42 THE ORIGIN OF bPECIES 



although (lilTiM-ing from each other, and from other spe- 

 cies of the same genus, in several trilling respects, they 

 often differ in an extreme degree in some one part, both 

 when compared one with another, and more especially 

 when compared with the species under 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 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 do- 

 mestic 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 interest- 



