principle of sexual selection. And certainly no one who has accepted
natural selection should reject sexual selection, for, not only do the two
processes rest upon the same basis, but they merge into one another, so
that it is often impossible to say how much of a particular character
depends on one and how much on the other form of selection.

(b) NATURAL SELECTION.

An actual proof of the theory of sexual selection is out of the question,
if only because we cannot tell when a variation attains to selection-value.
It is certain that a delicate sense of smell is of value to the male moth
in his search for the female, but whether the possession of one additional
olfactory hair, or of ten, or of twenty additional hairs leads to the
success of its possessor we are unable to tell. And we are groping even
more in the dark when we discuss the excitement caused in the female by
agreeable perfumes, or by striking and beautiful colours. That these do
make an impression is beyond doubt; but we can only assume that slight
intensifications of them give any advantage, and we MUST assume this SINCE
OTHERWISE SECONDARY SEXUAL CHARACTERS REMAIN INEXPLICABLE.

The same thing is true in regard to natural selection. It is not possible
to bring forward any actual proof of the selection-value of the initial
stages, and the stages in the increase of variations, as has been already
shown. But the selection-value of a finished adaptation can in many cases
be statistically determined. Cesnola and Poulton have made valuable
experiments in this direction. The former attached forty-five individuals
of the green, and sixty-five of the brown variety of the praying mantis
(Mantis religiosa), by a silk thread to plants, and watched them for
seventeen days. The insects which were on a surface of a colour similar to
their own remained uneaten, while twenty-five green insects on brown parts