The Factors of Evolution 



237 



availed nothing to increase or decrease the size of each * ' pure 

 line" as Professor Johannsen called the progeny of his nine- 

 teen seeds. ''Can a man add one cubit unto his stature," or 

 a bean one millimeter unto its length? Professor Johannsen 

 thinks not. 



POPULATION 



DIAGRAM SHOWING THE EFFI CT OF SELECTION UPON A MIXED LOT OP 



BEANS OR "POPULATION" 



If several thousand beans, selected at random, are sorted ac- 

 cording to size and placed in test tubes, the lower figure will result; 

 those beans of an average size will be greatest in number, while 

 the larger and smaller sizes will be fewest. If beans from different 

 test tubes be selected for planting the "pure lines" 1, 2, 3, etc. will 

 result. Further breeding of these ' ' pure lines ' ; will not serve to change 

 the average size of the population, selection serving merely to sort out 

 the varieties jumbled together in a mixed "population." From Walter, 

 after Johannsen. 



Similar results have been obtained by several American 

 workers, notably Professor Pearl, who, while at the Maine 

 Agricultural Experiment Station, attempted by selection to 

 increase the laying capacity of hens and failed; and Pro- 

 fessor Jennings of Johns Hopkins, who has likewise shown 

 that selection is powerless to change the size of the one-celled 

 animal Paramoecium. 



