254 HYBRIDISM. Chap. VIII. 



largely accepted by modern naturalists ; namely, that 

 most of our domestic animals have descended from two 

 or more aboriginal species, since commingled by inter- 

 crossing. On this view, the aboriginal species must 

 either at first have produced quite fertile hybrids, or 

 the hybrids must have become in subsequent genera- 

 tions quite fertile under domestication. This latter 

 alternative seems to me the most probable, and I am 

 inclined to believe in its truth, although it rests on no 

 direct evidence. I believe, for instance, that our dogs 

 have descended from several wild stocks ; yet, with per- 

 haps the exception of certain indigenous domestic dogs 

 of South America, all are quite fertile together; and 

 analogy makes me greatly doubt, whether the several 

 aboriginal species would at first have freely bred to- 

 gether and have produced quite fertile hybrids. So 

 again there is reason to believe that our European and 

 the humped Indian cattle are quite fertile together ; 

 but from facts communicated to me by Mr. Blyth, I 

 think they must be considered as distinct species. On 

 this view of the origin of many of our domestic animals, 

 we must either give up the belief of the almost uni- 

 versal sterility of distinct species of animals when 

 crossed ; or we must look at sterility, not as an in- 

 delible characteristic, but as one capable of being re- 

 moved by domestication. 



Finally, looking to all the ascertained facts on the 

 intercrossing of plants and animals, it may be con- 

 cluded that some degree of sterility, both in first crosses 

 and in hybrids, is an extremely general result ; but that 

 it cannot, under our present state of knowledge, be con- 

 sidered as absolutely universal. 



Laws governing the Sterility of first Crosses and of Hy- 

 brids. — We will now consider a little more in detail the 



