278 BTBRIDISM. 



themselves are perfect in structure, as far as the niicro- 

 scope 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 2)robably has been slurred 

 over, owing to the sterility in both cases being looked on 

 as a special endowment, beyond the province of our reason- 

 ing 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 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 when crossed and of 

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

 eral memoirs and works of those two conscientious and ad- 

 mirable observers, Kolreuter and Gartner, who almost 

 devoted their lives to this subject, without being deeply 

 impressed with the high generality of some degree of ster- 

 ility. Kolreuter makes the rule universal; but then he 

 cuts the knot, for in ten cases in which he found two forms, 

 considered by most authors as distinct species, quite fertile 

 together, he unhesitatingly ranks them as varieties. Gart- 

 ner, also, makes the rule equally universal; and he disputes 

 the entire fertility of Kolreuter's ten cases. But in these 

 and in many other cases, Gartner is obliged carefully to 

 count the seeds, in order to show that there is any degree 

 of sterility. He always compares the maximum number 

 of seeds produced by two species when first crossed, and 

 the maximum produced by their hybrid offspring, with 

 the average number produced by both pure parent-species 

 in a state of nature. But causes of serious error here in- 

 tervene: a plant, to be hybridized, must be castrated, and, 

 what is often more important, must be secluded in order 

 to prevent pollen being brought to it by insects from other 

 plants. Nearly all the plants experimented on by Gartner 



