_if_ they exist, must of logical necessity cooeperate in the
manner required by the theory. _We must accept it because the
phenomena of evolution and adaptation must have a natural basis, and
because it is the only possible explanation of them._[53]

Many people are willing to admit that selection explains adaptations,
but they maintain that only a part of the phenomena are thus
explained, because everything does not depend upon adaptation. They
regard adaptation as, so to speak, a special effort on the part of
Nature, which she keeps in readiness to meet particularly difficult
claims of the external world on organisms. But if we look at the
matter more carefully we shall find that adaptations are by no means
exceptional, but that they are present everywhere in such enormous
numbers, that it would be difficult in regard to any structure
whatever, to prove that adaptation had _not_ played a part in its
evolution.

How often has the senseless objection been urged against selection
that it can create nothing, it can only reject. It is true that it
cannot create either the living substance or the variations of it;
both must be given. But in rejecting one thing it preserves another,
intensifies it, combines it, and in this way _creates_ what is new.
_Everything_ in organisms depends on adaptation; that is to say,
everything must be admitted through the narrow door of selection,
otherwise it can take no part in the building up of the whole. But, it
is asked, what of the direct effect of external conditions,
temperature, nutrition, climate and the like? Undoubtedly these can
give rise to variations, but they too must pass through the door of
selection, and if they cannot do this they are rejected, eliminated
from the constitution of the species.

It may, perhaps, be objected that such external influences are often
of a compelling power, and that every animal must submit to them, and
that thus selection has no choice and can neither select nor reject.
There may be such cases; let us assume for instance that the effect
of the cold of the Arctic regions was to make all the mammals become
black; the result would be that they would all be eliminated by
selection, and that no mammals would be able to live there at all. But
in most cases a certain percentage of animals resists these strong
influences, and thus selection secures a foothold on which to work,
eliminating the unfavourable variation, and establishing a useful
colouri