138 SELECTION IN CLADOCERA ON THE BASIS OF 



previous to August-September 1913 period show fairly consistent 

 rises in reaction-time, but henceforth, in spite of fluctuations, some- 

 what lower levels are usually maintained. 



It is extremely improbable that these general increases in re- 

 activeness in the S. exspinosus strains are due to improvement in 

 the stock due to better conditions in the laboratory than in the out- 

 door ponds from which the material was obtained. The increased 

 reactiveness does not appear soon enough after selection was begun 

 with Lines 740 and 757 to render this interpretation probable, and 

 there was an actual decrease in reactiveness for approximately 12 

 months (Line 740) and 9 months (Line 757) after laboratory culture 

 was begun. Then, too, the increases in reactiveness for Lines 740 

 and 757 are simultaneous, in spite of the fact that Line 740 had been 

 in the laboratory for about 12 months and Line 757 for only 9 months. 

 Further, Lines 794, 795, and 796 were in the laboratory for 10 

 months before a pronounced general tendency to increased reactive- 

 ness is evident (figure 18b). 



Again, with the 3 lines of Simocephalus (Lines 794, 795, and 

 796), with which selection was begun more than 2 years later than 

 with the older S. exspinosus lines (Lines 740 and 757), the reaction- 

 time curves (figures lie, 12c, and 13c) start at near the same levels 

 as were attained by the two older lines (figures 15 and 18b) for the 

 same months, regardless of the fact that the older lines had already 

 lived under laboratory conditions and undergone selection for ap- 

 proximately 28 and 25 months. 1 The striking decrease in reaction- 

 time means which occurred during the last few months of selection 

 (except to a lesser degree in the minus strain of Line 757) is likewise 

 as pronounced with the newer as with the older S. exspinosus lines. 



A further and (to the writer's mind) a fatal criticism to the 

 assumption (to explain decreased reaction-time) of a progressive 

 improvement in the stock due to favorable laboratory conditions 

 lies in the fact that there are not changes in reproductive indices 

 coincident with changes in reaction-time and that there is no pro- 

 gressive increase in reproductive index. 



To explain the result within Line 757 as due to a differential 

 physiological improvement requires several assumptions: (l) that 

 such improvement is not shown in the reproductive index; (2) that 

 Line 757 was more susceptible to such improvement than the other 

 S. exspinosus lines; (3) that not only was the plus strain of Line 

 757 subjected to greater improvement than the minus strain, but to 

 greater improvement than the other S. exspinosus plus strains; (4) 



1 Omitting the minus strain of Line 757 — which because of the effect of selection had a 

 considerably higher mean for this period, 829 seconds (47 individual records) — the mean re- 

 action-time for the period December 11, 1914, to January 15, 1915, for the older S. exspinosus 

 strains was 645 seconds (124 individual reaction-time records). For the same period, the period 

 during which the selection was begun with the newer lines of <S. exspinosus, the mean for the 

 newer lines was 658 seconds (319 individual records). 



