64 



SELECTION IN CLADOCERA ON THE BASIS OF 



reduction in amount of this difference during the last year of the 

 experiment. 



Additional evidence of a significantly lowered reaction-time for 

 the minus strain of Line 719 is obtained by comparison with the 

 minus strains of the other D. pulex lines which showed no evidence 

 of modification in reactiveness during the selection experiments, 

 Lines 691, 695, 713, 714, and 751. A composite curve for the re- 

 action-time mean for the minus strains of these lines compared with 



1.50 

 1.00 

 .50 



00 



8 I 



4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-6 



1912 1913 1914 1915 



_4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-6 



600 h 



450 



300 



150 



FIGURE 8. Line 719. 



A. Reproductive indices, actual values. 



B. Reproductive indices, superiority. 



C. Reaction-time curves with a superimposed curve (in faint broken line) representing the 



combined reaction-time means for the minus strains of all Daphnia pulex lines in which 

 significant reaction-time differences probably did not arise. 



the mean for the minus strain of Line 719 (figure 8c) shows that 

 beginning with August 1912 and continuing to January 1914 the 

 mean for the minus strain of Line 719 was lower than that for the 

 combined mean for the other minus strains for the same two-month 

 periods in 8 of 9 two-month periods; 6 of these differences (ranging 

 from 48 to 197 seconds) were of statistical value and one other just 

 escaped statistical significance because of the small numbers of 

 reaction-time records involved. From February 1914 the minus 



