A PHYSIOLOGICAL CHARACTER. 103 



been conducted during any of the following months (under usual 

 environmental conditions), when the plus strain was more reactive, 

 the plus strain would have had a much lower mean and the minus 

 strain possibly a somewhat, but probably not much, lower mean, so 

 that the difference would have been much larger. 1 It is interesting, 

 however, to have this test series come at a time of relatively high 



450 



150 



5-10 11-4 5-10 11-4 5-IO 



1914 1915 1916 



FIGURE 19. Line 757. 



Reaction-time curves by six-month periods with similar curves for Line 740 superimposed . 



As noted on pages 87 and 88, the minus strain of Line 740 was exceptionally slightly reactive 

 compared with the other Simocephalus minus strains (Line 757 excepted) during the period 

 from April to September 1916, while the 740 plus strain was exceptionally reactive during Octo- 

 ber 1916 to January 1917. Hence the point representing the mean|for the minus strain for the 

 "5-10" 1916 period is exceptionally high, while the point for thejfplus^mean for^the "11-4" 

 period following is exceptionally low. 



reaction-time for the plus strain and yet find the difference in means 

 so markedly significant. 



In the final nine-month period (August 1916-April 1917) of the 

 experiment with Line 757, the mean for the minus strain was double 



1 The mean for the minus strain in this test series was only 5 seconds higher than during 

 the earlier test series conducted 2f years earlier (September 1913). On the other hand, the 

 mean for the plus strain was 288 seconds higher in the later than in the earlier test series. Yet, 

 in general, the plus strain during the latter part of the experiment was very much more reactive 

 than during the earlier portion of the experiment. The explanation of these peculiar means 

 compared with the means for the selection data lies in the following facts: (1) The plus strain 

 of Line 757 was more reactive than normally during the first test series. This was, in part at 

 least, due to local experimental conditions. Of the 15 broods tested in making selections (in the 

 laboratory strains of the different species) during the period when this test series was conducted, 

 the broods in 1 1 strains had lower reaction-time means than the combined means for the immedi- 

 ately preceding and next succeeding broods of the same strains and the differences were much 

 larger than in the 4 cases in which the reverse relation held. Hence it is clear that the first test 

 series for Line 757 was conducted at a time when local environmental conditions induced ab- 

 normally low reaction-time averages in most of the Cladocera strains. (2) During the second 

 test series the plus strain of Line 757 was relatively slightly reactive. The test-series mean for 

 the plus strain was 697 seconds, a larger mean than had occurred in the selection data for 14 

 months and larger than occurred at any later two-month period of the experiment. Two broods 

 used in selection tests in 757 plus soon after this test series was conducted had similar averages, 

 but of the other 27 broods of this strain tested during the remainder of the selection experiment 

 only one had an average as high as that for this entire test series. It seems clear that the 757 

 plus strain was in a relatively slightly reactive condition at the time of the second test series. 



