There are instructive applications of this Double Method in Wallace’s Darwinism.  In chap. viii., on Colour in Animals, he observes, that the usefulness of their coloration to animals is shown by the fact that, “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.”  Wild animals protect themselves by acquiring qualities adapted to their mode of life; and coloration is a very important one, its chief, though not its only use, being concealment.  Hence a useful coloration having been established in any species, individuals that occasionally may vary from it, will generally, perish; whilst, among domestic animals, variation of colour or marking is subject to no check except the taste of owners.  We have, then, two lists of instances; first, innumerable species of wild animals in which the coloration is constant and which depend upon their own qualities for existence; secondly, several species of domestic animals in which the coloration is not constant, and which do not depend upon their own qualities for existence.  In the former list two circumstances are present together (under all sorts of conditions); in the latter they are absent together.  The argument may be further strengthened by adding a third list, parallel to the first, comprising domestic animals in which coloration is approximately constant, but where (as we know) it is made a condition of existence by owners, who only breed from those specimens that come up to a certain standard of coloration.

Wallace goes on to discuss the colouring of arctic animals.  In the arctic regions, he says, some animals are wholly white all the year round, such as the polar bear, the American polar hare, the snowy owl and the Greenland falcon:  these live amidst almost perpetual snow.  Others, that live where the snow melts in summer, only turn white in winter, such as the arctic hare, the arctic fox, the ermine and the ptarmigan.  In all these cases the white colouring is useful, concealing the herbivores from their enemies, and also the carnivores in approaching their prey; this usefulness, therefore, is a condition of the white colouring.  Two other explanations have, however, been suggested:  first, that the prevalent white of the arctic regions directly colours the animals, either by some photographic or chemical action on the skin, or by a reflex action through vision (as in the chameleon); secondly, that a white skin checks radiation and keeps the animals warm.  But there are some exceptions to the rule of white colouring in arctic animals which refute these hypotheses, and confirm the author’s.  The sable remains