xceptions (and with that of the perfect fertility of
varieties when crossed--a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner as
do the closely allied species of the same genus in a state of nature,
but the differences in most cases are less in degree. This must be
admitted as true, for the domestic races of many animals and plants have
been ranked by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere varieties.
If any well marked distinction existed between a domestic race and a
species, this source of doubt would not so perpetually recur. It has
often been stated that domestic races do not differ from each other in
characters of generic value. It can be shown that this statement is not
correct; but naturalists differ much in determining what characters are
of generic value; all such valuations being at present empirical. When
it is explained how genera originate under nature, it will be seen that
we have no right to expect often to find a generic amount of difference
in our domesticated races.

In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are 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 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 closely
allied natural species--for instance, of the many foxes--inhabiting the
different quarters of the world. I do not believe, as we shall presently
see, that the whole amount of difference between the several breeds of
the dog has been produced under domestication; I believe that a small
part of the difference is due to their being descended from distinct
species. In the case of strongly marked races of some other domesticated
species, there is presumptive or even strong evidence that all are
descended from a single wild stock.

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