16 
VAKIATION 
Chap. T. 
from each other, and from other species of the same 
genus, in several trifling respects, they often differ in an 
extreme degree in some one part, both when compared 
one with another, and more especially when compared 
with all the species in nature to which they are nearest 
allied. With these exceptions (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, 
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 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 perpetu¬ 
ally 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 widely in deter¬ 
mining 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 difl’erences in our domesticated productions. 
When we attempt to estimate the amount of struc¬ 
tural 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 
