OKIGI]^ OF SPECIES. 



CHAPTER IX. 



HYBRIDISM. 



Distinction between the sterility of first crosses and of hybrids^ 

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

 terbreeding, 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 trjmorphism — 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 en- 

 dowed with sterility, in order to prevent their confusion. 

 This view certainly seems at first highly probable, for 

 species living together could hardly have l)een kept dis- 

 tinct had they been capable of freely crossing. The 

 subject is in many ways important for us, more especia l-^ 

 ly as the sterility _of sp ecies when firs t_ crossed,_ andthat 

 "oTtheir hybrid.afifspringj^cannot have been acquiredT^T" 

 shall show, by the preservation of suceissTve ^roStabl? 



