102 



SELECTION IN CLADOCERA ON THE BASIS OF 



During the year August 1915- July 1916 (36 generations and 

 505 individuals, and 39 generations and 486 individuals, respectively) 

 the reaction-time means for the two strains showed considerably 

 greater divergence (table 45). The difference was 3 12.0 11.91 

 seconds, 26.2 times the statistical probable error. The 8 same-day 

 broods for the same period (table 46) gave a mean difference of 

 270 21. 34 seconds, 12.65 times the probable error. With one 

 exception, each of the 8 same-day broods for the minus strain had a 



750 



600 



450 



300 



ISO 



00 



A 



A 



ll-l 

 912 



4-5 8-9 

 1913 



12-1 



4-5 8-9 

 1914 



12-1 



4-5 8-9 

 1915. 



12-1 



4-5 8-9 

 I9IQ 



12-1 



4 

 1917 



FIGURE 18. Line 757. 



A. Reproductive indices, superiority. 



B. Reaction-time curves with composite curves for all the other Simocephalus plus and 

 minus strains (from December 1914) superimposed. 



higher reaction-time by from 84 to 546 seconds. The test series 

 conducted during April 1916 consisted of 652 individuals of the plus 

 strain and 677 individuals of the minus strain (table 45). In every 

 one of these 12 pairs of broods the brood from the minus strain had 

 the higher reaction-time. The mean reaction-time for each strain 

 was relatively high, presumably because of unusual environmental 

 conditions. The means were 697.0 seconds for the plus strain and 

 864.6 seconds for the minus strain. The difference (-167.6 9.12 

 seconds) was 18.38 times the probable error, a difference unmistakably 

 significant. It can scarcely be doubted that if this test series had 



