240 THE ORIGIN OF SPECIES 



tinction 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 hy- 

 brid offspring. It is impossible to study the several memoirs and 

 works of those two conscientious and admirable observers, Kolreu- 

 ter 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 to- 

 gether, 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 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 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 Leguminosse, in which there is an acknowledged 

 difficulty in the manipulation) half of these twenty plants had 

 their fertility in some degree impaired. Moreover, as Gartner re- 

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

 pimpernels (Anagallis arvensis and coerulea), which the best 



