vement 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 presence of individuals of the other sex, but it soon became an
excitant, and as the individuals which caused the greatest degree of
excitement were preferred, it reached as high a pitch of perfection as
was possible to it. I shall confine myself here to the comparatively
recently discovered fragrance of butterflies. Since Fritz Muller found
out that certain Brazilian butterflies gave off fragrance "like a
flower," we have become acquainted with many such cases, and we now know
that in all lands, not only many diurnal Lepidoptera but nocturnal ones
also give off a delicate odour, which is agreeable even to man.
The ethereal oil to which this fragrance is due is secreted by the
skin-cells, usually of the wing, as I showed soon after the discovery
of the SCENT-SCALES. This is the case in the males; the females have no
SPECIAL scent-scales recognisable as such by their form, but they must,
nevertheless, give off an extremely delicate fragrance, although our
imperfect organ of smell cannot perceive it, for the males become aware
of the presence of a female, even at night, from a long distance off,
and gather round her. We may therefore conclude, that both sexes have
long given forth a very delicate perfume, which announced their presence
to others of the same species, and that in many species (NOT IN ALL)
these small beginnings became, in the males, particularly strong
scent-scales of char