2 HYBRIDISM. [Chap. IX. 



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 yrhen first 

 crossed, and the sterility of the hybrids produced from 

 them. 



Pure species have of course their organs of reproduc- 

 tion 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 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 distinc- 

 tion is important, when the cause of the sterility, which 

 is common to the two cases, has to be considered. The 

 distinction probably has been slurred over, owing to the 

 sterility in both cases being looked on as a special 

 endowment, bevond the province of our reasoning 

 powers. 



The fertility of varieties, that is of the forms known 

 or believed to be descended from common parents, when 

 crossed, and likewise the fertility of their mongrel 

 offspring, is, with reference to my theory, of equal 

 importance with the sterility of species; for it seems 

 to make a broad and clear distinction between varieties 

 and species. 



Degrees of Sterility. — First, for the sterility of species 



