selection-value of the initial stages and
the individual stages of increase. We have therefore to fall back on
PRESUMPTIVE EVIDENCE. This is to be found in THE INTERPRETATIVE VALUE OF
THE THEORY. Let us consider this point in greater detail.

In the first place, it is necessary to emphasise what is often
overlooked, namely, that the theory not only explains the
TRANSFORMATIONS of species, it also explains THEIR REMAINING THE SAME;
in addition to the principle of varying, it contains within itself that
of PERSISTING. It is part of the essence of selection, that it not
only causes a part to VARY till it has 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