100 THE DOCTRINE OF DESCENT. 



were pronounced ex cathedra to be varieties of the same 

 species, as their crosses are productive. But after read- 

 ing Darwin's careful comparison of the reports as to the 

 relations of certain species of wolves with the dogs of 

 savage nations, and of the European wolf with the Hun- 

 garian dog, we must agree with Darwin in thinking it as 

 extremely probable that in various parts of the world, and 

 at various periods, wild species of the genus Canis were 

 domesticated, of which the crosses produce fertile progeny 

 to an extent almost unlimited. 



It is the same with the domestic cat. With the forms 

 of the European domestic cat, the case is such that it 

 is scarcely possible to doubt its origin partly from a 

 Nubian species, and partly from the European wild-cat. 

 The inferences thus moved in a circle; forms belong 

 to the same species, because they may be fruitfully 

 crossed; and because they may be fruitfully crossed, 

 they belong to the same species; and, on the other hand, 

 because such and such forms, when crossed, produce no 

 fertile progeny, they constitute dififerent species; and be- 

 cause they are different species, they generate no fertile 

 offspring. The cases of persistent fertility in hybrids are 

 certainly not frequent, but they are nevertheless so well 

 certified that the contrary statement is in plain contra- 

 diction to the facts. But conversely, the proposition that 

 mongrels, the products of crosses among varieties, are 

 fertile, thus generally stated, is likewise untenable. The 

 variety which has been evolved in Paraguay from our 

 domestic cat, pairs no longer v^ith its ancestral stock, nor 

 does the tame European guinea-pig with the wild an- 

 cestral stock of Brazil. 



But even if, in general, crosses between varieties are 



