110 SELECTION IN CLADOCERA ON THE BASIS OF 



than the plus strain in one case and a practically equal reactiveness 

 in the other case — a thing which did not occur elsewhere in the 

 experiment — and yet the plus strain was markedly superior in vigor. 



3. There were 10 two-month periods during which the vigor of 

 the plus strain was quite above the average (figure 17c) the repro- 

 ductive index being more than 1.50. The mean reaction-time for 

 these periods of high vigor was 546.8 seconds, 36 seconds less than 

 the mean for the plus strain for the entire period of the experiment. 

 There were 9 corresponding periods of high vigor for the minus 

 strain, for which the mean reaction-time was 768 seconds, 23 seconds 

 greater than the mean for the entire period for this strain. There 

 were 9 two-month periods during which the plus strain had a repro- 

 ductive index under 1.25, somewhat below the average. For these 

 periods the general average reaction- time was 634.9 seconds, 52 

 seconds above the average for the entire experiment. For 12 periods 

 of correspondingly low reproductive indices in the minus strain the 

 mean reaction-time was 770.8 seconds, 26 seconds above the average 

 for the whole experiment. Hence in 3 of these 4 comparisons the 

 mean reaction-times varied in the direction to indicate some relation 

 between actual vigor and mean reaction-time. In one case the 

 reverse is true. 



4. A fairer method of making a comparison between relative 

 vigor and mean reaction-times is a detailed period-by-period exami- 

 nation of the data for reproductive index and the mean reaction- 

 time. Comparing the mean reaction-time for the plus strain for the 

 10 periods of high reproductive indices (above 1.50) with the reaction- 

 time means for the adjacent points in the curve (each representing 

 two-month periods), it is seen (figures 17c and 18b) that in 2 cases 

 the reaction- time means are higher than in the adjacent points in 

 the curve, in 3 cases intermediate, and in 5 cases lower. For the 9 

 periods of like high reproductive indices for the minus strain, the 

 mean reaction- times are higher than in the adjacent portions of the 

 curves in 5 cases, intermediate in 1 case, and lower in 3 cases. A 

 similar comparison for 9 periods of low reproductive index (below 

 1.25) in the plus strain shows that in 5 cases the mean reaction-time 

 is higher than in adjacent points in the curve, and in 4 cases it is 

 lower. For the 12 periods of like low reproductive indices in the 

 minus strain the mean reaction-times are higher than in adjacent 

 portions of the curve in 3 cases, intermediate in 5 cases, and lower in 

 4 cases. These comparisons indicate a possible slight relation be- 

 tween mean reaction-time and reproductive vigor for the plus strain, 

 high vigor being more often associated with greater reactiveness and 

 a lower reaction-time; but the reverse relation is more strongly 

 indicated for the minus strain, higher reproductive vigor being more 

 often associated with less reactiveness and a higher reaction-time. 



