A PHYSIOLOGICAL CHARACTER. 125 



means for Line 757 plus being exceptionally high, in each of these 

 periods. For the other 12 periods Line 757 plus was the more re- 

 active, the differences varying from 25 to 269 seconds and being 

 4.1, 1.1, 1.8, 5.1, 14.0, 1.0, 2.0, 4.6, 3.1, 9.3, 5.6, and 8.1 times the 

 large statistical probable errors (table 48). Considered by longer 

 periods, it is seen (table 48) that the differences are -5.1 ±13.97 

 seconds, +123.1 ±10.24 seconds, and +89.5 ±13.86 seconds, the 

 plus sign indicating greater reactiveness on the part of the plus 

 strain of Line 757. Reference to figure 18a, which shows the re- 

 action-time curves for Line 757 and combined reaction-time curves 

 for all other S. exspinosus lines, Lines 740 (in part), 794, 795, and 

 796, shows clearly to what extent the mean reaction-time for the 

 plus strain of Line 757 is prevailingly lower than the combined means 

 for all the other S. exspinosus plus strains. These curves show at a 

 glance that the plus strain of Line 757 was essentially lower in its 

 reaction-time than the other plus strains of the same species. The 

 3 two-month periods in which the reverse exists are there seen in 

 their true light — as exceptionally high points in the reaction-time 

 curve for 757 plus. 



Thus, from examination of the data for mean reaction-times 

 for contemporaneous periods, it is found that during the latter half 

 of the experiment the plus strain of Line 757 is more reactive than 

 the plus strains of the other S. exspinosus lines. That this is a result 

 of selection is indicated by the fact that in the beginning of the 

 experiments both strains of Line 757 were appreciably less reactive 

 than the two strains of Line 740, with which Line 757 may fairly be 

 compared. 



Comparison of the reaction-time means for the minus strain of 

 Line 757 with the 740 minus strain (table 49 and figures 15 and 19) 

 shows at once that the minus strain of Line 757 was from the be- 

 ginning the less reactive of the two, and that as the experiment 

 progressed the difference in reactiveness between the minus strains 

 of the two lines increased until at the close of the experiment the 

 divergence was fairly large. 1 Of 27 two-month periods the minus 

 strain of Line 757 had a lower reaction-time than Line 740 minus 

 only 2 times, a remarkable uniformity of differences when the 

 irregularities of the curves are considered. Of the 25 times when 

 the minus strain of Line 757 was the less reactive, the differences 

 were of statistical value 18 times, being 4.5, 5.1, 3.4, 10.1, 3.0, 5.7, 

 11.5, 6.5, 7.2, 3.3, 12.8, 9.7, 6.5, 4.4, 5.4, 5.7, 8.6, and 12.3 times their 

 probable errors (see table 49). Seven of the differences by shorter 

 periods and the 2 differences in the reverse direction were not of 

 statistical significance. Considered by longer periods, the mean 



x As is general, when comparing reaction-time means of two strains for contemporaneous 

 periods, there is seen an obvious rough parallelism between the two curves; but the uniformity 

 with which these two curves parallel each other is rather exceptional. 



