164 SELECTION IN CLADOCERA ON THE BASIS OF 



(d) Negatively reacting individuals appear under circumstances 

 suggesting that their occurrence is the result of peculiar local environ- 

 mental conditions affecting the water of the experimental tank. 



(e) Broad up-and-down movements in the reaction-time curves 

 are presumably largely reflections of underlying (as distinguished ' 

 from more temporary) changes in content of the pond-water, though 

 in some cases temperature influences are contributory. More local 

 contemporaneous changes are clearly produced by factors other than 

 temperature, presumably changed constituents of the culture-water 

 and the water used in the experimental tank. 



(/) Slight seasonal (winter and summer) changes in reactiveness 

 are attributed to direct temperature influences and to the indirect 

 influence of temperature as affecting the content of the culture-water 

 as well as the water used in the experimental tank. 



(g) Following July 1913, and extending throughout the re- 

 mainder of the experiment, there was a gradual increase in the re- 

 activeness of all the strains of Simocephalus. That this is not due 

 to the cumulative effect of favorable conditions in the laboratory is 

 indicated: (1) by the fact that for several months after selection was 

 begun with the older lines of Simocephalus there was a general 

 decrease in reactiveness; (2) that the increase in reactiveness affected 

 all strains at the same time, though they had been in the laboratory 

 for different periods of time; and (3) that the reaction-time curves for 

 the newer Simocephalus lines (Lines 794, 795, and 796) start at and 

 follow approximately the same levels as were attained by the older 

 lines for contemporaneous two-month periods. 



(h) Because of the fact that the two strains of a selected line 

 ordinarily reproduce on different days, culture-water from different 

 collections was usually used with the two related strains. This 

 unavoidably somewhat differential treatment is believed to account 

 for some of the independent shifts in reaction-time means affecting 

 the two strains of the same line. One such shift, for a time, materially 

 cut down the difference in reaction-time means between the two 

 strains of Line 757. 



(i) Relatively local environmental influences are believed to 

 explain certain abrupt shifts in the reaction-time curves and in 

 reproductive indices which some workers with Cladocera might be 

 inclined to ascribe to "depression periods." 



(j) Environmental conditions obviously influence the vigor of 

 Cladocera. 



(k) The "reproductive index" is believed to be a safe measure 

 of general vigor and probably of general muscular activity as well. 



(I) Similar effects of environmental conditions are seen in many 

 coincident fluctuations in vigor of the two strains of the same line, 

 of all or most of the strains of the same species, and even of many of 

 the strains of both Simocephalus and Daphnia. 



