3( 



A PHYSIOLOGICAL CHARACTER. 



69 



the more reactive, the means being 269 and 482 seconds and the 

 difference -213 30.42 seconds. There is, moreover, an interesting 

 difference noted between every plus brood and every minus brood 

 tested during this period. Without exception the plus broods were 

 considerably more reactive than the minus broods tested at the 

 nearest dates. Such a consistent difference in reactiveness involving 

 every individual brood 1 is not generally found in the data obtained 

 from these selection experiments, except in portions of the data for 

 Line 757, in which a pronounced effect of selection was obtained. 



750 r 



600 



450 



300 



12-1 



1913 



4-5 8-9 



12-1 



1914 

 4-5 8-9 



12-1 



\ 



1915 

 4-5 8-9 



12-1 



1916 

 4-5 8 



4-5 8-9 

 1912 



12-1 



4-5 8-9 

 1913 



12-1 



4-5 8-9 

 1914 



12-1 



4-5 8-9 

 1915 



12-1 



4-5 8-9 

 1916 



FIGURE 10. 



A, B, C. Reaction-time curves for Daphnia longispina Lines 762, 766, and 768, respectively. 

 D. Composite reaction-time curves for all plus and all minus strains of Daphnia pulex 

 with reaction-time curves for Line 751 superimposed. 



The divergence in reaction-time means for this last two-month 

 period and particularly the complete and unusual consistency of 

 differences in reaction-time, brood by brood, for this last period 

 suggest an effect of selection. The amount of these data is too small 

 to be more than slightly suggestive, but it seems quite possible that 

 a mutation occurred in the plus strain of Line 762, thus producing 



1 lt will be remembered that temporary enviromental conditions are very potent factors 

 in modifying the reaction-time means of individual broods and that consequently rather wide 

 fluctuations occur in all the data. Hence, only in cases in which the reactiveness of the two 

 strains differs greatly do we fail to find the reaction-time means for. different broods of the two 

 strains of a line overlapping to a wide degree. 



