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