50 



SELECTION IN CLADOCERA ON THE BASIS OF 



TABLE 14. Same-day broods. Summary of data for Line 691. 



mean was larger by 46 46. 8 seconds. 1 For the year, August 1912- 

 July 1913, the two means were 248.8 and 296.6 seconds (225 and 257 

 individuals). The difference, 47.8 11.9 seconds, was 4.02 times 

 the probable error. The same-day broods for this year-period, of 

 which there were 8, gave a mean of 192 seconds for the plus and of 

 348 seconds for the minus strain and a difference of 156 31. 00 

 seconds 5.03 times the probable error (table 14). In spite of the 

 irregular course of the curves, the data for this year might seem to 

 indicate a possible (though slight) effect of selection, except for the 

 fact that the further history of the line does not bear out this inter- 

 pretation. 



For the following year (nearly), which concludes the data for 

 this line, the mean for the plus strain was 391.0 seconds (285 indi- 

 viduals); for the minus strain 416.2 seconds (273 individuals). The 

 difference was 25. 2 15. 2 seconds (table 13) and is not of statisti- 

 cal significance. There were 6 same-day broods for this period, for 

 which the plus mean was 413 seconds and the minus mean 370 

 seconds (table 14). This difference (+43 seconds), while not of 

 statistical significance, is in the reverse of an effect of selection. No 

 test series were obtained for Line 691. 



Figure 4 shows the curves of the mean reaction-times for the 

 two strains for Line 691 by two-month periods. The curves are 

 quite irregular and indicate considerable fluctuation in mean re- 

 action-times, particularly for the minus strain, which, except for its 

 wide fluctuations, was persistently the less reactive of the two strains 

 after the first 4 months of selection.' The curves for the two strains 

 follow each other in a general way, but are so irregular that they 

 cross 8 times. The rather persistently (though only slightly) higher 

 reaction-time of the minus strain is suggestive of a possible slight 



1 There were only 2 same-day broods, involving totals of only 7 individuals in the plus and 

 8 individuals in the minus strain. The means were 634 and 303 seconds, the difference being 

 +331 seconds. These broods came in July, when the plus strain was relatively slightly reactive 



