44 SELECTION IN CLADOCERA ON THE BASIS OF 



given in the seventh vertical column of the tables of data summarized 

 by two-month periods (tables 5 and 6). 



The reproductive indices for the two strains are plotted in figure 

 Ic, in which the actual values are represented by the amount of 

 elevation above the base-line of the solid and open circles. The 

 solid circles represent the reproductive indices for the plus strain, 

 the open circles for the minus strain. The distance between the 

 solid circle and the open circle for any period represents the difference 

 in reproductive index, the solid circle or the open circle being elevated 

 the more above the base-line as the plus or the minus strain has the 

 larger reproductive index. In figure 2s the differences between the 

 reproductive indices for the plus and minus strains are shown, a 

 plus advantage being shown and its amount indicated by the eleva- 

 tion of the solid circle above the base-line and a minus superiority by 

 the amount of the depression of the open circle below the base-line. 



Throughout the first of the three periods of the curve for Line 

 695, during which there was a consistent difference between the 

 mean reaction-times of the two strains for as great a period as 6 

 months (August 1912- January 1913), the plus strain had the lower 

 reaction-time and was slightly, though only very slightly, the more 

 vigorous. But during the succeeding four months (February-May 

 1913; see figures 2s and 2c) the plus strain had a considerably 

 greater advantage over the minus strain in vigor and yet the mean 

 reaction- times of the two strains were very near together. The 

 fairly wide and rather consistent difference between the mean re- 

 action-times of the plus and minus strains for 6 months during 1912- 

 13 is not accounted for on this basis. 



For the ten-month period (August-September 1914 to April- 

 May 1915) during which time the plus strain continuously had a 

 higher reaction-time than the minus strain, the plus strain was 

 slightly the less vigorous of the two during 6 of the 10 months; but 

 during the other 4 months the reverse was true, and for the 10 months 

 as a whole the reproductive vigor of the two strains was practically 

 the same. For the seven-month period at the close of the experi- 

 ment, during which the plus strain had a considerably higher re- 

 action-time (was less reactive) than the minus strain, the plus strain, 

 judged by the measures applied to it, was actually considerably the 

 more vigorous of the two strains. 



Hence, of the three periods of the curve during which the two 

 strains differed consistently in reaction-time for as much as 6 months 

 at a time, only one of these differences, that for the second period, 

 and only a portion of it, could be accounted for on the basis of the 

 more vigorous strain having been the more reactive. 



Comparing by two-month periods for the entire duration of the 

 experiment, one finds that the strain of Line 695 which (on the basis 

 of its reproductive index) was the more vigorous had the lower re- 



