332 ORIGIN OF SPECIES 



the systematic affinity of the forms subjected to experiment; 

 for systematic affinity includes resemblances of all kinds. 



First crosses between forms known to be varieties, or suffi- 

 ciently alike to be considered as varieties, and their mon- 

 grel offspring, are very generally, but not, as is so often 

 stated, invariably fertile. Nor is this almost universal and 

 perfect fertility surprising, when it is remembered how 

 liable we are to argue in a circle with respect to varieties 

 in a state of nature ; and when we remember that the greater 

 number of varieties have been produced under domestication 

 by the selection of mere external differences, and that they 

 have not been long exposed to uniform conditions of life. It 

 should also be especially kept in mind, that long-continued 

 domestication tends to eliminate sterility, and is therefore 

 little likely to induce this same quality. Independently of the 

 question of fertility, in all other respects there is the closest 

 general resemblance between hybrids and mongrels, — in their 

 variability, in their power of absorbing each other by re- 

 peated crosses, and in their inheritance of characters from 

 both parent-forms. Finally, then, although we are as ig- 

 norant of the precise cause of the sterility of first crosses 

 and of hybrids as we are why animals and plants removed 

 from their natural conditions become sterile, yet the facts 

 given in this chapter do not seem to me opposed to the belief 

 that species aboriginally existed as varieties. 



