THE ORIGIN OF SPECIES 



CHAPTER IX 



HYBRIDISM 



Distinction between the sterility of first crosses and of hybrids — Sterility 

 various in degree, not universal, affected by close interbreeding, re- 

 moved by domestication — Laws governing the sterility of hybrids — 

 Sterility not a special endowment, but incidental on other differences, 

 not accumulated by natural selection — Causes of the sterility of first 

 crosses and of hybrids — Parallelism between the effects of changed con- 

 ditions of life and of crossing — Dimorphism and trimorphism — Fertility 

 of varieties when crossed and of their mongrel offspring not universal — 

 Hybrids and mongrels compared independently of their fertility — 

 Summary 



THE view commonly entertained by naturalists is that 

 species, when intercrossed, have been specially en- 

 dowed with sterility, in order to prevent their 

 confusion. This view certainly seems at first highly prob- 

 able, for species living together could hardly have been 

 kept distinct had they been capable of freely crossing. 

 The subject is in many ways important for us, more 

 especially as the sterility of species when first crossed, 

 and that of their hybrid offspring, cannot have been ac- 

 quired, as I shall show, by the preservation of successive 

 profitable degrees of sterility. It is an incidental result 

 of differences in the reproductive systems of the parent- 

 species. 



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

 extent fundamentally different, have generally been con- 



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