Chap. VIII. STERILITY. 247 



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 crossed and by their hybrid offspring, with the 

 average number produced 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 experimentised 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 

 artificially fertilised with their own pollen, and (exclud- 

 ing all cases such as the Leguininosae, 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 concluson 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 Gart- 

 ner believes. 



