eeds of
many of our domestic animals such as of the horse, pig, dog, sheep,
pigeon, and poultry, and of our plants have descended from more than one
aboriginal form. They leave it doubtful, whether such forms are to be
considered wild races, or true species, whose offspring are fertile when
crossed _inter se_. The main arguments for this view consist, firstly,
of the great difference between such breeds, as the Race-and Cart-Horse,
or the Greyhound and Bull-dog, and of our ignorance of the steps or
stages through which these could have passed from a common parent; and
secondly that in the most ancient historical periods, breeds resembling
some of those at present most different, existed in different countries.
The wolves of N. America and of Siberia are thought to be different
species; and it has been remarked that the dogs belonging to the
savages in these two countries resemble the wolves of the same country;
and therefore that they have probably descended from two different wild
stocks. In the same manner, these naturalists believe that the horse of
Arabia and of Europe have probably descended from two wild stocks both
apparently now extinct. I do not think the assumed fertility of these
wild stocks any very great difficulty on this view; for although in
animals the offspring of most cross-bred species are infertile, it is
not always remembered that the experiment is very seldom fairly tried,
except when two near species _both_ breed freely (which does not readily
happen, as we shall hereafter see) when under the dominion of man.
Moreover in the case of the China{206} and common goose, the canary and
siskin, the hybrids breed freely; in other cases the offspring from
hybrids crossed with either pure parent are fertile, as is practically
taken advantage of with the yak and cow; as far as the analogy of plants
serves, it is impossible to deny that some species are quite fertile
_inter se_; but to this subject we shall recur.

{205} Pallas' theory is discussed in the _Origin_, Ed. i. pp. 253,
254, vi. p. 374.

{206} See Darwin's paper on the fertility of hybrids from the
common and Chinese goose in _Nature_, Jan. 1, 1880.

On the other hand, the upholders of the view that the several breeds of
dogs, horses, &c., &c., have descended each from one stock, may aver
that their view removes all _difficulty about fertility_, and that the
main argument from the high antiquity of different breeds, somewhat