46 The Selection Theory 
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 Miiller 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 recog- 
nisable 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 zn all) these 
small beginnings became, 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 in this case also we see that characters, 
the original use of which was to bring the sexes together, and so to 
maintain the species, have been evolved in the males into means for 
exciting the female. And we can hardly doubt, that the females are 
most readily enticed to yield to the butterfly that sends out the 
strongest fragrance,—that is to say, that excites them to the highest 
degree. It is a pity that our organs of smell are not fine enough 
to examine the fragrance of male Lepidoptera in general, and to 
compare it with other perfumes which attract these insects. As far 
as we can perceive them they resemble the fragrance of flowers, but 
there are Lepidoptera whose scent suggests musk. A smell of musk 
is also given off by several plants: it is a sexual excitant in the 
musk-deer, the musk-sheep, and the crocodile. 
As far as we know, then, it is perfumes similar to those of flowers 
that the male Lepidoptera give off in order to entice their mates, 
and this is a further indication that animals, like plants, can to a 
1 See Poulton, Essays on Evolution, 1908, pp. 316, 317. 
