236 HYBRIDISM. [Chap. IX, 



two cases, has to be considered. Tie 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 gene- 

 rality 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 both pure parent- 

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

 vene : 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 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 fertilised with their own pollen, and (excluding all cases 

 such as the Leguminosse, in which there is an acknowledged diffi- 

 culty in the manipulation) half of these twenty plants had their 

 fertility in some degree impaired. Moreover, as Gartner repeatedly 

 crossed some forms, such as the common red and blue pimpernels 

 (Anagallis arvensis and ccerulea), which the best botanists rank as 

 varieties, and found them absolutely sterile, we may doubt whether 

 many species are really so sterile, when intercrossed, as he believed. 



It is certain, on the one hand, that the sterility of various speciea 

 when crossed is so different in degree and graduates away so in- 



