Chap. IX.] DEGEEES OF STEEILITY. 3 



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 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 

 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 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 (excluding all cases such as the Leguminosa3, 

 in which there is an acknowledged difficulty in the 

 manipulation) half of these twenty plants had their 

 fertility in some degree impaired. Moreover, as 



