44 HEREDITY [ch. 



processes which determine the transmission of char- 

 acters from one generation to another. 



Another argument that has been used against the 

 physiological validity of the law of ancestral heredity 

 is based on the work of Johannsen and others who 

 have obtained results similar to his in other cases. 

 Johannsen worked at the inheritance of weight of 

 seeds in beans and in barley, and self-fertilised the 

 plants investigated for a series of generations so as 

 to isolate what he calls ' pure lines.' He found that 

 in beans, for example, the seed-weights of a mixed 

 population gave a normal frequency curve — the 

 weights varied continuously and evenly about a mean 

 value. The beans on an individual plant when the 

 flowers are self-fertilised also form a normal curve 

 about a mean, but this mean is not necessarily identical 

 with that of the race in general. If now the flowers 

 on such an individual are self-fertilised, and the beans 

 produced are sown, the mean weight of the beans on 

 all the daughter plants will be identical with the 

 mean of the beans on the parent, i.e. among the 

 offspring produced by self-fertilisation there is no 

 regression towards the mean of the race. It thus 

 makes no difference whether large or small seeds are 

 chosen within the pure line ; the mean weight of the 

 seeds on plants grown from the smallest and largest of 

 the parental beans (seeds) is in each case equal to the 



