DEGREES OF STERILITY 299 



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 arc 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 prob- 

 ably 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 con- 

 scientious 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 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 their 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 from other plants. Nearly all the plants experi- 

 mented on by Gartner were potted, and were kept in a 

 chamber in his house. That these processes are often in- 

 jurious to the fertility of a plant cannot be doubted; for 



