344 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



The general term "differentiation" is used rather indiscrimi- 

 nately by Darwin, as it is quite generally, to apply to (a) different 

 parts of the same individual, (b) to individuals as a unit, (c) to 

 groups of individuals constituting a strain, a species, or a larger 

 group. The nature, interrelations and fluctuations of these 

 different kinds of differentiation involve, we may note, some of 

 the most interesting of the unsolved biological problems of today. 



It is clear, as will be noted more fully later, that the differ- 

 entiation is of different kinds and of different degrees, and that it 

 fluctuates in its expression through fertility both in self- and cross- 

 fertilization. It would appear that in the lesser fertility of inbred 

 stock compared with that of cross-bred stock, which did appear in 

 some instances, Darwin saw a lack of constitutional differentiation 

 due to the more intimate relationship of parts concerned. It is 

 clearly evident, however, that he did not place the development 

 of self-sterility (physiological incompatibility) on any such basis. 

 He attempts thus to distinguish complete sterility (from physi- 

 ological incompatibility) from that which he thought is associated 

 with decreased vegetative vigor due to inbreeding, and to ascribe 

 only the latter to lack of differentiation from close relationship. 



Darwin was not fully aware that, aside from "illegitimate 

 pollination," cross-sterility between plants of the same species 

 and of seed origin can exist quite as decidedly as does self-sterility. 

 He had quite clearly shown that intercrossing (inbreeding within 

 a variety), especially if the plants had been grown for some time 

 under similar conditions, may not give increased fertility and vigor, 

 and thus be quite similar to the assumed results of self-fertilization. 

 While in general Darwin assigned the causes of sterility to lack 

 of constitutional differentiation in sex, he did not relate the pecu- 

 liar condition of self-incompatibility to the decreased fertility 

 assumed to arise from close relationship, but to a sporadic response 

 to influences of en\ironment. Furthermore, he failed to see any 

 relation between the self-sterility in non-dimorphic species as 

 Eschscholtzia californica and that in dimorphic forms, a point 

 that will be discussed later. 



Attempts were made early in the study of self-sterility (self- 

 incompatibility) to determine the causes and to analyze the con- 

 stitutional conditions through a comparative study of the growth 



