the same time making provision for its own
offspring, since it is only after fertilisation that the seeds begin to
develop. There is thus nothing to prevent our referring this structural
adaptation in Pronuba yuccasella to processes of selection, which
have gradually transformed the maxillary palps of the female into the
sickle-shaped instrument for collecting the pollen, and which have at
the same time developed in the insect the instinct to press the pollen
into the pistil.

In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any
other explanation, which, now that the facts are so well known, could
be regarded as a serious rival to it. That selection is a factor, and
a very powerful 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