228 ON THE ORIGIN OF SPECIES 



whole flocks of these crossed geese are kept in various 

 parts of the country ; and as they are kept for profit, 

 where neither pure parent -species exists, they must 

 certainly be highly fertile. 



A doctrine which originated with Pallas, has been 

 largely accepted by modern naturalists ; namely, that 

 most of our domestic animals have descended from two 

 or more wild 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 

 perhaps 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 together 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 

 removed by domestication. 



Finally, looking to all the ascertained facts on the 

 intercrossing of plants and animals, it may be concluded 

 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 

 Hybrids. — We will now consider a little more in detail 



