A PHYSIOLOGICAL CHARACTER. 165 



(m) Independent fluctuations in vigor, like the independent 

 fluctuations in reaction-time affecting related strains, are believed to 

 be due to the somewhat differential treatment of the material. 



in) This factor is also believed to account for the generally- 

 greater divergence between the two strains of a line during later 

 periods than at the beginning of selection. 



(o) There is not a generally increased reproductive index in 

 S. exspinosus to correspond with the increased reactiveness of this 

 species. 



(p) Selection did not produce plus strains of increasingly greater 

 vigor as contrasted with minus strains of less vigor, either when 

 measured by the numbers of plus and minus strains which died out, 

 by the total numbers of generations of descent, or by the reproduc- 

 tive index. 



8. Sometimes the plus and minus strains of the same line repro- 

 duced on and were tested on the same day. The " same-day-brood " 

 data is more trustworthy, because of more uniform environmental 

 conditions, than data not simultaneously obtained, and is given 

 separate treatment. 



9. Reaction-time means were obtained of large numbers of 

 young from mothers of each of the two strains of a line reared simul- 

 taneously and tested on the same day. These "test" series gave 

 averages obtained from several hundred individuals reared and sub- 

 jected to stimulation by light under as nearly uniform conditions as 

 possible, and served to check up the effect or lack of effect of selection 

 within individual lines. 



10. Reaction-time is slightly, if at all, correlated with reproduc- 

 tive vigor. This is a point of interest, since the minus strains in 

 general had a somewhat lower reproductive index than the corre- 

 sponding plus strains. 



11. The possibility that the method of selection might have been 

 inadequate is discussed and evidence given which it is believed 

 vindicates the method used. Were the method of selection greatly 

 at fault the results obtained with some of the lines, particularly with 

 Line 757, could not have been possible. 



12. In general, the plus strains showed somewhat greater repro- 

 ductive vigor (measured by reproductive indices) than the corre- 

 sponding minus strains. But this effect was not cumulative and 

 selection did not result in plus strains of increasingly greater vigor 

 than corresponding minus strains. 



13. There are differences between the behavior of the different 

 species in the experimental tank. S. exspinosus was generally 

 relatively slightly reactive compared with the two species of Daphnia. 

 Many individuals of this species (and a few of the species of Daphnia) 

 failed to reach either end of the experimental tank during the 15- 

 minute period of the selection test. 



