ave all been lost and absorbed in a mongrel
race. It is probably owing to the freedom of crossing, that, in
uncivilised countries, where inclosures do not exist, we seldom meet
with more than one race of a species: it is only in enclosed countries,
where the inhabitants do not migrate, and have conveniences for
separating the several kinds of domestic animals, that we meet with a
multitude of races. Even in civilised countries, want of care for a few
years has been found to destroy the good results of far longer periods
of selection and separation.

This power of crossing will affect the races of all _terrestrial_
animals; for all terrestrial animals require for their reproduction the
union of two individuals. Amongst plants, races will not cross and blend
together with so much freedom as in terrestrial animals; but this
crossing takes place through various curious contrivances to a
surprising extent. In fact such contrivances exist in so very many
hermaphrodite flowers by which an occasional cross may take place, that
I cannot avoid suspecting (with Mr Knight) that the reproductive action
requires, at _intervals_, the concurrence of distinct individuals{204}.
Most breeders of plants and animals are firmly convinced that benefit is
derived from an occasional cross, not with another race, but with
another family of the same race; and that, on the other hand, injurious
consequences follow from long-continued close interbreeding in the same
family. Of marine animals, many more, than was till lately believed,
have their sexes on separate individuals; and where they are
hermaphrodite, there seems very generally to be means through the water
of one individual occasionally impregnating another: if individual
animals can singly propagate themselves for perpetuity, it is
unaccountable that no terrestrial animal, where the means of observation
are more obvious, should be in this predicament of singly perpetuating
its kind. I conclude, then, that races of most animals and plants, when
unconfined in the same country, would tend to blend together.

{204} The so-called Knight-Darwin Law is often misunderstood. See
Goebel in _Darwin and Modern Science_, 1909, p. 419; also F.
Darwin, _Nature_, Oct. 27, 1898.

_Whether our domestic races have descended from one or more wild
stocks._

Several naturalists, of whom Pallas{205} regarding animals, and Humboldt
regarding certain plants, were the first, believe that the br