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 the original purpose of the voice,
the announcing of the male's presence, became subsidiary, and the exciting
of the female became the chief goal to be aimed at. The loudest singers
awakened the strongest excitement, and the improvement resulted as a matter
of course. I conceive of the origin of bird-song in a somewhat similar
manner, first as a means of enticing, then of exciting the female.

One more kind of secondary sexual character must here be mentioned: the
odour which emanates from so many animals at the breeding season. It is
possible that this odour also served at first merely to give notice of the