and pugnacity. They are provided, either exclusively or in a much
higher degree than the females, with organs for vocal or instrumental
music, and with odoriferous glands. They are ornamented with infinitely
diversified appendages, and with the most brilliant or conspicuous
colours, often arranged in elegant patterns, whilst the females are
unadorned. When the sexes differ in more important structures, it is
the male which is provided with special sense-organs for discovering
the female, with locomotive organs for reaching her, and often with
prehensile organs for holding her. These various structures for
charming or securing the female are often developed in the male during
only part of the year, namely the breeding-season. They have in many
cases been more or less transferred to the females; and in the latter
case they often appear in her as mere rudiments. They are lost or never
gained by the males after emasculation. Generally they are not
developed in the male during early youth, but appear a short time
before the age for reproduction. Hence in most cases the young of both
sexes resemble each other; and the female somewhat resembles her young
offspring throughout life. In almost every great class a few anomalous
cases occur, where there has been an almost complete transposition of
the characters proper to the two sexes; the females assuming characters
which properly belong to the males. This surprising uniformity in the
laws regulating the differences between the sexes in so many and such
widely separated classes, is intelligible if we admit the action of one
common cause, namely sexual selection.
Sexual selection depends on the success of certain individuals over
others of the same sex, in relation to the propagation of the species;
whilst natural selection depends on the success of both sexes, at all
ages, in relation to the general conditions of life. The sexual
struggle is of two kinds; in the one it is between individuals of the
same sex, generally the males, in order to drive away or kill their
rivals, the females remaining passive; whilst in the other, the
struggle is likewise between the individuals of the same sex, in order
to excite or charm those of the opposite sex, generally the females,
which no longer remain passive, but select the more agreeable partners.
This latter kind of selection is closely analogous to that which man
unintentionally, yet effectually, brings to bear on his domesticated
productions, when he preserves during a long period the most pleasing
or useful individuals, without any wish to modify the breed.
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