USE AND DISUSE. REVERSION, AND HYBRIDISM. 75 



that the rule of sterility is not absolute. On 

 the other hand, races of the same species, 

 though widely separated, produce fertile, though 

 often degenerate offspring. 



Individuals resulting from the crossing of 

 species, are termed " hybrids," and those pro- 

 duced by the crossing of races, ''mongrels." 

 Darwin attributed the degeneracy of half-breeds, 

 to reversion to a more barbarous condition of 

 humanity,* attributable to the crossing ; though 

 moral causes must operate as well. Yet close 

 interbreeding is well known to be highly pre- 

 judicial, and the crossing of races not too 

 widely separated, is certain greatly to improve 

 the breed of any domestic anima], except so far 

 as it perils its more unstable and most recently 

 acquired characters, both by the direct influence 

 of the cross, and by the impetus which it gives 

 to reversion. 



Again, when a species is first domesticated, 

 the change of food and habits not unfrequently 

 affects its constitution so unfavourably as to 

 render it sterile ; though when an animal has 



* "Variation of Animals and Plants under Domestication," vol. i., pp. 

 46, 47. 



