118 



SELECTION IN CLADOCERA ON THE BASIS OF 



of Simocephalus during the course of the selection experiment. This 

 general lowering of the mean is probably attributable to environ- 

 mental conditions (see pages 137-1 39). l But in the case of the 

 757 minus strain, and particularly during the final 11 months of the 

 experiment, the mean was lowered much less than that for each of 

 the other 9 selected strains of this species. Selection here seems to 

 have operated to hold the mean much higher than it otherwise 

 would have been. 



REACTIVENESS OF BOTH STRAINS OF LINE 757 MODIFIED THROUGH SELECTION. 



Another method of checking up the effect of selection in Line 

 757 is by a detailed comparison of the mean reaction-times of the 

 plus and minus strains of Line 757 with corresponding data for 

 Line 740, with which it is most similar in point of time of beginning 

 the selections and in duration of the selection experiment; and by a 

 further comparison of the reaction-time means for the two strains 

 of Line 757 with corresponding mean reaction-times for the plus and 

 the minus strains of the other lines of the same species, Lines 794, 795, 

 and 796. 



This comparison is given in detail in tables 48 and 49 and serves 

 to show whether both the plus and minus strains of Line 757 became 

 modified during the selection or whether the divergence between the 

 two strains of this line resulted from a genetic change in reactiveness 

 of the minus strain alone. This would seem to have an important 

 bearing upon the question as to whether the results came from 

 mutation or gradual change in Line 757. It would seem less easy 

 to explain the result as due to larger mutations if both the plus and 

 minus strains of Line 757 were affected, i. e., departed from the 

 means for corresponding strains of the other lines of the same species, 

 than if only one of the two strains of Line 757 was affected. 



The comparison between the mean reaction-times of the Line 757 

 plus strain and the plus strains of the other S. exspinosus lines is 

 the more crucial test, for it is obvious, by mere inspection and com- 

 parison of the figures (figures lie, 12c, 13c, 15, and 18c) showing the 

 reaction-time curves for Line 757 and the other S. exspinosus lines, 

 that the Line 757 minus strain became much less reactive than any 

 of the other S. exspinosus strains. The following analysis seems to 

 show, however, that the plus strain of Line 757 likewise became 

 more reactive than the plus strain of Line 740 or than the plus 

 strains of the other S. exspinosus lines. 



The cyrves showing reaction-time means for the plus strain of 

 Line 757 (figures 18s and 19) are somewhat irregular, but their 



1 Possibly the general lowering of the means in both the plus and minus strains of all the 

 lines of S. exspinosus is to be accounted for through a better handling of the material as the exper- 

 iment progressed. No intentional changes in the handling of the material were made after about 

 July 1912, however, and we were not conscious of any better manipulation as the experiments 

 progressed. 



