124 Self-Incomimtlhility in Hermaphrodite Plants 



formed. The latter is especially involved in what Darwin calls "relative 

 fertility," the former is especially represented in " innate fertility." An 

 extreme type of loss of relative fertility is seen in self- and cross-incom- 

 patibility of plants of such species as Eschscholtzia cali/ornica, Nicotiana 

 Forgetiana and Cichorium Intyhus. 



In regard to the fertility of cross-bred stock we now have the state- 

 ment of East and Park (1918, p. 527) that "neither Shull nor East has 

 maintained that crossing increases fertility. The number of flowers and 

 fruits is often ilicreased but no data have appeared which indicate a 

 decreased percentage of non-functional gametes." East, who has had 

 wide experience in studies of inbred and cross-bred stock, thus considers 

 that increased seed production observed in certain cross-bred progeny 

 is due solely to an increase in the number of sex organs. If this be true 

 it appears that heterosis, at least in species in which no compatibilities 

 are already in evidence, does not influence compatibilities of sex organs 

 and does not raise relative fertility, a conclusion fully reached by the 

 more recent report by Jones (1918). 



One may well hesitate therefore before assuming that any observed 

 case of decreased productivity in selfed stock involves an actual loss in 

 relative fertility. Such cases may involve solely the number of sex 

 organs produced and the perfection of their development, as appears to 

 be the condition in certain inbred strains of corn. 



At the present time there is no direct or even conclusive indirect 

 evidence that physiological incompatibility with selective fertilization 

 (at least selective for definite hereditary units) and variations in relative 

 fertilities arise in a species as a direct result of inbreeding and repeated 

 self-fertilization. There is therefore some question regarding the validity 

 of the old doctrine that there is a tendency inherent in hermaphrodites 

 for the sex organs to be non-functional together simply because they 

 are produced by a single individual and that foreign pollen is naturally 

 prepotent. The accumulation of evidence that inbreeding is not neces- 

 sarily injurious has lead to the view that decreased vegetative and 

 reproductive vigour in inbred stock is due to an inherently weak 

 constitution existing before inbreeding was begun (see especially 

 Jones, 1918). This of course still leaves the question open as to the 

 sources and causes of the cases of weak constitution which do appear in 

 all sorts of species. 



A question of special consideration is whether inbreeding and self- 

 fertilization in species (homomorphic) in which incompatibilities are 

 already in evidence lead to an increase of incompatibilities. According 



