Chap. IX.] DEGREES OF STBEILITY. 3 



cies when crossed and of their hybrid offspring. It is 

 impossible to study the several memoirs and works of 

 those two conscientious and admirable observers, K61- 

 reuter 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 eases in which he found two forms, considered 

 by most authors as distinct species, quite fertile to- 

 gether, he unhesitatingly ranks them as varieties. Gart- 

 ner, also, makes the rule equally universal; and he dis- 

 putes 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 intervene: 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 (ex- 

 cluding 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 repeatedly 

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