108 SELECTION IN CLADOCERA ON THE BASIS OF 



Thus, for the moment ignoring the data for the less reactive 

 (over-time) individuals, one finds that the more reactive individuals 

 of the plus strain showed greater reactiveness than the more reactive 

 individuals of the minus strain. These differences are not large and 

 are not all in the same direction, but on the basis of this portion of 

 the data alone there is evidence for an effect of selection, though 

 naturally it is not as decisive as that obtained from the complete 

 data. 



Relation Between Relative Vigor and Mean Reaction-Time. 



There remains to be considered the relation, if any, between the 

 relative vigor of the two strains and their relative reactiveness. 

 This is an important point. While it has already been shown that 

 with the other lines subjected to selection in connection with which 

 this point was considered, there is little, if any, relation between 

 reactiveness and the general vigor of the stock, it still remains 

 desirable to know whether such is certainly the case with Line 757. 



There is to be noted with the minus strain of Line 757, as with 

 the minus strains of many other lioes of Cladocera subjected to 

 selection, a somewhat lower reproductive vigor than with the 

 corresponding plus strain. This is presumably the result of the 

 individuals selected in the minus strains being somewhat below the 

 average in vigor. That this at the same time has not operated 

 materially to increase the mean reaction-time of the minus strain is 

 evident enough in nearly every line. The data for vigor and mean 

 reaction-time considered for the other individual lines (and for all 

 the lines as a whole) seem to show conclusively that there is no 

 relation between vigor and reaction-time. While this suggests that 

 in Line 757 the result of selection is not due to less vigor in the minus 

 strain, yet it does not preclude the possibility that it may have been 

 a factor in the divergence in reactiveness in Line 757. Some attention 

 will now be directed toward an analysis of the data on mean reaction- 

 time as related to vigor in this line. The matter of concern is whether 

 there is a relation between the relative vigor of the two strains and 

 their mean reaction-times, and, if there is a relationship, to what 

 extent it is operative. Several more or less disconnected considera- 

 tions will be taken up in regard to this point before referring to 

 the statistical correlations. 



The two strains were compared as to general vigor as revealed 

 by the measures applied to them (see page 42) and stated in terms 

 of the reproductive index (tables 43 and 44). 



1 . The plus strain was on the whole the more vigorous of the two, 

 as will be seen by reference to tables 43 and 44 and figure 18a. Of 

 the 27 periods of 2 months each (except the first period of 3 months 

 and the last period of 1 month) the plus strain was of superior vigor 

 during 16 periods and the minus strain during 11 periods. The 



