ale.

Isolation of the group of individuals which is in process of varying
is undoubtedly of great value in sexual selection, for even a solitary
conspicuous variation will become dominant much sooner in a small
isolated colony, than among a large number of members of a species.

Anyone who agrees with me in deriving variations from germinal selection
will regard that process as an essential aid towards explaining the
selection of distinctive courtship-characters, such as coloured spots,
decorative feathers, horny 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