CHAPTER VIII 



HYBRIDISM 



Distinction between the sterility of first crosses and of hybrids- 

 Sterility various in degree, not universal, affected by close inter- 

 breeding, removed by domestication— Laws governing the sterility 

 of hybrids — Sterility not a special endowment, but incidental 

 on other differences— Causes of the sterility of first crosses and 

 of hybrids— Parallelism between the effects of changed con- 

 ditions of life and crossing— Fertility of varieties when crossed 

 and of their mongrel offspring not universal— Hybrids and 

 mongrels compared independently of their fertility— Summary. 



The view generally entertained by naturalists is that 

 species, when intercrossed, have been specially endowed 

 with the quality of sterility, in order to prevent the con- 

 fusion of all organic forms. This view certainly seems 

 at first probable, for species within the same country 

 could hardly have kept distinct had they been capable 

 of crossing freely. The importance of the fact that 

 hybrids are very generally sterile, has, I think, been 

 much underrated by some late writers. On the theory 

 of natural selection the case is especially important, inas- 

 much as the sterility of hybrids could not possibly be of 

 any advantage to them, and therefore could not have 

 been acquired by the continued preservation of succes- 

 sive profitable degrees of sterility. I hope, however, to 

 be able to show that sterility is not a specially acquired 

 or endowed quality, but is incidental on other acquired 

 differences. 



In treating this subject, two classes of facts, to a large 

 extent fundamentally different, have generally been con- 

 founded together ; namely, the sterility of two species 



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