SELECTION IN CLADOCERA ON THE BASIS OF 



tween the two strains of Line 740 for the period April 1916-January 

 1917 is due to exceptionally high means for the minus strain during 

 the period April-September and to exceptionally low means in the 

 plus strain from October to the following January, both relative to 

 the means for corresponding two-month periods for the other S. 

 exspinosus strains (Lines 794, 795, and 796). The following of one 

 of these two exceptional reaction-time periods just upon the other 

 produces the long period of divergence which at first sight seems very 

 suggestive of an effect of selection. Since it is seen in this case that 

 neither the plus nor the minus strain of Line 740 is consistently 

 different from the composite curve for the other plus and minus 

 strain of lines of S. exspinosus throughout this ten-month period, it 

 seems pretty evident that genetic change is not responsible for this 

 divergence. It is believed that the explanation lies in the unavoid- 

 able differential treatment of the two strains of the same line (see 

 pages 140-142). 



A final test series was conducted in April 1917. There were 756 

 individuals of each strain. The mean reaction-times were 314.5 

 seconds and 329.8 seconds. The difference ( 15. 3 9.1 seconds) 

 was only 1.68 times the probable error. For the contingent two- 

 month period the plus strain was the less reactive by 96.3 seconds. 

 This test series, fortunately conducted at the close of selection, 

 showed conclusively that there was no effect of selection in this 

 line; or, if there had actually been an effect of selection, that it 

 had been lost. The selections were somewhat relaxed during the 

 October-January period, but it does not seem probable that a real 

 selective difference would have been lost so quickly. Hence, every- 

 thing considered, the writer does not believe that the data for the 

 ten-month period in which there was a consistent reaction-time 

 difference between the two strains indicates an effect of selection. 



Line 740 is somewhat unique among the lines of Cladocera for 

 which the data for vigor has been worked out, in that the minus 

 strain was on the whole more vigorous. Of the 29 two-month periods, 

 the minus strain was the more vigorous as measured by the repro- 

 ductive index for 18 periods and the plus strain was the more vigorous 

 of the two for only 11 periods. The differences in favor of the plus 

 strain average somewhat larger than those favoring the minus strain, 

 however. Examining these data with reference to periods of superior- 

 ity in the reproductive index on the part of the two strains, one fails 

 to find any correlation between relatively greater reactiveness and 

 greater vigor. For the 5 two-month periods during which the re- 

 productive index for the plus strain was superior to that for the 

 minus strain by 0.25 or more, the plus strain had a higher reaction- 

 time twice and a lower reaction-time three times. For the 6 two- 

 month periods for which the reproductive index for the minus strain 

 was superior to that for the plus strain by 0.25 or more, the reaction- 



