A PHYSIOLOGICAL CHARACTER. 133 



1913-March 1914, a winter period during which relatively low levels 

 are generally seen. 



It is not difficult to believe that these environmental factors may 

 have been in part chemical differences in the pond-water, gradual 

 changes in carbon-dioxide or oxygen content, or other chemical 

 differences, since the effects observed are operative through longer 

 periods of time than the temporary environmental effects already 

 considered. Such gradual changes in the water of a pond are well 

 known, and the water used for culture-water and in the experimental 

 tank during these tests was pond- water. We know nothing about the 

 precise specific character of the changes involved in the present case. 



Coincident Changes in Reactiveness. 



The curves for Line 695 (figure 2c) show these general environ- 

 mental effects to a marked degree, particularly in the relatively 

 low levels of the curves for October 1912-November 1913 and in the 

 higher level from December 1913-September 1914. The general 

 coincidence of level of the two curves does not end here, but it is 

 not so marked through the later course of the experiment because 

 of the relatively wide fluctuations of the curve for the minus strain. 

 The correlation between the mean reaction-times of the plus and 

 minus strains of this line by two-month periods is 0.4962 ±0.0979. 

 With Line 689 (figure 3c) the environmental effect is observable 

 throughout the course of the curves. In Line 691 (figure 4), while 

 the curve for the minus strain is quite irregular, the general course 

 of the curves is very similar. The same may be said for Line 711 

 (figure 5). In Line 713 (figure 6), in spite of the very irregular curve 

 for the plus strain, the curves in general follow each other remark- 

 ably closely. The correlation for the two strains of this line is 

 0.6982 ±0.0773. With Line 714 (figure 7c) the curves are unusually 

 irregular for strains of Daphnia pulex, yet the environmental influence 

 is obvious from the general course of the curves. The correlation 

 is 0.5027 ±0.0971. In Line 719 (figure 8c), again, the curve for the 

 plus strain is very irregular, yet the general coincidence of the curves 

 is obvious. 



Line 751 (figure 9) shows the same general coincidence of the 

 curves. This line, within which selection was begun in November 

 1912, is the only D. pulex line with which selection was begun later 

 than March 1912. It is interesting to note that the curves for this 

 line start at virtually the same level as was attained by the other D. 

 pulex strains during the same two-month period (December 1912- 

 January 1913) and closely follow the composite curves for the other 

 D. pulex strains (see figure 10d). 



The curves (figure 10, a and b) for the relatively short exper- 

 iments with Lines 762 and 766, both Daphnia longispina, show the 

 same general indication of a parallel effect of environment upon the 



