220 HYBRIDISM, [CHAP. IX. 



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 

 perfect condition, yet when intercrossed they produce either few 

 or no offspring. Hybrids, on the other hand, have their repro- 

 ductive 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 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 when 

 crossed and of their hybrid offspring. It is impossible to study 

 the several 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 sterility. 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. Gartner, also, makes the rule equally universal; and 

 he disputes the entire fertility of Kb'lreuter'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 pro- 

 duced by both pure parent-species in a state of nature. But 

 causes of serious error here intervene : a plant, to be hybridised, 

 must be castrated, and, what is often more important, must be 

 secluded in order to prevent pollen being brought to it by insects 



