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SELECTION IN CLADOCERA ON THE BASIS OF 



ENVIRONMENTAL CONDITIONS AS AFFECTING REACTION- 

 TIME AND VIGOR OF STOCK. 



Any relation between environmental influences and reactive- 

 ness to light is a fact of general biological significance and is a matter 

 of importance in the present connection. In the light tests with 

 Cladocera little-understood environmental influences were important 

 factors in influencing reaction-time in all three species which were 

 subjected to selection. These environmental influences are dis- 

 cussed under the following heads: (1) temperature influences; (2) 

 influences of exhaled substances from the observer's breath; (3) 

 relatively temporary chemical (?) differences in the water used in 

 the experimental tank; (4) occurrence of negatively reacting indi- 

 viduals; (5) general influences operative through longer periods of 

 time; (6) influence of vigor. 



1. Temperature Influences. 



The temperature of the water in the experimental tank was a 

 minor factor influencing reaction-time. This influence was not 

 marked enough to be evident to the observer in the tests day by 

 day, but the somewhat lower reaction means during the winter 

 periods indicate and correlation between temperature and reaction- 

 time demonstrates that lower temperatures are to some extent 

 associated with greater reactiveness and a lower reaction-time mean. 



The temperature of the water in which the animals were tested 

 was always near that of the basement room in which the tests were 

 conducted. Fortunately this room was less affected by fluctuations 

 in summer temperatures than the remainder of the building, but in 

 winter temperatures ran fairly low, —10° to 15° C, inasmuch as 

 this room was not provided with much artificial heat. 



Correlations between individual reaction-times (50-second 

 classes) and temperature (0.5° C. classes) of the water for all the 

 tested individuals of strains of D. pulex cover the following periods 

 and give the following values : 



