152 SELECTION IN CLADOCERA ON THE BASIS OF 



months of the close of the experiment, when the effect of selection 

 was at its height.^ 



Considering the data by longer periods, those from the beginning 

 of the experiment to July 1914 show significant differences in mean 

 reaction-time, though the early differences were generally not large. 

 Following this was a period during which both strains, and particu- 

 larly the minus strain, fluctuated widely, though the difference for 

 this longer period is significant of a real difference in reactiveness 

 between the two strains. During the latter part of 1915 the dis- 

 turbances due to environmental factors became less marked and 

 the divergence in mean reaction-time appeared to increase greatly 

 and, with a single fluctuation, continued very large during the 

 remainder of the experiment. During the final 9 months of the 

 experiment the mean for the plus strain was only one-third that for 

 the minus strain. The form of the curves (figure 18b) suggests that 

 the divergence in reactiveness was still increasing at the close of the 

 experiment. 



Points of importance in connection with the results with Line 

 757 are that both the plus and minus strains became modified during 

 selection and that the divergence was permanent or at least persisted 

 for 112 generations. It is also important to note that there is nothing 

 to distinguish the two strains other than the difference in behavior 

 of the strains in the experimental tank in response to directive 

 stimulation by light, and a somewhat higher reproductive index for 

 the plus strain during most but not during the concluding months of 

 the experiment. 



DISCUSSION OF RESULTS. 



Consistent differences in reactions to light of animals of the same 

 or closely related stocks appear not to have been frequently found. 



McEwan (1918) presents a conclusive case of differences in 

 phototropism which are clearly genetic. A certain mutant strain, 

 "tan," of Drosophila melanogaster, unlike most of this species, 

 showed very little response to light. Tan is a very variable character 

 and can not always be distinguished by inspection of the flies. The 

 differences in phototropism between tan and normal flies were so 

 striking that it was found (and proven by breeding tests) that the 

 slight phototropism of tan stock was a more rehable means of diag- 

 nosing tan than inspection of the flies. The character was shown to 

 be a recessive sex-linked character. McEwan found differences in 



* Such a marked reduction in the difference between the means, coming at a time when 

 according to all criteria the two strains were so different in their reactiveness to light, seems 

 certainly due to unequal local environmental influences, which may also account for the relatively 

 wide fluctuations in the reaction-time curves for the period from June 1914 to July 1915 (see 

 pages 141 and 159-160). While during this earlier period the minus strain for 1 two-month 

 period had the lower reaction-time mean, this fact need perhaps excite no more surprise than 

 the fact that the mean for the minus strain was so near that for the plus strain at the later period 

 when the general reactiveness of the two strains was so widely different. 



