THEORY OF THE PURE LINE 119 



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, 



