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.]