90 SELECTION IN CLADOCERA ON THE BASIS OF 



The tabulations of the data and the diagrams employed are 

 practically identical with those used in presenting the data for Line 

 695 and the other lines to which rather careful treatment has been 

 accorded. They are: tabulations of the data by broods for the 757 

 plus strain and the 757 minus strain (tables 41 and 42) ; tabulation 

 by two-month periods (tables 43 and 44) ; a summary of the selection 

 data, including the results of the test series (table 45); a tabulation 

 of the data for the same-day broods (table 46) ; summary of data for 

 more reactive individuals (table 47) ; figure 18s, showing graphically 

 the mean reaction-times of the two strains by two-month periods; 

 figure 19, showing the reaction-time curves by six-month periods; 

 figure 16, showing the relative rates of descent of the two strains; 

 figure 17 A, representing the average brood-size for the two strains; 

 figure 17s, showing average age of mothers at the time the first 

 broods were produced; figure 17c, giving the reproductive indices 

 for the two strains; and figure ISA, showing graphically the differ- 

 ences in vigor of the two strains as indicated by the reproductive 

 indices. .Tables 48 and 49, comparing the data for Line 757 with that 

 for the other S. exspinosus lines, are also presented, and the data in 

 them are graphically presented in figures 15 and 18B. 



Examination of tables 41 and 42 shows that in Line 757, as with 

 all the lines subjected to selection, there are considerable fluctuations 

 in the reactiveness of individual broods. During the first 5 months 

 of selection the range in mean reaction-time by broods for the plus 

 strain was from 472 to 900 seconds and for the minus strain from 

 586 to 900 seconds. The fluctuations in reaction-time means by 

 broods were still large after the two strains became widely different 

 in their reactiveness to light. For the last 5 months of selection with 

 Line 757 the plus strain means ranged from 93 to 651 seconds and 

 the minus strain means from 133 to 814 seconds. 



Although brood-by-brood comparison (tables 41 and 42) shows 

 clearly that in general, after the early part of the selection experi- 

 ment with Line 757, the minus broods were very much the less re- 

 active, the rather wide fluctuations by brood averages made it desir- 

 able to treat the data by larger units. 



Tables of the data by one-month periods were made. 1 From 

 the averages in these tables it was found that for the first 9 months 

 of selection the plus strain of Line 757 was the more reactive for 7 

 months; for the second longer period (the year August 1913-July 

 1914) the plus was the more reactive during 10 months; for the third 

 longer period the plus was the more reactive during 10 months; for 

 the remaining 21 months of the experiment the plus was the more 

 reactive during each month. After the first 5 months of selection 



1 These are omitted from the paper for the sake of reducing the cost of publication and in 

 favor of retaining in the paper the tables of data by broods in which the original data are given 

 in sufficient detail to make possible their utilization by anyone who might wish to examine the 

 data statistically. 



