is 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. 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 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 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 are possessed by all the species;
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
their great difference in 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 being
the descendants of 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 in the same parts
having been accumulated by natural and sexual selection, and thus having
been adapted for secondary sexual, and for ordinary purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF
ONE SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE