reached its
highest pitch of adaptation, but that it _maintains it at this pitch.
This conserving influence of natural selection_ is of great
importance, and was early recognised by Darwin; it follows naturally
from the principle of the survival of the fittest.

We understand from this how it is that a species which has become
fully adapted to certain conditions of life ceases to vary, but
remains "constant," as long as the conditions of life _for_ it remain
unchanged, whether this be for thousands of years, or for whole
geological epochs. But the most convincing proof of the power of the
principle of selection lies in the innumerable multitude of phenomena
which cannot be explained in any other way. To this category belong
all structures which are only _passively_ of advantage to the
organism, because none of these can have arisen by the alleged
_Lamarckian principle_. These have been so often discussed that we
need do no more than indicate them here. Until quite recently the
sympathetic coloration of animals--for instance, the whiteness of
Arctic animals--was referred, at least in part, to the _direct_
influence of external factors, but the facts can best be explained by
referring them to the processes of selection, for then it is
unnecessary to make the gratuitous assumption that many species are
sensitive to the stimulus of cold and that others are not. The great
majority of Arctic land-animals, mammals and birds, are white, and
this proves that they were all able to present the variation which was
most useful for them. The sable is brown, but it lives in trees, where
the brown colouring protects and conceals it more effectively. The
musk-sheep (_Ovibos moschatus_) is also brown, and contrasts sharply
with the ice and snow, but it is protected from beasts of prey by its
gregarious habit, and therefore it is of advantage to be visible from
as great a distance as possible. That so many species have been able
to give rise to white varieties does not depend on a special
sensitiveness of the skin to the influence of cold, but to the fact
that Mammals and Birds have a general tendency to vary towards white.
Even with us, many birds--starlings, blackbirds, swallows,
etc.--occasionally produce white individuals, but the white variety
does not persist, because it readily falls a victim to the carnivores.
This is true of white fawns, foxes, deer, etc. The whiteness,
therefore, arises from internal causes, and only persists