18 



SELECTION IN CLADOCERA ON THE BASIS OF 



Such differences in response indicate environmental factors of 

 great influence, but what these influences were was in most cases 

 not determined. It was frequently noted, in the tests at the begin- 

 ning of these experiments, that after the observer had tested several 

 broods in succession the broods tested later reacted less quickly than 

 the earlier ones. This may have been due to the accumulation in 

 the water of CO2 from the observer's breath. The difficulty was 

 obviated by changing the water in the experimental tank after testing 

 each four broods. 



These and other factors were much too influential for one to 

 ignore them completely, but their influence was presumably a 

 horizontal one, affecting all the individuals of a brood and of the 

 different strains in the same manner and not in any way influencing 

 selections, except as affecting the reaction-itme and in causing a few 



Table 2. — Data from an early test series of Line 695 to illustrate differences in general 

 readiveness to light on successive days. 



'This is mother No. 9 of the fifty -seventh generation of the minus strain of Line 695. 



individuals which otherwise would have been positive to be rendered 

 negative in their reactions. It seems wise to postpone the detailed 

 analysis of the effects of environment upon reaction-time and upon 

 vigor until after the general analysis of the data has been presented 

 (see pages 128-149). 



Test Series. 



Because of the various factors instrumental in influencing the 

 reactions of the different broods of Cladocera, and in order to obtain 

 a critical measure of the effect of selection, if such occurred, so-called 



