meaning of this and of many analogous differences, is not at all
understood. Differences of structure between the two sexes in relation
to different habits of life are generally confined to the lower
animals; but with some few birds the beak of the male differs from that
of the female. In the Huia of New Zealand the difference is wonderfully
great, and we hear from Dr. Buller (3. Birds of New Zealand, 1872, p.
66.) that the male uses his strong beak in chiselling the larvae of
insects out of decayed wood, whilst the female probes the softer parts
with her far longer, much curved and pliant beak: and thus they
mutually aid each other. In most cases, differences of structure
between the sexes are more or less directly connected with the
propagation of the species: thus a female, which has to nourish a
multitude of ova, requires more food than the male, and consequently
requires special means for procuring it. A male animal, which lives for
a very short time, might lose its organs for procuring food through
disuse, without detriment; but he would retain his locomotive organs in
a perfect state, so that he might reach the female. The female, on the
other hand, might safely lose her organs for flying, swimming, or
walking, if she gradually acquired habits which rendered such powers
useless.
We are, however, here concerned only with sexual selection. This
depends on the advantage which certain individuals have over others of
the same sex and species solely in respect of reproduction. When, as in
the cases above mentioned, the two sexes differ in structure in
relation to different habits of life, they have no doubt been modified
through natural selection, and by inheritance limited to one and the
same sex. So again the primary sexual organs, and those for nourishing
or protecting the young, come under the same influence; for those
individuals which generated or nourished their offspring best, would
leave, ceteris paribus, the greatest number to inherit their
superiority; whilst those which generated or nourished their offspring
badly, would leave but few to inherit their weaker powers. As the male
has to find the female, he requires organs of sense and locomotion, but
if these organs are necessary for the other purposes of life, as is
generally the case, they will have been developed through natural
selection. When the male has found the female, he sometimes absolutely
requires prehensile organs to hold her; thus Dr. Wallace informs me
that the males of certain moths cannot unite with the females if their
tarsi or feet are broken. The males of many oceanic crustaceans, when
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