BREEDING SYSTEMS 



worked on the basis of a scries of multiple allelomorphs, and in fact 

 it depends on two unlinked genes. One decides the difference 

 between the short-style and the not-short-style. The other has no 

 action on short-style, but decides whether a not-short-style shall be 

 mid- or long-style. It is a case of epistasy. 



The same genetic and physiological organization as with hetero- 

 styly is revealed by CapscUa grandifora. The pollen depends for its 

 behaviour on its diploid parent, and two gene differences work 

 the device as in Lytlirnm. But morphologically the flowers of the 

 different types are all alike. The heterostyly is cryptic. Thus the 

 physiological difference, which in the other instances goes with a, 

 difference of position, can be achieved without the help of such 

 a difference. 



Mating Discrimination 



The diversity of outbreeding mechanism in plants, dioecyJ 

 monoecy, protandry, incompatibility in its various forms, is not] 

 matched in animals. With them dioecy, sexual differentiation of the] 

 diploid organism, is the basic device almost without exception. Even 

 the hermaphrodite oyster can separate its sexes in time by the cyclical! 

 succession of male and female phases, and thereby become effectively 

 dioecious. Dioecy excludes the extreme inbreeding mechanism of 

 self-fertilization so common in plants. But, as we saw, dioecy can! 

 be determined by many sex-chromosome and other mechanisms,] 

 and on it can be superimposed a number of devices working both 

 ways, favouring either inbreeding or outbreeding. We have already 

 noticed intra-uterine copulation and first cousin marriage as favour- 

 ing inbreeding. We can now consider the outbreeding mechanisms, j 



Animals move about, but their movement is limited. Dioecy 

 forbids self-fertilization, but it does not forbid brother-sister mating 

 such as is most likely where eggs are laid in clutches. In some 

 insects, e.g. Sciara, the dimg-fly, this incest is avoided by the unisexual 

 brood. By delayed action, as with heterostyly, the mother directly 

 determines the sex of her offspring, which may be all male or all 

 female. The brood of one sex is thus excluded from incest by the 

 generic properties of the mother. 



The habits of individuals or the laws of society may have the , 

 same effect on the breeding system as delayed gene action. Where- 



^5^ 



