical acquisitions of the parent, and is inherited with other
metaphysical qualities by the young, which, during the period of growth,
is much more susceptible to modifying influences, and is likely to
exhibit structural change in consequence."[204]
From these few examples of their teachings, it is clear that these
American evolutionists have departed very widely from the views of Mr.
Darwin, and in place of the well-established causes and admitted laws to
which he appeals have introduced theoretical conceptions which have not
yet been tested by experiments or facts, as well as metaphysical
conceptions which are incapable of proof. And when they come to
illustrate these views by an appeal to palaeontology or morphology, we
find that a far simpler and more complete explanation of the facts is
afforded by the established principles of variation and natural
selection. The confidence with which these new ideas are enunciated, and
the repeated assertion that without them Darwinism is powerless to
explain the origin of organic forms, renders it necessary to bestow a
little more time on the explanations they give us of well-known
phenomena with which, they assert, other theories are incompetent to
grapple.
As examples of use producing structural change, Mr. Cope adduces the
hooked and toothed beaks of the falcons and the butcher-birds, and he
argues that the fact of these birds belonging to widely different groups
proves that similarity of use has produced a similar structural result.
But no attempt is made to show any direct causal connection between the
use of a bill to cut or tear flesh and the development of a tooth on the
mandible. Such use might conceivably strengthen the bill or increase its
size, but not cause a special tooth-like outgrowth which was not present
in the ancestral thrush-like forms of the butcher-bird. On the other
hand, it is clear that any variations of the bill tending towards a hook
or tooth would give the possessor some advantage in seizing and tearing
its prey, and would thus be preserved and increased by natural
selection. Again, Mr. Cope urges the effects of a supposed "law of polar
or centrifugal growth" to counteract a tendency to unsymmetrical growth,
where one side of the body is used more than the other. But the
undoubted hurtfulness of want of symmetry in many important actions or
functions would rapidly eliminate any such tendency. When, however, it
has become useful, as in the case of the single enlarged claw of many
Crustacea, it has been preserved by natural selection.
_Origin of the Feet of the Ungulates._
Perhaps the most original and suggestive of Mr. Cope's applications of
the theory of use and effort in modifying structure are, his chapters
"On the Origin of the Foot-Structure of the Ungulates;" and that "On the
Effect of Impacts and Strains on the Feet of Mammalia;" and they will
serve also to show the comparative merits of this theory and that of
natural selection in explaining a difficult case of modification,
especially as it is an explanation claimed as new and original when
first enunciated in 1881. Let us, then, see how he deals with the
problem.
The remarkable progressive change of a four or five-toed ancestor into
the one-toed horse, and the equally remarkable division of the whole
group of ungulate animals into the odd-toed and even-toed divisions, Mr.
Cope attempts to explain by the effects of impact and use among animals
which frequented hard or swampy ground respectively. On hard ground, it
is urged, the long middle toe would be most used and subjected to the
greatest strains, and would therefore acquire both strength and
development. It would then be still more exclusively used, and the extra
nourishment required by it would be drawn from the adjacent less-used
toes, which would accordingly diminish in size, till, after a long
series of changes, the records of which are so well preserved in the
American tertiary rocks, the true one-toed horse was developed. In soft
or swampy ground, on the other hand, the tendency would be to spread out
the foot so that there were two toes on each side. The two middle toes
would thus be most used and most subject to strains, and would,
therefore, increase at the expense of the lateral toes. There would be,
no doubt, an advantage in these two functional toes being of equal size,
so as to prevent twisting of the foot while walking; and variations
tending to bring this about would be advantageous, and would therefore
be preserved. Thus, by a parallel series of changes in another
direction, adapted to a distinct set of conditions, we should arrive at
the symmetrical divided hoofs of our deer and cattle. The fact that
sheep and goats are specially mountain and rock-loving animals may be
explained by their being a later modification, since the divided hoof
once formed is evidently well adapted to secure a firm footing on rugged
and precipitous ground, although it could hardly have been first
developed in such localities. Mr. Cope thus concludes: "Certain it is
that the length of the bones in the feet of the ungulate orders has a
direct relation to the dryness of the ground they inhabit, and the
possibility of speed which their habit permits them or necessarily
imposes on them."[205]
If there is any truth in the explanation here briefly summarised, it
must entirely depend on the fact of individual modifications thus
produced being hereditary, and we yet await the proof of this. In the
meantime it is clear that the very same results could have been brought
about by variation and natural selection. For the toes, like all other
organs, vary in size and proportions, and in their degree of union or
separation; and if in one group of animals it was beneficial to have the
middle toe larger and longer, and in another set to have the two middle
toes of the same size, nothing can be more certain than that these
particular modifications would be continuously preserved, and the very
results we see ultimately produced.
The oft-repeated objections that the cause of variations is unknown,
that there must be something to determine variations in the right
direction; that "natural selection includes no actively progressive
principle, but must wait for the development of variation, and then,
after securing the survival of the best, wait again for the best to
project its own variations for selection," we have already sufficiently
answered by showing that variation--in abundant or typical species--is
always present in ample amount; that it exists in all parts and organs;
that these vary, for the most part, independently, so that any required
combination of variations can be secured; and finally, that all
variation is necessarily either in excess or defect of the mean
condition, and that, consequently, the right or favourable variations
are so frequently present that the unerring power of natural selection
never wants materials to work upon.
_Supposed Action of Animal Intelligence._
The following passage briefly summarises Mr. Cope's position:
"Intelligence is a conservative principle, and will always direct effort
and use into lines which will be beneficial to its possessor. Here we
have the source of the fittest, _i.e._ addition of parts by increase and
location of growth-force, directed by the influence of various kinds of
compulsion in the lower, and intelligent option among higher animals.
Thus intelligent choice, taking advantage of the successive evolution of
physical conditions, may be regarded as the _originator of the fittest_,
while natural selection is the tribunal to which all results of
accelerated growth are submitted. This preserves or destroys them, and
determines the new points of departure on which accelerated growth shall
build."[206]
This notion of "intelligence"--the intelligence of the animal
itself--determining its own variation, is so evidently a very partial
theory, inapplicable to the whole vegetable kingdom, and almost so to
all the lower forms of animals, amongst which, nevertheless, there is
the very same adaptation and co-ordination of parts and functions as
among the highest, that it is strange to see it put forward with such
confidence as necessary for the completion of Darwin's theory. If "the
various kinds of compulsion"--by which are apparently meant the laws of
variation, growth, and reproduction, the struggle for existence, and the
actions necessary to preserve life under the conditions of the animal's
environment--are sufficient to have developed the varied forms of the
lower animals and of plants, we can see no reason why the same
"compulsion" should not have carried on the development of the higher
animals also. The action of this "intelligent option" is altogether
unproved; while the acknowledgment that natural selection is the
tribunal which either preserves or destroys the variations submitted to
it, seems quite inconsistent with the statement that intelligent choice
is the "orginator of the fittest," since whatever is really "the
fittest" can never be destroyed by natural selection, which is but
another name for the survival of the fittest. If "the fittest" is always
definitely produced by some other power, then natural selection is not
wanted. If, on the other hand, both fit and unfit are produced, and
natural selection decides between them, that is pure Darwinism, and Mr.
Cope's theories have added nothing to it.
[Illustration: FIG. 35.--Transformation of Artemia salina to A.
Milhausenii; 1, tail-lobe of A. salina, and its transition through
2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A.
salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A.
Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)]
_Semper on the Direct Influence of the Environment._
Another eminent naturalist, Professor Karl Semper of Würzburg, also
adopts the view of the direct transforming power of the environment, and
has brought together an immense body of interesting facts showing the
influence of food, of light, of temperature, of still water and moving
water, of the atmosphere and its currents, of gravitation, and of other
organisms, in modifying the forms and other characteristics of
animals.[207] He believes that these various influences produce a direct
and important effect, and that this effect is accumulated by
inheritance; yet he acknowledges that we have no direct evidence of
this, and there is hardly a single case adduced in the book which is not
equally well explained by adaptation, brought about by the survival of
beneficial variations. Perhaps the most remarkable case he has brought
forward is that of the transformation of species of crustaceans by a
change in the saltness of the water (see Fig. 35). Artemia salina lives
in brackish water, while A. Milhausenii inhabits water which is much
salter. They differ greatly in the form of the tail-lobes, and in the
presence or absence of spines upon the tail, and had always been
considered perfectly distinct species. Yet either was transformed into
the other in a few generations, during which the saltness of the water
was gradually altered. Yet more, A. salina was gradually accustomed to
fresher water, and in the course of a few generations, when the water
had become perfectly fresh, the species was changed into Branchipus
stagnalis, which had always been considered to belong to a different
genus on account of differences in the form of the antennae and of the
posterior segments of the body (see Fig. 36). This certainly appears to
be a proof of change of conditions producing a change of form
independently of selection, and of that change of form, while remaining
under the same conditions, being inherited. Yet there is this
peculiarity in the case, that there is a chemical change in the water,
and that this water permeates the whole body, and must be absorbed by
the tissues, and thus affect the ova and even the reproductive
elements, and in this way may profoundly modify the whole organisation.
Why and how the external effects are limited to special details of the
structure we do not know; but it does not seem as if any far-reaching
conclusions as to the cumulative effect of external conditions on the
higher terrestrial animals and plants, can be drawn from such an
exceptional phenomenon. It seems rather analogous to those effects of
external influences on the very lowest organisms in which the vegetative
and reproductive organs are hardly differentiated, in which case such
effects are doubtless inherited.[208]
[Illustration: FIG. 36. _a._ Branchipus stagnalis. _b._ Artemia salina.]
_Professor Geddes's Theory of Variation in Plants._
In a paper read before the Edinburgh Botanical Society in 1886 Mr.
Patrick Geddes laid down the outlines of a fundamental theory of plant
variation, which he has further extended in the article "Variation and
Selection" in the _Encydopaedia Britannica_, and in a paper read before
the Linnaean Society but not yet published.
A theory of variation should deal alike with the origin of specific
distinctions and with those vaster differences which characterise the
larger groups, and he thinks it should answer such questions as--How an
axis comes to be arrested to form a flower? how the various forms of
inflorescence were evolved? how did perigynous or epigynous flowers
arise from hypogynous flowers? and many others equally fundamental.
Natural selection acting upon numerous accidental variations will not,
he urges, account for such general facts as these, which must depend on
some constant law of variation. This law he believes to be the
well-known antagonism of vegetative and reproductive growth acting
throughout the whole course of plant development; and he uses it to
explain many of the most characteristic features of the structure of
flowers and fruits.
Commencing with the origin of the flower, which all botanists agree in
regarding as a shortened branch, he explains this shortening as an
inevitable physiological fact, since the cost of the development of the
reproductive elements is so great as necessarily to check vegetative
growth. In the same manner the shortening of the inflorescence from
raceme to spike or umbel, and thence to the capitulum or dense
flower-head of the composite plants is brought about. This shortening,
carried still further, produces the flattened leaf-like receptacle of
Dorstenia, and further still the deeply hollowed fruity receptacle of
the fig.
The flower itself undergoes a parallel modification due to a similar
cause. It is formed by a series of modified leaves arranged round a
shortened axis. In its earlier stages the number of these modified
leaves is indefinite, as in many Ranunculaceae; and the axis itself is
not greatly shortened, as in Myosurus. The first advance is to a
definite number of parts and a permanently shortened axis, in the
arrangement termed hypogynous, in which all the whorls are quite
distinct from each other. In the next stage there is a further
shortening of the central axis, leaving the outer portion as a ring on
which the petals are inserted, producing the arrangement termed
perigynous. A still further advance is made by the contraction of the
axis, so as to leave the central part forming the ovary quite below the
flower, which is then termed epigynous.
These several modifications are said to be parallel and definite, and to
be determined by the continuous checking of vegetation by reproduction
along what is an absolute groove of progressive change. This being the
case, the importance of natural selection is greatly diminished. Instead
of selecting and accumulating spontaneous indefinite variations, its
function is to retard them after the stage of maximum utility has been
independently reached. The same simple conception is said to unlock
innumerable problems of vegetable morphology, large and small alike. It
explains the inevitable development of gymnosperm into angiosperm by the
checked vegetative growth of the ovule-bearing leaf or carpel; while
such minor adaptations as the splitting fruit of the geranium or the
cupped stigma of the pansy, can be no longer looked upon as achievements
of natural selection, but must be regarded as naturally traceable to
the vegetative checking of their respective types of leaf organ. Again,
a detailed examination of spiny plants practically excludes the
hypothesis of mammalian selection altogether, and shows spines to arise
as an expression of the diminishing vegetativeness--in fact, the ebbing
vitality of a species.[209]
_Objections to the Theory._
The theory here sketched out is enticing, and at first sight seems
calculated to throw much light on the history of plant development; but
on further consideration, it seems wanting in definiteness, while it is
beset with difficulties at every step. Take first the shortening of the
raceme into the umbel and the capitulum, said to be caused by arrest of
vegetative growth, due to the antagonism of reproduction. If this were
the whole explanation of the phenomenon, we should expect the quantity
of seed to increase as this vegetative growth diminished, since the seed
is the product of the reproductive energy of the plant, and its quantity
the best measure of that energy. But is this the case? The ranunculus
has comparatively few seeds, and the flowers are not numerous; while in
the same order the larkspur and the columbine have far more seeds as
well as more flowers, but there is no shortening of the raceme or
diminution of the foliage, although the flowers are large and complex.
So, the extremely shortened and compressed flower-heads of the
compositae produce comparatively few seeds--one only to each flower;
while the foxglove, with its long spike of showy flowers, produces an
enormous number.
Again, if the shortening of the central axis in the successive stages of
hypogynous, perigynous, and epigynous flowers were an indication of
preponderant reproduction and diminished vegetation, we should find
everywhere some clear indications of this fact. The plants with
hypogynous flowers should, as a rule, have less seed and more vigorous
and abundant foliage than those at the other extreme with epigynous
flowers. But the hypogynous poppies, pinks, and St. John's worts have
abundance of seed and rather scanty foliage; while the epigynous
dogwoods and honeysuckles have few seeds and abundant foliage. If,
instead of the number of the seeds, we take the size of the fruit as an
indication of reproductive energy, we find this at a maximum in the
gourd family, yet their rapid and luxuriant growth shows no diminution
of vegetative power. So that the statement that plant modifications
proceed "along an absolute groove of progressive change" is contradicted
by innumerable facts indicating advance and regression, improvement or
degradation, according as the ever-changing environment renders one form
more advantageous than the other. As one instance I may mention the
Anonaceae or custard-apple tribe, which are certainly an advance from
the Ranunculaceae; yet in the genus Polyalthea the fruit consists of a
number of separate carpels, each borne on a long stalk, as if reverting
to the primitive stalked carpellary leaves.
_On the Origin of Spines._
But perhaps the most extraordinary application of the theory is that
which considers spines to be an indication of the "ebbing vitality of a
species," and which excludes "mammalian selection altogether." If this
were true, spines should occur mainly in feeble, rare, and dying-out
species, instead of which we have the hawthorn, one of our most vigorous
shrubs or trees, with abundant vitality and an extensive range over the
whole Palaearctic region, showing that it is really a dominant species.
In North America the numerous thorny species of Crataegus are equally
vigorous, as are the false acacia (Robinia) and the honey-locust
(Gleditschia). Neither have the numerous species of very spiny Acacias
been noticed to be rarer or less vigorous than the unarmed kinds.
On the other point--that spines are not due to mammalian selection--we
are able to adduce what must be considered direct and conclusive
evidence. For if spines, admittedly produced by aborted branches,
petioles, or peduncles, are due solely or mainly to diminished
vegetativeness or ebbing vitality, they ought to occur in all countries
alike, or at all events in all whose similar conditions tend to check
vegetation; whereas, if they are, solely or mainly, developed as a
protection against the attacks of herbivorous mammals, they ought to be
most abundant where these are plentiful, and rare or absent where
indigenous mammalia are wanting. Oceanic islands, as compared with
continents, would thus furnish a crucial test of the two theories; and
Mr. Hemsley of Kew, who has specially studied insular floras, has given
me some valuable information on this point. He says: "There are no spiny
or prickly plants in the indigenous element of the St. Helena flora. The
relatively rich flora of the Sandwich Isles is not absolutely without a
prickly plant, but almost so. All the endemic genera are unarmed, and
the endemic species of almost every other genus. Even such genera as
Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, of which there are
many armed species in other countries, are only represented by unarmed
species. The two endemic Rubi have the prickles reduced to the setaceous
condition, and the two palms are unarmed.
"The flora of the Galapagos includes a number of prickly plants, among
them several cacti (these have not been investigated and may be American
species), but I do not think one of the known endemic species of any
family is prickly or spiny.
"Spiny and prickly plants are also rare in New Zealand, but there are
the formidably armed species of wild Spaniard (Aciphylla), one species
of Rubus, the pungent-leaved Epacrideae and a few others."
Mr. J.G. Baker of Kew, who has specially studied the flora of Mauritius
and the adjacent islands, also writes me on this point. He says: "Taking
Mauritius alone, I do not call to mind a single species that is a
spinose endemic tree or shrub. If you take the whole group of islands
(Mauritius, Bourbon, Seychelles, and Rodriguez), there will be about a
dozen species, but then nine of these are palms. Leaving out palms, the
trees and shrubs of that part of the world are exceptionally
non-spinose."
These are certainly remarkable facts, and quite inexplicable on the
theory of spines being caused solely by checked vegetative growth, due
to weakness of constitution or to an arid soil and climate. For the
Galapagos and many parts of the Sandwich Islands are very arid, as is a
considerable part of the North Island of New Zealand. Yet in our own
moist climate and with our very limited number of trees and shrubs we
have about eighteen spiny or prickly species, more, apparently, than in
the whole endemic floras of the Mauritius, Sandwich Islands, and
Galapagos, though these are all especially rich in shrubby and arboreal
species. In New Zealand the prickly Rubus is a leafless trailing plant,
and its prickles are probably a protection against the large snails of
the country, several of which have shells from two to three and a half
inches long.[210] The "wild Spaniards" are very spiny herbaceous
Umbelliferae, and may have gained their spines to preserve them from
being trodden down or eaten by the Moas, which, for countless ages, took
the place of mammals in New Zealand. The exact use or meaning of the
spines in palms is more doubtful, though they are, no doubt, protective
against some animals; but it is certainly an extraordinary fact that in
the entire flora of the Mauritius, so largely consisting of trees and
shrubs, not a single endemic species should be thorny or spiny.
If now we consider that every continental flora produces a considerable
proportion of spiny and thorny species, and that these rise to a maximum
in South Africa, where herbivorous mammalia were (before the settlement
of the country), perhaps, more abundant and varied than in any other
part of the world; while another district, remarkable for well-armed
vegetation, is Chile, where the camel-like vicugnas, llamas, and
alpacas, and an abundance of large rodents wage perpetual war against
shrubby vegetation, we shall see the full significance of the almost
total absence of thorny and spiny plants in the chief oceanic islands;
and so far from "excluding the hypothesis of mammalian selection
altogether," we shall find in this hypothesis the only satisfactory
explanation of the facts.
From the brief consideration of Professor Geddes's theory now given, we
conclude that, although the antagonism between vegetative and
reproductive growth is a real agency, and must be taken account of in
our endeavour to explain many of the fundamental facts in the structure
and form of plants, yet it is so overpowered and directed at every step
by the natural selection of favourable variations, that the results of
its exclusive and unmodified action are nowhere to be found in nature.
It may be allowed to rank as one of those "laws of growth," of which so
many have now been indicated, and which were always recognised by Darwin
as underlying all variation; but unless we bear in mind that its action
must always be subordinated to natural selection, and that it is
continually checked, or diverted, or even reversed by the necessity of
adaptation to the environment, we shall be liable to fall into such
glaring errors as the imputing to "ebbing vitality" alone such a
widespread phenomenon as the occurrence of spines and thorns, while
ignoring altogether the influence of the organic environment in their
production.[211]
The sketch now given of the chief attempts that have been made to prove
that either the direct action of the environment or certain fundamental
laws of variation are independent causes of modification of species,
shows us that their authors have, in every case, failed to establish
their contention. Any direct action of the environment, or any
characters acquired by use or disuse, can have no effect whatever upon
the race unless they are inherited; and that they are inherited in any
case, except when they directly affect the reproductive cells, has not
been proved. On the other hand, as we shall presently show, there is
much reason for believing that such acquired characters are in their
nature non-heritable.
_Variation and Selection Overpower the Effects of Use and Disuse._
But there is another objection to this theory arising from the very
nature of the effects produced. In each generation the effects of use or
disuse, or of effort, will certainly be very small, while of this small
effect it is not maintained that the whole will be always inherited by
the next generation. How small the effect is we have no means of
determining, except in the case of disuse, which Mr. Darwin investigated
carefully. He found that in twelve fancy breeds of pigeons, which are
often kept in aviaries, or if free fly but little, the sternum had been
reduced by about one-seventh or one-eighth of its entire length, and
that of the scapula about one-ninth. In domestic ducks the weight of the
wing-bones in proportion to that of the whole skeleton had decreased
about one-tenth. In domestic rabbits the bones of the legs were found to
have increased in weight in due proportion to the increased weight of
the body, but those of the hind legs were rather less in proportion to
those of the fore legs than in the wild animal, a difference which may
be imputed to their being less used in rapid motion. The pigeons,
therefore, afford the greatest amount of reduction by
disuse--one-seventh of the length of the sternum. But the pigeon has
certainly been domesticated four or five thousand years; and if the
reduction of the wings by disuse has only been going on for the last
thousand years, the amount of reduction in each generation would be
absolutely imperceptible, and quite within the limits of the reduction
due to the absence of selection, as already explained. But, as we have
seen in Chapter III, the fortuitous variation of every part or organ
usually amounts to one-tenth, and often to one-sixth of the average
dimensions--that is, the fortuitous variation in one generation among a
limited number of the individuals of a species is as great as the
cumulative effects of disuse in a thousand generations! If we assume
that the effects of use or of effort in the individual are equal to the
effects of disuse, or even ten or a hundred times greater, they will
even then not equal, in each generation, the amount of the fortuitous
variations of the same part. If it be urged that the effects of use
would modify all the individuals of a species, while the fortuitous
variations to the amount named only apply to a portion of them, it may
be replied, that that portion is sufficiently large to afford ample
materials for selection, since it often equals the numbers that can
annually survive; while the recurrence in each successive generation of
a like amount of variation would render possible such a rapid adjustment
to new conditions that the effects of use or disuse would be as nothing
in comparison. It follows, that even admitting the modifying effects of
the environment, and that such modifications are inherited, they would
yet be entirely swamped by the greater effects of fortuitous variation,
and the far more rapid cumulative results of the selection of such
variations.
_Supposed Action of the Environment in Initiating Variations._
It is, however, urged that the reaction of the environment initiates
variations, which without it would never arise; such, for instance, as
the origin of horns through the pressures and irritations caused by
butting, or otherwise using the head as a weapon or for defence.
Admitting, for the sake of argument, that this is so, all the evidence
we possess shows that, from the very first appearance of the rudiment of
such an organ, it would vary to a greater extent than the amount of
growth directly produced by use; and these variations would be subject
to selection, and would thus modify the organ in ways which use alone
would never bring about. We have seen that this has been the case with
the branching antlers of the stag, which have been modified by
selection, so as to become useful in other ways than as a mere weapon;
and the same has almost certainly been the case with the variously
curved and twisted horns of antelopes. In like manner, every conceivable
rudiment would, from its first appearance, be subject to the law of
variation and selection, to which, thenceforth, the direct effect of the
environment would be altogether subordinate.
A very similar mode of reasoning will apply to the other branch of the
subject--the initiation of structures and organs by the action of the
fundamental laws of growth. Admitting that such laws have determined
some of the main divisions of the animal and vegetable kingdom, have
originated certain important organs, and have been the fundamental cause
of certain lines of development, yet at every step of the process these
laws must have acted in entire subordination to the law of natural
selection. No modification thus initiated could have advanced a single
step, unless it were, on the whole, a useful modification; while its
entire future course would be necessarily subject to the laws of
variation and selection, by which it would be sometimes checked,
sometimes hastened on, sometimes diverted to one purpose, sometimes to
another, according as the needs of the organism, under the special
conditions of its existence, required such modification. We need not
deny that such laws and influences may have acted in the manner
suggested, but what we do deny is that they could possibly escape from
the ever-present and all-powerful modifying effects of variation and
natural selection.[212]
_Weismann's Theory of Heredity._
Professor August Weismann has put forth a new theory of heredity founded
upon the "continuity of the germ-plasm," one of the logical consequences
of which is, that acquired characters of whatever kind are not
transmitted from parent to offspring. As this is a matter of vital
importance to the theory of natural selection, and as, if well founded,
it strikes away the foundations of most of the theories discussed in the
present chapter, a brief outline of Weismann's views must be attempted,
although it is very difficult to make them intelligible to persons
unfamiliar with the main facts of modern embryology.[213]
The problem is thus stated by Weismann: "How is it that in the case of
all higher animals and plants a single cell is able to separate itself
from amongst the millions of most various kinds of which an organism is
composed, and by division and complicated differentiation to reconstruct
a new individual with marvellous likeness, unchanged in many cases even
throughout whole geological periods?" Darwin attempted to solve the
problem by his theory of "Pangenesis," which supposed that every
individual cell in the body gave off gemmules or germs capable of
reproducing themselves, and that portions of these germs of each of the
almost infinite number of cells permeate the whole body and become
collected in the generative cells, and are thus able to reproduce the
whole organism. This theory is felt to be so ponderously complex and
difficult that it has met with no general acceptance among
physiologists.
The fact that the germ-cells _do_ reproduce with wonderful accuracy not
only the general characters of the species, but many of the individual
characteristics of the parents or more remote ancestors, and that this
process is continued from generation to generation, can be accounted
for, Weismann thinks, only on two suppositions which are physiologically
possible. Either the substance of the parent germ-cell, after passing
through a cycle of changes required for the construction of a new
individual, possesses the capability of producing anew germ-cells
identical with those from which that individual was developed, or _the
new germ-cells arise, as far as their essential and characteristic
substance is concerned, not at all out of the body of the individual,
but direct from the parent germ-cell_. This latter view Weismann holds
to be the correct one, and, on this theory, heredity depends on the fact
that a substance of special molecular composition passes over from one
generation to another. This is the "germ-plasm," the power of which to
develop itself into a perfect organism depends on the extraordinary
complication of its minutest structure. At every new birth a portion of
the specific germ-plasm, which the parent egg-cell contains, is not used
up in producing the offspring, but is reserved unchanged to produce the
germ-cells of the following generation. Thus the germ-cells--so far as
regards their essential part the germ-plasm--are not a product of the
body itself, but are related to one another in the same way as are a
series of generations of unicellular organisms derived from one another
by a continuous course of simple division. Thus the question of heredity
is reduced to one of growth. A minute portion of the very same
germ-plasm from which, first the germ-cell, and then the whole organism
of the parent, were developed, becomes the starting-point of the growth
of the child.
_The Cause of Variation._
But if this were all, the offspring would reproduce the parent exactly,
in every detail of form and structure; and here we see the importance of
sex, for each new germ grows out of the united germ-plasms of two
parents, whence arises a mingling of their characters in the offspring.
This occurs in each generation; hence every individual is a complex
result reproducing in ever-varying degrees the diverse characteristics
of his two parents, four grandparents, eight great-grandparents, and
other more remote ancestors; and that ever-present individual variation
arises which furnishes the material for natural selection to act upon.
Diversity of sex becomes, therefore, of primary importance as _the cause
of variation_. Where asexual generation prevails, the characteristics of
the individual alone are reproduced, and there are thus no means of
effecting the change of form or structure required by changed conditions
of existence. Under such changed conditions a complex organism, if only
asexually propagated, would become extinct. But when a complex organism
is sexually propagated, there is an ever-present cause of change which,
though slight in any one generation, is cumulative, and under the
influence of selection is sufficient to keep up the harmony between the
organism and its slowly changing environment.[214]
_The Non-Heredity of Acquired Characters._
Certain observations on the embryology of the lower animals are held to
afford direct proof of this theory of heredity, but they are too
technical to be made clear to ordinary readers. A logical result of the
theory is the impossibility of the transmission of acquired characters,
since the molecular structure of the germ-plasm is already determined
within the embryo; and Weismann holds that there are no facts which
really prove that acquired characters can be inherited, although their
inheritance has, by most writers, been considered so probable as hardly
to stand in need of direct proof.
We have already shown, in the earlier part of this chapter, that many
instances of change, imputed to the inheritance of acquired variations,
are really cases of selection; while the very fact that _use_ implies
_usefulness_ renders it almost impossible to eliminate the action of
selection in a state of nature. As regards mutilations, it is generally
admitted that they are not hereditary, and there is ample evidence on
this point. When it was the fashion to dock horses' tails, it was not
found that horses were born with short tails; nor are Chinese women born
with distorted feet; nor are any of the numerous forms of racial
mutilation in man, which have in some cases been carried on for hundreds
of generations, inherited. Nevertheless, a few cases of apparent
inheritance of mutilations have been recorded,[215] and these, if
trustworthy, are difficulties in the way of the theory. The undoubted
inheritance of disease is hardly a difficulty, because the
predisposition to disease is a congenital, not an acquired character,
and as such would be the subject of inheritance. The often-quoted case
of a disease induced by mutilation being inherited (Brown-Sequard's
epileptic guinea-pigs) has been discussed by Professor Weismann, and
shown to be not conclusive. The mutilation itself--a section of certain
nerves--was never inherited, but the resulting epilepsy, or a general
state of weakness, deformity, or sores, was sometimes inherited. It is,
however, possible that the mere injury introduced and encouraged the
growth of certain microbes, which, spreading through the organism,
sometimes reached the germ-cells, and thus transmitted a diseased
condition to the offspring. Such a transference of microbes is believed
to occur in syphilis and tuberculosis, and has been ascertained to occur
in the case of the muscardine silkworm disease.[216]
_The Theory of Instinct._
The theory now briefly outlined cannot be said to be proved, but it
commends itself to many physiologists as being inherently probable, and
as furnishing a good working hypothesis till displaced by a better. We
cannot, therefore, accept any arguments against the agency of natural
selection which are based upon the opposite and equally unproved theory
that acquired characters are inherited; and as this applies to the whole
school of what may be termed Neo-Lamarckians, their speculations cease
to have any weight.
The same remark applies to the popular theory of instincts as being
inherited habits; though Darwin gave very little weight to this, but
derived almost all instincts from spontaneous useful variations which,
like other spontaneous variations, are of course inherited. At first
sight it appears as if the acquired habits of our trained
dogs--pointers, retrievers, etc.--are certainly inherited; but this need
not be the case, because there must be some structural or psychical
peculiarities, such as modifications in the attachments of muscles,
increased delicacy of smell or sight, or peculiar likes and dislikes,
which are inherited; and from these, peculiar habits follow as a natural
consequence, or are easily acquired. Now, as selection has been
constantly at work in improving all our domestic animals, we have
unconsciously modified the structure, while preserving only those
animals which best served our purpose in their peculiar faculties,
instincts, or habits.
Much of the mystery of instinct arises from the persistent refusal to
recognise the agency of imitation, memory, observation, and reason as
often forming part of it. Yet there is ample evidence that such agency
must be taken into account. Both Wilson and Leroy state that young birds
build inferior nests to old ones, and the latter author observes that
the best nests are made by birds whose young remain longest in the nest.
So, migration is now well ascertained to be effected by means of vision,
long flights being made on bright moonlight nights when the birds fly
very high, while on cloudy nights they fly low, and then often lose
their way. Thousands annually fly out to sea and perish, showing that
the instinct to migrate is imperfect, and is not a good substitute for
reason and observation.
Again, much of the perfection of instinct is due to the extreme severity
of the selection during its development, any failure involving
destruction. The chick which cannot break the eggshell, the caterpillar
that fails to suspend itself properly or to spin a safe cocoon, the bees
that lose their way or that fail to store honey, inevitably perish. So
the birds that fail to feed and protect their young, or the butterflies
that lay their eggs on the wrong food-plant, leave no offspring, and the
race with imperfect instincts perishes. Now, during the long and very
slow course of development of each organism, this rigid selection at
every step of progress has led to the preservation of every detail of
structure, faculty, or habit that has been necessary for the
preservation of the race, and has thus gradually built up the various
instincts which seem so marvellous to us, but which can yet be shown to
be in many cases still imperfect. Here, as everywhere else in nature, we
find comparative, not absolute perfection, with every gradation from
what is clearly due to imitation or reason up to what seems to us
perfect instinct--that in which a complex action is performed without
any previous experience or instruction.[217]
_Concluding Remarks._
Having now passed in review the more important of the recent objections
to, or criticisms of, the theory of natural selection, we have arrived
at the conclusion that in no one case have the writers in question been
able materially to diminish its importance, or to show that any of the
laws or forces to which they appeal can act otherwise than in strict
subordination to it. The direct action of the environment as set forth
by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit
that its effects on the individual are transmitted by inheritance, are
so small in comparison with the amount of spontaneous variation of every
part of the organism that they must be quite overshadowed by the latter.
And if such direct action may, in some cases, have initiated certain
organs or outgrowths, these must from their very first beginnings have
been subject to variation and natural selection, and their further
development have been almost wholly due to these ever-present and
powerful causes. The same remark applies to the views of Professor
Geddes on the laws of growth which have determined certain essential
features in the morphology of plants and animals. The attempt to
substitute these laws for those of variation and natural selection has
failed in cases where we can apply a definite test, as in that of the
origin of spines on trees and shrubs; while the extreme diversity of
vegetable structure and form among the plants of the same country and of
the same natural order, of itself affords a proof of the preponderating
influence of variation and natural selection in keeping the many diverse
forms in harmony with the highly complex and ever-changing environment.
Lastly, we have seen that Professor Weismann's theory of the continuity
of the germ-plasm and the consequent non-heredity of acquired
characters, while in perfect harmony with all the well-ascertained facts
of heredity and development, adds greatly to the importance of natural
selection as the one invariable and ever-present factor in all organic
change, and that which can alone have produced the temporary fixity
combined with the secular modification of species. While admitting, as
Darwin always admitted, the co-operation of the fundamental laws of
growth and variation, of correlation and heredity, in determining the
direction of lines of variation or in the initiation of peculiar organs,
we find that variation and natural selection are ever-present agencies,
which take possession, as it were, of every minute change originated by
these fundamental causes, check or favour their further development, or
modify them in countless varied ways according to the varying needs of
the organism. Whatever other causes have been at work, Natural Selection
is supreme, to an extent which even Darwin himself hesitated to claim
for it. The more we study it the more we are convinced of its
overpowering importance, and the more confidently we claim, in Darwin's
own words, that it "has been the most important, but not the exclusive,
means of modification."
FOOTNOTES:
[Footnote 198: See the Duke of Argyll's letter in _Nature_, vol. xxxiv.
p. 336.]
[Footnote 199: _Journal of the Anthropological Institute,_ vol. xv. pp.
246-260.]
[Footnote 200: The idea of the non-heredity of acquired variations was
suggested by the summary of Professor Weismann's views, in _Nature_,
referred to later on. But since this chapter was written I have, through
the kindness of Mr. E.B. Poulton, seen some of the proofs of the
forthcoming translation of Weismann's Essays on Heredity, in which he
sets forth an explanation very similar to that here given. On the
difficult question of the almost entire disappearance of organs, as in
the limbs of snakes and of some lizards, he adduces "a certain form of
correlation, which Roux calls 'the struggle of the parts in the
organism,'" as playing an important part. Atrophy following disuse is
nearly always attended by the corresponding increase of other organs:
blind animals possess more developed organs of touch, hearing, and
smell; the loss of power in the wings is accompanied by increased
strength of the legs, etc. Now as these latter characters, being useful,
will be selected, it is easy to understand that a congenital increase of
these will be accompanied by a corresponding congenital diminution of
the unused organ; and in cases where the means of nutrition are
deficient, every diminution of these useless parts will be a gain to the
whole organism, and thus their complete disappearance will, in some
cases, be brought about directly by natural selection. This corresponds
with what we know of these rudimentary organs.
It must, however, be pointed out that the non-heredity of acquired
characters was maintained by Mr. Francis Galton more than twelve years
ago, on theoretical considerations almost identical with those urged by
Professor Weismann; while the insufficiency of the evidence for their
hereditary transmission was shown, by similar arguments to those used
above and in the work of Professor Weismann already referred to (see "A
Theory of Heredity," in _Journ. Anthrop. Instit._, vol. v. pp.
343-345).]
[Footnote 201: This explanation is derived from Weismann's Theory of the
Continuity of the Germ-Plasm as summarised in _Nature_.]
[Footnote 202: See a collection of his essays under the title, _The
Origin of the Fittest: Essays on Evolution_, D. Appleton and Co. New
York. 1887.]
[Footnote 203: _Origin of the Fittest_, p. 174.]
[Footnote 204: _Ibid._ p. 29. It may be here noted that Darwin found
these theories unintelligible. In a letter to Professor E.T. Morse in
1877, he writes: "There is one point which I regret you did not make
clear in your Address, namely, what is the meaning and importance of
Professors Cope and Hyatt's views on acceleration and retardation? I
have endeavoured, and given up in despair, the attempt to grasp their
meaning" (_Life and Letters_, vol. iii. p. 233).]
[Footnote 205: _Origin of the Fittest_, p. 374.]
[Footnote 206: _Origin of the Fittest_, p. 40.]
[Footnote 207: _The Natural Conditions of Existence as they Affect
Animal Life._ London, 1883.]
[Footnote 208: In Dr. Weismann's essay on "Heredity," already referred
to, he considers it not improbable that changes in organisms produced by
climatic influences may be inherited, because, as these changes do not
affect the external parts of an organism only, but often, as in the case
of warmth or moisture permeate the whole structure, they may possibly
modify the germ-plasm itself, and thus induce variations in the next
generation. In this way, he thinks, may possibly be explained the
climatic varieties of certain butterflies, and some other changes which
seem to be effected by change of climate in a few generations.]
[Footnote 209: This brief indication of Professor Geddes's views is
taken from the article "Variation and Selection" in the _Encyclopedia
Britannica_, and a paper "On the Nature and Causes of Variation in
Plants" in _Trans. and Proc. of the Edinburgh Botanical Society_, 1886;
and is, for the most part, expressed in his own words.]
[Footnote 210: Placostylis bovinus, 3½ inches long; Paryphanta Busbyi, 3
in. diam.; P. Hochstetteri, 2¾ in. diam.]
[Footnote 211: The general arguments and objections here set forth will
apply with equal force to Professor G. Henslow's theory of the origin of
the various forms and structures of flowers as due to "the responsive
actions of the protoplasm in consequence of the irritations set up by
the weights, pressures, thrusts, tensions, etc., of the insect visitors"
(_The Origin of Floral Structures through Insect and other Agencies_, p.
340). On the assumption that acquired characters are inherited, such
irritations may have had something to do with the initiation of
variations and with the production of certain details of structure, but
they are clearly incompetent to have brought about the more important
structural and functional modifications of flowers. Such are, the
various adjustments of length and position of the stamens to bring the
pollen to the insect and from the insect to the stigma; the various
motions of stamens and styles at the right time and the right direction;
the physiological adjustments bringing about fertility or sterility in
heterostyled plants; the traps, springs, and complex movements of
various parts of orchids; and innumerable other remarkable phenomena.
For the explanation of these we have no resource but variation and
selection, to the effects of which, acting alternately with regression
or degradation as above explained (p. 328) must be imputed the
development of the countless floral structures we now behold. Even the
primitive flowers, whose initiation may, perhaps, have been caused, or
rendered possible, by the irritation set up by insects' visits, must,
from their very origin, have been modified, in accordance with the
supreme law of utility, by means of variation and survival of the
fittest.]
[Footnote 212: In an essay on "The Duration of Life," forming part of
the translation of Dr. Weismann's papers already referred to, the author
still further extends the sphere of natural selection by showing that
the average duration of life in each species has been determined by it.
A certain length of life is essential in order that the species may
produce offspring sufficient to ensure its continuance under the most
unfavourable conditions; and it is shown that the remarkable
inequalities of longevity in different species and groups may be thus
accounted for. Yet more, the occurrence of death in the higher
organisms, in place of the continued survival of the unicellular
organisms however much they may increase by subdivision, may be traced
to the same great law of utility for the race and survival of the
fittest. The whole essay is of exceeding interest, and will repay a
careful perusal. A similar idea occurred to the present writer about
twenty years back, and was briefly noted down at the time, but
subsequently forgotten.]
[Footnote 213: The outline here given is derived from two articles in
_Nature_, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which
Weismann's papers are summarised and partly translated.]
[Footnote 214: There are many indications that this explanation of the
cause of variation is the true one. Mr. E.B. Poulton suggests one, in
the fact that parthenogenetic reproduction only occurs in isolated
species, not in groups of related species; as this shows that
parthenogenesis cannot lead to the evolution of new forms. Again, in
parthenogenetic females the complete apparatus for fertilisation remains
unreduced; but if these varied as do sexually produced animals, the
organs referred to, being unused, would become rudimentary.
Even more important is the significance of the "polar bodies," as
explained by Weismann in one of his _Essays_; since, if his
interpretation of them be correct, variability is a necessary
consequence of sexual generation.]
[Footnote 215: Darwin's _Animals and Plants_, vol. ii. pp. 23, 24.]
[Footnote 216: In his essay on "Heredity," Dr. Weismann discusses many
other cases of supposed inheritance of acquired characters, and shows
that they can all be explained in other ways. Shortsightedness among
civilised nations, for example, is due partly to the absence of
selection and consequent regression towards a mean, and partly to its
individual production by constant reading.]
[Footnote 217: Weismann explains instinct on similar lines, and gives
many interesting illustrations (see _Essays on Heredity_). He holds
"that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences, but
upon variations of the germ." Many interesting and difficult cases of
instinct are discussed by Darwin in Chapter VIII of the _Origin of
Species_, which should be read in connection with the above remarks.
Since this chapter was written my attention has been directed to Mr.
Francis Galton's _Theory of Heredity_ (already referred to at p. 417)
which was published thirteen years ago as an alternative for Darwin's
theory of pangenesis.
Mr. Galton's theory, although it attracted little attention, appears to
me to be substantially the same as that of Professor Weismann. Galton's
"stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements
in the offspring to be directly formed from the residue of the _stirp_
not used up in the development of the body of the parent--Weismann's
"continuity of the germ-plasm." Galton also draws many of the same
conclusions from his theory. He maintains that characters acquired by
the individual as the result of external influences cannot be inherited,
unless such influences act directly on the reproductive
elements--instancing the possible heredity of alcoholism, because the
alcohol permeates the tissues and may reach the sexual elements. He
discusses the supposed heredity of effects produced by use or disuse,
and explains them much in the same manner as does Weismann. Galton is an
anthropologist, and applies the theory, mainly, to explain the
peculiarities of hereditary transmission in man, many of which
peculiarities he discusses and elucidates. Weismann is a biologist, and
is mostly concerned with the application of the theory to explain
variation and instinct, and to the further development of the theory of
evolution. He has worked it out more thoroughly, and has adduced
embryological evidence in its support; but the views of both writers are
substantially the same, and their theories were arrived at quite
independently. The names of Galton and Weismann should therefore be
associated as discoverers of what may be considered (if finally
established) the most important contribution to the evolution theory
since the appearance of the _Origin of Species_.]
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.
Our review of modern Darwinism might fitly have terminated with the
preceding chapter; but the immense interest that attaches to the origin
of the human race, and the amount of misconception which prevails
regarding the essential teachings of Darwin's theory on this question,
as well as regarding my own special views upon it, induce me to devote a
final chapter to its discussion.
To any one who considers the structure of man's body, even in the most
superficial manner, it must be evident that it is the body of an animal,
differing greatly, it is true, from the bodies of all other animals, but
agreeing with them in all essential features. The bony structure of man
classes him as a vertebrate; the mode of suckling his young classes him
as a mammal; his blood, his muscles, and his nerves, the structure of
his heart with its veins and arteries, his lungs and his whole
respiratory and circulatory systems, all closely correspond to those of
other mammals, and are often almost identical with them. He possesses
the same number of limbs terminating in the same number of digits as
belong fundamentally to the mammalian class. His senses are identical
with theirs, and his organs of sense are the same in number and occupy
the same relative position. Every detail of structure which is common to
the mammalia as a class is found also in man, while he only differs from
them in such ways and degrees as the various species or groups of
mammals differ from each other. If, then, we have good reason to believe
that every existing group of mammalia has descended from some common
ancestral form--as we saw to be so completely demonstrated in the case
of the horse tribe,--and that each family, each order, and even the
whole class must similarly have descended from some much more ancient
and more generalised type, it would be in the highest degree
improbable--so improbable as to be almost inconceivable--that man,
agreeing with them so closely in every detail of his structure, should
have had some quite distinct mode of origin. Let us, then, see what
other evidence bears upon the question, and whether it is sufficient to
convert the probability of his animal origin into a practical certainty.
_Rudiments and Variations as Indicating the Relation of Man to other
Mammals._
All the higher animals present rudiments of organs which, though useless
to them, are useful in some allied group, and are believed to have
descended from a common ancestor in which they were useful. Thus there
are in ruminants rudiments of incisor teeth which, in some species,
never cut through the gums; many lizards have external rudimentary legs;
while many birds, as the Apteryx, have quite rudimentary wings. Now man
possesses similar rudiments, sometimes constantly, sometimes only
occasionally present, which serve intimately to connect his bodily
structure with that of the lower animals. Many animals, for example,
have a special muscle for moving or twitching the skin. In man there are
remnants of this in certain parts of the body, especially in the
forehead, enabling us to raise our eyebrows; but some persons have it in
other parts. A few persons are able to move the whole scalp so as to
throw off any object placed on the head, and this property has been
proved, in one case, to be inherited. In the outer fold of the ear there
is sometimes a projecting point, corresponding in position to the
pointed ear of many animals, and believed to be a rudiment of it. In the
alimentary canal there is a rudiment--the vermiform appendage of the
caecum--which is not only useless, but is sometimes a cause of disease
and death in man; yet in many vegetable feeding animals it is very long,
and even in the orang-utan it is of considerable length and convoluted.
So, man possesses rudimentary bones of a tail concealed beneath the
skin, and, in some rare cases, this forms a minute external tail.
The variability of every part of man's structure is very great, and many
of these variations tend to approximate towards the structure of other
animals. The courses of the arteries are eminently variable, so that for
surgical purposes it has been necessary to determine the probable
proportion of each variation. The muscles are so variable that in fifty
cases the muscles of the foot were found to be not strictly alike in any
two, and in some the deviations were considerable; while in thirty-six
subjects Mr. J. Wood observed no fewer than 558 muscular variations. The
same author states that in a single male subject there were no fewer
than seven muscular variations, all of which plainly represented muscles
proper to various kinds of apes. The muscles of the hands and
arms--parts which are so eminently characteristic of man--are extremely
liable to vary, so as to resemble the corresponding muscles of the lower
animals. That such variations are due to reversion to a former state of
existence Mr. Darwin thinks highly probable, and he adds: "It is quite
incredible that a man should, through mere accident, abnormally resemble
certain apes in no less than seven of his muscles, if there had been no
genetic connection between them. On the other hand, if man is descended
from some ape-like creature, no valid reason can be assigned why certain
muscles should not suddenly reappear after an interval of many thousand
generations, in the same manner as, with horses, asses, and mules, dark
coloured stripes suddenly reappear on the legs and shoulders, after an
interval of hundreds, or more probably of thousands of
generations."[218]
_The Embryonic Development of Man and other Mammalia._
The progressive development of any vertebrate from the ovum or minute
embryonic egg affords one of the most marvellous chapters in Natural
History. We see the contents of the ovum undergoing numerous definite
changes, its interior dividing and subdividing till it consists of a
mass of cells, then a groove appears marking out the median line or
vertebral column of the future animal, and thereafter are slowly
developed the various essential organs of the body. After describing in
some detail what takes place in the case of the ovum of the dog,
Professor Huxley continues: "The history of the development of any other
vertebrate animal, lizard, snake, frog, or fish tells the same story.
There is always to begin with, an egg having the same essential
structure as that of the dog; the yelk of that egg undergoes division or
segmentation, as it is called, the ultimate products of that
segmentation constitute the building materials for the body of the young
animal; and this is built up round a primitive groove, in the floor of
which a notochord is developed. Furthermore, there is a period in which
the young of all these animals resemble one another, not merely in
outward form, but in all essentials of structure, so closely, that the
differences between them are inconsiderable, while in their subsequent
course they diverge more and more widely from one another. And it is a
general law that the more closely any animals resemble one another in
adult structure, the larger and the more intimately do their embryos
resemble one another; so that, for example, the embryos of a snake and
of a lizard remain like one another longer than do those of a snake and
a bird; and the embryos of a dog and of a cat remain like one another
for a far longer period than do those of a dog and a bird, or of a dog
and an opossum, or even than those of a dog and a monkey."[219]
We thus see that the study of development affords a test of affinity in
animals that are externally very much unlike each other; and we
naturally ask how this applies to man. Is he developed in a different
way from other mammals, as we should certainly expect if he has had a
distinct and altogether different origin? "The reply," says Professor
Huxley, "is not doubtful for a moment. Without question, the mode of
origin and the early stages of the development of man are identical with
those of the animals immediately below him in the scale." And again he
tells us: "It is very long before the body of the young human being can
be readily discriminated from that of the young puppy; but at a
tolerably early period the two become distinguishable by the different
forms of their adjuncts, the yelk-sac and the allantois;" and after
describing these differences he continues: "But exactly in those
respects in which the developing man differs from the dog, he resembles
the ape.... So that it is only quite in the latter stages of development
that the young human being presents marked differences from the young
ape, while the latter departs as much from the dog in its development as
the man does. Startling as this last assertion may appear to be, it is
demonstrably true, and it alone appears to me sufficient to place beyond
all doubt the structural unity of man with the rest of the animal world,
and more particularly and closely with the apes."[220]
A few of the curious details in which man passes through stages common
to the lower animals may be mentioned. At one stage the os coccyx
projects like a true tail, extending considerably beyond the rudimentary
legs. In the seventh month the convolutions of the brain resemble those
of an adult baboon. The great toe, so characteristic of man, forming the
fulcrum which most assists him in standing erect, in an early stage of
the embryo is much shorter than the other toes, and instead of being
parallel with them, projects at an angle from the side of the foot, thus
corresponding with its permanent condition in the quadrumana. Numerous
other examples might be quoted, all illustrating the same general law.
_Diseases Common to Man and the Lower Animals._
Though the fact is so well known, it is certainly one of profound
significance that many animal diseases can be communicated to man, since
it shows similarity, if not identity, in the minute structure of the
tissues, the nature of the blood, the nerves, and the brain. Such
diseases as hydrophobia, variola, the glanders, cholera, herpes, etc.,
can be transmitted from animals to man or the reverse; while monkeys are
liable to many of the same non-contagious diseases as we are. Rengger,
who carefully observed the common monkey (Cebus Azarae) in Paraguay,
found it liable to catarrh, with the usual symptoms, terminating
sometimes in consumption. These monkeys also suffered from apoplexy,
inflammation of the bowels, and cataract in the eye. Medicines produced
the same effect upon them as upon us. Many kinds of monkeys have a
strong taste for tea, coffee, spirits, and even tobacco. These facts
show the similarity of the nerves of taste in monkeys and in ourselves,
and that their whole nervous system is affected in a similar way. Even
the parasites, both external and internal, that affect man are not
altogether peculiar to him, but belong to the same families or genera as
those which infest animals, and in one case, scabies, even the same
species.[221] These curious facts seem quite inconsistent with the idea
that man's bodily structure and nature are altogether distinct from
those of animals, and have had a different origin; while the facts are
just what we should expect if he has been produced by descent with
modification from some common ancestor.
_The Animals most nearly Allied to Man._
By universal consent we see in the monkey tribe a caricature of
humanity. Their faces, their hands, their actions and expressions
present ludicrous resemblances to our own. But there is one group of
this great tribe in which this resemblance is greatest, and they have
hence been called the anthropoid or man-like apes. These are few in
number, and inhabit only the equatorial regions of Africa and Asia,
countries where the climate is most uniform, the forests densest, and
the supply of fruit abundant throughout the year. These animals are now
comparatively well known, consisting of the orang-utan of Borneo and
Sumatra, the chimpanzee and the gorilla of West Africa, and the group of
gibbons or long-armed apes, consisting of many species and inhabiting
South-Eastern Asia and the larger Malay Islands. These last are far
less like man than the other three, one or other of which has at various
times been claimed to be the most man-like of the apes and our nearest
relations in the animal kingdom. The question of the degree of
resemblance of these animals to ourselves is one of great interest,
leading, as it does, to some important conclusions as to our origin and
geological antiquity, and we will therefore briefly consider it.
If we compare the skeletons of the orang or chimpanzee with that of man,
we find them to be a kind of distorted copy, every bone corresponding
(with very few exceptions), but altered somewhat in size, proportions,
and position. So great is this resemblance that it led Professor Owen to
remark: "I cannot shut my eyes to the significance of that all-pervading
similitude of structure--every tooth, every bone, strictly
homologous--which makes the determination of the difference between
_Homo_ and _Pithecus_ the anatomist's difficulty."
The actual differences in the skeletons of these apes and that of
man--that is, differences dependent on the presence or absence of
certain bones, and not on their form or position--have been enumerated
by Mr. Mivart as follows:--(1) In the breast-bone consisting of but two
bones, man agrees with the gibbons; the chimpanzee and gorilla having
this part consisting of seven bones in a single series, while in the
orang they are arranged in a double series of ten bones. (2) The normal
number of the ribs in the orang and some gibbons is twelve pairs, as in
man, while in the chimpanzee and gorilla there are thirteen pairs. (3)
The orang and the gibbons also agree with man in having five lumbar
vertebrae, while in the gorilla and the chimpanzee there are but four,
and sometimes only three. (4) The gorilla and chimpanzee agree with man
in having eight small bones in the wrist, while the orang and the
gibbons, as well as all other monkeys, have nine.[222]
The differences in the form, size, and attachments of the various bones,
muscles, and other organs of these apes and man are very numerous and
exceedingly complex, sometimes one species, sometimes another agreeing
most nearly with ourselves, thus presenting a tangled web of affinities
which it is very difficult to unravel. Estimated by the skeleton alone,
the chimpanzee and gorilla seem nearer to man than the orang, which last
is also inferior as presenting certain aberrations in the muscles. In
the form of the ear the gorilla is more human than any other ape, while
in the tongue the orang is the more man-like. In the stomach and liver
the gibbons approach nearest to man, then come the orang and chimpanzee,
while the gorilla has a degraded liver more resembling that of the lower
monkeys and baboons.
_The Brains of Man and Apes._
We come now to that part of his organisation in which man is so much
higher than all the lower animals--the brain; and here, Mr. Mivart
informs us, the orang stands highest in rank. The height of the orang's
cerebrum in front is greater in proportion than in either the chimpanzee
or the gorilla. "On comparing the brain of man with the brains of the
orang, chimpanzee, and baboon, we find a successive decrease in the
frontal lobe, and a successive and very great increase in the relative
size of the occipital lobe. Concomitantly with this increase and
decrease, certain folds of brain substance, called 'bridging
convolutions,' which in man are conspicuously interposed between the
parietal and occipital lobes, seem as utterly to disappear in the
chimpanzee, as they do in the baboon. In the orang, however, though much
reduced, they are still to be distinguished.... The actual and absolute
mass of the brain is, however, slightly greater in the chimpanzee than
in the orang, as is the relative vertical extent of the middle part of
the cerebrum, although, as already stated, the frontal portion is higher
in the orang; while, according to M. Gratiolet, the gorilla is not only
inferior to the orang in cerebral development, but even to his smaller
African congener, the chimpanzee."[223]
On the whole, then, we find that no one of the great apes can be
positively asserted to be nearest to man in structure. Each of them
approaches him in certain characteristics, while in others it is widely
removed, giving the idea, so consonant with the theory of evolution as
developed by Darwin, that all are derived from a common ancestor, from
which the existing anthropoid apes as well as man have diverged. When,
however, we turn from the details of anatomy to peculiarities of
external form and motions, we find that, in a variety of characters, all
these apes resemble each other and differ from man, so that we may
fairly say that, while they have diverged somewhat from each other, they
have diverged much more widely from ourselves. Let us briefly enumerate
some of these differences.
_External Differences of Man and Apes._
All apes have large canine teeth, while in man these are no longer than
the adjacent incisors or premolars, the whole forming a perfectly even
series. In apes the arms are proportionately much longer than in man,
while the thighs are much shorter. No ape stands really erect, a posture
which is natural in man. The thumb is proportionately larger in man, and
more perfectly opposable than in that of any ape. The foot of man
differs largely from that of all apes, in the horizontal sole, the
projecting heel, the short toes, and the powerful great toe firmly
attached parallel to the other toes; all perfectly adapted for
maintaining the erect posture, and for free motion without any aid from
the arms or hands. In apes the foot is formed almost exactly like our
hand, with a large thumb-like great toe quite free from the other toes,
and so articulated as to be opposable to them; forming with the long
finger-like toes a perfect grasping hand. The sole cannot be placed
horizontally on the ground; but when standing on a level surface the
animal rests on the outer edge of the foot with the finger and
thumb-like toes partly closed, while the hands are placed on the ground
resting on the knuckles. The illustration on the next page (Fig. 37)
shows, fairly well, the peculiarities of the hands and feet of the
chimpanzee, and their marked differences, both in form and use, from
those of man.
The four limbs, with the peculiarly formed feet and hands, are those of
arboreal animals which only occasionally and awkwardly move on level
ground. The arms are used in progression equally with the feet, and the
hands are only adapted for uses similar to those of our hands when the
animal is at rest, and then but clumsily. Lastly, the apes are all hairy
animals, like the majority of other mammals, man alone having a smooth
and almost naked skin. These numerous and striking differences, even
more than those of the skeleton and internal anatomy, point to an
enormously remote epoch when the race that was ultimately to develop
into man diverged from that other stock which continued the animal type
and ultimately produced the existing varieties of anthropoid apes.
[Illustration: FIG. 37.--Chimpanzee (Troglodytes niger).]
_Summary of the Animal Characteristics of Man._
The facts now very briefly summarised amount almost to a demonstration
that man, in his bodily structure, has been derived from the lower
animals, of which he is the culminating development. In his possession
of rudimentary structures which are functional in some of the mammalia;
in the numerous variations of his muscles and other organs agreeing with
characters which are constant in some apes; in his embryonic
development, absolutely identical in character with that of mammalia in
general, and closely resembling in its details that of the higher
quadrumana; in the diseases which he has in common with other mammalia;
and in the wonderful approximation of his skeleton to those of one or
other of the anthropoid apes, we have an amount of evidence in this
direction which it seems impossible to explain away. And this evidence
will appear more forcible if we consider for a moment what the rejection
of it implies. For the only alternative supposition is, that man has
been specially created--that is to say, has been produced in some quite
different way from other animals and altogether independently of them.
But in that case the rudimentary structures, the animal-like variations,
the identical course of development, and all the other animal
characteristics he possesses are deceptive, and inevitably lead us, as
thinking beings making use of the reason which is our noblest and most
distinctive feature, into gross error.
We cannot believe, however, that a careful study of the facts of nature
leads to conclusions directly opposed to the truth; and, as we seek in
vain, in our physical structure and the course of its development, for
any indication of an origin independent of the rest of the animal world,
we are compelled to reject the idea of "special creation" for man, as
being entirely unsupported by facts as well as in the highest degree
improbable.
_The Geological Antiquity of Man._
The evidence we now possess of the exact nature of the resemblance of
man to the various species of anthropoid apes, shows us that he has
little special affinity for any one rather than another species, while
he differs from them all in several important characters in which they
agree with each other. The conclusion to be drawn from these facts is,
that his points of affinity connect him with the whole group, while his
special peculiarities equally separate him from the whole group, and
that he must, therefore, have diverged from the common ancestral form
before the existing types of anthropoid apes had diverged from each
other. Now, this divergence almost certainly took place as early as the
Miocene period, because in the Upper Miocene deposits of Western Europe
remains of two species of ape have been found allied to the gibbons, one
of them, Dryopithecus, nearly as large as a man, and believed by M.
Lartet to have approached man in its dentition more than the existing
apes. We seem hardly, therefore, to have reached, in the Upper Miocene,
the epoch of the common ancestor of man and the anthropoids.
The evidence of the antiquity of man himself is also scanty, and takes
us but very little way back into the past. We have clear proof of his
existence in Europe in the latter stages of the glacial epoch, with many
indications of his presence in interglacial or even pre-glacial times;
while both the actual remains and the works of man found in the
auriferous gravels of California deep under lava-flows of Pliocene age,
show that he existed in the New World at least as early as in the
Old.[224] These earliest remains of man have been received with doubt,
and even with ridicule, as if there were some extreme improbability in
them. But, in point of fact, the wonder is that human remains have not
been found more frequently in pre-glacial deposits. Referring to the
most ancient fossil remains found in Europe--the Engis and Neanderthal
crania,--Professor Huxley makes the following weighty remark: "In
conclusion, I may say, that the fossil remains of Man hitherto
discovered do not seem to me to take us appreciably nearer to that lower
pithecoid form, by the modification of which he has, probably, become
what he is." The Californian remains and works of art, above referred
to, give no indication of a specially low form of man; and it remains an
unsolved problem why no traces of the long line of man's ancestors, back
to the remote period when he first branched off from the pithecoid type,
have yet been discovered.
It has been objected by some writers--notably by Professor Boyd
Dawkins--that man did not probably exist in Pliocene times, because
almost all the known mammalia of that epoch are distinct species from
those now living on the earth, and that the same changes of the
environment which led to the modification of other mammalian species
would also have led to a change in man. But this argument overlooks the
fact that man differs essentially from all other mammals in this
respect, that whereas any important adaptation to new conditions can be
effected in them only by a change in bodily structure, man is able to
adapt himself to much greater changes of conditions by a mental
development leading him to the use of fire, of tools, of clothing, of
improved dwellings, of nets and snares, and of agriculture. By the help
of these, without any change whatever in his bodily structure, he has
been able to spread over and occupy the whole earth; to dwell securely
in forest, plain, or mountain; to inhabit alike the burning desert or
the arctic wastes; to cope with every kind of wild beast, and to provide
himself with food in districts where, as an animal trusting to nature's
unaided productions, he would have starved.[225]
It follows, therefore, that from the time when the ancestral man first
walked erect, with hands freed from any active part in locomotion, and
when his brain-power became sufficient to cause him to use his hands in
making weapons and tools, houses and clothing, to use fire for cooking,
and to plant seeds or roots to supply himself with stores of food, the
power of natural selection would cease to act in producing modifications
of his body, but would continuously advance his mind through the
development of its organ, the brain. Hence man may have become truly
man--the species, Homo sapiens--even in the Miocene period; and while
all other mammals were becoming modified from age to age under the
influence of ever-changing physical and biological conditions, he would
be advancing mainly in intelligence, but perhaps also in stature, and by
that advance alone would be able to maintain himself as the master of
all other animals and as the most widespread occupier of the earth. It
is quite in accordance with this view that we find the most pronounced
distinction between man and the anthropoid apes in the size and
complexity of his brain. Thus, Professor Huxley tells us that "it may be
doubted whether a healthy human adult brain ever weighed less than 31
or 32 ounces, or that the heaviest gorilla brain has exceeded 20
ounces," although "a full-grown gorilla is probably pretty nearly twice
as heavy as a Bosjes man, or as many an European woman."[226] The
average human brain, however, weighs 48 or 49 ounces, and if we take the
average ape brain at only 2 ounces less than the very largest gorilla's
brain, or 18 ounces, we shall see better the enormous increase which has
taken place in the brain of man since the time when he branched off from
the apes; and this increase will be still greater if we consider that
the brains of apes, like those of all other mammals, have also increased
from earlier to later geological times.
If these various considerations are taken into account, we must conclude
that the essential features of man's structure as compared with that of
apes--his erect posture and free hands--were acquired at a comparatively
early period, and were, in fact, the characteristics which gave him his
superiority over other mammals, and started him on the line of
development which has led to his conquest of the world. But during this
long and steady development of brain and intellect, mankind must have
continuously increased in numbers and in the area which they
occupied--they must have formed what Darwin terms a "dominant race." For
had they been few in numbers and confined to a limited area, they could
hardly have successfully struggled against the numerous fierce carnivora
of that period, and against those adverse influences which led to the
extinction of so many more powerful animals. A large population spread
over an extensive area is also needed to supply an adequate number of
brain variations for man's progressive improvement. But this large
population and long-continued development in a single line of advance
renders it the more difficult to account for the complete absence of
human or pre-human remains in all those deposits which have furnished,
in such rich abundance, the remains of other land animals. It is true
that the remains of apes are also very rare, and we may well suppose
that the superior intelligence of man led him to avoid that extensive
destruction by flood or in morass which seems to have often overwhelmed
other animals. Yet, when we consider that, even in our own day, men are
not unfrequently overwhelmed by volcanic eruptions, as in Java and
Japan, or carried away in vast numbers by floods, as in Bengal and
China, it seems impossible but that ample remains of Miocene and
Pliocene man do exist buried in the most recent layers of the earth's
crust, and that more extended research or some fortunate discovery will
some day bring them to light.
_The Probable Birthplace of Man._
It has usually been considered that the ancestral form of man originated
in the tropics, where vegetation is most abundant and the climate most
equable. But there are some important objections to this view. The
anthropoid apes, as well as most of the monkey tribe, are essentially
arboreal in their structure, whereas the great distinctive character of
man is his special adaptation to terrestrial locomotion. We can hardly
suppose, therefore, that he originated in a forest region, where fruits
to be obtained by climbing are the chief vegetable food. It is more
probable that he began his existence on the open plains or high plateaux
of the temperate or sub-tropical zone, where the seeds of indigenous
cereals and numerous herbivora, rodents, and game-birds, with fishes and
molluscs in the lakes, rivers, and seas supplied him with an abundance
of varied food. In such a region he would develop skill as a hunter,
trapper, or fisherman, and later as a herdsman and cultivator,--a
succession of which we find indications in the palaeolithic and
neolithic races of Europe.
In seeking to determine the particular areas in which his earliest
traces are likely to be found, we are restricted to some portion of the
Eastern hemisphere, where alone the anthropoid apes exist, or have
apparently ever existed.
There is good reason to believe, also, that Africa must be excluded,
because it is known to have been separated from the northern continent
in early tertiary times, and to have acquired its existing fauna of the
higher mammalia by a later union with that continent after the
separation from it of Madagascar, an island which has preserved for us a
sample, as it were, of the early African mammalian fauna, from which not
only the anthropoid apes, but all the higher quadrumana are
absent.[227] There remains only the great Euro-Asiatic continent; and
its enormous plateaux, extending from Persia right across Tibet and
Siberia to Manchuria, afford an area, some part or other of which
probably offered suitable conditions, in late Miocene or early Pliocene
times, for the development of ancestral man.
It is in this area that we still find that type of mankind--the
Mongolian--which retains a colour of the skin midway between the black
or brown-black of the negro, and the ruddy or olive-white of the
Caucasian types, a colour which still prevails over all Northern Asia,
over the American continents, and over much of Polynesia. From this
primary tint arose, under the influence of varied conditions, and
probably in correlation with constitutional changes adapted to peculiar
climates, the varied tints which still exist among mankind. If the
reasoning by which this conclusion is reached be sound, and all the
earlier stages of man's development from an animal form occurred in the
area now indicated, we can better understand how it is that we have as
yet met with no traces of the missing links, or even of man's existence
during late tertiary times, because no part of the world is so entirely
unexplored by the geologist as this very region. The area in question is
sufficiently extensive and varied to admit of primeval man having
attained to a considerable population, and having developed his full
human characteristics, both physical and mental, before there was any
need for him to migrate beyond its limits. One of his earliest important
migrations was probably into Africa, where, spreading westward, he
became modified in colour and hair in correlation with physiological
changes adapting him to the climate of the equatorial lowlands.
Spreading north-westward into Europe the moist and cool climate led to a
modification of an opposite character, and thus may have arisen the
three great human types which still exist. Somewhat later, probably, he
spread eastward into North-West America and soon scattered himself over
the whole continent; and all this may well have occurred in early or
middle Pliocene times. Thereafter, at very long intervals, successive
waves of migration carried him into every part of the habitable world,
and by conquest and intermixture led ultimately to that puzzling
gradation of types which the ethnologist in vain seeks to unravel.
_The Origin of the Moral and Intellectual Nature of Man._
From the foregoing discussion it will be seen that I fully accept Mr.
Darwin's conclusion as to the essential identity of man's bodily
structure with that of the higher mammalia, and his descent from some
ancestral form common to man and the anthropoid apes. The evidence of
such descent appears to me to be overwhelming and conclusive. Again, as
to the cause and method of such descent and modification, we may admit,
at all events provisionally, that the laws of variation and natural
selection, acting through the struggle for existence and the continual
need of more perfect adaptation to the physical and biological
environments, may have brought about, first that perfection of bodily
structure in which he is so far above all other animals, and in
co-ordination with it the larger and more developed brain, by means of
which he has been able to utilise that structure in the more and more
complete subjection of the whole animal and vegetable kingdoms to his
service.
But this is only the beginning of Mr. Darwin's work, since he goes on to
discuss the moral nature and mental faculties of man, and derives these
too by gradual modification and development from the lower animals.
Although, perhaps, nowhere distinctly formulated, his whole argument
tends to the conclusion that man's entire nature and all his faculties,
whether moral, intellectual, or spiritual, have been derived from their
rudiments in the lower animals, in the same manner and by the action of
the same general laws as his physical structure has been derived. As
this conclusion appears to me not to be supported by adequate evidence,
and to be directly opposed to many well-ascertained facts, I propose to
devote a brief space to its discussion.
_The Argument from Continuity._
Mr. Darwin's mode of argument consists in showing that the rudiments of
most, if not of all, the mental and moral faculties of man can be
detected in some animals. The manifestations of intelligence, amounting
in some cases to distinct acts of reasoning, in many animals, are
adduced as exhibiting in a much less degree the intelligence and reason
of man. Instances of curiosity, imitation, attention, wonder, and memory
are given; while examples are also adduced which may be interpreted as
proving that animals exhibit kindness to their fellows, or manifest
pride, contempt, and shame. Some are said to have the rudiments of
language, because they utter several different sounds, each of which has
a definite meaning to their fellows or to their young; others the
rudiments of arithmetic, because they seem to count and remember up to
three, four, or even five. A sense of beauty is imputed to them on
account of their own bright colours or the use of coloured objects in
their nests; while dogs, cats, and horses are said to have imagination,
because they appear to be disturbed by dreams. Even some distant
approach to the rudiments of religion is said to be found in the deep
love and complete submission of a dog to his master.[228]
Turning from animals to man, it is shown that in the lowest savages many
of these faculties are very little advanced from the condition in which
they appear in the higher animals; while others, although fairly well
exhibited, are yet greatly inferior to the point of development they
have reached in civilised races. In particular, the moral sense is said
to have been developed from the social instincts of savages, and to
depend mainly on the enduring discomfort produced by any action which
excites the general disapproval of the tribe. Thus, every act of an
individual which is believed to be contrary to the interests of the
tribe, excites its unvarying disapprobation and is held to be immoral;
while every act, on the other hand, which is, as a rule, beneficial to
the tribe, is warmly and constantly approved, and is thus considered to
be right or moral. From the mental struggle, when an act that would
benefit self is injurious to the tribe, there arises conscience; and
thus the social instincts are the foundation of the moral sense and of
the fundamental principles of morality.[229]
The question of the origin and nature of the moral sense and of
conscience is far too vast and complex to be discussed here, and a
reference to it has been introduced only to complete the sketch of Mr.
Darwin's view of the continuity and gradual development of all human
faculties from the lower animals up to savages, and from savage up to
civilised man. The point to which I wish specially to call attention is,
that to prove continuity and the progressive development of the
intellectual and moral faculties from animals to man, is not the same as
proving that these faculties have been developed by natural selection;
and this last is what Mr. Darwin has hardly attempted, although to
support his theory it was absolutely essential to prove it. Because
man's physical structure has been developed from an animal form by
natural selection, it does not necessarily follow that his mental
nature, even though developed _pari passu_ with it, has been developed
by the same causes only. To illustrate by a physical analogy. Upheaval
and depression of land, combined with sub-aerial denudation by wind and
frost, rain and rivers, and marine denudation on coastlines, were long
thought to account for all the modelling of the earth's surface not
directly due to volcanic action; and in the early editions of Lyell's
_Principles of Geology_ these are the sole causes appealed to. But when
the action of glaciers was studied and the recent occurrence of a
glacial epoch demonstrated as a fact, many phenomena--such as moraines
and other gravel deposits, boulder clay, erratic boulders, grooved and
rounded rocks, and Alpine lake basins--were seen to be due to this
altogether distinct cause. There was no breach of continuity, no sudden
catastrophe; the cold period came on and passed away in the most gradual
manner, and its effects often passed insensibly into those produced by
denudation or upheaval; yet none the less a new agency appeared at a
definite time, and new effects were produced which, though continuous
with preceding effects, were not due to the same causes. It is not,
therefore, to be assumed, without proof or against independent evidence,
that the later stages of an apparently continuous development are
necessarily due to the same causes only as the earlier stages. Applying
this argument to the case of man's intellectual and moral nature, I
propose to show that certain definite portions of it could not have been
developed by variation and natural selection alone, and that, therefore,
some other influence, law, or agency is required to account for them.
If this can be clearly shown for any one or more of the special
faculties of intellectual man, we shall be justified in assuming that
the same unknown cause or power may have had a much wider influence, and
may have profoundly influenced the whole course of his development.
_The Origin of the Mathematical Faculty._
We have ample evidence that, in all the lower races of man, what may be
termed the mathematical faculty is, either absent, or, if present, quite
unexercised. The Bushmen and the Brazilian Wood-Indians are said not to
count beyond two. Many Australian tribes only have words for one and
two, which are combined to make three, four, five, or six, beyond which
they do not count. The Damaras of South Africa only count to three; and
Mr. Galton gives a curious description of how one of them was hopelessly
puzzled when he had sold two sheep for two sticks of tobacco each, and
received four sticks in payment. He could only find out that he was
correctly paid by taking two sticks and then giving one sheep, then
receiving two sticks more and giving the other sheep. Even the
comparatively intellectual Zulus can only count up to ten by using the
hands and fingers. The Ahts of North-West America count in nearly the
same manner, and most of the tribes of South America are no further
advanced.[230] The Kaffirs have great herds of cattle, and if one is
lost they miss it immediately, but this is not by counting, but by
noticing the absence of one they know; just as in a large family or a
school a boy is missed without going through the process of counting.
Somewhat higher races, as the Esquimaux, can count up to twenty by using
the hands and the feet; and other races get even further than this by
saying "one man" for twenty, "two men" for forty, and so on, equivalent
to our rural mode of reckoning by scores. From the fact that so many of
the existing savage races can only count to four or five, Sir John
Lubbock thinks it improbable that our earliest ancestors could have
counted as high as ten.[231]
When we turn to the more civilised races, we find the use of numbers
and the art of counting greatly extended. Even the Tongas of the South
Sea islands are said to have been able to count as high as 100,000. But
mere counting does not imply either the possession or the use of
anything that can be really called the mathematical faculty, the
exercise of which in any broad sense has only been possible since the
introduction of the decimal notation. The Greeks, the Romans, the
Egyptians, the Jews, and the Chinese had all such cumbrous systems, that
anything like a science of arithmetic, beyond very simple operations,
was impossible; and the Roman system, by which the year 1888 would be
written MDCCCLXXXVIII, was that in common use in Europe down to the
fourteenth or fifteenth centuries, and even much later in some places.
Algebra, which was invented by the Hindoos, from whom also came the
decimal notation, was not introduced into Europe till the thirteenth
century, although the Greeks had some acquaintance with it; and it
reached Western Europe from Italy only in the sixteenth century.[232] It
was, no doubt, owing to the absence of a sound system of numeration that
the mathematical talent of the Greeks was directed chiefly to geometry,
in which science Euclid, Archimedes, and others made such brilliant
discoveries. It is, however, during the last three centuries only that
the civilised world appears to have become conscious of the possession
of a marvellous faculty which, when supplied with the necessary tools in
the decimal notation, the elements of algebra and geometry, and the
power of rapidly communicating discoveries and ideas by the art of
printing, has developed to an extent, the full grandeur of which can be
appreciated only by those who have devoted some time (even if
unsuccessfully) to the study.
The facts now set forth as to the almost total absence of mathematical
faculty in savages and its wonderful development in quite recent times,
are exceedingly suggestive, and in regard to them we are limited to two
possible theories. Either prehistoric and savage man did not possess
this faculty at all (or only in its merest rudiments); or they did
possess it, but had neither the means nor the incitements for its
exercise. In the former case we have to ask by what means has this
faculty been so rapidly developed in all civilised races, many of which
a few centuries back were, in this respect, almost savages themselves;
while in the latter case the difficulty is still greater, for we have to
assume the existence of a faculty which had never been used either by
the supposed possessors of it or by their ancestors.
Let us take, then, the least difficult supposition--that savages
possessed only the mere rudiments of the faculty, such as their ability
to count, sometimes up to ten, but with an utter inability to perform
the very simplest processes of arithmetic or of geometry--and inquire
how this rudimentary faculty became rapidly developed into that of a
Newton, a La Place, a Gauss, or a Cayley. We will admit that there is
every possible gradation between these extremes, and that there has been
perfect continuity in the development of the faculty; but we ask, What
motive power caused its development?
It must be remembered we are here dealing solely with the capability of
the Darwinian theory to account for the origin of the _mind_, as well as
it accounts for the origin of the _body_ of man, and we must, therefore,
recall the essential features of that theory. These are, the
preservation of useful variations in the struggle for life; that no
creature can be improved beyond its necessities for the time being; that
the law acts by life and death, and by the survival of the fittest. We
have to ask, therefore, what relation the successive stages of
improvement of the mathematical faculty had to the life or death of its
possessors; to the struggles of tribe with tribe, or nation with nation;
or to the ultimate survival of one race and the extinction of another.
If it cannot possibly have had any such effects, then it cannot have
been produced by natural selection.
It is evident that in the struggles of savage man with the elements and
with wild beasts, or of tribe with tribe, this faculty can have had no
influence. It had nothing to do with the early migrations of man, or
with the conquest and extermination of weaker by more powerful peoples.
The Greeks did not successfully resist the Persian invaders by any aid
from their few mathematicians, but by military training, patriotism, and
self-sacrifice. The barbarous conquerors of the East, Timurlane and
Gengkhis Khan, did not owe their success to any superiority of intellect
or of mathematical faculty in themselves or their followers. Even if the
great conquests of the Romans were, in part, due to their systematic
military organisation, and to their skill in making roads and
encampments, which may, perhaps, be imputed to some exercise of the
mathematical faculty, that did not prevent them from being conquered in
turn by barbarians, in whom it was almost entirely absent. And if we
take the most civilised peoples of the ancient world--the Hindoos, the
Arabs, the Greeks, and the Romans, all of whom had some amount of
mathematical talent--we find that it is not these, but the descendants
of the barbarians of those days--the Celts, the Teutons, and the
Slavs--who have proved themselves the fittest to survive in the great
struggle of races, although we cannot trace their steadily growing
success during past centuries either to the possession of any
exceptional mathematical faculty or to its exercise. They have indeed
proved themselves, to-day, to be possessed of a marvellous endowment of
the mathematical faculty; but their success at home and abroad, as
colonists or as conquerors, as individuals or as nations, can in no way
be traced to this faculty, since they were almost the last who devoted
themselves to its exercise. We conclude, then, that the present gigantic
development of the mathematical faculty is wholly unexplained by the
theory of natural selection, and must be due to some altogether distinct
cause.
_The Origin of the Musical and Artistic Faculties._
These distinctively human faculties follow very closely the lines of the
mathematical faculty in their progressive development, and serve to
enforce the same argument. Among the lower savages music, as we
understand it, hardly exists, though they all delight in rude musical
sounds, as of drums, tom-toms, or gongs; and they also sing in
monotonous chants. Almost exactly as they advance in general intellect,
and in the arts of social life, their appreciation of music appears to
rise in proportion; and we find among them rude stringed instruments and
whistles, till, in Java, we have regular bands of skilled performers
probably the successors of Hindoo musicians of the age before the
Mahometan conquest. The Egyptians are believed to have been the earliest
musicians, and from them the Jews and the Greeks, no doubt, derived
their knowledge of the art; but it seems to be admitted that neither the
latter nor the Romans knew anything of harmony or of the essential
features of modern music.[233] Till the fifteenth century little
progress appears to have been made in the science or the practice of
music; but since that era it has advanced with marvellous rapidity, its
progress being curiously parallel with that of mathematics, inasmuch as
great musical geniuses appeared suddenly among different nations, equal
in their possession of this special faculty to any that have since
arisen.
As with the mathematical, so with the musical faculty, it is impossible
to trace any connection between its possession and survival in the
struggle for existence. It seems to have arisen as a _result_ of social
and intellectual advancement, not as a _cause_; and there is some
evidence that it is latent in the lower races, since under European
training native military bands have been formed in many parts of the
world, which have been able to perform creditably the best modern music.
The artistic faculty has run a somewhat different course, though
analogous to that of the faculties already discussed. Most savages
exhibit some rudiments of it, either in drawing or carving human or
animal figures; but, almost without exception, these figures are rude
and such as would be executed by the ordinary inartistic child. In fact,
modern savages are, in this respect hardly equal to those prehistoric
men who represented the mammoth and the reindeer on pieces of horn or
bone. With any advance in the arts of social life, we have a
corresponding advance in artistic skill and taste, rising very high in
the art of Japan and India, but culminating in the marvellous sculpture
of the best period of Grecian history. In the Middle Ages art was
chiefly manifested in ecclesiastical architecture and the illumination
of manuscripts, but from the thirteenth to the fifteenth centuries
pictorial art revived in Italy and attained to a degree of perfection
which has never been surpassed. This revival was followed closely by the
schools of Germany, the Netherlands, Spain, France, and England, showing
that the true artistic faculty belonged to no one nation, but was fairly
distributed among the various European races.
These several developments of the artistic faculty, whether manifested
in sculpture, painting, or architecture, are evidently outgrowths of the
human intellect which have no immediate influence on the survival of
individuals or of tribes, or on the success of nations in their
struggles for supremacy or for existence. The glorious art of Greece did
not prevent the nation from falling under the sway of the less advanced
Roman; while we ourselves, among whom art was the latest to arise, have
taken the lead in the colonisation of the world, thus proving our mixed
race to be the fittest to survive.
_Independent Proof that the Mathematical, Musical, and Artistic
Faculties have not been Developed under the Law of Natural Selection._
The law of Natural Selection or the survival of the fittest is, as its
name implies, a rigid law, which acts by the life or death of the
individuals submitted to its action. From its very nature it can act
only on useful or hurtful characteristics, eliminating the latter and
keeping up the former to a fairly general level of efficiency. Hence it
necessarily follows that the characters developed by its means will be
present in all the individuals of a species, and, though varying, will
not vary very widely from a common standard. The amount of variation we
found, in our third chapter, to be about one-fifth or one-sixth of the
mean value--that is, if the mean value were taken at 100, the variations
would reach from 80 to 120, or somewhat more, if very large numbers were
compared. In accordance with this law we find, that all those characters
in man which were certainly essential to him during his early stages of
development, exist in all savages with some approach to equality. In the
speed of running, in bodily strength, in skill with weapons, in
acuteness of vision, or in power of following a trail, all are fairly
proficient, and the differences of endowment do not probably exceed the
limits of variation in animals above referred to. So, in animal instinct
or intelligence, we find the same general level of development. Every
wren makes a fairly good nest like its fellows; every fox has an average
amount of the sagacity of its race; while all the higher birds and
mammals have the necessary affections and instincts needful for the
protection and bringing-up of their offspring.
But in those specially developed faculties of civilised man which we
have been considering, the case is very different. They exist only in a
small proportion of individuals, while the difference of capacity
between these favoured individuals and the average of mankind is
enormous. Taking first the mathematical faculty, probably fewer than one
in a hundred really possess it, the great bulk of the population having
no natural ability for the study, or feeling the slightest interest in
it.[234] And if we attempt to measure the amount of variation in the
faculty itself between a first-class mathematician and the ordinary run
of people who find any kind of calculation confusing and altogether
devoid of interest, it is probable that the former could not be
estimated at less than a hundred times the latter, and perhaps a
thousand times would more nearly measure the difference between them.
The artistic faculty appears to agree pretty closely with the
mathematical in its frequency. The boys and girls who, going beyond the
mere conventional designs of children, draw what they _see_, not what
they _know_ to be the shape of things; who naturally sketch in
perspective, because it is thus they see objects; who see, and represent
in their sketches, the light and shade as well as the mere outlines of
objects; and who can draw recognisable sketches of every one they know,
are certainly very few compared with those who are totally incapable of
anything of the kind. From some inquiries I have made in schools, and
from my own observation, I believe that those who are endowed with this
natural artistic talent do not exceed, even if they come up to, one per
cent of the whole population.
The variations in the amount of artistic faculty are certainly very
great, even if we do not take the extremes. The gradations of power
between the ordinary man or woman "who does not draw," and whose
attempts at representing any object, animate or inanimate, would be
laughable, and the average good artist who, with a few bold strokes, can
produce a recognisable and even effective sketch of a landscape, a
street, or an animal, are very numerous; and we can hardly measure the
difference between them at less than fifty or a hundred fold.
The musical faculty is undoubtedly, in its lower forms, less uncommon
than either of the preceding, but it still differs essentially from the
necessary or useful faculties in that it is almost entirely wanting in
one-half even of civilised men. For every person who draws, as it were
instinctively, there are probably five or ten who sing or play without
having been taught and from mere innate love and perception of melody
and harmony.[235] On the other hand, there are probably about as many
who seem absolutely deficient in musical perception, who take little
pleasure in it, who cannot perceive discords or remember tunes, and who
could not learn to sing or play with any amount of study. The
gradations, too, are here quite as great as in mathematics or pictorial
art, and the special faculty of the great musical composer must be
reckoned many hundreds or perhaps thousands of times greater than that
of the ordinary "unmusical" person above referred to.
It appears then, that, both on account of the limited number of persons
gifted with the mathematical, the artistic, or the musical faculty, as
well as from the enormous variations in its development, these mental
powers differ widely from those which are essential to man, and are, for
the most part, common to him and the lower animals; and that they could
not, therefore, possibly have been developed in him by means of the law
of natural selection.
* * * * *
We have thus shown, by two distinct lines of argument, that faculties
are developed in civilised man which, both in their mode of origin,
their function, and their variations, are altogether distinct from those
other characters and faculties which are essential to him, and which
have been brought to their actual state of efficiency by the necessities
of his existence. And besides the three which have been specially
referred to, there are others which evidently belong to the same class.
Such is the metaphysical faculty, which enables us to form abstract
conceptions of a kind the most remote from all practical applications,
to discuss the ultimate causes of things, the nature and qualities of
matter, motion, and force, of space and time, of cause and effect, of
will and conscience. Speculations on these abstract and difficult
questions are impossible to savages, who seem to have no mental faculty
enabling them to grasp the essential ideas or conceptions; yet whenever
any race attains to civilisation, and comprises a body of people who,
whether as priests or philosophers, are relieved from the necessity of
labour or of taking an active part in war or government, the
metaphysical faculty appears to spring suddenly into existence,
although, like the other faculties we have referred to, it is always
confined to a very limited proportion of the population.
In the same class we may place the peculiar faculty of wit and humour,
an altogether natural gift whose development appears to be parallel with
that of the other exceptional faculties. Like them, it is almost unknown
among savages, but appears more or less frequently as civilisation
advances and the interests of life become more numerous and more
complex. Like them, too, it is altogether removed from utility in the
struggle for life, and appears sporadically in a very small percentage
of the population; the majority being, as is well known, totally unable
to say a witty thing or make a pun even to save their lives.[236]
_The Interpretation of the Facts._
The facts now set forth prove the existence of a number of mental
faculties which either do not exist at all or exist in a very
rudimentary condition in savages, but appear almost suddenly and in
perfect development in the higher civilised races. These same faculties
are further distinguished by their sporadic character, being well
developed only in a very small proportion of the community; and by the
enormous amount of variation in their development, the higher
manifestations of them being many times--perhaps a hundred or a thousand
times--stronger than the lower. Each of these characteristics is totally
inconsistent with any action of the law of natural selection in the
production of the faculties referred to; and the facts, taken in their
entirety, compel us to recognise some origin for them wholly distinct
from that which has served to account for the animal
characteristics--whether bodily or mental--of man.
The special faculties we have been discussing clearly point to the
existence in man of something which he has not derived from his animal
progenitors--something which we may best refer to as being of a
spiritual essence or nature, capable of progressive development under
favourable conditions. On the hypothesis of this spiritual nature,
superadded to the animal nature of man, we are able to understand much
that is otherwise mysterious or unintelligible in regard to him,
especially the enormous influence of ideas, principles, and beliefs over
his whole life and actions. Thus alone we can understand the constancy
of the martyr, the unselfishness of the philanthropist, the devotion of
the patriot, the enthusiasm of the artist, and the resolute and
persevering search of the scientific worker after nature's secrets. Thus
we may perceive that the love of truth, the delight in beauty, the
passion for justice, and the thrill of exultation with which we hear of
any act of courageous self-sacrifice, are the workings within us of a
higher nature which has not been developed by means of the struggle for
material existence.
It will, no doubt, be urged that the admitted continuity of man's
progress from the brute does not admit of the introduction of new
causes, and that we have no evidence of the sudden change of nature
which such introduction would bring about. The fallacy as to new causes
involving any breach of continuity, or any sudden or abrupt change, in
the effects, has already been shown; but we will further point out that
there are at least three stages in the development of the organic world
when some new cause or power must necessarily have come into action.
The first stage is the change from inorganic to organic, when the
earliest vegetable cell, or the living protoplasm out of which it arose,
first appeared. This is often imputed to a mere increase of complexity
of chemical compounds; but increase of complexity, with consequent
instability, even if we admit that it may have produced protoplasm as a
chemical compound, could certainly not have produced _living_
protoplasm--protoplasm which has the power of growth and of
reproduction, and of that continuous process of development which has
resulted in the marvellous variety and complex organisation of the whole
vegetable kingdom. There is in all this something quite beyond and
apart from chemical changes, however complex; and it has been well said
that the first vegetable cell was a new thing in the world, possessing
altogether new powers--that of extracting and fixing carbon from the
carbon-dioxide of the atmosphere, that of indefinite reproduction, and,
still more marvellous, the power of variation and of reproducing those
variations till endless complications of structure and varieties of form
have been the result. Here, then, we have indications of a new power at
work, which we may term _vitality_, since it gives to certain forms of
matter all those characters and properties which constitute Life.
The next stage is still more marvellous, still more completely beyond
all possibility of explanation by matter, its laws and forces. It is the
introduction of sensation or consciousness, constituting the fundamental
distinction between the animal and vegetable kingdoms. Here all idea of
mere complication of structure producing the result is out of the
question. We feel it to be altogether preposterous to assume that at a
certain stage of complexity of atomic constitution, and as a necessary
result of that complexity alone, an _ego_ should start into existence, a
thing that _feels_, that is _conscious_ of its own existence. Here we
have the certainty that something new has arisen, a being whose nascent
consciousness has gone on increasing in power and definiteness till it
has culminated in the higher animals. No verbal explanation or attempt
at explanation--such as the statement that life is the result of the
molecular forces of the protoplasm, or that the whole existing organic
universe from the amaeba up to man was latent in the fire-mist from
which the solar system was developed--can afford any mental
satisfaction, or help us in any way to a solution of the mystery.
The third stage is, as we have seen, the existence in man of a number of
his most characteristic and noblest faculties, those which raise him
furthest above the brutes and open up possibilities of almost indefinite
advancement. These faculties could not possibly have been developed by
means of the same laws which have determined the progressive development
of the organic world in general, and also of man's physical
organism.[237]
These three distinct stages of progress from the inorganic world of
matter and motion up to man, point clearly to an unseen universe--to a
world of spirit, to which the world of matter is altogether subordinate.
To this spiritual world we may refer the marvellously complex forces
which we know as gravitation, cohesion, chemical force, radiant force,
and electricity, without which the material universe could not exist for
a moment in its present form, and perhaps not at all, since without
these forces, and perhaps others which may be termed atomic, it is
doubtful whether matter itself could have any existence. And still more
surely can we refer to it those progressive manifestations of Life in
the vegetable, the animal, and man--which we may classify as
unconscious, conscious, and intellectual life,--and which probably
depend upon different degrees of spiritual influx. I have already shown
that this involves no necessary infraction of the law of continuity in
physical or mental evolution; whence it follows that any difficulty we
may find in discriminating the inorganic from the organic, the lower
vegetable from the lower animal organisms, or the higher animals from
the lowest types of man, has no bearing at all upon the question. This
is to be decided by showing that a change in essential nature (due,
probably, to causes of a higher order than those of the material
universe) took place at the several stages of progress which I have
indicated; a change which may be none the less real because absolutely
imperceptible at its point of origin, as is the change that takes place
in the curve in which a body is moving when the application of some new
force causes the curve to be slightly altered.
_Concluding Remarks._
Those who admit my interpretation of the evidence now adduced--strictly
scientific evidence in its appeal to facts which are clearly what ought
_not_ to be on the materialistic theory--will be able to accept the
spiritual nature of man, as not in any way inconsistent with the theory
of evolution, but as dependent on those fundamental laws and causes
which furnish the very materials for evolution to work with. They will
also be relieved from the crushing mental burthen imposed upon those
who--maintaining that we, in common with the rest of nature, are but
products of the blind eternal forces of the universe, and believing also
that the time must come when the sun will lose his heat and all life on
the earth necessarily cease--have to contemplate a not very distant
future in which all this glorious earth--which for untold millions of
years has been slowly developing forms of life and beauty to culminate
at last in man--shall be as if it had never existed; who are compelled
to suppose that all the slow growths of our race struggling towards a
higher life, all the agony of martyrs, all the groans of victims, all
the evil and misery and undeserved suffering of the ages, all the
struggles for freedom, all the efforts towards justice, all the
aspirations for virtue and the wellbeing of humanity, shall absolutely
vanish, and, "like the baseless fabric of a vision, leave not a wrack
behind."
As contrasted with this hopeless and soul-deadening belief, we, who
accept the existence of a spiritual world, can look upon the universe as
a grand consistent whole adapted in all its parts to the development of
spiritual beings capable of indefinite life and perfectibility. To us,
the whole purpose, the only _raison d'être_ of the world--with all its
complexities of physical structure, with its grand geological progress,
the slow evolution of the vegetable and animal kingdoms, and the
ultimate appearance of man--was the development of the human spirit in
association with the human body. From the fact that the spirit of
man--the man himself--_is_ so developed, we may well believe that this
is the only, or at least the best, way for its development; and we may
even see in what is usually termed "evil" on the earth, one of the most
efficient means of its growth. For we know that the noblest faculties of
man are strengthened and perfected by struggle and effort; it is by
unceasing warfare against physical evils and in the midst of difficulty
and danger that energy, courage, self-reliance, and industry have become
the common qualities of the northern races; it is by the battle with
moral evil in all its hydra-headed forms, that the still nobler
qualities of justice and mercy and humanity and self-sacrifice have been
steadily increasing in the world. Beings thus trained and strengthened
by their surroundings, and possessing latent faculties capable of such
noble development, are surely destined for a higher and more permanent
existence; and we may confidently believe with our greatest living
poet--
That life is not as idle ore,
But iron dug from central gloom,
And heated hot with burning fears,
And dipt in baths of hissing tears,
And batter'd with the shocks of doom
To shape and use.
We thus find that the Darwinian theory, even when carried out to its
extreme logical conclusion, not only does not oppose, but lends a
decided support to, a belief in the spiritual nature of man. It shows us
how man's body may have been developed from that of a lower animal form
under the law of natural selection; but it also teaches us that we
possess intellectual and moral faculties which could not have been so
developed, but must have had another origin; and for this origin we can
only find an adequate cause in the unseen universe of Spirit.
FOOTNOTES:
[Footnote 218: _Descent of Man_, pp. 41-43; also pp. 13-15.]
[Footnote 219: _Man's Place in Nature_, p. 64.]
[Footnote 220: _Man's Place in Nature_, p. 67. See Figs. of Embryos of
Man and Dog in Darwin's _Descent of Man_, p. 10.]
[Footnote 221: _The Descent of Man_, pp. 7, 8.]
[Footnote 222: _Man and Apes._ By St. George Mivart, F.R.S., 1873. It is
an interesting fact (for which I am indebted to Mr. E.B. Poulton) that
the human embryo possesses the extra rib and wrist-bone referred to
above in (2) and (4) as occurring in some of the apes.]
[Footnote 223: _Man and Apes_, pp. 138, 144.]
[Footnote 224: For a sketch of the evidence of Man's Antiquity in
America, see _The Nineteenth Century_ for November 1887.]
[Footnote 225: This subject was first discussed in an article in the
_Anthropological Review_, May 1864, and republished in my _Contributions
to Natural Selection_, chap, ix, in 1870.]
[Footnote 226: _Man's Place in Nature_, p. 102.]
[Footnote 227: For a full discussion of this question, see the author's
_Geographical Distribution of Animals_, vol. i. p. 285.]
[Footnote 228: For a full discussion of all these points, see _Descent
of Man_, chap. iii.]
[Footnote 229: _Descent of Man_, chap. iv.]
[Footnote 230: Lubbock's _Origin of Civilisation_, fourth edition, pp.
434-440; Tylor's _Primitive Culture_, chap. vii.]
[Footnote 231: It has been recently stated that some of these facts are
erroneous, and that some Australians can keep accurate reckoning up to
100, or more, when required. But this does not alter the general fact
that many low races, including the Australians, have no words for high
numbers and never require to use them. If they are now, with a little
practice, able to count much higher, this indicates the possession of a
faculty which could not have been developed under the law of utility
only, since the absence of words for such high numbers shows that they
were neither used nor required.]
[Footnote 232: Article Arithmetic in _Eng. Cyc. of Arts and Sciences_.]
[Footnote 233: See "History of Music," in _Eng. Cyc._, Science and Arts
Division.]
[Footnote 234: This is the estimate furnished me by two mathematical
masters in one of our great public schools of the proportion of boys who
have any special taste or capacity for mathematical studies. Many more,
of course, can be drilled into a fair knowledge of elementary
mathematics, but only this small proportion possess the natural faculty
which renders it possible for them ever to rank high as mathematicians,
to take any pleasure in it, or to do any original mathematical work.]
[Footnote 235: I am informed, however, by a music master in a large
school that only about one per cent have real or decided musical talent,
corresponding curiously with the estimate of the mathematicians.]
[Footnote 236: In the latter part of his essay on Heredity (pp. 91-93 of
the volume of _Essays_), Dr. Weismann refers to this question of the
origin of "talents" in man, and, like myself, comes to the conclusion
that they could not be developed under the law of natural selection. He
says: "It may be objected that, in man, in addition to the instincts
inherent in every individual, special individual predispositions are
also found, of such a nature that it is impossible they can have arisen
by individual variations of the germ-plasm. On the other hand, these
predispositions--which we call talents--cannot have arisen through
natural selection, because life is in no way dependent on their
presence, and there seems to be no way of explaining their origin except
by an assumption of the summation of the skill attained by exercise in
the course of each single life. In this case, therefore, we seem at
first sight to be compelled to accept the transmission of acquired
characters." Weismann then goes on to show that the facts do not support
this view; that the mathematical, musical, or artistic faculties often
appear suddenly in a family whose other members and ancestors were in no
way distinguished; and that even when hereditary in families, the talent
often appears at its maximum at the commencement or in the middle of the
series, not increasing to the end, as it should do if it depended in any
way on the transmission of acquired skill. Gauss was not the son of a
mathematician, nor Handel of a musician, nor Titian of a painter, and
there is no proof of any special talent in the ancestors of these men of
genius, who at once developed the most marvellous pre-eminence in their
respective talents. And after showing that such great men only appear at
certain stages of human development, and that two or more of the special
talents are not unfrequently combined in one individual, he concludes
thus--
"Upon this subject I only wish to add that, in my opinion,
talents do not appear to depend upon the improvement of any
special mental quality by continued practice, but they are the
expression, and to a certain extent the bye-product, of the
human mind, which is so highly developed in all directions."
It will, I think, be admitted that this view hardly accounts for the
existence of the highly peculiar human faculties in question.]
[Footnote 237: For an earlier discussion of this subject, with some
wider applications, see the author's _Contributions to the Theory of
Natural Selection_, chap. x.]
INDEX
=A=
Abbott, Dr. C.C., instability of habits of birds, 76
on American water-thrushes (Seiurus), 117
Mr., drawings of caterpillars and their food plants, 203
Accessory plumes, development and display of, 293
Acclimatisation, 94
Achatinellidae, Gulick on variations in, 147
Acquired characters, non-heredity of, 440
Acraeidae, mimicry of, 247
Adaptation to conditions at various periods of life, 112
Adolias dirtea, sexual diversity of, 271
Aegeriidae, mimicry by, 240
Agaristidae, mimicry of, 246
Agassiz, on species, 5
on North American weeds, 15.
Agelaeus phoeniceus, diagram showing variations of, 56;
proportionate numbers which vary, 64
Albatross, courtship of great, 287
Allen, Mr. Grant, on forms of leaves, 133
on degradation of wind-fertilised from insect-fertilised flowers,
325 (note)
on insects and flowers, 332
on production of colour through the agency of the colour sense, 334
Mr. J.A., on the variability of birds, 50
Allen, Mr. J.A., on colour as influenced by climate, 228
Alluring coloration, 210
American school of evolutionists, 420
Anemone nemorosa, variability of, 78
Animal coloration, a theory of, 288
general laws of, 296
intelligence, supposed action of, 425
characteristics of man, 454
Animals, the struggle among, 18
wild, their enjoyment of life, 39
usually die painless deaths, 38
constitutional variation of, 94
uses of colours of, 134
supposed effects of disuse in wild, 415
most allied to man, 450
Antelopes, recognition marks of, 219
Anthrocera filipendula inedible, 235
Apples, variations of, 87
Arctic animals, supposed causes of white colour of, 191
Argyll, Duke of, on goose reared by a golden eagle, 75
Artemia salina and A. milhausenii, 426
Asclepias curassavica, spread of, 28
Asses running wild in Quito, 28
Attractive fruits, 306
Australia, spread of the Cape-weed in, 29
fossil and recent mammals of, 392
Azara, on cause of horses and cattle not running wild in Paraguay, 19
Azores, flora of, supports aerial transmission of seeds, 368
=B=
Baker, Mr. J.G., on rarity of spiny plants in Mauritius, 432
Ball, Mr., on cause of late appearance of exogens, 400
Barber, Mrs., on variable colouring of pupae of Papilio nireus, 197
on protective colours of African sun-birds, 200
Barbs, 91
Barriers, importance of, in questions of distribution, 341
Bates, Mr. H.W., on varieties of butterflies, 44
on inedibility of Heliconidae, 234
on a conspicuous caterpillar, 236
on mimicry, 240, 243, 249
Bathmism or growth-force, Cope on, 421
Beddard, Mr. F.E., variations of earthworms, 67
on plumes of bird of paradise, 292
Beech trees, aggressive in Denmark, 21
Beetle and wasp (figs.), 259
Beetle, fossil in coal measures of Silesia, 404
Beginnings of important organs, 128
Belt, Mr., on leaf-like locust, 203
on birds avoiding Heliconidae, 234
Belt's frog, 266
Birds, rate of increase of, 25
how destroyed, 26
variation among, 49
variation of markings of, 52
variation of wings and tails of, 53
diagram showing variation of tarsus and toes, 60
use of structural peculiarities of, 135
eggs, coloration of, 212
recognition marks of, 222
and butterflies, white in tropical islands, 230
sometimes seize inedible butterflies, 255
mimicry among, 263
Birds, sexual coloration of, 275
cause of dull colour of female, 277
choice of female not known to be determined by colour, etc., 285
decorative plumage of, 285
antics of unornamented, 287
which fertilise flowers, 319
colours of, not dependent on the colours of flowers, 336
no proof of aesthetic tastes in, 336
dispersal of, 355
and insects at sea, 357
of oceanic islands, 358
carrying seeds on their feet, 361
ancestral forms of, 407
Birthplace, probable, of man, 459
Bombyx regia, protective form of larva of, 210
Boyd Dawkins, on development of deer's horns, 389
on origin of man, 456
Brady, Mr. George, on protective colouring of starfishes, 209
Brain development, progressive, 390
Brains of man and apes, 452
Branner, Mr. J.C., on supposed proofs of glaciation in Brazil, 370
Brazil, supposed proof of glaciation in, 370
Brewer, Professor W.H., on want of symmetry in colours of animals, 217
Bromelia, animals inhabiting leaves of, 118
Bronn, Professor, on supposed uselessness of variations of ears
and tails, 136
Butler, Mr. A.G., on inedibility of conspicuous caterpillars, 237
Butterflies, varieties of, 44
small, of Isle of Man, 106
special protective colouring of, 206
recognition by, 226
inedibility of some, 234
mimicry among, 240, 249
colour development of, 274
sexual coloration of, 271
=C=
Caddis-fly larvae inhabiting bromelia leaves, 118
Callophis, harmless mimicking poisonous species, 262
Candolle, Alp. de, on variation in oaks, 77
on variability of Papaver bracteatum, 79
Cardinalis virginianus, diagram showing proportionate numbers
which vary, 65;
variations of, 58
Carpenter, Dr. W.B., on variation in the Foraminifera, 43
Carriers, 91
Caterpillars, resemblance of, to their food plants, 203-205
inedible, 236
Cattle, how they prevent the growth of trees, 18
increase of, in St. Domingo, Mexico, and the pampas, 27
Ceylon, spread of Lantana mixta in, 29
Chaffinch, change of habit of, in New Zealand, 76
Chambers, Robert, on origin of species, 3
Chance rarely determines survival, 123
Change of conditions, utility of, 326
Characters, non-adaptive, 131
transferred from useless to useful class, 132
Charaxes psaphon persecuted by a bird, 235
Chile, numerous red tubular flowers in, 320
Chimpanzee, figure of, 454
Clark, Mr. Edwin, on cause of absence of forests on the pampas, 23
on the struggle for life in the South American valleys, 24
Cleistogamous flowers, 322
Close interbreeding, supposed evil results of, 326
Clover, white, spread of, in New Zealand, 28
Co-adaptation of parts by variation, no real difficulty, 418
Cobra, use of hood of, 262
Coccinella mimicked by grasshopper, (figure), 260
Collingwood, Mr., on butterflies recognising their kind, 226
Coloration, alluring, 210
of birds' eggs, 212
a theory of animal, 288
Colour correlated with sterility, 169
correlated with constitutional peculiarities, 170
in nature, the problem to be solved, 188
constancy, in animals indicates utility, 189
and environment, 190
general theories of animal, 193
animal, supposed causes of, 193
obscure, of many tropical animals, 194
produced by surrounding objects, 195
adaptations, local, 199
for recognition, 217
of wild animals not quite symmetrical, 217 (note)
as influenced by locality or climate, 228
development in butterflies, 274
more variable than habits, 278
and nerve distribution, 290
and tegumentary appendages, 291
of flowers, 308
change of, in flowers when fertilised, 317
in nature, concluding remarks on, 299, 333
of fruits, 304
of flowers growing together contrasted, 318
Complexity of flowers due to alternate adaptation to insect
and self-fertilisation, 328
Composite, a, widely dispersed without pappus, 367
Confinement, affecting fertility, 154
Continental and oceanic areas, 346
Continents and oceans cannot have changed places, 345
possible connections between, 349
Continuity does not prove identity of origin, 463
Cope, Dr. E.D., on non-adaptive characters, 131
on fundamental laws of growth, 420
on bathmism or growth-force, 421
on use producing structural change, 422
on law of centrifugal growth, 422
on origin of the feet of ungulates, 423
on action of animal intelligence, 425
Correlations in pigeons, horses, etc., 140
Corvus frugilegus, 2
corone, 2
Coursers, figures of secondary quills, 224
Cowslip, two forms of, 157
Crab, sexual diversity of colour of, 269
Cretaceous period, dicotyledons of, 400
Crisp, Dr., on variations of gall bladder and alimentary canal, 69
Crosses, a cause of variation, 99
reciprocal, 155
Cross-fertilisation, modes of securing, 310
difference in, 155
Crossing and changed conditions,
parallelism of, 166
Cruciferae, variations of structure in, 80
Cuckoo, eggs of, 216
Cuckoos mimick hawks, 263
Cultivated plants, origin of useful, 97
Curculionidae mimicked by various insects (figs.), 260
Curves of variation, 64
=D=
Dana, Professor, on the permanence of continents, 342
Danaidae little attacked by mites, 235
mimicry of, 246
Darwin, change of opinion effected by, 8
the Newton of Natural History, 9
his view of his own work, 10
on the enemies of plants, 16
on fir-trees destroyed by cattle, 17
on change of plants and animals caused by planting, 18
on absence of wild cattle in Paraguay, 19
on cats and red clover, 20
on variety of plants in old turf, 35
on the beneficent action of the struggle for existence, 40
on variability of wild geraniums, 79
on variability of common species, 80
his non-recognition of extreme variability of wild species, 82
on races of domestic pigeon, 90
on constitutional variation in plants, 95
on unconscious selection, 96
on a case of divergence, 105
on advantage of diversification of structure in inhabitants
of one region, 110
on species of plants in turf, 110
on isolation, 119
on origin of mammary glands, 129
on eyes of flatfish, 129
on origin of the eye, 130
on useless characters, 131
on use of ears and tails, 136
on disappearance of sports, 140
on tendency to vary in one direction, 141
on rare perpetuation of sports, 142
on utility of specific characters, 142 (note)
on importance of biological environment, 148
on variable fertility of plants, 155
on fertile hybrids among plants, 164
Darwin, on correlation of sterility and colour, 169
on selective association, 172
on infertility and natural selection, 174
on cause of infertility of hybrids, 185
on white tail of rabbit, 218
on conspicuous caterpillars, 236
on sexual selection in insects, 274
on decorative plumage of male birds, 285
on development of ocelli, 290
on value of cross-fertilisation, 309
on limits to utility of intercrossing, 326
on flowers due to insects, 332
on oceanic islands, 342
on effects of disuse in domestic animals, 415, 435
on direct action of environment, 419
on unintelligibility of theory of retardation and acceleration,
421 (note)
on origin of man's moral nature, 461
Mr. George, on intermarriages of British aristocracy, 326
Darwinian theory, statement of, 10
not opposed to spiritual nature of man, 478
Dawkins, Professor Boyd, on development of deer's horns, 389
on recent origin of man, 456
Dawson, Sir W., on determination of fossil plants by leaves, 398 (note)
Death of wild animals usually painless, 38
De Candolle, definition of species, 1
on difficulty of naturalising plants, 15
on war between plants, 16
on origin of useful cultivated plants, 97
Deer's horns, development of, 389
Degeneration, 121
Delboeuf's law of variation, 141
Dendraeca coronata, variation of wing-feathers of, 51
Denmark, struggle between trees in, 20
Denudation, evidences of, 379
Desert animals, colour of, 192
Deserts, effect of goats and camels in destroying vegetation in, 17
Development and display of accessory plumes, 293
Diadema anomala, 271
misippus, great diversity of sexes in, 271
Diaphora mendica mimics Spilosoma menthrasti, 249
Difficulties in the facts of fertilisation of flowers, 325
Dimorphism and trimorphism, 156
Dippers, probable origin of, 116
Disease and markings, 290
Diseases common to man and animals, 449
Display of decorative plumage, 287
Distribution of organisms should be explained by theory of descent, 338
conditions which have determined the, 341
of marsupials, 350
of tapirs, 352
Disuse, effects of, among wild animals, 415
no proof that the effects of, are inherited, 417
Divergence of character, 105-109
leads to maximum of forms of life in each area, 109
Diversity of fauna and flora with geographical proximity, 339
Dixon, Mr. C, changed habits of chaffinch in New Zealand, 76
Dogs, origin of, 88
varieties of, 89
Dolichonyx oryzivorus, diagram showing variations of, 55
Domestic animals, varieties of, 88
Draba verna, varieties of, 77
Dress of men not determined by female choice, 286
Dust from Krakatoa, size of particles of, 363
=E=
Eastern butterflies, variation of, 45
Eaton, Rev. A.E., on Kerguelen insects, 106
Edwards, Mr. W.H., on dark forms of Papilio turnus, 248
Eggs protectively coloured, 214, 215
theory of varied colours of, 216
Elaps mimicked by harmless snakes, 261
Embryonic development of man and other mammalia, 448
Ennis, Mr. John, on willows driving out watercresses from
rivers of New Zealand, 24
Entomostraca, in bromelia leaves, 118
Environment never identical for two species, 149
direct action of, 418
direct influence of, 426
as initiator of variations, 436
action of, overpowered by natural selection, 437
Ethical aspect of the struggle for existence, 36
Euchelia jacobeae inedible, 235
Everett, Mr. A., on a caterpillar resembling moss, 205
Evidence of evolution that may be expected among fossil forms, 380
Evolutionists, American school of, 420
Exogens, possible cause of sudden late appearance of, 400
External differences of man and apes, 453
Extinct animals, number of species of, 376
Extinction of large animals, cause of, 394
Eye, origin of, 130
Eyes, explanation of loss of in cave animals, 416
=F=
Facts of natural selection, summary of, 122
Falcons illustrating divergence, 108
and butcher birds, hooked and toothed beaks of, 422
Fantails, 91
Female birds, why often dull coloured, 277
Female birds, what their choice of mates is determined by, 286
butterflies, why dull coloured, 272
brighter than male bird, 281
choice a doubtful agent in selection, 283
preference neutralised by natural selection, 294
Fertility of domestic animals, 154
Flatfish, eyes of, 129
Flesh-fly, enormous increase of, 25
Floral structure, great differences of, in allied genera and species, 329
Flowers, variations of, 88
colours of, 308
with sham nectaries, 317
changing colour when fertilised, 317
adapted to bees or to butterflies, 318
contrasted colours of, at same season and locality, 318
fertilisation of, by birds, 319
self-fertilisation of, 321
once insect-fertilised now self-fertile, 323
how the struggle for existence acts among, 328
repeatedly modified during whole Tertiary period, 331
the product of insect agency, 332
Forbes, Mr. H.O., on protective colour of a pigeon, 200
on spider imitating birds' dropping, 211
Fossil shells, complete series of transitional forms of, 381
crocodiles afford evidence of evolution, 383
horses in America, 386
and living animals, local relations of, 391
Fowl, early domestication of, 97
Frill-back, Indian, 93
Frog inhabiting bromelia leaves, 118
Fruits, use of characters of, 133
colours of, 304
edible or attractive, 306
poisonous, 307
Fulica atra, protectively coloured eggs of, 215
Fulmar petrel, abundance of, 30
=G=
Gallinaceae, ornamental plumes of, 292
Galton, Mr. F., diagrams of variability used by, 74
on markings of zebra, 220 (note)
on regression towards mediocrity, 414
theory of heredity by, 443 (note)
on imperfect counting of the Damaras, 464
Gaudry on extinct animals at Pikermi, 377
Gay, Mons. T., on variations of structure in Cruciferae, 80
Gazella soemmerringi (figure), 219
Gazelles, recognition marks of, 218
Geddes, Professor, on variation in plants, 428
objection to theory of, 430
Geikie, Dr. Archibald, on formation of marine stratified rocks, 344
Geoffroy St. Hilaire, on species, 6
Geological evidences of evolution, 376, 381
record, causes of imperfection of, 379
distribution of insects, 403
antiquity of man, 455
Ghost-moth, colours of, 270
Glaciation, no proofs of, in Brazil, 370
Glow-worm, light a warning of inedibility, 287
Gomphia oleaefolia, variability of, 79
Goose eating flesh, 75
Gosse, Mr. P.H., on variation in the sea-anemones, 43
on sea-anemone and bullhead, 265
Gould, Mr., on colours of coast and inland birds, 228
Grant Allen, on forms of leaves, 133
on insects and flowers, 332
Graphite in Laurentian implies abundant plant life, 398
Gray, Dr. Asa, on naturalised plants in the United States, 110
Dr. J.E., on variation of skulls of mammalia, 71
Great fertility not essential to rapid increase, 30
Great powers of increase of animals, 27
Green colour of birds in tropical forests, 192
Grouse, red, recent divergence of, 106
Gulick, Rev. J.T., on variation of land-shells, 43
on isolation and variation, 147, 150
on divergent evolution, 148
=H=
Habits of animals, variability of, 74
Hairy caterpillars inedible, 237
Hanbury, Mr. Thomas, on a remarkable case of wind
conveyance of seed, 373 (note)
Hansten-Blangsted, on succession of trees in Denmark, 21
Harvest mice, prehensile tails of young, 136
Hawkweed, species and varieties of British, 77
Hector, Sir James, use of horns of deer, 137
Heliconidae, warning colours of, 234
mimicry of, 240
Helix nemoralis, varieties of, 43
hortensis, varieties of, 43
Hemsley, Mr., on rarity of spines in oceanic islands, 432
Henslow, Professor G., on vigour of self-fertilised plants, 323
on wind-fertilised as degradations from insect-fertilised flowers, 324
on origin of forms and structures of flowers, 434 (note)
Herbert, Dean, on species, 6
on plant hybrids, 164
Herbivora, recognition marks of, 218
Heredity, 11
Weismann's theory of, 437
Herschel, Sir John, on species, 3
Hooker, Sir Joseph, on attempts at naturalising Australian
plants in New Zealand, 16
Home, Mr. C, on inedibility of an Indian locust, 267
Horns of deer, uses of, 136
Horse tribe, pedigree of, 384
ancestral forms of, 386
Humming-birds, recognition marks of, 226
Huth, Mr., on close interbreeding, 160
Huxley, Professor on the struggle for existence, 37
on fossil crocodiles, 383
on anatomical peculiarities of the horse tribe, 384
on development of vertebrates, 448
on early man, 456
on brains of man and the gorilla, 457
Hybridity, remarks on facts of, 166
summary on, 184
Hybrids, infertility of, supposed test of distinct species, 152
fertility of, 159
fertile among animals, 162
between sheep and goat, 162
fertile between distinct species of moths, 163
fertile among plants, 163
Hymenopus bicornis, resembling flower, 212
=I=
Icterus Baltimore, diagram showing proportionate numbers which
vary, 63
Imitative resemblances, how produced, 205
Increase of organisms in a geometrical ratio, 25
Inedible fruits rarely coloured, 308
Insect and self-fertilisation, alternation of, in flowers, 328
Insect-fertilisation, facts relating to, 316
Insects, coloured for recognition, 226
warning colours of, 233
sexual coloration of, 269
importance of dull colours to female, 272
visiting one kind of flower at a time, 318
and flowers, the most brilliant not found together, 335
Insects, no proof of love of colour by, 336
and birds at sea, 357
in mid-ocean, 359
at great altitudes, 360
geological distribution of, 403
ancestral in Silurian, 405
fossil support evolution, 405
Instability of useless characters, 138
Instinct, the theory of, 441
Insular organisms illustrate powers of dispersal, 354
Interbreeding, close, injurious effects of, 160
supposed evil results of close, 326
Intercrossing, swamping effects of, 142
not necessarily useful, 325
Intermediate forms, why not found, 380
Islands, all oceanic are volcanic or coralline, 342
Isle of Man, small butterflies of, 106
Isolation, the importance of, 119
to prevent intercrossing, 144
by variations of habits, etc., 145
Rev. J.G. Gulick on, 147
when ineffective, 150
Ituna Ilione and Thyridia megisto, figures of wings of, 251
=J=
Jacobin, 93
Jenyns, Rev. L., on internal variations of mammalia, 69
Jordan, Mons. A., on varieties of Draba verna, 77
Judd, Professor, on dust fallen at Genoa, 363
on Hungarian fossil lacustrine shells, 381
=K=
Kerguelen Island, wingless insects of, 106
Kerivoula picta, protective colour of, 201
Kerner, Professor, on use of external characters of plants, 133
on seeds found on glaciers, 366
Kingfishers illustrating divergence of character, 109
=L=
Lacerta muralis, diagram of variation of, 47
Lagopus scoticus, origin of, 107
Lamarck, on origin of species, 3
Land debris deposited near coasts, 343
and ocean, diagram showing comparative height and depth of, 345
Large animals, cause of extinction of, 394
Larvae of moths, variability of, 46
Laughers, Frill-backs, Nuns, Spots, and Swallows, 93
Law of relation of colour and nest, 278, 279
Laws of animal coloration, 296
Lawson Tait, on uses of tails, 136
Leaf-butterflies, 207
Leguminosae, rare in oceanic islands, 368
Lemuria, an unsound hypothesis, 354
Lepidoptera, variation of, 44
Leyden Museum, diagram showing variability of birds in, 61
Life, Weismann on duration of, 437 (note)
Limenitis misippus mimics Danais archippus, 248
ursula mimics Papilio philenor, 248
Linnaeus, on rapid increase of the flesh-fly, 25
Livingstone, his sensations when seized by a lion, 38
Lizards, variation among, 46
diagram of variation of, 48
sexual colours of, 281
Local colour adaptations, 199
Locusts with warning colours inedible, 267
Longicorns mimic Malacoderms, 257
Low, Mr., on effects of close interbreeding, 160
Low, Mr., on fertile crosses between sheep and goat, 162
on selective association, 172
Low forms of life, continued existence of, explained, 114
forms, persistence of, 121
temperature of tropics not needed to explain plant dispersal, 370
Lower types, extinction of, among the higher animals, 114
Lubbock, Sir John, on forms of leaves, 133
on imperfect counting of early man, 464
Lyell, Sir Charles, on variation of species, 4
on the shifting of continents, 342
=M=
Madagascar and New Zealand, 347
Madeira, wingless beetles of, 105
Maize, origin of, 98
Male rivalry, a real cause of selection, 283
Males of many animals fights together, 282
Malm, on eyes of flatfish, 129
Mammalia, variation of, 65
sexual colours of, 281, 282
afford crucial tests of theories of distribution, 353
early forms of, 407
geological distribution of, 408
Mammary glands, supposed origin of, 129
Man, summary of animal characteristics of, 454
geological antiquity of, 455
early remains of, in California, 456
probably as old as the Miocene, 457
probable birthplace of, 459
origin of moral and intellectual nature of, 461
possesses mental qualities not derived exclusively
from his animal progenitors, 474
Man's body that of an animal, 444
development similar to that of animals, 449
structure compared with that of the anthropoid apes, 451
Mania typica refused by lizards, 238
Mantidae resembling flowers, 212
Marcgravia nepenthoides fertilised by birds (woodcut), 320
Marine animals, protective resemblance among, 208
with warning colours, 266
Marsh, Mr., on destructiveness to vegetation of goats and camels, 17
Professor O., on the development of the horse tribe, 386
on brain development of Tertiary mammals, 391
on specialised forms dying out, 395
Marsupials, distribution of, 350
Mathematical faculty, the origin of the, 464
how developed, 466
not developed by law of natural selection, 469
Mathematics, late development of, 465
Meldola, Professor R., on variable protective colouring, 196
on mimicry among British moths, 249 (note)
on an extension of the theory of mimicry, 255 (note)
Melons, variations of, 87
Methona psidii and Leptalis orise (figs.), 241
Meyer, Dr. A.B., on mimicry of snakes, 262
Milne Edwards, on variation of lizards, 46
Mimicking birds deceive naturalists, 264
butterfly, figure of, 241
Mimicry, 239
how it has been produced, 242
among protected genera, 249
extension of, 255
in various orders of insects, 257
among vertebrata, 261
among birds, 263
objections to theory of, 264
Mineral particles carried by wind, 363
Miocene fossils of North America, 378
Missing links, character of, 380
Mivart, Dr. St. George, on variation of ribs and dorsal vertebrae, 69
on supposed useless characters, 138 (note)
on resemblance of man and apes, 451
Modifications for special purposes, 113
Mongrels, sterility of, 165
Monkeys affected by medicines as are men, 450
Monocotyledons degradations from dicotyledons, 325 (note)
scarcity of, in Rocky Mountains, 401
scarcity of, in Alpine flora, 401
Moral nature of man, origin of, 461
Morse, Professor E.T., on protective colouring of marine mollusca, 209
Moseley, Professor, on protective resemblance among marine
animals, 208
on courtship of Great Albatross, 287
Moths, protected groups of, 235
Mountains, remote, with identical plants, 369
Müller, Dr. Fritz, on inhabitants of bromelia leaves, 118
on butterfly, deceived by its mimic, 245
his explanation of mimicry among protected genera, 252
Dr. Hermann, on variability of Myosurus minimus, 78
Murray, Mr. John, on bulk of land and ocean, 344
on quartz particles on ocean floor, 363
Rev. R.P., variation in the neuration of butterflies' wings, 45
Musical and artistic faculties, origin of, 467
Myosurns minimus, variability of, 78
=N=
Natural selection with changed and unchanged conditions, 103
and sterility, 173
overpowers effects of use and disuse, 435
the most important agency in modifying species, 444
Naturalist deceived by a mimicking insect, 259
by mimicking birds, 264
Naudin, M., on varieties of melons, 87
Nectarinea amethystina, protective colouring of, 201
Nestor notabilis, variation of habits of, 75
Nests of birds influence the colour of females, 278
New species, conditions favourable to origin of, 115
Newton, Professor A., on fertile hybrid ducks, 162
New Zealand, European plants in, 15
spread of white clover in, 28
effects of introduced plants in, 29
native rat and fly exterminated by European species, 34
many plants of, incapable of self-fertilisation, 321
fauna of, 348
few spiny plants in, 433
Nocturnal animals, colours of, 193
Non-adaptive characters, instability of, 138
Normandy pigs, fleshy appendages to jaws of, 139
North America, Miocene fossils of, 378
Northern plants in southern hemisphere, 368
Nostus Borbonicus, variability of, 80
Number of individuals which vary, 62
Nutmeg, how dispersed, 307
Nuts, not meant to be eaten, 305
=O=
Oaks, great variability of, 78
Objections to Darwin's theory, 126
Ocean floor, deposits on, 343
Oceanic animals, colours of, 193
and continental areas, 346
islands have no mammals or batrachia, 342
Oceans, the permanence of, 341
Oedicnemus, figures of wings of, 223
Opthalmis lincea and Artaxa simulans (figs.), 247
Orang-utans, variations of skull of, 69
Orchideae, why scarce on oceanic islands, 367
Orchis pyramidalis, mode of fertilisation of, 314
figures illustrating fertilisation of, 315
Organic development, three stages of, involving new cause or power, 474
world, the development of, implies a spiritual world, 476
Organisation, advance of, by natural selection, 120
degradation in, 121
Origin of species, objections, 7
of accessory plumes, 291
Orioles mimicking honey-suckers, 263
Ornamental plumes and vitality, 293
=P=
Pachyrhynchi subjects of mimicry, 261
Pampas, effects of drought in, 23
Papaver bracteatum, variability of, 79
Papilio, use of forked tentacle of larva of, 210
protected groups of, 235
mimicry of, 247
Paraguay, absence of wild cattle and horses, 19
Parnassia palustris, sham nectaries of, 317
Parrot, change of habits of New Zealand, 75
Parus, species of, illustrate divergence, of character, 107
Passenger-pigeon, account of its breeding-places and numbers, 31
Pelagic animals, colours of, 193
Phasmidae, resemblance of, to sticks and leaves, 203
Physiological selection, 180
Pickard-Cambridge, Rev. O., on sexual selection, 296 (note)
Pieridae, sexual diversity among, 271
Pigeons, varieties of, 89
domestic, derived from wild rock-pigeons, 90
curious correlations in, 140
white eggs of, protective, 213
Pigs, great increase of, in South America and New Zealand, 28
Pikermi, extinct animals of, 377
Pipits as illustrating divergence, 108
Planorbidae, variations of, 44
Plants, the enemies of, 16
variability of, 76
constitutional variation of, 94
colour relations of, 302
true mimicry rare in, 303
exotic rarely naturalised in Europe, 356
dispersal of, 361
northern, in southern hemisphere, 368
identical on summits of remote mountains, 369
progressive development of, 397
geological development of (diagram), 402
Plovers, recognition marks of (figure), 221
Plumes, origin of accessory, 291
muscular relation of ornamental, 292
Poisonous fruits, 307
Porto Santo, rabbits of, 326
Poulton, Mr. E.B., on variable colours of larvae and pupae, 196, 198
on concealments of insects by resemblance to environment, 202
on protective form of Notodonta ziczac, 210
on inedibility of conspicuous larvae, 237
Pouters, 90
Primulaceae, variations of structure in, 79
Problem, the, before Darwin, 6
Problems in variation and heredity, 410
Progression in plants and animals, 395
Protection by terrifying enemies, 209
Protective colouring, variable, 195
of white-headed fruit-pigeon, 200
of African sun-birds, 200
of Kerivoula picta, 201
of sloths, 201
of larva of Sphinx ligustri, 202
of stick and leaf insects, 203
of caterpillars, 203, 205
of butterflies, 206
Ptilopus cinctus, protective colour of, 200
Pugnacity of birds with accessory plumes, 294
=R=
Rabbit, use of white tail of, 218
Rapid increase of plants, 28
Raspail, M., on variability in a grass, 80
Rat, black, spread of, 34
Rattlesnake, use of rattle of, 262
Raven, why black in arctic regions, 191
Reciprocal crosses, 155
Recognition marks of herbivora, 218
of birds, 222
of tropical forest birds, 224
of insects, 226
Reproductive functions, susceptibility of, 153
Reptiles, geological distribution of, 406
Rhinoceroses, evidence of evolution afforded by fossil, 383
Rocks, all stratified formed in shallow water, 344
Rocky Mountains, scarcity of monocotyledons in, 401
Rodents, prevent woody vegetation in the pampas, 23
Romanes, Professor G.J., on useless characters, 131, 139
on meaningless peculiarities of structure, 140
on supposed absence of simultaneous variations, 142
on physiological selection, 180
Rook and crow, 2
Roses, Mr. Baker on varieties of, 77
Rubus, Bentham and Babington on species and varieties of, 77
Rudiments and variations in man, 446
Runts, 91
Rutaceae, variation of structure in, 79
=S=
St. Helena, destruction of forests by goats, 17
St. Hilaire, M. Aug., variability of Gomphia oleaefolia, 79
Saxicola, divergence of character in species of, 108
recognition marks of, 222
Scientific opinion before Darwin, 4
Scolopax, figures of tails of, 225
Scudder, Mr. S.H., on inedibility of Danais archippus, 238
on fossil insects, 403
Seebohm, Mr., on swamping effects of intercrossing, 143
Seeds, how dispersed, 306
how protected, 307
floating great distances, 361
dispersal of, by wind, 362
weight and dimensions of, 364
importance of wind-carriage of, 372
remarkable case of wind-carriage of, 373
Seiurus carolinensis, diagram of variation, 67
sp., habits of, 117
Selection, artificial, 84
by man, circumstances favourable to, 96
unconscious, 96
Selective association, isolation by, 171
Self-fertilisation of flowers, 321
Semper, Professor, on casting hairs of reptiles and Crustacea, 137 (note)
on direct influence of environment, 426
Sesiidae, mimicry by, 240
Sex colour and nests of birds, 277
Sex, colours characteristic of, 269
Sexual colours of insects, probable causes of, 273
of birds, 275
characters due to natural selection, 283
diversity the cause of variation, 439
Sexual selection and colour, 274
by struggles of males, 282
neutralised by natural selection, 294-296
restricted to male struggles, 296
Shetland Islands, variety of ghost-moth in, 270
Shrews and field-mice, internal variations of, 69
Shrikes, recognition marks of, 222
Similarity of forms of life not due to similarity of conditions, 339
Singing of male birds, use of, 284
Skull of wolf, diagram of variations of, 70
of Ursus labiatus, diagram of variations of, 72
of Sus cristatus, diagram of variations of, 73
Skunk an illustration of warning colour, 233
Slack, Baron von, on protective markings of sloths, 201
Sloth, protective colour and marking of, 201
Snakes, mimicry of poisonous, 261
Snipe, tails of two species (figs.), 225
Sounds and odours peculiar to male,
how useful, 284
produced by peculiar feathers, 284
South America, fossil and recent mammals of, 393
Species, definition of, 1, 2
origin of, 2, 6
Lyell on, 4
Agassiz on, 5
transmutation of, 6
Geoffroy St. Hilaire on, 6
Dean Herbert on, 6
Professor Grant on, 6
Von Buch on, 6
allied, found in distinct areas, 36
Species, which vary little, 80
closely allied inhabit distinct areas, 111
vigour and fertility of, how kept up, 327
Spencer, Mr. Herbert, on factors of organic evolution, 411
on effects of disuse, 413
on difficulty as to co-adaptation of parts, 417
on direct action of environment, 418
Sphingidae, protective attitudes of larvae, 210
Sphinx ligustri, general resemblance of larva to food plant, 202
Spider, alluring coloration of, 211
Spines, on origin of, 431
rarity of, in oceanic islands, 432
Spiny plants abundant in South Africa and Chile, 433
Spots a primitive ornamentation of animals, 289
Sprengel on flowers and insects, 309
Staphylinidae, protective habit of, 210
Sterility of mongrels, 165
correlated with colour, etc., 168
and natural selection, 173
of hybrids produced by natural selection, 179
Struggle for existence, 14
among plants, 15
for life, illustrations of, 18
for existence on the pampas, 22
for life between closely allied forms most severe, 33
for existence, ethics of, 36
how it acts among flowers, 328
Summary of facts of colouring for protection and recognition, 227
Survival of the fittest, 11, 122, 123
Swainson, definition of species, 2
Swamping effects of intercrossing, 142
Sweden, destruction of grass by larvae of moths in, 17
Swinhoe, Mr., on protective colouring of a bat, 201
Symmetry, bilateral in colours of animals needful for recognition, 217
=T=
Tails used as respirators, 136
Tapirs, distribution of, 352
Tegetmeier, Mr., on feeding habits of pigeons and fowls, 75
on sparrows and crocuses, 75
on curious correlations in pigeons, 140
Tegumentary appendages and colour, 291
Thousand-fathom line divides oceanic from continental islands, 347
the teachings of, 348
map showing, 349
Thwaites, Mr., on spread of Lantana mixta in Ceylon, 30
Tiger, use of stripes of, 199
Titmice as illustrating divergence, 107
Transformation of species of crustacea, 427
Transmutationists, the early, 3
Travers, Mr. W.L., on effects of introduced plants in New Zealand, 29
Trees, great variety of, in many forests, 36
Trimen, Mr., on butterfly deceived by its mimic, 245
on mimicry, 247
Tropical animals, why brilliantly coloured, 299
Tropics, no proof of lower temperature of, 369
Tropidorhynchi mimicked by orioles, 263
Trumpeter, 93
Tumblers, 91
Turbits and owls, 91
Tylor, Mr. A., on _Coloration in Animals and Plants_, 285
=U=
Ungulates, origin of feet of, 423
Use and disuse, effects of, overpowered by natural selection, 435
Useless characters, 131
not specific, 132
Useless specific characters, no proof of existence of, 141
Utriculariae inhabiting bromelias, 118
=V=
Vanessa callirhoe, small variety in Porto Santo, 106
Variability of the lower animals, 42
of the Foraminifera, 43
of sea-anemones, 43
of land mollusca, 43
of insects, 44
of lizards, 46
of birds, 49
of primary wing-feathers, 51
of wings and tail, 53
of Dolichonyx oryzivorus, 55
of Agelaeus phoeniceus, 56
of Cardinalis virginianus, 58
of tarsus and toes, 60
of birds in Leyden Museum, 61
of Sciurus carolinensis, 67
of skulls of wolf, 70
of skulls of a bear, 72
of skulls of Sus cristatus, 73
of plants, 76
of oaks, 77
Variation, Lyell on, 4
in internal organs, 66
the facts of, 83
proofs of generality of, 85
of vegetables and fruits, 86
of apples and melons, 87
under domestication accords with that under nature, 100
coincident not necessary, 127
and heredity, problems of, 410
Professor Geddes's theory of, 428
the cause of, 439
Variations of flowers, 88
of domestic animals, 88
of domestic pigeons, 89
conditions favourable to production of, 98
beneficial, 143
Varieties, importance of, 41
of same species adapted to self or to insect-fertilisation, 330
Vegetables, variation of, 86
Vegetation and reproduction, antagonism of, 428
Vertebrata, mimicry among, 261
geological succession of, 405
_Vestiges of Creation_, 3
Viola odorata, 2
canina, 2
Violets, as illustrating species, 2
Von Buch on species, 6
=W=
Wallace, Dr. Alexander, on absence of choice by female moths, 275
Ward, Mr. Lester F., on progressive development of plants, 398
Warning coloration, 232
Warning colours of marine animals, 265
Wasps and bees, mimicry of, 258
poisonous with warning colours, 287
Water-cress, chokes rivers in New Zealand, 24
driven out by willows, 24
Water-ouzels, probable origin of, 116
Weale, Mr. Mansel, on protective colours of butterflies, 206
Weeds of United States, 15
Weir, Mr. Jenner, on deceptive resemblance of a caterpillar to
a twig, 204
on inedibility of conspicuous caterpillars, 236
on birds disregarding inedible larvae, 254
Weismann on progressive adaptation of colours of larva, 206
on non-heredity of acquired characters, 440
and Galton's theories of heredity almost identical, 443 (note)
on origin of the mathematical faculty, 472 (note)
Weismann's theory of heredity, 437
Westwood, Professor, on variation of insects, 44
deceived by a mimicking cricket, 259
White coloration of insular birds and butterflies, 230
Whymper, Mr., his sensations when falling on the Matterhorn, 38
Willows, species and varieties of British, 77
Wilson, Alexander, his account of the passenger-pigeon in North
America, 31
Wind-carriage of seeds explains many facts of plant distribution,
371
Wind-dispersal of seeds, objections to, 365
Wind-fertilised degraded from insect-fertilised flowers, 324
Wings of stone-curlews (figure), 223 why small but useless are
retained, 416
Wit and humour, origin of faculties of, 472
Wollaston, Mr. T.W., on variation of beetles, 44 on small
butterfly in Porto Santo, 106
Wolves, varieties of in Catskill Mountains, 105
Wood, Mr. J., on muscular variations, 447
Mr. T.W., on variable colouring of pupae of cabbage butterflies,
197
Woodward, Dr. S.P., on variation of mollusca, 43
=Y=
Youatt, on breeds of sheep, 97
Young animals often spotted, 289
=Z=
Zebra, markings for recognition and protection, 220 (note)
THE END
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