134 SELECTION IN CLADOCERA ON THE BASIS OF 



mean reaction-times of the plus and minus strains. Likewise, with 

 the other D. longispina Hne, Line 768, the curves (figure 10c) for the 

 two strains, despite irregularities, follow the same general course. 



With the S. exspinosus material the general influence of environ- 

 ment upon the mean reaction-times of the two strains of the same 

 line is less conspicuous than with the lines of the two species of 

 Daphnia, but it is evident in most of the curves and is particularly- 

 clear for the curves for the final several months of the experiments. 



The curves for Line 794 (figure lie) show this influence to a 

 marked degree, in spite of the irregular portions of the curve for the 

 minus strain. Because of the unusually irregular curve in Line 795 

 (figure 12c), the environmental influence is less in evidence, but 

 there probably is a small positive correlation between the reaction- 

 time means for the two strains by the two-month periods of the 

 experiment. In Line 796 (figure 13c) the curves follow each other 

 in a general way. 



With Line 740 (figure 15) the curves on the whole follow each 

 other to a remarkable degree. A clearer demonstration of the effect 

 of environmental influence working through relatively long periods 

 would be hard to find. The correlation between the mean reac- 

 tion-times for the plus and minus strain by two-month periods is 

 0.6437 ±0.0733. The relatively shorter range of the up-and-down 

 movements of the curves for this line renders this correlation 

 smaller than one would otherwise expect from the relatively close 

 parallelism of the two curves, but, considering the limited num- 

 ber of terms involved in the correlation, it is very large. 



In Line 757 (figure 18b) the environmental effect is somewhat 

 in evidence in the courses of the two curves, but naturally it is 

 obscured by the marked effect of selection, particularly during the 

 last 21 months of the experiment; yet, even in this latter period, in 

 spite of the wide divergence of the curves, there is an evident rela- 

 tion between the two which seems intelligible only as an effect of 

 environmental influence. The correlation for the reaction-time 

 means for the two strains is 0.4283 ±0.1041. 



A composite curve plotting the mean reaction-times for all the 

 plus strains and another for all the minus strains of D. pulex is shown 

 in figure IOd. The courses of the curves for the strains of any single 

 D. pulex line, despite irregularities, strikingly resemble the corre- 

 sponding portions of this composite curve. Likewise, the few re- 

 action-time curves for strains of D. longispina (see figure 10, a, b, 

 and c) to some extent follow the same course as the composite re- 

 action-time curve (figure IOd) for the D. pulex strains. 



A composite curve of the reaction-time means for all the plus 

 strains and one for all the minus strains of S. exspinosus (except 

 Line 757) is shown in figure 18b. The environmental influence is well 

 shown in the generally fairly coincidental courses of the two curves, 



