y at the present day. 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. 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 readily have succeeded in giving
to the species of the same group a greater amount of difference in their
sexual characters, than in other parts of their structure.

It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of
the same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature,
the relation can hardly be accidental. The same number of joints in the
tarsi is a character generally common to very large groups of beetles,
but in the Engidae, as Westwood