stout: pollinations in cichorium intybus 341 



bred origin. Darwin concluded that the sporadic character of 

 the phenomenon is an evidence of an incidental and abnormal 

 condition which he says "we may attribute to some change in the 

 conditions of life acting on the plants themselves or in their 

 parents." 



Darwin developed no formal hypothesis of sterility (physiologi- 

 cal incompatibility) like that of Jost or of Correns, but contented 

 himself with such vague statements as: "in most cases it is deter- 

 mined by the conditions to which the plants have been sub- 

 jected" ('77, p. 343). He emphasizes the variations in the self- 

 fertility of Eschscholtzia in different climates, but just how climatic 

 influences operate he does not attempt to say. Of the causes in 

 Reseda odorata, in which plants of the same parentage grown in 

 the same culture were self-sterile or self- fertile, he states "we are 

 forced in our ignorance to speak of the cause as due to spon- 

 taneous variability; but we should remember that the progenitors 

 of these plants, either on the male or the female side, may have 

 been exposed to somewhat different conditions" ('77, p. 344). 

 His main interest in the whole subject was undoubtedly from the 

 standpoint of the search for the causes of variations which may 

 become the material for natural selection in producing evolutionary 

 changes, but it is to be specifically noted that he considers that 

 the view that self-sterility (physiological self-incompatibility) "is 

 a quality which has been gradually acquired for the special 

 purposes of preventing fertilization must, I believe, be rejected" 

 ('77, p. 345), and that "we must look at it as an incidental result 

 dependent on the conditions to which the plants have been sub- 

 jected" ('77, p. 346). 



(c) From the above it is clear that Darwin distinguished the 

 phenomena of self-sterility from the decreased sterility which he 

 conceived to result from close genetic relationship. Darwin ('77) 

 presents data from extensive experiments with cross- and self- 

 fertilization of plants which he considers proof that the degree of 

 self-fertility in species regularly self-fertile is less than that of 

 cross-fertility, and that close inbreeding decreases the fertility. 

 This effect he considers is especially evident in the "innate fer- 

 tility" (general or potential power to produce seed judged by the 

 fertility of a plant when exposed to open pollination by insects), 

 which he finds is, as a rule, decidedly greater in plants raised from 



