The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

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.

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._

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._

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.

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._

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]

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.

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.

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.

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.

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]

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._

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.

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