150 SELECTION IN CLADOCERA ON THE BASIS OF 



STATEMENT OF RESULTS OF SELECTION. 



The results of this series of experiments may be stated briefly: 



With two lines, Lines 689 and 719, there were found reaction- 

 time differences in the reverse of selection. These differences were 

 so consistent as to suggest real genetic differences affecting reactive- 

 ness in the two strains of these two lines, though Line 689 was carried 

 for a relatively short period of time. In the case of Line 719 the 

 reaction-times differed less toward the end of the experiment, and it 

 is suggested that selection may have operated to produce this effect. 



The results with Line 689 and Line 719 suggest that in addition 

 to genetic differences in the direction sought in selection genetic 

 changes may occur in the opposite direction. This is strongly sug- 

 gestive of mutation. It does not, however, obviate the theoretical 

 difficulty of explaining by mutation the plural genetic changes in 

 Line 757. 



Lines 695, 713, 714, 751, 766, 768, and 796 show no evidence 

 (or slight evidence as in Line 713) of significant differences in reactive- 

 ness between their plus and minus strains. Some of these lines were 

 carried for relatively short periods of time, but Lines 695, 713, 714, 

 and perhaps 768 and 796 were subjected to selection for what would 

 seem adequate periods of time. 



Line 691 shows a possible effect of selection, though such an 

 effect is not fully supported by the last year's data and may therefore 

 be disregarded. Selection was conducted for 27 months, 84 and 85 

 generations in the two strains. 



Line 711 apparently shows an effect of selection, but the differ- 

 ences are small and the evidence is not entirely conclusive, though 

 the case is as convincing as the result in the opposite direction for 

 Lines 689 and 719. Selection was continued for 45 and 46 genera- 

 tions, covering a period of about 16 months. 



Line 762 gave a possible effect of selection during the last two 

 months of the experiment. In view of the wide fluctuations in the 

 reaction-time curves of many of the lines, this is not sufficient data 

 upon which to base a conclusion, but the entire consistency in the 

 differences between broods of the plus and minus strains during this 

 late portion of the experiment is unusual enough to be highly sug- 

 gestive. However, in view of the local differences in reactiveness 

 seen in other lines (see pages 140-142), an effect of selection in Line 

 762 must be considered problematical. The plus strain was lost, 

 thereby terminating the experiment after 36 and 35 generations of 

 selection. 



Line 794 may have undergone a divergence in reactiveness during 

 selection. For the first 6 months the differences were in the reverse 

 of selection, but the remaining 12 two-month periods gave differences 

 in the opposite direction, with 3 exceptions, and 8 of these differences 



