INBREEDING AND OUTBREEDING DEVICES 



in the grass-mite, Pediculopsis, copulation of brothers and sisters 

 occurs in the uterus of the mother with the same regularity as it 

 can do in experimental breeding. Further, in man, where brother- 

 sister mating is normally prohibited by social conventions, the same 

 conventions in some societies ensure a regular first-cousin marriage. 

 This, too, when starting with heterozygotes, will bring about 

 homozygosis, though it will do so only very slowly. 



In short, heterozygosity will at once follow any lapse from 

 haploid or diploid self-fertilization, brother-sister mating or cousin 

 marriage. Homozygosity will be gradually recovered when the 

 system of inbreeding is restored. But it will be recovered at different 

 rates, each characteristic of its own system. Hence, where a propor- 

 tion of sexual unions habitually depart from the strict inbreeding 

 habit (as they do even in peas and barley) homozygosity and 

 heterozygosity will always exist side by side : they will be in equili- 

 brium. This liyhridity eqiiilihriiiin will depend on (i) the rate of 

 recovery of homozygosity, which, as we have seen, depends in turn 

 on the type of inbreeding, (ii) the frequency of lapses from 

 inbreeding, and (iii) the amount of heterozygosity produced by 

 lapses, which depends in turn on (iv) the amount of genetical 

 variation in the breeding group, which of course is influenced by 

 mutation. These varying factors jointly constitute what has been 

 called the closeness of inbreeding of the group. 



Inbreeding and Outbreeding Devices 



The hybridity equilibrium can be controlled in a great variety 

 of ways, so great a variety that their common effect is usually over- 

 looked. This is particularly true of the flowering plants. To begin 

 with, the primary function of the pollen mechanism is that it allows 

 crossing; other secondary functions we shall see later. Superimposed 

 on this mechanism we fmd devices favouring both selfmg and 

 crossing;. 



Mechanisms favouring close inbreeding generally depend on the 

 development and structure of the flowers. The failure of flowers 

 to open (deistogamy), best known in Viola, is widespread, and 

 obviously favours self-fertilization. In other cases (like our peas and 

 barley) the flower opens, but only after the pollen has been shed 



LUmciitsofGendiiS 24.1 O 



