148 SELECTION IN CLADOCERA ON THE BASIS OF 



action-time means are neither increased nor decreased. On the other 

 hand, reaction-time means are decreased for S. exspinosus lines, 

 while there are no consistent general changes in reproductive indices. 



Perhaps no better evidence bearing on the relation between 

 reproductive vigor and reaction-time can be had (or desired) than 

 that obtained from the following data: The total numbers of indi- 

 viduals tested in making the selections in all the plus strains of D. 

 pulex for the entire series of the experiments was 6,991; the mean 

 reaction-time of these 6,991 individuals was 343.76 seconds. The 

 corresponding figures for all the minus strains of D. pulex were 6,973 

 individuals, with a mean reaction-time of 345.92 seconds. The 

 difference was only 2.16 seconds. For the S. exspinosus plus strains 

 (except Line 757 plus), the total number of individuals was 4,722 

 and the mean reaction-time 531.56 seconds. For the 8. exspinosus 

 minus strains (except 757 minus) the figures were 3,943 individuals 

 and 532.55 seconds. The difference is 0.99 second. 



Since in both species the plus strains in general were somewhat 

 more vigorous than the corresponding minus strains, these negligible 

 differences in reaction-time means for the entire series of experiments 

 indicate a lack of relation between reproductive index and reaction- 

 time. 



Possible Divergence in Vigor between the Two" Strains of a Selected Line. 



The effect of selection on the basis of reaction to light might 

 conceivably result in plus strains of greater vigor and minus strains 

 of reduced vigor. It would indeed seem possible that the minus 

 strains might become very much reduced in vitality and perhaps 

 eventually die out if selection were continued long enough. In view 

 of this theoretical possibility it is interesting to note that of the 

 selection experiments discontinued because of loss of one or both of 

 the strains (see table 1, column 8), 2 lines were discontinued because 

 of the loss of both strains within a few days of each other, 1 line 

 suffered the loss of the minus strain only, and 5 lines suffered the 

 loss of the plus strain only. 1 Hence this significant bit of data does 

 not indicate reduced vitality in the minus strains as compared with 

 the corresponding plus strains. 



Column 9 of table 1 likewise throws light upon a point inti- 

 mately associated with the general vitality of corresponding plus and 

 minus strains, the relative numbers of generations of descent of the 

 plus and corresponding minus strains. This column shows the total 

 number of generations of descent of the different strains at the close 

 of selection within the different lines. In 8 lines the plus strain had 

 descended for a greater number of generations than the corresponding 

 minus strains; in 7 the minus had descended farther; and in 1 line 



1 Two of the last were lost through accident, however, rather than because of general weak- 

 ness or poor food conditions, and one of the minus strains suffered the accidental loss of part 

 of its stock, except for which it might have survived. 



