Chap. IX. Hybridism. 235, 



CHAPTEE 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 r 

 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 

 universal — 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 suc- 

 cessive 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 confounded ; 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 per- 

 fect 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 perfect in structure, as far as the microscope 

 reveals. In the first case the two sexual elements which go to form 

 the embryo are perfect ; in the second case they are either not at all 

 developed, or are imperfectly developed. This distinction is im- 

 portant, when the cause of the sterility, which is common to the 



