8 THE ORIGIN OF SPECIES 



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 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 distinction 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, beyond the prov- 

 ince 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 off- 

 spring, is, with reference to my theory, of equal impor- 

 tance 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 when crossed and of 

 tbeir hybrid offspring. It is impossible to study the sev- 

 eral memoirs and works of those two conscientious and 

 admirable observers, Kolreuter and Gartner, who almost 

 devoted their lives to this subject, without being deeply 

 impressed with the high generality of some degree of 



