THEORY OF THE PURE LINE IIg 
produce a change in the average character of a popula- 
tion taken as a whole. Selection within a pure line 
produces no effect of this kind. The average character 
of the offspring of typical members of the line is the 
same as that of the offspring of members which show 
the widest deviations from the type. 
Selection in a population consists in the partial 
separation of those lines the types of which differ in 
the required direction from the average character of 
the population. This effect must of necessity come to 
an end when the most eccentric line is completely 
isolated. The great complications introduced when 
the lines are intermingled through mixed breeding may 
make this process of isolation a very tedious one. 
It will be seen that the values calculated by Pearson 
to represent the result of selection in a population 
agree quite well with Johannsen’s explanation of the 
constitution of such a population out of a number 
of pure lines. The result of Professor Johannsen’s 
further experiments will therefore be awaited with 
great interest by biologists and biometricians alike. 
On the theory of pure lines it is to be noticed that 
the personal character of a particular ancestor has 
no influence upon his descendants; it is only the 
type of the line to which he belongs which influences 
the offspring, so that this theory is in perfect agreement 
with Weismann’s theory of inheritance as described 
on p. 74. 
It is also to be observed that the principle of the 
pure line applies only to quantitative characters— 
such characters of size, or of weight, or of proportion, 
