THE PURE LINE AND SELECTION 129 



of individuals composing a pure line from a group 

 making up a population, since both may be phenotypi- 

 cally alike. Fluctuations about the average occur in 

 both cases with no appreciable difference in character, 

 although such fluctuations, when they occur within a 

 pure line, are simply somatic differences caused in 

 general probably by modifications in nutrition or some 

 other external factor of environment, while fluctua- 

 tions in a population include not only modifications of 

 this transient nature, but also permanent hereditary 

 differences due to germinal differences in the various 

 pure lines of which the population is composed. 



Johannsen has made the distinction between pure 

 lines and populations clear by the following figure 

 (Fig. 26), in which five pure lines of beans are com- 

 bined artificially to form a population. 



The beans which make up the pure lines noted in 

 this figure are represented inclosed within inverted 

 test tubes. The beans in any single tube are all of 

 one size. Tubes vertically superimposed upon each 

 other also contain only beans of one size. 



Thus it is seen that what may be a rare size of 

 bean in one line, for instance that in the left-hand 

 tube of pure line 3, may be identical with the com- 

 monest size in another line, as pure line 2. The five 

 pure lines represented in Figure 26 are combined in 

 a population at the bottom of the figure. In this 

 population array the five pure lines are hidden. 



Hence, while selection within a pure line has no he- 

 reditary influence, it is evident that selection within a 

 population may shift or move over the type of the 



