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The Descent of Man and Selection in Relation to Sex

Principles Of Sexual Selection
Secondary sexual characters--Sexual selection--Manner of action--Excess of males--
Polygamy--The male alone generally modified through sexual selection--Eagerness of the
male--Variability of the male--Choice exerted by the female--Sexual compared with
natural selection--Inheritance, at corresponding periods of life, at corresponding seasons
of the year, and as limited by sex--Relations between the several forms of inheritance--
Causes why one sex and the young are not modified through sexual selection--
Supplement on the proportional numbers of the two sexes throughout the animal
kingdom--The proportion of the sexes in relation to natural selection.
With animals which have their sexes separated, the males necessarily differ from the
females in their organs of reproduction; and these are the primary sexual characters. But
the sexes often differ in what Hunter has called secondary sexual characters, which are
not directly connected with the act of reproduction; for instance, the male possesses
certain organs of sense or locomotion, of which the female is quite destitute, or has them
more highly-developed, in order that he may readily find or reach her; or again the male
has special organs of prehension for holding her securely. These latter organs, of
infinitely diversified kinds, graduate into those which are commonly ranked as primary,
and in some cases can hardly be distinguished from them; we see instances of this in the
complex appendages at the apex of the abdomen in male insects. Unless indeed we
confine the term "primary" to the reproductive glands, it is scarcely possible to decide
which ought to be called primary and which secondary.
The female often differs from the male in having organs for the nourishment or protection
of her young, such as the mammary glands of mammals, and the abdominal sacks of the
marsupials. In some few cases also the male possesses similar organs, which are wanting
in the female, such as the receptacles for the ova in certain male fishes, and those
temporarily developed in certain male frogs. The females of most bees are provided with
a special apparatus for collecting and carrying pollen, and their ovipositor is modified
into a sting for the defence of the larvae and the community. Many similar cases could be
given, but they do not here concern us. There are, however, other sexual differences quite
unconnected with the primary reproductive organs, and it is with these that we are more
especially concerned--such as the greater size, strength, and pugnacity of the male, his
weapons of offence or means of defence against rivals, his gaudy colouring and various
ornaments, his power of song, and other such characters.
Besides the primary and secondary sexual differences, such as the foregoing, the males
and females of some animals differ in structures related to different habits of life, and not
at all, or only indirectly, to the reproductive functions. Thus the females of certain flies
(Culicidae and Tabanidae) are blood-suckers, whilst the males, living on flowers, have
mouths destitute of mandibles. (1. Westwood, 'Modern Classification of Insects,' vol. ii.
1840, p. 541. For the statement about Tanais, mentioned below, I am indebted to Fritz
Muller.) The males of certain moths and of some crustaceans (e.g. Tanais) have
imperfect, closed mouths, and cannot feed. The complemental males of certain Cirripedes
live like epiphytic plants either on the female or the hermaphrodite form, and are destitute
of a mouth and of prehensile limbs. In these cases it is the male which has been modified,
and has lost certain important organs, which the females possess. In other cases it is the
female which has lost such parts; for instance, the female glow-worm is destitute of