Page:Darwinism by Alfred Wallace 1889.djvu/252

 So many cases are adduced from so many different islands, both in the eastern and western hemisphere, that it is impossible to doubt the existence of some common cause; and it seems probable to me now, after a fuller consideration of the whole subject of colour, that here too we have one of the almost innumerable results of the principle of protective coloration. White is, as a rule, an uncommon colour in animals, but probably only because it is so conspicuous. Whenever it becomes protective, as in the case of arctic animals and aquatic birds, it appears freely enough; while we know that white varieties of many species occur occasionally in the wild state, and that, under domestication, white or parti-coloured breeds are freely produced. Now in all the islands in which exceptionally white-marked birds and butterflies have been observed, we find two features which would tend to render the conspicuous white markings less injurious—a luxuriant tropical vegetation, and a decided scarcity of rapacious mammals and birds. White colours, therefore, would not be eliminated by natural selection; but variations in this direction would bear their part in producing the recognition marks which are everywhere essential, and which, in these islands, need not be so small or so inconspicuous as elsewhere.

On a review of the whole subject, then, we must conclude that there is no evidence of the individual or prevalent colours of organisms being directly determined by the amount of light, or heat, or moisture, to which they are exposed; while, on the other hand, the two great principles of the need of concealment from enemies or from their prey, and of recognition by their own kind, are so wide-reaching in their application that they appear at first sight to cover almost the whole ground of animal coloration. But, although they are indeed wonderfully general and have as yet been very imperfectly studied, we are acquainted with other modes of coloration which have a different origin. These chiefly appertain to the very singular class of warning colours, from which arise the yet more extraordinary phenomena of mimicry; and they open up so curious a field of inquiry and present so many interesting problems, that a chapter must be devoted to them. Yet another chapter will