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.[43] 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 large
extent meet the claims made upon them by life, and produce the
adaptations which are most purposive,--a further proof, too, of my
proposition that the useful variations, so to speak, are _always
there_. The flowers developed the perfumes which entice their
visitors, and the male Lepidoptera developed the perfumes which entice
and excite their mates.

There are many pretty little problems to be solved in this connection,
for there are insects, such as some flies, that are attracted by
smells which are unpleasant to us, like those from decaying flesh and
carrion. But there are also certain flowers, some orchids for
instance, which give forth no very agreeable odour, but one which is
to us repulsive and disgusting; and we should therefore expect that
the males of such insects would give off a smell unpleasant to us, but
there is no case known to me in which this has been demonstrated.

In cases such as we have discussed, it is obvious that there is no
possible explanation except through selection. This brings us to the
last kind of secondary sexual characters, and the one in regard to
which doubt has been most frequently expressed,--decorative colours
and decorative forms, the brilliant plumage of the male pheasant, the
humming-birds, and the bird of Paradise, as well as the bright colours
of many species of butterfly, from the beautiful blue of our little
Lycaenidae to the magnificent azure of the