98 



SELECTION IN CLADOCERA ON THE BASIS OF 



individual reaction-time records, respectively) were: for the plus 

 strain 788.3 seconds, and for the minus strains 838.9 seconds (table 

 45). The difference (-50.6 11. 18 seconds) is 4.53 times the 

 statistical probable error. These data include those for the 5 months 

 at the beginning of the experiment, when selection was only slightly 

 if at all effective. For this nine-month period there are 9 same-day 

 broods, containing 114 and 99 individuals, with the reaction-time for 

 the minus strain averaging 58 16. 24 seconds greater than that 



TABLE 43. Selection data summarized by two-month periods for Line 757 plus. 



for the plus strain. This difference is 3.59 times its probable error. 

 Only one of these minus same-day broods had a mean reaction-time 

 lower than that for the corresponding plus brood. 



For the year August 1913- July 1914 (44 generations and 504 

 individual reaction-time records, and 42 generations and 412 indi- 

 viduals in the plus and minus strains, respectively), the mean re- 

 action-times were 683.7 seconds and 783.0 seconds (table 45). The 

 difference in mean reaction-time was 99.3 11. 77 seconds, or 8.44 

 times the probable error. The 1 1 same-day broods for this period 

 (145 and 123 individuals, table 46) gave a difference in reaction- 



