CHAPTER IX 



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 ster- 

 ility 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 — Parallel- 

 ism between the effects of changed conditions 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 con- 

 fusion. This view certainly seems at first highly probable, 

 for species living together could hardly have been kept dis- 

 tinct 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 acquired, as I shall show, by the 

 preservation of successive profitable degrees of sterility. It 

 is an incidental result of differences in the reproductive sys- 

 tems of the parent-species. 



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

 extent fundamentally different, have generally been con- 

 founded; namely, the sterility of species when first crossed, 

 and the sterility of the hybrids produced from them. 



Pure species have of course their organs of reproduction 

 in a perfect condition, yet when intercrossed they produce 

 either few or no offspring. Hybrids, on the other hand, have 

 their reproductive organs functionally impotent, as may be 

 clearly seen in the state of the male element in both plants 

 and animals; though the formative organs themselves are 



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