A PHYSIOLOGICAL CHARACTER. 149 



the 2 strains had descended an equal number of generations. The 

 plus strains which had descended a greater number of generations 

 averaged an advance of 2.9 generations ahead of their respective 

 minus strains; the minus strains which had descended farther than 

 their corresponding plus strains averaged an advance of 4.3 genera- 

 tions beyond the plus strains. The D. pulex and S. exspinosus lines 

 were pretty well divided as to whether the plus or minus strains had 

 descended the greater number of generations. In all the D. longispina 

 lines, however, the plus strain had descended farther by a single 

 generation in two of the lines, but by 5 generations (almost 11 per 

 cent) in one. Hence, there was possibly some real loss of vigor in 

 the minus strains of D. longispina as compared with 'their respective 

 plus strains (though three lines are too small a number from which to 

 reach a safe conclusion). But with S. exspinosus and D. pulex there 

 was no loss in vigor of the minus as compared with their respective 

 plus strains, so far as was revealed by their total numbers of general 

 tions of descent. In fact (so far as this point is concerned), the minus 

 strains in 5 of the 8 D. pulex lines descended farther and by larger 

 margins (an average of 5.2 generations as compared with 3.7 genera- 

 tions for the plus strains), which affords almost as much ground for 

 assuming that the plus strains of the D. pulex lines became of inferior 

 vigor compared with the corresponding minus strains as is afforded for 

 the opposite conclusion with reference to the D. longispina lines. 



Some effect of the selections upon the general vigor of the 

 corresponding plus and minus strains is evident, however, in some 

 cases in the generally lower reproductive indices for some of the 

 minus strains as compared with their corresponding plus strains. In 

 only 1 (Line 740) of the 8 lines for which the reproductive indices 

 have been worked out was the minus strain in general conspicuously 

 the more vigorous of the 2 strains of the same line (figure MB). It 

 is significant, however, that (except possibly for parts of the data 

 for Lines 795 and 796, and these differences were not maintained) 

 the differences in vigor of the two strains of the same line were not 

 cumulative; and in no case were they greater during the later than 

 during the early portions of the experiments, except for the very 

 first months of selection. It would seem, then, that in some cases 

 there was a tendency for the selected individual (the most reactive 

 individual) in the plus strain to be more vigorous than the selected 

 individual (the least reactive individual) in the corresponding minus 

 strains, but that this difference is neither universal, uniform, nor 

 cumulative in its effect. As is shown elsewhere, differences in re- 

 productive indices, even when relatively large, are not associated with 

 corresponding differences in reaction-time. 1 



x The lack of relation between reproductive index and reaction-time has been referred to 

 in several places (see pages 40-45, 49, 60, 65, 72, 76, 80, 83, 88-89, and 108-115) and need 

 not be further discussed here. 



