78 



SELECTION IN CLADOCERA ON THE BASIS OF 



Casual examination seems to reveal at once the fact that there 

 is no effect of selection, and this is probably correct, but on careful 

 analysis it is not altogether certain that an effect of selection is 

 entirely lacking. The plus strain is generally the less reactive, but 

 the curve for the minus strain is extremely fluctuating. 



For the first longer period (8 months) the mean for the plus 

 strain was 703.3 seconds, for the minus strain 571.4 seconds (table 



Table 35. — Selection summary for Line 795. 



Table 36. — Same-day broods. Summary of data for Line 795. 



35). The difference was +131.9 ±15.7 seconds, or 8.4 times the 

 probable error. The 12 same-day broods for this period likewise gave 

 a mean for the plus strain in excess of the minus strain (table 36). 

 The difference was +63 ±23.89 seconds, 2.63 times the probable error. 

 For the second longer period (August 1915 — July 1916) of the 

 data for this line the means were 625.0 and 530.7 seconds for the 

 plus and minus strains. The difference was +94.3 ±14.2 seconds. 

 This difference was 6.64 times the probable error. Twelve same- 



