If the numbers be wholly kept down by the
causes just indicated, as will often have been the case, natural selection
will be powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we are
far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, so no doubt it will be under
nature. Thus it is rendered possible for the two sexes to be modified
through natural selection in relation to different habits of life, as is
sometimes the case; or for one sex to be modified in relation to the other
sex, as commonly occurs. This leads me to say a few words on what I have
called sexual selection. This form of selection depends, not on a struggle
for existence in relation to other organic beings or to external
conditions, but on a struggle between the individuals of one sex, generally
the males, for the possession of the other sex. The result is not death to
the unsuccessful competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally, the most
vigorous males, those which are best fitted for their places in nature,
will leave most progeny. But in many cases victory depends not so much on
general vigour, but on having special weapons, confined to the male sex.
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