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

here set forth, and the very constant conditions under which they occur,

we shall see how utterly inadequate are these causes, either singly or

combined. These constant conditions are--

1. That the imitative species occur in the same area and occupy

the very same station as the imitated.
2. That the imitators are always the more defenceless.
3. That the imitators are always less numerous in individuals.
4. That the imitators differ from the bulk of their allies.
5. That the imitation, however minute, is _external_ and

_visible_ only, never extending to internal characters or to

such as do not affect the external appearance.

These five characteristic features of mimicry show us that it is really

an exceptional form of protective resemblance. Different species in the

same group of organisms may obtain protection in different ways: some by

a general resemblance to their environment; some by more exactly

imitating the objects that surround them--bark, or leaf, or flower;

while others again gain an equal protection by resembling some species

which, from whatever cause, is almost as free from attack as if it were

a leaf or a flower. This immunity may depend on its being uneatable, or

dangerous, or merely strong; and it is the resemblance to such creatures

for the purpose of sharing in their safety that constitutes mimicry.

_Concluding Remarks on Warning Colours and Mimicry._

of inedibility, or of the possession of dangerous offensive weapons, are

probably more numerous than have been hitherto supposed; and, if so, we

shall be able to explain a considerable amount of colour in nature for

which no use has hitherto been conjectured. The brilliant and varied

colours of sea-anemones and of many coral animals will probably come

under this head, since we know that many of them possess the power of

ejecting stinging threads from various parts of their bodies which

render them quite uneatable to most animals. Mr. Gosse describes how, on

putting an Anthea into a tank containing a half-grown bullhead (Cottus

bubalis) which had not been fed for some time, the fish opened his mouth

and sucked in the morsel, but instantly shot it out again. He then

seized it a second time, and after rolling it about in his mouth for a

moment shot it out again, and then darted away to hide himself in a

hole. Some tropical fishes, however, of the genera Tetrodon,

Pseudoscarus, Astracion, and a few others, seem to have acquired the

power of feeding on corals and medusae; and the beautiful bands and

spots and bright colours with which they are frequently adorned, may be

either protective when feeding in the submarine coral groves, or may, in

some cases, be warning colours to show that they themselves are

poisonous and uneatable.
A remarkable illustration of the wide extension of warning colours, and

their very definite purpose in nature, is afforded by what may now be

termed "Mr. Belt's frog." Frogs in all parts of the world are, usually,

protectively coloured with greens or browns; and the little tree-frogs

are either green like the leaves they rest upon, or curiously mottled to

imitate bark or dead leaves. But there are a certain number of very

gaily coloured frogs, and these do not conceal themselves as frogs

usually do. Such was the small toad found by Darwin at Bahia Blanca,

which was intense black and bright vermilion, and crawled about in the

sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt

found a little frog gorgeously dressed in a livery of red and blue,

which did not attempt concealment and was very abundant, a combination

of characters which convinced him that it was uneatable. He, therefore,

took a few specimens home with him and gave them to his fowls and ducks,

but none would touch them. At last, by throwing down pieces of meat, for

which there was a great competition among the poultry, he managed to

entice a young duck into snatching up one of the little frogs. Instead

of swallowing it, however, the duck instantly threw it out of its mouth,

and went about jerking its head as if trying to get rid of some

unpleasant taste.[114]

considered to be a crucial test of a true theory, and if so, the theory

of warning colours, and with it that of mimicry, must be held to be well

established. Among the creatures which probably have warning colours as

a sign of inedibility are, the brilliantly coloured nudibranchiate

molluscs, those curious annelids the Nereis and the Aphrodite or

sea-mouse, and many other marine animals. The brilliant colours of the

scallops (Pecten) and some other bivalve shells are perhaps an

indication of their hardness and consequent inedibility, as in the case

of the hard beetles; and it is not improbable that some of the

phosphorescent fishes and other marine organisms may, like the

glow-worm, hold out their lamp as a warning to enemies.[115] In

Queensland there is an exceedingly poisonous spider, whose bite will

kill a dog, and cause severe illness with excruciating pain in man. It

is black, with a bright vermilion patch on the middle of the body; and

it is so well recognised by this conspicuous coloration that even the

spider-hunting wasps avoid it.[116]
Locusts and grasshoppers are generally of green protective tints, but

there are many tropical species most gaudily decorated with red, blue,

and black colours. On the same general grounds as those by which Mr.

Belt predicted the inedibility of his conspicuous frog, we might safely

predict the same for these insects; but we have fortunately a proof that

they are so protected, since Mr. Charles Home states that one of the

bright coloured Indian locusts was invariably rejected when offered to

birds and lizards.[117]

for the purpose of serving as a danger-signal to enemies are very

widespread in nature, and, with the corresponding colours of the species

which mimic them, furnish us with a rational explanation of a

considerable portion of the coloration of animals which is outside the

limits of those colours that have been acquired for either protection or

recognition. There remains, however, another set of colours, chiefly

among the higher animals, which, being connected with some of the most

interesting and most disputed questions in natural history, must be

discussed in a separate chapter.

that specimens of Danais and Euplaea in collections were less subject to

the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was

corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.]

1869, p. 21.]

93; E.B. Poulton, _Proceedings of the Zool. Soc. of London_, 1887, pp.

191-274.]
[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp.

495-566, coloured plates.]

one of which is placed with the Danaidae; but to avoid confusion I shall

always speak of the American genera under the old term Heliconidae.]
[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878,

p. 158.]
[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation

of Malayan Papilionidae; and, Wallace's _Contributions to Natural

Selection_ chaps. iii. and iv., where full details are given.]

plates illustrating cases of mimicry.]

part vi.]

case of mimicry among British moths, in which Acidalia subsericata

imitates Asthena candidata. See _Ent. Mo. Mag._, vol. iv. p. 163.]
[Footnote 105: From Professor Meldola's translation of Dr. F. Müller's

paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]

by Professor Meldola in the paper already referred to; and he has

answered the objections to Dr. F. Müller's theory with great force in

the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]

Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]

series ii., No. 59, 1878.]

Lampyridae (fireflies and glow-worms)--_Naturalist in Nicaragua_, p.

320. Mr. Verrill and Professor Meldola made the same suggestion in the

case of medusae and other phosphorescent marine organisms (_Nature_,

vol. xxx. pp. 281, 289).]
[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]
[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]

CHAPTER X

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.

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.

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._

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.

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.

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.

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