eneral. In this case, again, the selection-value of the initial
stages hardly admits of doubt; better vision _directly_ secures
reproduction.

In many cases _the organ of smell_ shows a similar improvement. Many
lower Crustaceans (Daphnidae) have better developed organs of smell in
the male sex. The difference is often slight and amounts only to one
or two olfactory filaments, but certain species show a difference of
nearly a hundred of these filaments (Leptodora). The same thing occurs
among insects.

We must briefly consider the clasping or grasping organs which have
developed in the males among many lower Crustaceans, but here natural
selection plays its part along with sexual selection, for the union of
the sexes is an indispensable condition for the maintenance of the
species, and as Darwin himself pointed out, in many cases the two
forms of selection merge into each other. This fact has always seemed
to me to be a proof of natural selection, for, in regard to sexual
selection, it is quite obvious that the victory of the best-equipped
could have brought about the improvement only of the organs concerned,
the factors in the struggle, such as the eye and the olfactory organ.

We come now to the _excitants_; that is, to the group of sexual
characters whose origin through processes of selection has been most
frequently called in question. We may cite the _love-calls_ produced
by many male insects, such as crickets and cicadas. These could only
have arisen in animal groups in which the female did not rapidly flee
from the male, but was inclined to accept his wooing from the first.
Thus, notes like the chirping of the male cricket serve to entice the
females. At first they were merely the signal which showed the
presence of a male in the neighbourhood, and the female was gradually
enticed nearer and nearer by the continued chirping. The male that
could make himself heard to the greatest distance would obtain the
largest following, and would transmit the beginnings, and, later, the
improvement of his voice to the greatest number of descendants. But
sexual excitement in the female became associated with the hearing of
the love-call, and then the sound-producing organ of the male began to
improve, until it attained to the emission of the long-drawn-out soft
notes of the mole-cricket or the maenad-like cry of the cicadas. I
cannot here follow the process of development in detail, but will call
attention to the fact that t