58 



SELECTION IN CLADOCERA ON THE BASIS OF 



2.00 

 1.50 

 1.00 

 .50 

 00 



ceding long period, there is a reversal in this longer period of the 

 direction of the difference in the mean reaction-times for the entire 

 data and in that for the same-day broods. 



For the next year-period (August 1, 19 14- July 31, 1915), the 

 means were 378.1 and 362.3 seconds. The difference, which was 

 + 15.8 dz 12.3 seconds, is not statistically significant and is again in 

 the reverse direction from that for the preceding period. During 

 this year-period the same-day-brood data consisted of 13 broods and 



4-5 8-9 I2-I 4-5 8-9 12-1 4-5 8-9 ^ 12-1 4-5 8-9 12-1 4-5 8-9 

 1912 1913 1914- 



I 



1915 1916 



1913 



rT ^ * VI] 



J 



A u 



^ ' 



J- 



T 1 1 B r . 



_ 6 



-^-^ 



HI 



-.75 



-.50 



-.25 



00 



500 



4-50 



300 



.25 

 .50 

 .75 



150 h 1912 



4-5 8-9 12-1 



Figure 7. — Line 714. 



A. Reproductive indices, actual values. 



B. Reproductive indices, superiority. 



C. Reaction-time curves. 



the mean for the minus strain was 1 14 dz 19.44 seconds higher than 

 that for the plus strain. This is a difference almost 6 times the 

 statistical probable error. ^ 



A test series conducted during October 1914, containing 249 

 individuals in the plus strain and 289 in the minus strain, gave 



* These same-day broods are pretty well scattered through the year period, except that none 

 occurred during the last 4 months. The averages obtained for the same-day broods are more 

 dependable than the averages for the data as a whole when there are sufficient numbers, and 129 

 and 105 individuals constitute fairly good numbers. It is unusual that the reaction-time means 

 for the complete data and for the same-day-brood data for the same period should differ so widely 

 as in the present case. 



