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 brown throughout the winter;