THEORY OF THE PURE LINE 117 



inheritance. Barley and kidney beans were among 

 the plants examined, and the simplest character con- 

 sidered was the size of the seeds of the latter as 

 measured by weighing. In this particular series of 

 experiments each plant was regarded as being 

 characterized by the average weight of the seeds 

 which it produced. 



All the descendants arising from a single plant by self- 

 fertilization are spoken of by Johannsen as making up 

 a * pure line. 1 And the members of such a line showed, 

 in respect of the weight of their seeds, normal varia- 

 bility about a mean or type value. The general 

 population of bean plants, made up of a great number 

 of such pure lines, also exhibited a normal curve when 

 the weights of the seeds were plotted. The pure 

 lines composing such a population showed various 

 types, some of them close to the modal value of the 

 population, but others differing widely from it. If 

 now a somewhat widely deviating member of a par- 

 ticular line was selected for propagation, its off- 

 spring showed regression to the type of this par- 

 ticular line, and not to the mean value of the general 

 population. 



The case is indeed precisely similar to the supposed 

 example of a mixture of races of peas, which was made 

 use of as an illustration at the beginning of the present 

 chapter. In other words, a pure line consists of a 

 group of individuals which has a normal variability of 

 its own, and the offspring of which by self-fertilization 

 breed true to the type of their own particular group, 



