The brilliant or conspicuous colours which characterise many birds in
the present class, can rarely or never be of service to them as a
protection; so that they have probably been gained by the males through
sexual selection, and then transferred to the females and the young. It
is, however, possible that the males may have selected the more
attractive females; and if these transmitted their characters to their
offspring of both sexes, the same results would follow as from the
selection of the more attractive males by the females. But there is
evidence that this contingency has rarely, if ever, occurred in any of
those groups of birds in which the sexes are generally alike; for, if
even a few of the successive variations had failed to be transmitted to
both sexes, the females would have slightly exceeded the males in
beauty. Exactly the reverse occurs under nature; for, in almost every
large group in which the sexes generally resemble each other, the males
of some few species are in a slight degree more brightly coloured than
the females. It is again possible that the females may have selected
the more beautiful males, these males having reciprocally selected the
more beautiful females; but it is doubtful whether this double process
of selection would be likely to occur, owing to the greater eagerness
of one sex than the other, and whether it would be more efficient than
selection on one side alone. It is, therefore, the most probable view
that sexual selection has acted, in the present class, as far as
ornamental characters are concerned, in accordance with the general
rule throughout the animal kingdom, that is, on the males; and that
these have transmitted their gradually-acquired colours, either equally
or almost equally, to their offspring of both sexes.
Another point is more doubtful, namely, whether the successive
variations first appeared in the males after they had become nearly
mature, or whilst quite young. In either case sexual selection must
have acted on the male when he had to compete with rivals for the
possession of the female; and in both cases the characters thus
acquired have been transmitted to both sexes and all ages. But these
characters if acquired by the males when adult, may have been
transmitted at first to the adults alone, and at some subsequent period
transferred to the young. For it is known that, when the law of
inheritance at corresponding ages fails, the offspring often inherit
characters at an earlier age than that at which they first appeared in
their parents. (30. Variation of Animals and Plants under
506