A PHYSIOLOGICAL CHARACTER. 147 



the fact of the identity of environmental conditions before selection 

 was begun and the diversity of environmental conditions as affecting 

 the two strains later (or as due to their being in different portions of 

 their cycles at the same time later). However, some of the strains, 

 e. g., Lines 695 and 689, on the whole show greater differences in 

 reproductive indices during the first 4 months than later (see figures 

 Ic and 3a). 



Is THE Increased Reactiveness for Lines of S. exspinosus Associated with 



Increased Vigor? 



In view of the increase in reactiveness in the S. exspinosus lines 

 during the course of the experiments, it is of interest to examine the 

 reproductive indices to see if similar or other changes are seen in 

 them. 



With the 4 lines of Daphnia pulex (Lines 689, 695, 714, and 719) 

 for which reproductive indices are worked out, there are in general 

 reproductive indices somewhat below the average for the first 2 

 two-month periods, very low reproductive indices for the following 

 2 periods (during the obviously poor food conditions), and there- 

 after the reproductive indices are somewhat fluctuating, but in 

 general fall about slightly higher means throughout the remainder 

 of the experiment (figures Ic, 3 a, 7a, and 8a). Hence with the lines 

 of D. pulex the reproductive index increased somewhat during the 

 course of the experiment. 



The reproductive indices for Line 740 show marked increases 

 for the two-month periods just following the first 4 months of selec- 

 tion. These early low reproductive indices are clearly related to 

 the poor food conditions prevalent during the early months of 

 selection with this line and reflected even more strongly in the D. 

 pulex lines which had been in the laboratory for several months 

 previously. There is also a rise in reproductive index for Line 757 

 after the first 2 months, and this rise is only partly coincident with 

 recognized differences in food conditions in the laboratory. This 

 rise is capable of explanation as due to the cumulative effect of 

 better food conditions in the laboratory than in the outdoor ponds, 

 but practically the maximum rise in reproductive index for this line 

 was realized at once (figure 17c) ^ and there is no evidence for a 

 cumulative effect of better food conditions. The reproductive 

 indices for Line 795 are much higher during the latter part of the 

 experiment than earlier (figure 12a), but the reverse is true for Line 

 796 (figure 13a). On the whole, the reproductive indices for S. 

 exspinosus do not show any general increase or decrease during the 

 course of the experiment. 



Thus it is seen that reproductive indices for the D. pulex strains 

 increase somewhat during the course of the experiment, while re- 



'The general level for the plus strain was not higher for the remainder of the experiment, 

 though the general level for the minus strain increased somewhat further. 



