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
this law. "In Pontella, for instance, the sexual characters are afforded
mainly by the anterior antennae and by the fifth pair of legs: the
specific differences also are principally given by these organs." This
relation has a clear meaning on my view: I look at all the species of the
same genus as having as certainly descended from the same progenitor, as
have the two sexes of any one species.
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