72 SELECTION IN CLADOCERA ON THE BASIS OF 



This experiment was continued during the most of another 

 month, but the data consists of the records for only 3 broods from 

 each strain. The means are 350 and 198.4 seconds. The difference 

 ( + 151.6 seconds), while large, is based upon entirely too few 

 individual reaction-times to be assigned significance. It is obvious 

 that with Line 768 there was no effect of selection. 



The reproductive indices for this line have not been worked out, 

 but the plus strain was apparently somewhat the more vigorous. 

 That this difference in vigor has not influenced reaction-time, to any 

 considerable extent at any rate, is evident from the fact that the 

 same-day broods of the plus strain were on the whole less reactive 

 than those of the minus strain. 



There is again, as with many other lines, an obvious effect of 

 environmental conditions upon reaction- time. In spite of rather 

 wide local fluctuations, the means for the two strains, on the whole, 

 follow each other rather closely, as is shown in figure lOc. 



Reference to figure 10 (in which are given, separately, the re- 

 action-time curves for the three selection lines of D. longispina and 

 composite curves for the reaction-times of all the D. pulex selection 

 lines) shows that there is a striking, though somewhat rough, paral- 

 lelism between the reaction-time curves for the D. longispina and 

 the D. pulex lines. This is an expression of similar influences of the 

 same environmental factors upon the reactiveness to light of the 

 two species. 



