ereafter to be discussed), domestic races of the
same species differ from each other in the same manner as, only in most
cases in a lesser degree than, do closely-allied species of the same
genus in a state of nature. I think this must be admitted, when we find
that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as
mere varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it could
be shown that this statement is hardly correct; but naturalists differ
most widely in determining what characters are of generic value; all
such valuations being at present empirical. Moreover, on the view of
the origin of genera which I shall presently give, we have no right
to expect often to meet with generic differences in our domesticated
productions.

When we attempt to estimate the amount of structural difference between
the domestic races of the same species, we are soon involved in doubt,
from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the offspring of any single species, then such
facts would have great weight in making us doubt about the immutability
of the many very closely allied and natural species--for instance, of
the many foxes--inhabiting different quarters of the world. I do not
believe, as we shall presently see, that all our dogs have descended
from any one wild species; but, in the case of some other domestic
races, there is presumptive, or even strong, evidence in favour of this
view.

It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates