234 DEGKEES OF STERILITY. Chap. YIII. 



feet ; in the second case they are either not at all developed, 

 or are imperfectly developed. This distinction 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, owiniii; to the sterility in botli cases being' looked 

 on as. a special endowment, Ijoyond the province of our reason- 

 ing powers. 



The fertility of vaiietics, that is of the forms known or be- 

 lieved to have descended from common parents, when inter- 

 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 dis- 

 tinction between varieties and species. 



Defjrees of Sterility. — First, for the sterility of species when 

 crossed and of their hybrid offspring. It is impossible to study 

 the several memoirs and works of those two conscientious and 

 admirable observers, Kolrcuter and Gartner, who almost de- 

 voted their lives to this subject, without being deeply im- 

 pressed with the high generality of some degree of steriUty. 

 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 un- 

 hesitatingly ranks them as varieties. Gartner, also, makes the 

 rule equally universal ; and he disputes the entire fertility of 

 Kolrcuter'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 off- 

 spring, with the average number produced by both pure par- 

 ent-species in a state of nature, lint a serious cause of error 

 seems to me to be here introduced : a plant, to be hybridized, 

 nuist be castrated, and Avhat is often more imjiortant, must be 

 secluded in order to prevent pollen being brought to it by in- 

 sects 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 Giirtner gives in his table 

 about a score of cases of plants Avhich he castrated, and arti- 

 ficially fertilized with their own pollen, and (excluding all cases 

 such as the Leguminosa', in which there is an acknowledged 

 difliculty in the manipidation) half of these twenty plants had 

 their fertility in some degree impaired. Moreover, as Giirlncr 



