130 SELECTION IN CLADOCERA ON THE BASIS OF 



water was ordinarily used soon after it reached approximately the 

 room temperature; hence it was probably near saturation for the 

 principal gases which had been thrown out of solution. Whether 

 the gases retained in solution were equally oxygen and other gases 

 would of course depend upon the actual amounts of individual gases 

 within the water when taken from the pond. Assuming that the 

 pond-water was ordinarily near saturation for oxygen when brought 

 into the laboratory, after it had given up a portion of its contained 

 gases (as it always did) it would presumably be saturated for oxygen. 

 Since the amount of oxygen at saturation at 25° C. is about 26 per 

 cent less than at 10° C, this is a basis for the suggestion of an ex- 

 planation for the greater reactiveness at the lower temperatures. 

 No specific tests have been made to determine if this is the correct 

 interpretation, and the results of the summer correlations for D. 

 pulex do not fit in with this explanation. 



There is also the possibility that temperature influenced reaction- 

 time indirectly through its effect upon the food in the culture-water. 

 The water used as culture-water went through the changes in tem- 

 perature (and chemical constitutents) to which a shallow out-door 

 pond is subject in the climate of Cold Spring Harbor and the develop- 

 ment of the protista upon which the Cladocera feed is naturally 

 influenced by temperature as well as other environmental factors. 

 Before being used the culture-water (during its successive strainings) 

 was kept for approximately 6 days indoors in jars surrounded by 

 running water. The temperature of this running water varied with 

 out-door temperatures, but was much less extreme. Hence there is 

 also room for the assumption of influences of temperature upon the 

 Cladocera stock, not directly as a temperature influence per se, but 

 indirectly through the culture medium, which is a veritable micro- 

 cosm. 



2. Influence of Substances Exhaled from Observer's Breath. 



That the amount of carbon dioxide was actually an important 

 factor in influencing reaction-time is suggested by the fact that if 

 several broods were tested successively in the experimental tank 

 without change of water the later broods reacted markedly less 

 promptly than the ones tested in the same water an hour earlier. 2 

 The young Cladocera are so small as to be just visible in the illumina- 

 tion in the experimental tank, so that one must get the eyes at about 

 reading distance from the water of the tank in order to observe the 

 young animals. As a consequence, the exhaled air very largely 



1 The spring water is known to contain relatively a large amount of CO2. 



2 This was observed a number of times, after which the plan of frequently changing the 

 water in the experimental tank was adopted to obviate this disturbing influence. It seemed 

 impracticable to change the water oftener than after testing each four broods, although it was 

 sometimes noted that the last brood or two of the four tested between changes of water was less 

 reactive than broods tested just after the change. 



