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? Has the