THE PURE LINE 115 



would be arranged historically from a genetic point 

 of view to show their true origin one from another. 



9. THE DISTINCTION BETWEEN A POPULATION 

 AND A PURE LINE 



A mixture of pure lines has been called a popula- 

 tion. 



It is not possible to distinguish a pure line from a 

 population by inspection, since both may be pheno- 

 typically 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 differ- 

 ences caused in general probably by modifications 

 in nutrition or some other external factor of environ- 

 ment, while fluctuations 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. 40), 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 



