130 EVOLUTION AND GENETICS 



smallest changes that add to or subtract from a part 

 in the smallest measurable degree may also arise 

 by mutation. We identify these smaller mutational 

 changes as the most probable variants that make a 

 theory of evolution possible both because they do 

 transcend the original types, and because they are 

 inherited. If there are other kinds of heritable varia- 

 tions than mutants, it seems scarcely possible that they 

 should have been overlooked; for, many thorough- 

 going studies of variation have now been made. 



Pure Lines 



The work of the Danish botanist, Johannsen, pub- 

 lished in 1909, furnishes the most critical evidence re- 

 latins' to the inheritance of variations that has as vet 

 been obtained. There are, moreover, special reasons 

 why the material that he used is better suited to give 

 definite information than any other so far studied. 



Johannsen worked with a garden bean (Phaseolus 

 vulgaris nana ) , weighing the seeds or else measur- 

 ing them. The plant multiplies by self-fertilization. 

 Taking advantage of this fact Johannsen kept the 

 seeds of each plant separate from the others, and 

 raised from them a new generation. When curves 

 were made of these new groups it was found that 

 some of them had different modes from that of the 

 original general population {fig. 62, A-E, bottom 

 group) . They are shown in the upper groups {A,B, 



