170 



CARL DOWNEY LA RUE 



From the nature of the method of selection, spores near the extremes 

 of the range of variation were chosen, and accordingly the difference 

 between the selected spore of the minus line, and that for the plus line in a 

 given generation, was always large. The lengths of the selected spores 

 for the experiments are shown in figure 8. The differences between the 

 parent spores of the two groups does not increase significantly during the 

 experiment, nor does it decrease. At the time the cultures were transferred 

 to America the lengths of the parent spores decreased considerably but the 

 deviations between them were unchanged. To what extent the decrease 

 in spore length was due to change of culture medium, and to what extent 

 due to change of climate, is unknown. 



FIGURE 8. Graphs of the lengths of individual spores selected as parents of the plus and 

 minus line of experiment 3. P, the plus selections; M, the minus selections. The ordinates are 

 lengths in /*; the abscissae, the successive generations. 



The mean spore lengths of the generations of this experiment are shown 

 in table 8. The removal of the cultures to America resulted in a decrease 

 in mean spore length which is clearly shown in figure 9. The plus and 

 minus lines did not parallel each other so closely as in experiments 1 and 2. 

 This is to be expected because the selections in the two lines were not 

 usually made at the same time. The two lines were consequently not in 

 harmony in regard to the conditions under which they were developed and 

 their fluctuations were not parallel. In five of the ten generations the 

 minus line has a greater spore length than the plus-selected one. That 

 this result is due to selection is unthinkable. It serves as a warning that 

 small changes, apparently in the direction of a selection effect, should not 

 too readily be considered as due to selection, though they have usually 

 been so interpreted. 



