i dele) THE DOCTRINE OF DESCENT. 
were pronounced ex cathedré 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 
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 progeay 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 
