MENDEL1SM AND ANCESTRAL INHERITANCE 371 



may pair, and, in various ways, assortative mating may come 

 about naturally. And whenever inbreeding sets in prepotency 

 develops i.e. peculiarities, even if trivial, gain great staying- 

 power in inheritance. (5) But even more important are the 

 facts disclosed by Mendel and his school, that crossing does not 

 tend to swamp new features, for if the hybrids be inbred there is 

 a persistent segregation of the parental type. A new mutant 

 crossed with a related form of contrasted character may be 

 dominant or recessive in the immediate hybrid (F 1 ), but in 

 either case, if the hybrids are inbred, it will reappear in pure 

 form in the next generation (F 2 ), and so forth. There is, how- 

 ever, no warrant for the common belief that hybridisation in 

 itself gives rise to new races. 



Mendelism in Relation to Ancestral Inheritance. It may 

 be that the conception of ancestral inheritance and the conception 

 of segregate parental inheritance apply to different sets of cases. 



1. At one extreme we may perhaps place cases of sterility, 

 where the fertilised egg-cell fails to develop, owing perhaps to 

 mutual incompatibility between the paternal and maternal 

 contributions. "The sterility of distinct species when crossed 

 is probably due to the confusion and disruption of the systems 

 of forces in the pronuclei of the germ-cells by antagonising 

 ancestral stimuli" (Dendy, 1903). 



2. It is possible that in some cases where a spermatozoon 

 enters an egg it fulfils one of its functions acting as a liberating 

 stimulus prompting the egg to develop and yet does not fulfil 

 its other function of contributing half of the inheritance. It is 

 possible that it is sometimes only the egg-nucleus which develops. 

 This possibility is suggested by some of the results of experi- 

 mental embryology e.g. that an egg may develop with only a 

 sperm-nucleus (merogony), or with only its own nucleus (artificial 

 parthenogenesis). 



3. Dendy suggests that those remarkable abnormal insects 

 (see Darwin, Variation of Animals and Plants under Domestica- 



