48 



SELECTION IN CLADOCERA ON THE BASIS OF 



1.00- 



50- 



00 



There were more over-time individuals in the plus strain, 24 as 

 compared with 7 in the minus strain. The average minimum and 

 maximum reaction-times Hkewise indicate greater reactiveness in 

 the minus strain for the last 12 of the total 16 months of the experi- 

 ment. The average minimum reaction-time for all the broods tested 

 in the plus strain for this last year was 174 seconds (table 11), while 

 the corresponding average for the minus strain was 136 seconds, 

 indicating that the most reactive individuals in the minus strain had 

 a lower reaction-time by 38 seconds 

 than the corresponding individuals 

 in the plus strain. A corresponding 

 comparison for the maximum re- 

 action-times for the two strains 

 gives 514 seconds as the average 

 maximum for the plus and 392 

 seconds as the average for the minus 

 strain, the difference (122 seconds) 

 indicating that the individuals 

 with the highest reaction-times in 

 the plus strain were 31 per cent 

 slower in reacting than the corre- 

 sponding individuals in the minus 

 strains. These portions of data 

 tend to confirm a greater re- 

 activeness for the minus strain 

 than for the plus strain. 



Comparison of the curves for 

 the two strains of Line 689 may 

 be made with the combined mean 



^50 



300 



150 



00 



4-5 



8-9 



12-1 4-5 



1912 



1913 



Figure 3. — Line 689. 



A. Reproductive indices, actual values. 



B. Reproductive indices, superiority. 



C. Reaction-time curves with superim- 



reaction-time curves for all the 

 plus and for all the minus strains 

 of those Daphnia pulex lines be- 

 tween the two strains of which 



there were no POnsistPTlfdiffprpnpps posed curves (in faint lines) representing the 

 Lliei e were no COnSlSieni amerenceS combined reaction-time means for the plus and 



m reactiveness Lines 691, 695, minus strains of allDaMnmp«iexlines in which 



71 Q 71/1 r^r^A 7K1 /r^^^ f^^,,^^ 0^^ significant reaction-time differences did not 

 713. 714, and 751 (see figure 3c). arise-Lines 691, 695. 713. 714. and 751. 



Inspection of these curves shows 



that for 3 two-month periods (August 1912-January 1913) the 

 plus strain of Line 689 was abnormally high, while the minus 

 strain of Line 689 was abnormally low in reaction-time, and that 

 aside from these 3 two-month periods the two strains of Line 689 

 did not differ so greatly in their reactiveness. It would seem possible 

 that the pronounced divergence in reactiveness between the two 

 strains of this line for the period indicated may have been due 

 to the coincidence of unusual reactiveness of the minus strain and 

 slight reactiveness of the plus strain brought about by differential 



