y outgrowths in birds and
reptiles, combs, feather-tufts, and the like, since the beginnings of
these would be presented with relative frequency in the struggle
between the determinants within the germ-plasm. The process of
transmission of decorative feathers to the female results, as Darwin
pointed out and illustrated by interesting examples, in the
_colour-transformation of a whole species_, and this process, as the
phyletically older colouring of young birds shows, must, in the course
of thousands of years, have repeated itself several times in a line of
descent.

If we survey the wealth of phenomena presented to us by secondary
sexual characters, we can hardly fail to be convinced of the truth of
the 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 th