A PHYSIOLOGICAL CHARACTER. 105 



ences are more instructive if stated in percentages. The average 

 minima are 37, 54, 35, 146, and 161 per cent higher for the minus 

 than for the plus strain. 



These differences clearly point to a marked and cumulative di- 

 vergence in reactiveness of the most reactive individuals of the two 

 strains, a divergence which became in the final period more than 

 3 times as great as the average minimum for the plus strain. It is 

 a striking fact that for the last 9 months of the experiment the mean 

 reaction-time for the plus strain was considerably lower than the 

 average minimum reaction-time for the broods of the same strain for 

 the first 21 months of the experiment. 



A comparison of the average maximum reaction-times is less 

 instructive, for a large percentage of the broods of S. exspinosus 

 contain over-time individuals; but even here comparison indicates 

 increasingly greater reactiveness for the least reactive individuals of 

 the broods of the plus strain toward the end of the experiment. For 

 the first 9 months every brood in the plus strain as well as the minus 

 strain contained over-time individuals and the average maximum 

 reaction-time for each strain is (the arbitrary) 900 seconds. For the 

 longer periods of the entire experiment the maxima for the plus strain 

 declined from 900 to 879, 829, 811, and 632 seconds (table 45). 

 There is a slight decline for the minus strain, the averages being 900, 

 884, 854, 888, and 854 seconds (table 45). The difference in average 

 maximum reaction-time for the two strains increased from to 5, 

 25, 77, and 222 seconds. The average maximum reaction-times are 

 0, 1, 3, 10, and 35 per cent higher for the minus than for the plus 

 strain. For the minus strain the total lowering of the average maxi- 

 mum reaction-time was only 46 seconds, or 5.1 per cent as compared 

 with 268 seconds, or 29.8 per cent, for the plus strain. 



The data for the same-day broods may be referred to again. 

 They, too, show an interesting decline in the mean for the plus 

 strain through the various periods (from 798 to 670, 676, 538, and 93 

 seconds, table 46). The average for the last period is for a single 

 brood and is abnormally low. The general trend is unmistakable, 

 however. There is no marked lowering for the minus strain, the 

 means being 857, 805, 876, 808, and 814 seconds (table 46). The in- 

 creased divergence between the means for the two strains is striking — 

 from 58 to 135, 200, 270, and 720 seconds. Stated in percentages, 

 these differences for the several longer periods of the experiment, 

 compared with the mean for the plus strain, are 7.3, 20.2, 29.6, 50.1, 

 and 774.2 per cent. Again, although allowing that the difference for 

 the last period was obtained from too meager data, the divergence 

 is marked and continuous, and these differences arise from the most 

 strictly comparable data obtainable. 



Considering the number of " over-time" individuals for the two 

 strains in the different periods of the selection experiment is another 



