MEN DELI SM 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. Mendelian ex- 

 periments show the possibilities of arriving at novelties by com- 

 binations of unit- characters, and it is interesting here to refer 

 to the extreme view of Lotsy (see p. 604) that all new races arise 

 by hybridisation. 



Mendelian and Statistical Methods. There seems no sound 

 reason for pitting these two methods against one another. In 

 his Modes oj Research in Genetics (1915), Dr. Raymond Pearl 

 writes as follows : " Hereditary differences behave, in the main, 

 as discrete units, which are shuffled about and re-distributed to 

 individuals, in the course of the hereditary process, to a con- 

 siderable extent independently of each other." Statistical or 

 biometric methods study the distribution of the hereditary 

 differences in one way, Mendelian methods in another. " The 

 biometric method studies the ancestry of the individual, while 

 the Mendelian method studies the individual's progeny. One 

 goes backward on the pedigree ; the other goes forward. The 

 network of descent may be likened to two pencils of light rays 

 both of which focus in the individual. The ancestral pencil 

 converges upon the individual. The progenial pencil diverges 

 from the individual." " The statistical method is a logically 

 necessary adjunct to the experimental method." 



When we pass, however, from biometric methods in general 

 to particular formulations, such as Galton's Law (see p. 328), 

 we have to recognise that blending inheritance and Mendelian 

 or alternative inheritance cannot be forced into one generalised 

 statement, and that the Law of Ancestral Inheritance really 



