Chap. VIII. HYBRIDISM. 245 



CHAPTEE VIII. 



Hybeidism. 



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 

 confusion 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 im- 

 portant, inasmuch as the sterility of hybrids could not 

 possibly be of any advantage to them, and therefore 

 could not have been acquired by the continued preser- 

 vation of successive profitable degrees of sterility. I 

 hope, however, to be able to show that sterility is not 

 a specially acquired or endowed quality, but is inci- 

 dental on other acquired differences. 



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

 large extent fundamentally different, have generally 

 been confounded together ; namely, the sterility of two 



