under
domestication{203}. So marked is this variability in cross-bred
descendants, that Pallas and some other naturalists have supposed that
all variation is due to an original cross; but I conceive that the
history of the potato, Dahlia, Scotch Rose, the guinea-pig, and of many
trees in this country, where only one species of the genus exists,
clearly shows that a species may vary where there can have been no
crossing. Owing to this variability and tendency to reversion in
cross-bred beings, much careful selection is requisite to make
intermediate or new permanent races: nevertheless crossing has been a
most powerful engine, especially with plants, where means of propagation
exist by which the cross-bred varieties can be secured without incurring
the risk of fresh variation from seminal propagation: with animals the
most skilful agriculturalists now greatly prefer careful selection from
a well-established breed, rather than from uncertain cross-bred stocks.

{202} The effects of crossing is much more strongly stated here
than in the _Origin_. See Ed. i. p. 20, vi. p. 23, where indeed the
opposite point of view is given. His change of opinion may be due
to his work on pigeons. The whole of the discussion on crossing
corresponds to Chapter VIII of the _Origin_, Ed. i. rather than to
anything in the earlier part of the book.

{203} The parallelism between the effects of a cross and the
effects of conditions is given from a different point of view in
the _Origin_, Ed. i. p. 266, vi. p. 391. See the experimental
evidence for this important principle in the author's work on
_Cross and Self-Fertilisation_. Professor Bateson has suggested
that the experiments should be repeated with gametically pure
plants.

Although intermediate and new races may be formed by the mingling of
others, yet if the two races are allowed to mingle quite freely, so that
none of either parent race remain pure, then, especially if the parent
races are not widely different, they will slowly blend together, and the
two races will be destroyed, and one mongrel race left in its place.
This will of course happen in a shorter time, if one of the parent
races exists in greater number than the other. We see the effect of this
mingling, in the manner in which the aboriginal breeds of dogs and pigs
in the Oceanic Islands and the many breeds of our domestic animals
introduced into S. America, h