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 v/olves with the dogs 

 of savage nations, and of the European wolf with the 

 Hungarian 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 be- 

 long 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 different species; and 

 because 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 with its ancestral stock, 

 nor does the tame European guinea-pig with the wild 

 ancestral stock of Brazil. 



But even if, in general, crosses between varieties are 



