DEGREES OF STERILITY. 261 



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 01- 

 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 distin«tion 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 admira- 

 ble 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. Kol- 

 reuter 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 fer- 

 tility 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 pro- 

 duced 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 intervene ; 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 were potted, and were 

 kept in a chamber in his house. That these processes are 

 often injurious to the fertility of a plant, cannot be doubted; 

 for Gartner gives in his table about a score of cases of plants 

 which he castrated, and artificially fertilized with their own 

 pollen, and (excluding all cases such as the Leguminosee, in 



