136 SELECTION IN CLADOCERA ON THE BASIS OF 



curves. In general, the temperature of the water in the experimental 

 tank was very much the same for August-September 1912 as for 

 June-July preceding. It is interesting to note that this August- 

 September period, because of poor food conditions, was a period of 

 great difficulty in maintaining our stock; mortality was high; most 

 broods were excessively small, usually 1 to 3 individuals; and sterility 

 was relatively common. Yet this is one of the periods of the greatest 

 drops in the curves (decrease in reaction-time) which occurred at any 

 part of the experiment with D. pulex. 



A second interesting contemporaneous change in the reaction- 

 time curves was a marked rise for all of the ten strains of D. pulex 

 for December 1913 to January 1914. Though the temperatures were 

 somewhat lower for this period than for the preceding period, it 

 should be noted that the reaction-time curves for D. pulex strains 

 remained very close to this high level for 8 months, namely, from 

 December 1913 to July 1914 (from early winter to midsummer). 

 Hence it seems impossible to attribute this fluctuation in the D. 

 pulex curves for the December 1913-January 1914 period to tem- 

 perature influences. 



Another local drop is seen in every one of the 10 curves for S. 

 exspinosus strains for the February-March 1915 period. This drop 

 is very large for 5 of these strains. Temperatures for this two- 

 month period were in general slightly lower than during the preceding 

 and succeeding periods, but as compared with other periods these 

 are comparatively slight temperature differences and the drops in 

 the curves can not be considerd primarily, if at all, due to the slight 

 temperature differences between these two periods. 



It is worthy of note that the contemporaneous shift in the re- 

 action-time curves affecting every one of the 10 strains of D. pulex 

 for December 1913-January 1914 period is not markedly reflected 

 in the 4 S. exspinosus strains (see figures 15 and 18b) and that the 

 profound drop in the curves for strains of Simocephalus for the last 

 period mentioned above is not in evidence for the strains of D. pulex. 

 This in itself would seem to indicate clearly enough that the effect 

 is not due to temperature influences per se. 1 



One might be tempted to assume that the first and third of 

 these profound shifts in the reaction-time curves were due to some 

 improvement in Cladocera stock due to favorable laboratory condi- 

 tions, inasmuch as the first appears after 4 months of selection in the 

 D. pulex lines and the third after 2 months of selection in 3 of the S. 

 exspinosus lines, except for the facts that (1) the D. pulex lines had 

 been under laboratory conditions for 33^ months before selection 

 began and hence a total of 7% months before these drops occurred, 

 and further, that they did not maintain these lower levels; (2) that 



1 The lack of contemporaneous shifts in the curves for strains of D. pulex and <S. exspinosus 

 is in line with physiological differences between the two species observed in other connections. 



