406 CONCLUDING REMARKS. Chap. XXVIII. 



tendency to sterility when crossed, has been shown to be 

 highly probable or almost certain. We cannot avoid this 

 conclusion when we reflect on the parentage and present 

 fertility of the several breeds of the dog, of the Indian or 

 humped and European cattle, and of the two chief kinds of 

 pigs. Hence it would be unreasonable to expect that races 

 formed under domestication should acquire sterility when 

 crossed, whilst at the same time we admit that domestication 

 eliminates the normal sterility of crossed species. Why with 

 closely -allied species their reproductive systems should almost 

 invariably have been modified in so peculiar a manner as to 

 be mutually incapable of acting on each other — though in un- 

 equal degrees in the two sexes, as shown by the difference in 

 fertility between reciprocal crosses of the same species — we 

 do not know, but may with much probability infer the cause 

 to be as follows. Most natural species have been habituated to 

 nearly uniform conditions of life for an incomparably longer 

 time than have domestic races; and we positively know that 

 changed conditions exert an especial and powerful influence 

 on the reproductive system. Hence this difference may well 

 account for the difference in the power of reproduction between 

 domestic races when crossed and species when crossed. It is 

 probably in chief part owing to the same cause that domestic 

 races can be suddenly transported from one climate to 

 another, or placed under widely different conditions, and yet 

 retain in most cases their fertility unimpaired ; whilst a 

 multitude of species subjected to lesser changes are rendered 

 incapable of breeding. 



The offspring of crossed domestic races and of crossed 

 species resemble each other in most respects, with the one im- 

 portant exception of fertility ; they often partake in the same 

 unequal degree of the characters of their parents, one of 

 which is often prepotent over the other ; and they are liable 

 to reversion of the same kind. By successive crosses one 

 species may be made to absorb completely another, and so 

 it notoriously is with races. The latter resemble species in 

 many other ways. They sometimes inherit their newly- 

 acquired characters almost or even quite as firmly as species. 

 The conditions leading to variability and the laws governing 



