60 SELECTION IN CLADOCERA ON THE BASIS OF 



The minus strain of Line 714 was on the whole slightly more 

 vigorous than the plus strain. 1 The plus strain was superior in vigor 

 during 13 two-month periods; the minus strain more vigorous during 

 14 two-month periods. The differences in favor of the minus strain 

 were somewhat greater than those in favor of the plus strain. 



There is no apparent relation between vigor and reaction-time. 

 Casual inspection of the reaction curves and the reproductive index 

 figure (figure 7) gives this impression, and the following facts bear 

 it out: During 11 periods the temporarily more vigorous strain was 

 the more reactive, during 15 periods the more vigorous strain was 

 the less reactive, and during 1 period the reaction-times were the 

 same. Selecting the 9 periods during which the mean reproductive 

 indices differed by more than 0.30, it is found that in 5 periods the 

 more vigorous strain is the less reactive, in 3 periods the more 

 vigorous strain is the more reactive, and for 1 period the mean re- 

 action-times were the same. Selecting the 9 two-month periods 

 during which the reaction-time means differed by more than 100 

 seconds, it is found that in 2 cases these large differences in the mean 

 reaction-time correspond with periods in which the more reactive 

 strain was also the more vigorous, but that in 7 cases the more re- 

 active strain was the less vigorous of the two. 



There is an interesting feature of the curves (figure 7c) in the 

 marked influence of environment upon the mean reaction-time of the 

 two strains. In spite of considerable irregularities of the curves for 

 the two strains, they follow each other to a remarkable extent. 

 Nothing other than environmental influences would seem to account 

 for such a coincidence. 



Line 719. 



The data for Line 719 will be found in tables 23, 24, 25, and 26 

 and in figure 8. This is one of the original lines of Daphnia pulex 

 taken into the laboratory in November 1911. Selection was begun 

 (March 28, 1912) in the eighth laboratory generation and continued 

 for 130 and 138 generations, until both strains were lost in June 1915. 



Consultation of the curves (figure 8c) shows that there is no 

 effect of selection; some interesting features of this data, however, 

 will be brought out by a somewhat detailed analysis. 



For the first 4 months the mean reaction-times for the plus and 

 minus strains were 450.1 and 493.1 seconds. The difference is 

 — 43 ± 31.8 seconds (table 25). For the same-day broods (only 3) the 

 corresponding data are 718 and 539 seconds, the difference being 

 + 179 ± 76.00 seconds. 



1 The average reproductive index (obtained by dividing the sum of the reproductive indices 

 by two-month periods by the number of two-month periods) for the plus strain was 1.006, for 

 the minus strain 1.034. 



