esculent
vegetables, for an almost infinite number of generations, would be opposed
to all experience. I may add, that when under nature the conditions of life
do change, variations and reversions of character probably do occur; but
natural selection, as will hereafter be explained, will determine how far
the new characters thus arising shall be preserved.

When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with closely allied species, we generally
perceive in each domestic race, as already remarked, less uniformity of
character than in true species. Domestic races of the same species, also,
often have a somewhat monstrous character; by which I mean, that, although
differing {16} from each other, and from other species of the same genus,
in several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially
when compared with all the species in nature to which they are nearest
allied. With these exceptions (and with that of the perfect fertility of
varieties when crossed,--a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner as,
only in most cases in a lesser degree than, do closely-allied species of
the same genus in a state of nature. I think this must be admitted, when we
find that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by competent judges as mere varieties,
and by other competent judges as the descendants of aboriginally distinct
species. If any marked distinction existed between domestic races and
species, this source of doubt could not so perpetually recur. It has often
been stated that domestic races do not differ from each other in characters
of generic value. I think it could be shown that this statement is hardly
correct; but naturalists differ widely in determining what characters are
of generic value; all such valuations being at present empirical. Moreover,
on the view of the origin of genera which I shall presently give, we have
no right to expect often to meet with generic differences in our
domesticated productions.

When we attempt to estimate the amount of structural difference between the
domestic races of the same species, we are soon involved in doubt, from not
knowing whether they have descended from one or several parent-species.
This point, if it could be