142 SELECTION IN CLADOCERA ON THE BASIS OF 



those means. This fact would seem a further confirmation of the 

 effect of selection within Line 757. 



Fluctuations for single two-month periods are common in the 

 data. They are presumably due to the same factors which produce 

 the fluctuations in reaction-time for longer periods. It is also true 

 that the number of reaction-time records for single two-month 

 periods is not sufficient to obtain highly reliable averages, and hence 

 fluctuations for single two-month periods are less significant than 

 those covering 3 or more such periods. 



Purely local differences in reactiveness might be expected to be 

 seen occasionally in results obtained from the different test series, 

 and such proves to be the case. The first test series for Line 695 

 gave a mean reaction-time for the plus strain 32 seconds greater than 

 that for the minus strain. The second test series (9 months later) 

 gave a mean for the minus strain 62 seconds the greater. The other 

 2 test series for Line 695 were in consecutive months, yet the differ- 

 ence in one was —1 second and the other +51 seconds. Such local 

 differences lend caution to one's interpretation of the result of test 

 series, and the results of test series are given credit only when they 

 show pronounced agreement and there is other confirmatory evidence. 



In the first test series for Line 757 the mean for the plus strain is 

 282 seconds lower than the mean for the selection data for the three- 

 month period during which this test series was conducted (table 45) ; 

 while during the second test series the mean for the plus strain was 

 133 seconds higher than that for the selection data for the contingent 

 three-month period. It is extremely probable that if another test 

 series had been conducted within a few weeks after the first one, the 

 mean for the plus strain would have been more nearly within the 

 range of the selection data for the plus strain for that period, and that 

 another test series conducted a few weeks after the second test series 

 would have given a much lower mean for the Line 757 plus strain. 

 The reaction-time differences between the two strains of Line 757 are 

 so large, however, that a wide departure from the usual reactiveness 

 of a strain still leaves a large difference in mean reactiveness between 

 the two strains. 1 



Whether "Depression Periods" Occur. 



These periods of relatively temporary drops or rises in the 

 reaction-time curves are probably in no case due to genetic influences. 

 In cases in which the reaction-time curves rise, the large and some- 

 what continuous lifts in the level of the curves may be ascribed by 



1 For the benefit of any who (in view of the fact that local environmental effects are seen 

 even in test series) might be inclined to wonder if additional test series would have shown such 

 large divergences between the two strains of Line 757, it may be added that in July 1919 and 

 again in December 1919 (27 and 32 months after selection was discontinued) extensive test series 

 were conducted in Line 757, each covering a period of and consisting of material from three 

 generations. In extremely few cases in these large series did a brood of the plus strain fail to 

 show markedly greater reactiveness than the corresponding brood of the minus strain. 



