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THE AMERICAN NATURALIST [Vol. L 



Such strong environmental influences would, for the most 

 part, effectively confuse the various combinations of ger- 

 minal factors, and selection would continue to produce 

 slight advances for a long time. 



5. Castle has explained (Castle and Phillips, :14, p. 

 24) the significance of the . . observed reduction of 

 variability" for the multiple factor interpretation; he 

 stated at the same time that "... extensive modifica- 

 tion through selection is possible without any marked 

 falling off in variability." Since the observed reduction 



in variability is not considered to be marked, Fig. 1 is pre- 

 sented to show the facts graphically. The standard devi- 

 ations plotted in this graph have all been calculated di- 

 rectly from the data, and in several cases they differ 

 slightly from those given by Castle. The decrease in 

 variability that is shown by this figure is the expected re- 

 sult of reduced heterozygosity accompanying continued 

 selection. 



6. The question of the rate of advance has been an- 



