SOCIAL AND WORLD ASPECTS 209 



transform any heteroz3"gous population into a homo- 

 zygous one, the rapidity with which the completel}' 

 homozygous condition is reached depending on the 

 system of inbreeding employed and the degree of 

 heterozygosity in the original population. Self- fertili- 

 sation, which is not uncommon in plants, is, of course, 

 the most stringent form of inbreeding, and it quickly 

 reduces any heterogeneous heterozygous population 

 to a completely homoz^'gous (but heterogeneous) con- 

 dition, unless some condition in the germ plasm, such 

 as balanced lethal factors, or some method of repro- 

 duction, such as apogamy. prevents the gametic 

 representatives of the different characters being freel}' 

 redistributed to the next generation. In wild species, 

 also, some of the Mendelian characters in the popula- 

 tion may be subject to natural selection, positiveh' 

 or negatively, and thus alter their proportions in the 

 population. This would lead to the elimination of 

 certain characters, even in a population mating at 

 random, although such characters are more dithcult 

 to eliminate from the germ plasm when recessive than 

 when dominant. In the same way the action of 

 natural selection would lead to the more rapid 

 multiplication and spread in the population of a 

 favoured character whether dominant or recessive. 



Laughlin (1920) has worked out the conditions of 

 ancestral inheritance in man, assuming twelve pairs 

 of chromosomes representing as man}^ pairs of linked 

 genes. Galton's original law of ancestral inheritance 

 is well known — namely, that the two parents con- 

 tribute 50 per cent. (0-5) of the inheritance, the four 

 grandparents 25 per cent, or (0-5)^, the eight great- 

 grandparents 12*5 per cent (0-5)^, etc., the total 

 heritage being (0-5) +(0-5)^ + (0-5)^+ . . . = 1. Karl 

 Pearson later modified this scheme, on the basis 

 of statistical data of correlation between father and 

 son, finding that the resemblance varied from 30 per 



