he 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 Mueller 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 become, in the males, particularly strong scent-scales of
characteristic form (lute, brush, or lyre-shaped). At first these
scales were scattered over the surface of the wing, but gradually they
concentrated themselves, and formed broad, velvety bands, or strong,
prominent brushes, and they attained their highest pitch of evolution
when they became enclosed within pits or folds of the skin, which
could be opened to let the delicious fragrance stream forth suddenly
towards the female. Thus