222 ON THE ORIGIN OF SPECIES 



sterility. He always compares the maximum number 

 of seeds produced by two species when crossed and by 

 their hybrid offspring, with the average number pro- 

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

 But a serious cause of error seems to me to be here 

 introduced : 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 experiment- 

 ised on by Gartner were potted, and apparently 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 artifi- 

 cially fertilised with their own pollen, and (excluding 

 all cases such as the Leguminosae, 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 during several years 

 repeatedly crossed the primrose and cowslip, which 

 we have such good reason to believe to be varieties, 

 and only once or twice succeeded in getting fertile 

 seed ; as he found the common red and blue pim- 

 pernels (Anagallis arvensis and ccerulea), which the 

 best botanists rank as varieties, absolutely sterile to- 

 gether ; and as he came to the same conclusion in 

 several other analogous cases ; it seems to me that 

 we may well be permitted to doubt whether many 

 other species are really so sterile, when intercrossed, 

 as Gartner believes. 



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

 various species when crossed is so different in degree 

 and graduates away so insensibly, and, on the other 

 hand, that the fertility of pure species is so easily 

 affected by various circumstances, that for all practical 

 purposes it is most difficult to say where perfect fer- 

 tility ends and sterility begins. I think no better 

 evidence of this can be required than that the two most 

 experienced observers who have ever lived, namely, 

 Kolreuter and Gartner, should have arrived at dia- 



