crossing, that only a single variety should be turned loose in
its new home. Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancestral forms, it seems to me
not improbable, that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance,
of the cabbage, in very poor soil (in which case, however, some effect
would have to be attributed to the direct action of the poor soil),
that they would to a large extent, or even wholly, revert to the wild
aboriginal stock. Whether or not the experiment would succeed, is not of
great importance for our line of argument; for by the experiment itself
the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion,--that
is, to lose their acquired characters, whilst kept under unchanged
conditions, and whilst kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations
of structure, in such case, I grant that we could deduce nothing from
domestic varieties in regard to species. But there is not a shadow of
evidence in favour of this view: to assert that we could not breed
our cart and race-horses, long and short-horned cattle, and poultry of
various breeds, and 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 species closely allied
together, 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 from each other,
and from the 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 h