56 SELECTION IN CLADOCERA ON THE BASIS OF 



620 and 380 seconds, the plus mean being 240 34. 44 seconds the 

 larger. 1 



A small test series (268 and 273 individuals in the two strains) 

 conducted during March 1914, gave 265.9 and 247.3 seconds as the 

 plus and minus mean reaction-times (table 19). The difference 

 ( + 18.6 11.62 seconds) was 1.58 times the probable error. 



For the final year of the experiment with Line 713 (August 

 1914- July 7, 1915) the means were 306.0 and 349.6 seconds. The 

 difference was 43. 6 10.1 seconds, or 4.32 times the probable 

 error. For the 14 same-day broods the means were 305 and 333 

 seconds, the difference being only 28 seconds. Two test series were 

 conducted during March of this year. The first consisted of a com- 

 paratively small number of individuals (180 and 178). The means 

 were '342.9 and 384.4 seconds. The difference (-41.5 18.15 

 seconds) is 2.28 times the probable error and is not a significant 

 difference. A much larger and more adequate test series was con- 

 ducted after that just referred to. The numbers were 1,266 and 

 1,268 for the plus and minus strains, respectively. The means were 

 372.7 and 371.7 seconds. The difference was only +1.0 6.88 

 seconds. This test series was conducted with every care and it is 

 believed may safely be regarded as a crucial test. It brings out 

 clearly enough that, in spite of the small but rather consistent dif- 

 ference in mean reaction-times during the last year of the experi- 

 ment, there is not an effect of selection in Line 713. 



LINE 714. 



Line 714 was one of the original lines of D. pulex. Selection 

 was begun in the eighth generation and continued for more than 53 

 months, 184 generations in the plus strain and 196 generations in 

 the minus strain. The data for Line 714 are given in tables 21 and 

 22 and in figure 7. 



For the first 4 months of the experiment the mean reaction-time 

 for the plus strain was 495.4 and for the minus strain 618.9 seconds 

 (table 21). The difference was rather large (-123. 5 32.0 seconds, 

 3.86 times the probable error). There were only two small same- 

 day broods for this period (table 22), for which the mean for the 

 minus strain was 231 seconds the larger. 



For the year-period (August 1, 1912- July 31, 1913) the mean 

 reaction-time for the plus strain was 309.2 seconds and that for 

 the minus strain 278.5 seconds. The difference was +30.7 12.4 

 seconds (2.48 times the probable error). This difference is in the 

 reverse direction to that for the earlier (four-month) period. The 3 

 same-day broods had a mean for the plus strain 226 27.62 seconds 

 the larger. 



1 Of these 7 same-day broods, 4 occurred during May, when the plus strain for some un- 

 accounted-for reason was rather non-reactive for D. pulex. 



