68 



SELECTION IN CLADOCERA ON THE BASIS OF 



a given species tend to have approximately the same reaction-time 

 means, regardless of the length of their previous laboratory history. 



Line 762. 



Line 762 was one of the two lineally distinct line's of D. longispina 

 subjected to selection. It was obtained October 19, 1912, from Pond 

 I, one of the surface-water ponds on the upland, and was subjected 

 to selection at once. This is a less favorable species for selection on 

 the basis of reactiveness to light than the other species used. While 

 it is moderately reactive, it does not so readily withstand the hand- 

 ling, and individuals sometimes become disabled or become lodged 

 in the surface film of the water. 



The summary of the data for Line 762 is found in table 29 and 

 in figure 10a. The experiment with this line was of relatively short 

 duration (10 months) as compared with the other lines, due to the 



Table 29. — Selection summary for Line 762. 



accidental loss of the plus strain in September 1913. Shortly after 

 this loss the experiment was resumed by selecting in both directions 

 from the minus strain. The line was henceforth designated Line 

 766. This was in effect a return selection within the minus strain. 1 



The data for the first nine-month period (November 1912- 

 July 1913, table 29) gives reaction-time means of 366.2 and 377.3 

 seconds. The difference ( — 11.1 ±16.19 seconds) is not significant. 

 The 10 same-day broods for this line, comprised mostly of broods 

 produced during June and July, gave the following means: 355 

 seconds for the plus strain (57 individuals) and 349 seconds for the 

 minus strain (54 individuals). The same-day-brood data rather 

 clearly substantiate the conclusion which the curves (figure 10a) 

 suggest, that until that time there was no effect of selection. 



For the final portion of the experiment, a little more than a 

 month (until the plus strain was lost), the plus strain was markedly 



1 It is unfortunate that return selections were not conducted in other lines, particularly in 

 Line 757. The return selection in Line 762 (which then became Line 766) is not significant as a 

 return selection, because presumably there was not an effect of selection within Line 762. 



