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

colours of red snow and other low algae and fungi, or even in the

universal mantle of green which clothes so large a portion of the

earth's surface. The presence of some colour, or even of many brilliant

colours, in animals and plants would require no other explanation than

does that of the sky or the ocean, of the ruby or the emerald--that is,

it would require a purely physical explanation only. It is the wonderful

individuality of the colours of animals and plants that attracts our

attention--the fact that the colours are localised in definite patterns,

sometimes in accordance with structural characters, sometimes altogether

independent of them; while often differing in the most striking and

fantastic manner in allied species. We are thus compelled to look upon

colour not merely as a physical but also as a biological characteristic,

which has been differentiated and specialised by natural selection, and

must, therefore, find its explanation in the principle of adaptation or

utility.

fundamental law of utility is indicated by a general fact which has

received very little attention. As a rule, colour and marking are

constant in each species of wild animal, while, in almost every

domesticated animal, there arises great variability. We see this in our

horses and cattle, our dogs and cats, our pigeons and poultry. Now, the

essential difference between the conditions of life of domesticated and

wild animals is, that the former are protected by man, while the latter

have to protect themselves. The extreme variations in colour that

immediately arise under domestication indicate a tendency to vary in

this way, and the occasional occurrence of white or piebald or other

exceptionally coloured individuals of many species in a state of nature,

shows that this tendency exists there also; and, as these exceptionally

coloured individuals rarely or never increase, there must be some

constant power at work to keep it in check. This power can only be

natural selection or the survival of the fittest, which again implies

that some colours are useful, some injurious, in each particular case.

With this principle as our guide, let us see how far we can account both

for the general and special colours of the animal world.

_Colour and Environment._

animals as a whole, is the close relation that exists between these

colours and the general environment. Thus, white prevails among arctic

animals; yellow or brown in desert species; while green is only a common

colour in tropical evergreen forests. If we consider these cases

somewhat carefully we shall find, that they afford us excellent

materials for forming a judgment on the various theories that have been

suggested to account for the colours of the animal world.

white all the year round, or which only turn white in winter. Among the

former are the polar bear and the American polar hare, the snowy owl and

the Greenland falcon; among the latter the arctic fox, the arctic hare,

the ermine, and the ptarmigan. Those which are permanently white remain

among the snow nearly all the year round, while those which change their

colour inhabit regions which are free from snow in summer. The obvious

explanation of this style of coloration is, that it is protective,

serving to conceal the herbivorous species from their enemies, and

enabling carnivorous animals to approach their prey unperceived. Two

other explanations have, however, been suggested. One is, that the

prevalent white of the arctic regions has a direct effect in producing

the white colour in animals, either by some photographic or chemical

action on the skin or by a reflex action through vision. The other is,

that the white colour is chiefly beneficial as a means of checking

radiation and so preserving animal heat during the severity of an arctic

winter. The first is part of the general theory that colour is the

effect of coloured light on the objects--a pure hypothesis which has, I

believe, no facts whatever to support it. The second suggestion is also

an hypothesis merely, since it has not been proved by experiment that a

white colour, _per se_, independently of the fur or feathers which is so

coloured, has any effect whatever in checking the radiation of low-grade

heat like that of the animal body. But both alike are sufficiently

disproved by the interesting exceptions to the rule of white coloration

in the arctic regions, which exceptions are, nevertheless, quite in

harmony with the theory of protection.

require protection by the white colour, then neither the cold nor the

snow-glare has any effect upon their coloration. The sable retains its

rich brown fur throughout the Siberian winter; but it frequents trees at

that season and not only feeds partially on fruits or seeds, but is able

to catch birds among the branches of the fir-trees, with the bark of

which its colour assimilates. Then we have that thoroughly arctic

animal, the musk-sheep, which is brown and conspicuous; but this animal

is gregarious, and its safety depends on its association in small herds.

It is, therefore, of more importance for it to be able to recognise its

kind at a distance than to be concealed from its enemies, against which

it can well protect itself so long as it keeps together in a compact

body. But the most striking example is that of the common raven, which

is a true arctic bird, and is found even in mid-winter as far north as

any known bird or mammal. Yet it always retains its black coat, and the

reason, from our point of view, is obvious. The raven is a powerful bird

and fears no enemy, while, being a carrion-feeder, it has no need for

concealment in order to approach its prey. The colour of the raven and

of the musk-sheep are, therefore, both inconsistent with any other

theory than that the white colour of arctic animals has been acquired

for concealment, and to that theory both afford a strong support. Here

we have a striking example of the exception proving the rule.

accordance of colour with surroundings. The lion, the camel, and all the

desert antelopes have more or less the colour of the sand or rock among

which they live. The Egyptian cat and the Pampas cat are sandy or earth

coloured. The Australian kangaroos are of similar tints, and the

original colour of the wild horse is supposed to have been sandy or clay

coloured. Birds are equally well protected by assimilative hues; the

larks, quails, goatsuckers, and grouse which abound in the North African

and Asiatic deserts are all tinted or mottled so as closely to resemble

the average colour of the soil in the districts they inhabit. Canon

Tristram, who knows these regions and their natural history so well,

says, in an often quoted passage: "In the desert, where neither trees,

brushwood, nor even undulations of the surface afford the slightest

protection to its foes, a modification of colour which shall be

assimilated to that of the surrounding country is absolutely necessary.

Hence, without exception, the upper plumage of every bird, whether lark,

chat, sylvain, or sand-grouse, and also the fur of all the smaller

mammals, and the skin of all the snakes and lizards, is of one uniform

isabelline or sand colour."
Passing on to the tropical regions, it is among their evergreen forests

alone that we find whole groups of birds whose ground colour is green.

Parrots are very generally green, and in the East we have an extensive

group of green fruit-eating pigeons; while the barbets, bee-eaters,

turacos, leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many

other groups, have so much green in their plumage as to tend greatly to

their concealment among the dense foliage. There can be no doubt that

these colours have been acquired as a protection, when we see that in

all the temperate regions, where the leaves are deciduous, the ground

colour of the great majority of birds, especially on the upper surface,

is a rusty brown of various shades, well corresponding with the bark,

withered leaves, ferns, and bare thickets among which they live in

autumn and winter, and especially in early spring when so many of them

build their nests.

dusky colours of mice, rats, bats, and moles, and in the soft mottled

plumage of owls and goatsuckers which, while almost equally

inconspicuous in the twilight, are such as to favour their concealment

in the daytime.
An additional illustration of general assimilation of colour to the

surroundings of animals, is furnished by the inhabitants of the deep

oceans. Professor Moseley of the Challenger Expedition, in his British

Association lecture on this subject, says: "Most characteristic of

pelagic animals is the almost crystalline transparency of their bodies.

So perfect is this transparency that very many of them are rendered

almost entirely invisible when floating in the water, while some, even

when caught and held up in a glass globe, are hardly to be seen. The

skin, nerves, muscles, and other organs are absolutely hyaline and

transparent, but the liver and digestive tract often remain opaque and

of a yellow or brown colour, and exactly resemble when seen in the water

small pieces of floating seaweed." Such marine organisms, however, as

are of larger size, and either occasionally or habitually float on the

surface, are beautifully tinged with blue above, thus harmonising with

the colour of the sea as seen by hovering birds; while they are white

below, and are thus invisible against the wave-foam and clouds as seen

by enemies beneath the surface. Such are the tints of the beautiful

nudibranchiate mollusc, Glaucus atlanticus, and many others.

_General Theories of Animal Colour._
We are now in a position to test the general theories, or, to speak more

correctly, the popular notions, as to the origin of animal coloration,

before proceeding to apply the principle of utility to the explanation

of some among the many extraordinary manifestations of colour in the

animal world. The most generally received theory undoubtedly is, that

brilliancy and variety of colour are due to the direct action of light

and heat; a theory no doubt derived from the abundance of

bright-coloured birds, insects, and flowers which are brought from

tropical regions. There are, however, two strong arguments against this

theory. We have already seen how generally bright coloration is wanting

in desert animals, yet here heat and light are both at a maximum, and if

these alone were the agents in the production of colour, desert animals

should be the most brilliant. Again, all naturalists who have lived in

tropical regions know that the proportion of bright to dull coloured

species is little if any greater there than in the temperate zone, while

there are many tropical groups in which bright colours are almost

entirely unknown. No part of the world presents so many brilliant birds

as South America, yet there are extensive families, containing many

hundreds of species, which are as plainly coloured as our average

temperate birds. Such are the families of the bush-shrikes and

ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the

American creepers (Dendrocolaptidae), together with a large proportion

of the wood-warblers (Mniotiltidae), the finches, the wrens, and some

other groups. In the eastern hemisphere, also, we have the

babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the

honey-suckers (Meliphagidae), and several other smaller groups which are

certainly not coloured above the average standard of temperate birds.
Again, there are many families of birds which spread over the whole

world, temperate and tropical, and among these the tropical species

rarely present any exceptional brilliancy of colour. Such are the

thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named

group it is the temperate and arctic zones that afford the most

brilliant coloration.

tropical insects present some of the most gorgeous coloration in the

whole realm of nature, yet there are thousands and tens of thousands of

species which are as dull coloured as any in our cloudy land. The

extensive family of the carnivorous ground-beetles (Carabidae) attains

its greatest brilliancy in the temperate zone; while by far the larger

proportion of the great families of the longicorns and the weevils, are

of obscure colours even in the tropics. In butterflies, there is

undoubtedly a larger proportion of brilliant colour in the tropics; but

if we compare families which are almost equally developed over the

globe--as the Pieridae or whites and yellows, and the Satyridae or

ringlets--we shall find no great disproportion in colour between those

of temperate and tropical regions.
The various facts which have now briefly been noticed are sufficient to

indicate that the light and heat of the sun are not the direct causes of

the colours of animals, although they may favour the production of

colour when, as in tropical regions, the persistent high temperature

favours the development of the maximum of life. We will now consider the

next suggestion, that light reflected from surrounding coloured objects

tends to produce corresponding colours in the animal world.
This theory is founded on a number of very curious facts which prove,

that such a change does sometimes occur and is directly dependent on the

colours of surrounding objects; but these facts are comparatively rare

and exceptional in their nature, and the same theory will certainly not

apply to the infinitely varied colours of the higher animals, many of

which are exposed to a constantly varying amount of light and colour

during their active existence. A brief sketch of these dependent changes

of colour may, however, be advantageously given here.

_Variable Protective Colouring._
There are two distinct kinds of change of colour in animals due to the

colouring of the environment. In one case the change is caused by reflex

action set up by the animal _seeing_ the colour to be imitated, and the

change produced can be altered or repeated as the animal changes its

position. In the other case the change occurs but once, and is probably

not due to any conscious or sense action, but to some direct influence

on the surface tissues while the creature is undergoing a moult or

change to the pupa form.

which changes to white, brown, yellowish, or green, according to the

colour of the object on which it rests. This change is brought about by

means of two layers of pigment cells, deeply seated in the skin, and of

bluish and yellowish colours. By suitable muscles these cells can be

forced upwards so as to modify the colour of the skin, which, when they

are not brought into action, is a dirty white. These animals are

excessively sluggish and defenceless, and the power of changing their

colour to that of their immediate surroundings is no doubt of great

service to them. Many of the flatfish are also capable of changing their

colour according to the colour of the bottom they rest on; and frogs

have a similar power to a limited extent. Some crustacea also change

colour, and the power is much developed in the Chameleon shrimp (Mysis

Chamaeleon) which is gray when on sand, but brown or green when among

brown or green seaweed. It has been proved by experiment that when this

animal is blinded the change does not occur. In all these cases,

therefore, we have some form of reflex or sense action by which the

change is produced, probably by means of pigment cells beneath the skin

as in the chameleon.
The second class consists of certain larvae, and pupae, which undergo

changes of colour when exposed to differently coloured surroundings.

This subject has been carefully investigated by Mr. E.B. Poulton, who

has communicated the results of his experiments to the Royal

Society.[65] It had been noticed that some species of larvae which fed

on several different plants had colours more or less corresponding to

the particular plant the individual fed on. Numerous cases are given in

Professor Meldola's article on "Variable Protective Colouring" (_Proc.

Zool. Soc._, 1873, p. 153), and while the general green coloration was

attributed to the presence of chlorophyll beneath the skin, the

particular change in correspondence to each food-plant was attributed to

a special function which had been developed by natural selection. Later

on, in a note to his translation of Weissmann's _Theory of Descent_,

Professor Meldola seemed disposed to think that the variations of colour

of some of the species might be phytophagic--that is, due to the direct

action of the differently coloured leaves on which the insect fed. Mr.

Poulton's experiments have thrown much light on this question, since he

has conclusively proved that, in the case of the sphinx caterpillar of

Smerinthus ocellatus, the change of colour is not due to the food but to

the coloured light reflected from the leaves.

exposed to differently coloured surroundings, obtained by sewing the

leaves together, so that in one case only the dark upper surface, in the

other the whitish under surface was exposed to view. The result in each

case was a corresponding change of colour in the larvae, confirming the

experiments on different individuals of the same batch of larvae which

had been supplied with different food-plants or exposed to a different

coloured light.

of pupae, which in many cases were known to be affected by the material

on which they underwent their transformations. The late Mr. T.W. Wood

proved, in 1867, that the pupae of the common cabbage butterflies

(Pieris brassicae and P. rapae) were either light, or dark, or green,

according to the coloured boxes they were kept in, or the colours of the

fences, walls, etc., against which they were suspended. Mrs. Barber in

South Africa found that the pupae of Papilio Nireus underwent a similar

change, being deep green when attached to orange leaves of the same

tint, pale yellowish-green when on a branch of the bottle-brush tree

whose half-dried leaves were of this colour, and yellowish when attached

to the wooden frame of a box. A few other observers noted similar

phenomena, but nothing more was done till Mr. Poulton's elaborate series

of experiments with the larvae of several of our common butterflies were

the means of clearing up several important points. He showed that the

action of the coloured light did not affect the pupa itself but the

larva, and that only for a limited period of time. After a caterpillar

has done feeding it wanders about seeking a suitable place to undergo

its transformation. When this is found it rests quietly for a day or

two, spinning the web from which it is to suspend itself; and it is

during this period of quiescence, and perhaps also the first hour or two

after its suspension, that the action of the surrounding coloured

surfaces determines, to a considerable extent, the colour of the pupa.

By the application of various surrounding colours during this period,

Mr. Poulton was able to modify the colour of the pupa of the common

tortoise-shell butterfly from nearly black to pale, or to a brilliant

golden; and that of Pieris rapae from dusky through pinkish to pale

green. It is interesting to note, that the colours produced were in all

cases such only as assimilated with the surroundings usually occupied by

the species, and also, that colours which did not occur in such

surroundings, as dark red or blue, only produced the same effects as

dusky or black.

produced through the sight of the caterpillar. The ocelli were covered

with black varnish, but neither this, nor cutting off the spines of the

tortoise-shell larva to ascertain whether they might be sense-organs,

produced any effect on the resulting colour. Mr. Poulton concludes,

therefore, that the colour-action probably occurs over the whole surface

of the body, setting up physiological processes which result in the

corresponding colour-change of the pupa. Such changes are, however, by

no means universal, or even common, in protectively coloured pupae,

since in Papilio machaon and some others which have been experimented

on, both in this country and abroad, no change can be produced on the

pupa by any amount of exposure to differently coloured surroundings. It

is a curious point that, with the small tortoise-shell larva, exposure

to light from gilded surfaces produced pupae with a brilliant golden

lustre; and the explanation is supposed to be that mica abounded in the

original habitat of the species, and that the pupae thus obtained

protection when suspended against micaceous rock. Looking, however, at

the wide range of the species and the comparatively limited area in

which micaceous rocks occur, this seems a rather improbable explanation,

and the occurrence of this metallic appearance is still a difficulty. It

does not, however, commonly occur in this country in a natural state.
The two classes of variable colouring here discussed are evidently

exceptional, and can have little if any relation to the colours of those

more active creatures which are continually changing their position with

regard to surrounding objects, and whose colours and markings are nearly

constant throughout the life of the individual, and (with the exception

of sexual differences) in all the individuals of the species. We will

now briefly pass in review the various characteristics and uses of the