factor in the evolution of organisms, can no longer be doubted. Even
although we cannot bring forward formal proofs of it IN DETAIL, cannot
calculate definitely the size of the variations which present themselves,
and their selection-value, cannot, in short, reduce the whole process to a
mathematical formula, yet we must assume selection, because it is the only
possible explanation applicable to whole classes of phenomena, and because,
on the other hand, it is made up of factors which we know can be proved
actually to exist, and which, IF they exist, must of logical necessity
cooperate 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. (This has been
discussed in many of my earlier works. See for instance "The All-
Sufficiency of Natural Selection, a reply to Herbert Spencer", London,
1893.)

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