8 Permanence and Evolution. 



to have pointed out, for instance, that the 

 existence on the two sides of Behring's Straits 

 of the American and Old World forms of fox, 

 and the American and Old World forms of 

 wolf, are to all appearance facts of the same 

 kind. But a clear and firm conception of this 

 is just as consistent with a tendency to make 

 stricter as to make looser our notions of per- 

 manence of type. There is no difficulty in 

 believing that at the first formation of animals 

 there existed independently, not only what we 

 happen to call distinct species, but also sub- 

 breeds, and strains still more faintly charac- 

 terised. 



It may be taken as generally known and 

 admitted, that the power to produce fertile 

 hybrids often does not run in close conformity 

 with morphological affinity. The causes of 

 sterility are in the present state of our know- 

 ledge very obscure. We know that some fertile 

 individuals of the same species are incapable of 



