STERILITY OF HYBRIDS 239 



to be a simple recessive character. Nor can we now easily 

 suppose that the attempt there made by Darwin to suggest 

 resemblance between the sterility produced by unnatural condi- 

 tions and that of hybrids has any physiological justification. 



In regarding the power to produce a sterile or partially 

 sterile hybrid as a distinction in kind, of a nature other than 

 those which we perceive among our varieties, I am aware that 

 I am laying stress on an impression which may hereafter prove 

 false. The distinction nevertheless is so striking and so con- 

 tinually before the eyes of a practical breeder that he can 

 scarcely avoid the inference that when he meets a considerable 

 degree of sterility in a cross-bred he is dealing with something 

 belonging to a distinct category, and not merely a varietal feature 

 of an exceptional kind. 



Besides the sterility of hybrids appeal has often been made to 

 the phenomenon of incompatibility, in its several stages of 

 completeness, as distinguishing species. No one doubts that in- 

 compatibility may arise from a variety of causes of most diverse 

 degrees of importance, but though sometimes referred to as an 

 extreme case of interspecific sterility, it is really a very different 

 matter. In regard to one phase of this incompatibility, that 

 associated with self-sterility, some progress has been made, and 

 we are not wholly without experimental evidence of its being 

 within the range of contemporary variation. 



Given the outline of Mendelian teaching as to gametic dif- 

 ferentiation and the classification of individuals in a mixed 

 population, it seemed highly probable that what we call self- 

 sterility must mean that the species really consisted of classes, 

 some of which are capable of interbreeding with others while 

 others are not. According to the received account every indi- 

 vidual, though incapable of fertilising itself, was supposed to be 

 able both to fertilise and to be fertilised by any other individual. 

 This notion has always seemed to me a self-evident absurdity, 

 for it would imply that there can be as many categories as 

 individuals. Such experiments, however, as I made did cer- 

 tainly give results consistent with that belief. I first tried 

 Cinerarias, which are usually self-sterile, but I found no in- 



