ecies 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.

It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts
of the organisation in which the species of the same genus differ from
each other. Of this fact I will give in illustration the first two
instances 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 common
to very large groups of beetles, but in the Engidae, as Westwood has
remarked, the number varies greatly and the number likewise differs in
the two sexes of the same species. Again in the fossorial hymenoptera,
the neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the
same species. Sir J. Lubbock has recently remarked, that several minute
crustaceans offer excellent illustrations of th