A PHYSIOLOGICAL CHARACTER. 137 



Lines 740 and 757 had been in the laboratory for more than 2 years 

 before these fluctuations occurred in the S. exspinosus lines; (3) 

 that no such improvement is shown in the reproductive indices. ^ 



While with the older lines of D. pulex subjected to selection the 

 reaction-time means decreased markedly almost from the beginning 

 of selection, and the means for these lines reached a low point in 

 the curves (see figures 10d,2c, 3c, 4, 5, 6, 7c, and 8c) within 8 months, 

 they afterward fluctuated upward and downward through the same 

 general range, so that there is no ground for assuming that this early 

 fluctuation was anything other than one of the wide general fluctua- 

 tions of the curves. This decrease in mean reaction-time was due 

 to environmental conditions, i. e., factors in the environment which 

 made for changed conditions under which the animals were tested 

 in the experimental tank. 



Further, comparison of the curves for mean reaction-times for 

 Line 751 (figure 9) with the curves for the other D. pulex lines shows 

 that the reduced reaction-time means, which occurred with all the 

 older D. pulex lines simultaneously soon after selection was begun, 

 were not attributable to any improvement in the general vigor or 

 quality of the daphnid stock due to its possibly being subjected to 

 more advantageous conditions in the laboratory than in the ponds 

 from which the stock was obtained. This is clear because the curves 

 for the two strains of Line 751 not only do not show a drop soon after 

 laboratory culture was begun, but start at very nearly the same 

 level as was attained during the same months (December 1912- 

 January 1913) by the other D. pulex lines (figures 2 to 8) which had 

 previously been subjected to laboratory culture for about 11 months 

 and selection for approximately 7 months; and these curves for Line 

 751 follow the same general course as those for the older lines (during 

 the same two-month periods) regardless of the fact that Line 751 

 had been under laboratory culture for a period 11 months shorter 

 than that for the older lines. 



General Increase in Reactiveness of Simocephalus exspinosus. 



With the S. exspinosus hnes, Lines 740 and 757, ^ there is a 

 slight simultaneous increase in reactiveness of both the plus and the 

 minus strains beginning after the June- July 1913 period (figure 15) 

 and continuing, with fluctuations, throughout the rest of the exper- 

 iment. This increase is more marked after the August-September 

 1915 period (figures 18b, 11c, 12c, 13c, and 15), where it appears 

 in all the 10 >S. exspinosus strains, though to a less degree in the 

 minus strain of Line 757. The curves for S. exspinosus strains 



^ Not only is there no improvement reflected in the reproductive indices, but those indices 

 for the D. pulex lines are abnormally low (indicating reduced vigor) for the very months (in the 

 autumn of 1912) during which this increase in reactiveness is most marked. 



^ No other Simocephalus lines were then in the laboratory. Lines 794, 795, and 796 were 

 begun in December 1914. 



