114 SELECTION IN CLADOCERA ON THE BASIS OF 



correlation obtained for the same data for the minus strain is positive 

 and is too small to be of statistical significance. 1 



Considered on all points, there is little in the data or the course 

 of the curves to indicate any appreciable relationship between the 

 relative vigor of the two strains and their mean reaction-times. The 

 comparisons made and the correlations calculated show that there 

 is at most only a possible slight relation between average vigor 

 (reproductive index) and mean reaction-time. The relation (if 

 actually significant) is so small, however, that when the data are 

 examined in many detailed ways it usually quite fails to appear and 

 statistical correlations do not establish it. This possible relation is 

 large enough at most to account for only a small portion of the fluc- 

 tuations of the curves and at best for only a small fraction of the 

 divergence between the mean reaction-times of the two strains of 

 Line 757. Hence it becomes clear that fluctuations of the mean 

 reaction-time curves of the two strains were controlled neither in 

 direction nor in amount (to any appreciable extent at any rate) 

 by the relative vigor of the two strains. 2 



While the writer is convinced that in Line 757 there is at most 

 only a very slight relation between vigor and reaction-time and has 

 already laboriously presented the evidence on this point from the 

 data for Line 757 itself, this conclusion is perhaps even better justi- 

 fied from consideration of the data for the plus and minus strains 

 of the other Cladocera lines subjected to selection. Inspection of the 

 curves showing mean reaction-times and reproductive indices for the 

 other lines of Simocephalus exspinosus 3 brings some crucial evidence 

 to bear directly on this point. 



The data for Line 794 (tables 33 and 34, and figures 11, A, B, 

 and c) show considerably greater reproductive superiority on the part 

 of the plus strain (as compared with the corresponding minus strain) 

 than in the plus strain of Line 757, yet the reaction-time means for the 

 two strains of Line 794 run much nearer together than in Line 757. 



The data for Lines 795 (figures 12, A, B, and c) and 796 (figures 13, 

 A, B, and c) show still greater differences in mean reproductive vigor of 

 their plus and minus strains, yet there are no corresponding differences 

 in mean reaction-time. The differences in mean reaction- time which 

 occur in different parts of the curves for these lines do not at all lend 



1 The correlations between age of mother and size of brood have no apparent bearing upon 

 reactiveness, but they may be mentioned for their biological interest. The correlations obtained 

 are -0.05785 0.0525 for the plus strain and - 0.01732 0.0527 for the minus strain. If one 

 eliminates the data for mothers which did not produce their first broods until more than 11 days 

 of age, thus eliminating the obviously weak mothers (since more than 1 1 days is an abnor- 

 mally late reproductive age), the correlations are 0.11618 0.0531 for the plus strain and 

 0.12409 0.0533 for the minus strain. 



2 It is obvious that if the vigor of the stock were extremely low the mean reaction-times 

 should become higher, but this stock was always maintained at the highest possible vigor and 

 the varying reproductive indices merely indicate various degrees of the always (except for very 

 limited periods) successful maintenance of high levels of reproductive vigor. 



3 See pages 44-45, 60 and 65 and figures 2, B and c, 3, B and c, 7, B and c, and 8, B and c, 

 for treatment of this data for the D. pulex lines. 



