146 SELECTION IN CLADOCERA ON THE BASIS OF 



the wide fluctuations in the four periods enumerated above; cer- 

 tainly it is not true for consistent differences in reproductive index 

 between the two strains of a species for several months in succession, 

 such as are seen in four different periods for Line 757 (figures 17c 

 and 18a) and in parts of the data for nearly every line for which 

 reproductive indices are given. 



Such independent fluctuations are believed to be due to the 

 somewhat differential treatment resulting from the transfer of young 

 of the two strains of a line on different days and into culture-water 

 of different collections. The cumulative effect of continued trans- 

 fers into somewhat different culture media seems very readily to 

 account for local differences in reproductive vigor between the two 

 strains of a line. 



Some of these independent fluctuations are capable of explana- 

 tion as due to variations in an internal cycle which most workers 

 believe exists in Cladocera. But differences in environmental con- 

 ditions explain these cases so readily, some of the contemporaneous 

 changes are associated with recognized changes in environmental 

 conditions, and it is well known that such culture media do have 

 great variation, so that any assumption of an internal cycle seems 

 quite unnecessary. Further, in Line 689 (figure 3a and b) the plus 

 strain, which had much the higher reproductive index for the 

 first two months of selection, had a considerably lower reproductive 

 index for the following 2 months. This would seem a short time for 

 the internal cycles of these two sister strains to have diverged. The 

 minus strain of Line 714 (figure 7, a and b) had a reproductive index 

 nearly three times as large as that for the plus strain for the first 

 two-month period. Can these strains just derived from the same 

 individual have immediately passed into different periods of an 

 internal cycle? There are other cases (less striking, however) of 

 significantly different reproductive indices immediately or very soon 

 after the origin of related strains. 



However, in spite of the unavoidably somewhat dissimilar treat- 

 ment of the two strains of a line and the consequent local differential 

 changes in reproductive indices, there is in general a very close 

 correlation between the reproductive indices of kindred strains, a 

 result to be ascribed, in large part at least, to the general influence of 

 similar environmental conditions. 



Differences between Reproductive Indices during Different Parts op Experiment. 



The reproductive indices for the two strains of the different 

 lines in general differed somewhat less at the beginning of selection 

 (for the first 4 months), when the two strains had just been separated, 

 than during the later parts of the experiment, after the two strains 

 had been separate for considerable periods of time (figures Ic, 3a, 

 7a, 8a, 11a, 12a, 13a, 14a, and 17c). This is explainable as due to 



