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 within the period of the branching off of the species
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.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will 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 their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been 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.
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