probable that they should vary 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 {157} 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 beetle