The laws of inheritance determine whether characters gained through
sexual selection by either sex shall be transmitted to the same sex, or
to both; as well as the age at which they shall be developed. It
appears that variations arising late in life are commonly transmitted
to one and the same sex. Variability is the necessary basis for the
action of selection, and is wholly independent of it. It follows from
this, that variations of the same general nature have often been taken
advantage of and accumulated through sexual selection in relation to
the propagation of the species, as well as through natural selection in
relation to the general purposes of life. Hence secondary sexual
characters, when equally transmitted to both sexes can be distinguished
from ordinary specific characters only by the light of analogy. The
modifications acquired through sexual selection are often 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. Certain characters are confined to one sex;
and this alone renders it probable that in most cases they are
connected with the act of reproduction. In innumerable instances these
characters 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 better armed, and are rendered the more
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 the marked results of man s
unconscious selection, when applied to domesticated animals and
cultivated plants, 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, characterised
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