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.” 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, 
