70 



SELECTION IN CLADOCERA ON THE BASIS OF 



the great lowering of the plus mean and the marked differences in 

 reactiveness between the two strains not present earlier. 



Line 766. 



As noted above, the two strains of this line were derived from 

 the minus strain of Line 762. The minus strain was a continuation 

 of the minus strain of Line 762, the selection having been inter- 

 rupted for three generations. The plus strain of Line 766 was de- 

 rived from the minus strain of Line 762 by selecting the most reactive 

 individual from the thirty-ninth generation of the minus strain of that 

 line. The summary of the data is given in table 30 and figure 10b. 

 Selection was continued for only 7 months, when it was interrupted 

 by the loss of the plus strain. 



A rather wide difference in mean reaction-times for the first 

 two-month period is not continued during the remainder of the ex- 



periment. The mean reaction-times for the two strains for the 

 entire period of the experiment were 327 and 385.8 seconds (table 

 30). While the difference is -58.8 seconds and is 2.72 times the 

 probable error, it is largely due to the difference in mean-reaction 

 time obtaining for the first two months. There were two same-day 

 broods containing totals of only 1 1 and 9 individuals in the plus and 

 minus strains, respectively. The averages were 222 and 250 seconds 

 and the difference was - 28 seconds, a difference which, with so few 

 individuals, is not significant. There is no effect of selection. 



The effect of environmental influences upon reaction-time is 

 indicated by the generall}^ coincident up-and-down courses of the 

 curves of the two strains of Line 762 and Line 766 (see figure 10, 

 A and b). 



Line 768. 



Line 768 was the second of the two genetically distinct lines of 

 D. longispina. It was taken into the laboratory from Pond IV in 

 November 1913, but selection was not begun until December 1914 



