so strongly pronounced that
the two sexes have frequently been ranked as distinct species, or even
as distinct genera. Such strongly marked differences must be in some
manner highly important; and we know that they have been acquired in
some instances at the cost not only of inconvenience, but of exposure to
actual danger.

The belief in the power of sexual selection rests chiefly on the
following considerations: The characters which we have the best reasons
for supposing to have been thus acquired are confined to one sex; and
this alone renders it probable that in most cases they are connected
with the act of reproduction. These characters in innumerable instances
are fully developed only at maturity; and often during only a part of
the year, which is always the breeding season. The males (passing over a
few exceptional cases) are the more active in courtship; they are the
best armed, and are rendered the most attractive in various ways. It is
to be especially observed that the males display their attractions with
elaborate care in the presence of the females; and that they rarely or
never display them excepting during the season of love. It is incredible
that all this should be purposeless. Lastly, we have distinct evidence
with some quadrupeds and birds that the individuals of one sex are
capable of feeling a strong antipathy or preference for certain
individuals of the other sex.

Bearing in mind these facts and not forgetting the marked results of
man's unconscious selection, it seems to me almost certain that if the
individuals of one sex were during a long series of generations to
prefer pairing with certain individuals of the other sex, characterized
in some peculiar manner, the offspring would slowly but surely become
modified in this same manner. I have not attempted to conceal that,
excepting when the males are more numerous than the females, or when
polygamy prevails, it is doubtful how the more attractive males succeed
in leaving a larger number of offspring to inherit their superiority in
ornaments or other charms than the less attractive males; but I have
shown that this would probably follow from the females--especially the
more vigorous ones, which would be the first to breed--preferring not
only the more attractive but at the same time the more vigorous and
victorious males.

Although we have some positive evidence that birds appreciate bright and
beautiful objects, as with the bower-birds of Austr