OEIGIN OF SPECIES. 



CHAPTEE IX. 



Hybeidism. 



Distinction between the sterility of first crosses and of hybrids — 

 Sterility various in degree, not univei'sal, affected by close inter- 

 breeding, removed 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 conditions of life 

 and of crossing — Dimorphism and trimorphism — Fertility of 

 varieties when crossed and of their mongrel offspring not uni- 

 versal — Hybrids and mongrels compared independently of their 

 fertility — Summary. 



The view commonly entertained by naturalists is that 

 species, when intercrossed, have been specially endowed 

 with sterility, in order to prevent their confusion. This 

 view certainly seems at first highly probable, 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 acquired, as I shall 

 show, by the preservation of successive profitable 



