A PHYSIOLOGICAL CHARACTER. 



67 



broods. The mean reaction-time for those of the plus strain was 

 325 seconds, for the minus strain 288 seconds. The difference 

 (+37 ±16.02 seconds) is in the same direction as that for the entire 

 data for this period. For the remaining period of selection with this 

 line, 10>2 months, the mean reaction-times were 375.4 seconds and 

 400.7 seconds for the plus and minus strains, respectively. The dif- 

 ference was -25.3 ±13.46 seconds, a difference not of statistical sig- 

 nificance. There was again a considerable number of same-day broods 

 (13), for which the mean reaction-times were 350 and 345 seconds and 

 the difference +5 seconds. 



A test series conducted in January 1914, consisting of 328 indi- 

 viduals in the plus strain and 327 individuals in the minus strain, gave 

 a mean reaction-time of 416.1 seconds for the plus strain and 433.9 

 seconds for the minus strain. The difference was -17.8 ±13.9 

 seconds. This difference like the others is not of statistical value. 



A curve for the reaction-time 

 means of this line is shown in figure 

 9 and, like the numerical summaries 

 just considered, shows that there is 

 probably no effect of selection. 

 There is just a possible suggestion 

 of an effect of selection, however, in 

 the last year's data, inasmuch as 

 during this period the plus strain 

 was continuously the more reactive, 

 except for one two-month period. 

 But in view of the great fluctuation 

 in the reaction-time curves for the other lines subjected to selection, 

 this suggestion has little weight. 



There is a feature of interest in the curves for this line, as with 

 many other lines, the general close correspondence in the levels of 

 the plus and minus curves in the different two-month periods; on 

 the whole they sweep upward and downward together, an expression 

 of a parallel effect of environment upon the two strains. 



It is a matter of interest and some importance to note that the 

 means for the two strains of Line 751 start at approximately the 

 same levels which were held by the other D. pulex selection lines 

 (Lines 689, 691, 695, 711, 713, 714, and 719) during the same two- 

 month period, although the other lines had been subjected to labora- 

 tory culture for approximately a year and to selection for 8 months 

 before Line 751 was brought into the laboratory. Comparison of the 

 curves for Line 751 (figure 9) and for the other D. pulex lines (figures 

 2c, 3c, 4, 5, 6, 7c, and 8c) and with the composite curves for all the 

 D. pulex plus and minus strains (figure 10d, on which the curves for 

 Line 751 are superimposed), strikingly brings out this fact. This 

 is important as showing that under laboratory conditions strains of 



Figure 9. — Line 751. 

 Reaction-time curves. 



