t 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 large Morphinae of Brazil. In
a great many cases, though not by any means in all, the male butterflies
are "more beautiful" than the females, and in the Tropics in particular
they shine and glow in the most superb colours. I really see no reason
why we should doubt the power of sexual selection, and I myself stand
wholly on Darwin's side. Even though we certainly cannot assume that
the females exercise a conscious choice of the "handsomest" mate, and
deliberate like the judges in a court of justice over the perfections
of their wooers, we have no reason to doubt that distinctive forms
(decorative feathers) and colours have a particularly exciting effect
upon the female, just as certain odours have among animals of so many
different groups, including the butterflies. The doubts which existed
for a considerable time, as a result of fallacious experiments, as to
whether the colours of flowers really had any influence in attracting
butterflies have now been set at rest through a series of more careful
investigations; we now know that the colours of flowers are there on
account of the butterflies, as Sprengel first showed, and that the
blossoms of Phanerogams are selected in relation to them, as Darwin
pointed out.

Certainly it is not possible to bring forward any convincing proof of
the origin of decorative colours through sexual selection, but there
are many weighty arguments in favour of it, and these form a body of
presumptive evidence so strong that it almost amounts to certainty.

In the first place, there is the analogy with other secondary sexual
characters. If the song of birds and the chirping of the cricket have
been evolved through sexual selection, if the penetrating odours of male
animals,--the crocodile, the musk-deer, the beaver, the carnivores, and,
finally, the flower-like fragrances of the butterflies have been evolved
to their present pitch in this way, why should decorative colours
have arisen in some other way? Why should th