HERITAGE OF THE INDIVIDUAL 303 



Pure Lines 



The importance of this point was discovered by careful 

 experiments on the inheritance of characters in single pure 

 lines; particularly those of Johannsen on inheritance in a 

 brown variety of the common garden Bean. For example, by 

 keeping the progeny of each individual bean separate from 

 that of all the rest, he was able to isolate a number of pure 

 lines which differed in regard to the average weight of the 



Fia. 156. Schematic representation of the effect of selection from the viewpoint- 

 of Galton's 'law of filial regression.' (/) Mode before selection; 2, 3, 4, new (successive) 

 modes, the results of selections of individuals at 2', 3', 4'. The mode has been shifted 

 in the direction of selection (toward the right) . But there has been each time an amount 

 of regression indicated by the length of the arrows. 



beans. Selection did nothing but resolve the species, or the 

 bean 'population' with which he began, into its constituent 

 'weight types,' or lines, each of which exhibited a characteris- 

 tic variability curve of its own with a mode departing more 

 or less from that of the population. But when Johannsen 

 selected within a pure line (ruled out combinations) nothing 

 at all resulted; he was unable to shift the mode because he 

 was dealing with nonheritable characters. In other words, 

 the effect of selection is one of isolation and not creation. As 

 a rule it sorts out pre-existing pure lines (lines with homo- 

 geneous germinal constitution) from a population and then 

 stops though if selection is stopped the isolated lines usually 

 soon merge again into the original population. A mutation 



