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.
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