86 Self-Incompatibility in Hermaphrodite Plants 



descent. There are all grades in the degree to which both self-compati- 

 bility and cross-compatibility may appear, and even the reciprocal 

 matings of two hermaphrodites may give opposite results. 



It is clear, as it has been since the time of Darwin at least, that 

 within the species compatibility and fertility, both self and cross, is the 

 rule, and represents the primitive condition, and that incompatibilities 

 are the special cases. Cross-sterility within a species is a relatively 

 rare phenomenon, often accompanied by morphological modifications 

 in the sex organs such as dimorphism. Self-incompatibility is more 

 common, and has evidently a selection or hereditary value which leads 

 to its continuation and intensification whenever it appears. 



The very wide distribution of homomorphic species showing self- 

 incompatibilities (and also cross-incompatibilities to some degree) among 

 the families of flowering plants (Gramineae, Liliaceae, Papaveraceae, 

 Cruciferae, Rosaceae, Pomaceae, Amygdalaceae, Solanaceae, Scrophu- 

 lariaceae, Cichoriaceae, and Compositae, for example) suggests that the 

 condition has arisen many times independently, and may be one of 

 fundamental significance. The generally accepted view that inbreeding 

 and continued self-fertilization are of themselves injurious is certainly 

 favoured by the evidence that self-incompatibility is so obviously a pro- 

 gressive character. The application of this doctrine in these cases is 

 to some extent subject to experimental test, for it would seem that a 

 species whose members exhibit wide variation in self-compatibility affords 

 opportunity to test whether. continued selfing is of itself injurious. 



Furthermore the intensive study of the fertility of individuals that 

 are feebly self-compatible gives positive evidence as to how the condition 

 of self-sterility has arisen in species joriginally self-fertile. The marked 

 fluctuation of the character in practically all self-sterile species is 

 especially illuminating on this point. 



In continuing my experimental studies (1916, 1917, 1918) of physio- 

 logical incompatibilities further evidence has been obtained in support 

 of the following general conclusions, 



1, In all so-called self-incompatible species investigated, with the 

 exception of Raphamis sativus and the dimorphic species Linum grandi- 

 florum (which, however, have not been so extensively* studied), self- 

 incompatibility and self-compatibility are highly variable in their be- 

 haviour ; although the variations are generally indiscriminate, there are 

 cases of marked correlation with conditions of age or vegetative vigour 

 operating during the period of bloom, giving such results as "end- 

 season " and " mid-season " self-fertility. 



