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