Page:Darwinism by Alfred Wallace 1889.djvu/215

Rh they live in autumn and winter, and especially in early spring when so many of them build their nests.

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.

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,