s, 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 fossorial
hymenoptera, the manner of 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. This relation has a clear meaning on my view of
the subject: 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 of the species. Consequently, whatever part of the structure of the
common progenitor, or of its early descendants, became variable; variations
of this part would, it is highly probable, be taken advantage of by natural
and sexual selection, in order to fit {158} the several species to their
several places in the economy of nature, and likewise to fit the two sexes
of the same species to each other, or to fit the males and females to
different habits of life, or the males to struggle with other males for the
possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;--that the
frequent extreme variability of any part which is developed in a species in
an extraordinary manner in comparison with the same part in its congeners;
and the slight degree of variability in a part, however extraordinarily it
may be developed, if it be common to a whole group of species;--that the
great variability of secondary sexual characters, and the great amount of
difference in these same characters between closely allied species;--that
secondary sexual and ordinary specific differences are generally displayed
in the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same group
having descended from a common progenitor, from whom they have inherited
much in common,--to parts which have recently and largely varied being more
likely still to go on varying than parts which have long been inherited and
have not varied,--to natural selection having more or less completely,
according to the lapse of time, overmastered the tendency to reversion and
to further variability,--to sexual selection being less rigid than ord