t 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. The extreme variations in colour that
immediately arise under domestication indicate a tendency to vary in
this way, and the occasional occurrence of white or piebald or other
exceptionally coloured individuals of many species in a state of nature,
shows that this tendency exists there also; and, as these exceptionally
coloured individuals rarely or never increase, there must be some
constant power at work to keep it in check. This power can only be
natural selection or the survival of the fittest, which again implies
that some colours are useful, some injurious, in each particular case.
With this principle as our guide, let us see how far we can account both
for the general and special colours of the animal world.

_Colour and Environment._

The fact that first strikes us in our examination of the colours of
animals as a whole, is the close relation that exists between these
colours and the general environment. Thus, white prevails among arctic
animals; yellow or brown in desert species; while green is only a common
colour in tropical evergreen forests. If we consider these cases
somewhat carefully we shall find, that they afford us excellent
materials for forming a judgment on the various theories that have been
suggested to account for the colours of the animal world.

In the arctic regions there are a number of animals which are wholly
white all the year round, or which only turn white in winter. Among the
former are the polar bear and the American polar hare, the snowy owl and
the Greenland falcon; among the latter the arctic fox, the arctic hare,
the ermine, and the ptarmigan. Those which are permanently white remain
among the snow nearly all the year round, while those which change their
colour inhabit regions which are free from snow in summer. The obvious
explanation of this style of coloration is, that it is protective,
serving to conceal the herbivorous species from their enemies, and
enabling carnivorous animals to approach their prey unperceived. Two
other explanations have, however, been suggested. One is, that the
prevalent white of the arctic regions has a direct effect in producing
the white colour in animals, either by some p