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 



