Darwinism (1889) / An exposition of the theory of natural selection, with some of its applications

by Alfred Russel Wallace

Nocturnal animals supply another illustration of the same rule, in the dusky colours of mice, rats, bats, and moles, and in the soft mottled plumage of owls and goatsuckers which, while almost equally inconspicuous in the twilight, are such as to favour their concealment in the daytime.

An additional illustration of general assimilation of colour to the surroundings of animals, is furnished by the inhabitants of the deep oceans. Professor Moseley of the Challenger Expedition, in his British Association lecture on this subject, says: "Most characteristic of pelagic animals is the almost crystalline transparency of their bodies. So perfect is this transparency that very many of them are rendered almost entirely invisible when floating in the water, while some, even when caught and held up in a glass globe, are hardly to be seen. The skin, nerves, muscles, and other organs are absolutely hyaline and transparent, but the liver and digestive tract often remain opaque and of a yellow or brown colour, and exactly resemble when seen in the water small pieces of floating seaweed." Such marine organisms, however, as are of larger size, and either occasionally or habitually float on the surface, are beautifully tinged with blue above, thus harmonising with the colour of the sea as seen by hovering birds; while they are white below, and are thus invisible against the wave-foam and clouds as seen by enemies beneath the surface. Such are the tints of the beautiful nudibranchiate mollusc, Glaucus atlanticus, and many others.

General Theories of Animal Colour.

We are now in a position to test the general theories, or, to speak more correctly, the popular notions, as to the origin of animal coloration, before proceeding to apply the principle of utility to the explanation of some among the many extraordinary manifestations of colour in the animal world. The most generally received theory undoubtedly is, that brilliancy and variety of colour are due to the direct action of light and heat; a theory no doubt derived from the abundance of bright-coloured birds, insects, and flowers which are brought from tropical regions. There are, however, two strong arguments against this theory. We have already seen how generally bright coloration is wanting in desert animals, yet here heat and light are both at a maximum, and if these alone were the agents in the production of colour, desert animals should be the most brilliant. Again, all naturalists who have lived in tropical regions know that the proportion of bright to dull coloured species is little if any greater there than in the temperate zone, while there are many tropical groups in which bright colours are almost entirely unknown. No part of the world presents so many brilliant birds as South America, yet there are extensive families, containing many hundreds of species, which are as plainly coloured as our average temperate birds. Such are the families of the bush-shrikes and ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the American creepers (Dendrocolaptidae), together with a large proportion of the wood-warblers (Mniotiltidae), the finches, the wrens, and some other groups. In the eastern hemisphere, also, we have the babbling-thrushes (Timaliidae), the cuckoo-shrikes (Campephagidae), the honey-suckers (Meliphagidae), and several other smaller groups which are certainly not coloured above the average standard of temperate birds.

Again, there are many families of birds which spread over the whole world, temperate and tropical, and among these the tropical species rarely present any exceptional brilliancy of colour. Such are the thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named group it is the temperate and arctic zones that afford the most brilliant coloration.

The same general facts are found to prevail among insects. Although tropical insects present some of the most gorgeous coloration in the whole realm of nature, yet there are thousands and tens of thousands of species which are as dull coloured as any in our cloudy land. The extensive family of the carnivorous ground-beetles (Carabidae) attains its greatest brilliancy in the temperate zone; while by far the larger proportion of the great families of the longicorns and the weevils, are of obscure colours even in the tropics. In butterflies, there is undoubtedly a larger proportion of brilliant colour in the tropics; but if we compare families which are almost equally developed over the globe—as the Pieridae or whites and yellows, and the Satyridae or ringlets—we shall find no great disproportion in colour between those of temperate and tropical regions.

The various facts which have now briefly been noticed are sufficient to indicate that the light and heat of the sun are not the direct causes of the colours of animals, although they may favour the production of colour when, as in tropical regions, the persistent high temperature favours the development of the maximum of life. We will now consider the next suggestion, that light reflected from surrounding coloured objects tends to produce corresponding colours in the animal world.

This theory is founded on a number of very curious facts which prove, that such a change does sometimes occur and is directly dependent on the colours of surrounding objects; but these facts are comparatively rare and exceptional in their nature, and the same theory will certainly not apply to the infinitely varied colours of the higher animals, many of which are exposed to a constantly varying amount of light and colour during their active existence. A brief sketch of these dependent changes of colour may, however, be advantageously given here.

Variable Protective Colouring.

There are two distinct kinds of change of colour in animals due to the colouring of the environment. In one case the change is caused by reflex action set up by the animal seeing the colour to be imitated, and the change produced can be altered or repeated as the animal changes its position. In the other case the change occurs but once, and is probably not due to any conscious or sense action, but to some direct influence on the surface tissues while the creature is undergoing a moult or change to the pupa form.