On the other hand, the points in which
species differ from other species of the same genus are called specific
characters; and as these specific characters have varied and come to differ
since the period when the species branched off from a common progenitor, it
is probable that they should still often be in some degree variable--at
least more variable than those parts of the organisation which have for a
very long period remained constant.
SECONDARY SEXUAL CHARACTERS VARIABLE.
I think it will be admitted by naturalists, without my entering on details,
that secondary sexual characters are highly variable. It will also be
admitted that species of the same group differ from each other more widely
in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the
males of gallinaceous birds, in which secondary sexual characters are
strongly displayed, with the amount of difference between the females. The
cause of the original variability of these characters is not manifest; but
we can see why they should not have been rendered as constant and uniform
as others, for they are accumulated by sexual selection, which is less
rigid in its action than ordinary selection, as it does not entail death,
but only gives fewer offspring to the less favoured males. Whatever the
cause may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the same group a
greater amount of difference in these than in other respects.
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