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 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 not great 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 ordinary
selection,--and to variations i