from several species will have not only to admit
that their offspring after a long course of domestication generally lose
all tendency to sterility when crossed together; but that between certain
breeds of dogs and some of their supposed aboriginal parents a certain
degree of sterility has been retained or possibly even acquired.

Notwithstanding the difficulties in regard to fertility given in the last
two paragraphs, when we reflect on the inherent improbability of man having
domesticated throughout the world one single species alone of so widely
distributed, so easily tamed, and so useful a group as the Canidae; when we
reflect on the extreme antiquity of the different breeds; and especially
when we reflect on the close similarity, both in external structure and
habits, between the domestic dogs of various countries and the wild species
still inhabiting these same countries, the balance of evidence is strongly
in favour of the multiple origin of our dogs.

* * * * *

_Differences between the several Breeds of the Dog._--If the several breeds
have descended from several wild stocks, their difference can obviously in
part be explained by that of their parent-species. For instance, the form
of the greyhound may be partly accounted for by descent from some such
animal as the slim Abyssinian _Canis simensis_,[55] with its elongated
muzzle; that of the larger dogs from the larger wolves, and the smaller and
slighter dogs from jackals: and thus perhaps we may account for certain
constitutional and climatal differences. But it would be a great error to
suppose that there has not been in addition[56] a large amount of
variation. The intercrossing of the several aboriginal wild stocks, and of
the subsequently formed {34} races, has probably increased the total number
of breeds, and, as we shall presently see, has greatly modified some of
them. But we cannot explain by crossing the origin of such extreme forms as
thoroughbred greyhounds, bloodhounds, bulldogs, Blenheim spaniels,
terriers, pugs, &c., unless we believe that forms equally or more strongly
characterised in these different respects once existed in nature. But
hardly any one has been bold enough to suppose that such unnatural forms
ever did or could exist in a wild state. When compared with all known
members of the family of Canidae they betray a distinct and abnormal origin.
No instance is on record of such dogs as bloodhounds, spanie