A PHYSIOLOGICAL CHARACTER. 



53 



750 



This is one of the original lines of D. pulex obtained from Pond 

 II, the spring-fed pond in the woods, during November 1911. Se- 

 lection was begun in the sixth laboratory generation and continued 

 for 15 months, for 45 generations in the plus and 46 generations in 

 the minus strains. 



For the first 4 months of the experiment with this line the 

 means were, for the plus strain 488.7 seconds (52 individuals) and 

 for the minus strain 588.2 seconds (49 individuals). The difference 

 ( 99. 5 43.1 seconds) was 2.31 times the probable error (table 17). 

 The mean reaction-time for the plus strain for the first two-month 

 period was 244 seconds less than that for the minus strain, but for 



the second two-month period the 

 minus strain had the lower reaction- 

 time by 63 seconds (tables 15 and 

 16). There were 3 same-day broods, 

 for which those of the plus strain had 

 a higher mean reaction-time by 42 

 seconds (table 18). The data for this 

 first 4 months of selection are not as 

 satisfactory as that obtained later 

 and wide fluctuations are less sur- 

 prising. 



For the remaining year (nearly) 

 of the experiment with this line, the 

 mean for the plus strain was 241.2 

 seconds (235 individuals) ; that for the 

 minus strain was 286.9 seconds (190 

 individuals). The same-day broods, 

 of which there were 6, have means 

 differing by only 7 seconds. Exam- 

 ination of the data (tables 15 and 16) 

 and the curves (figure 5) for this line 

 shows consistent differences in mean 

 reaction-times during this year-period. The wide divergence in mean 

 reaction-times for August-September is a chance result due to there 

 being extremely few tested individuals during this period. But for 

 the following 5 two-month periods the numbers of tested individuals 

 were greater and the rather uniform courses of the curves are sugges- 

 tive of a difference due to selection. 



Comparison of the curves for the means of the two strains of 

 this line with the combined mean reaction-time curves for Lines 

 691, 695, 713, 714, and 751 (in which genetic differences presumably 

 did not arise) shows (figure 5) that the plus strain of Line 711 after 

 the first 4 months of the experiment was somewhat more reactive 

 than the other plus strains. The means for the minus strain in 

 general were higher than the combined means for the other minus 



00 



190 



FIGURE 5. Line 711. 

 Reaction-time curves with superim- 

 posed curves representing the combined 

 reaction-time means for the plus and 

 minus strains of all Daphnia pulex lines 

 in which significant reaction-time differ- 

 ences did not arise. 



