144 SELECTION IN CLADOCERA ON THE BASIS OF 



Conversely, when the reproductive index is high, mortality is low, 

 sterility is extremely rare, there are few failures to produce a second 

 brood, and swimming activity is at its maximum. 



Coincident Fluctuations in Vigor. 



If vigor is related to environmental conditions, coincident fluc- 

 tuations in reproductive indices for the two strains of the same line 

 and for the strains of different lines should be in evidence. They are 

 present and most conspicuous when the two strains of the same line 

 are compared. 



In the data for Line 757 (figure 17c) for example, the reproduc- 

 tive indices for the two strains, compared in each case with the repro- 

 ductive index for the preceding period, rise or decline together 16 

 times and move in reverse directions only 10 times. Of these 16 

 coincident fluctuations, 13 are large enough to have a probable real 

 meaning, while only 4 of the 10 independent fluctuations are large 

 enough to be considered significant. 



There is not more coincident fluctuation in reproductive index 

 in Line 757 than with most of the other lines (see figures lc, 3a, 7a, 

 8a, 11a, 12a, 13a, and 17c), so that in general there is a rather close 

 correlation between the reproductive indices of the two strains of a 

 line. This was clearly seen to be a reflection of environmental in- 

 fluences in some cases, e. g., the August-November 1912 period 

 (see page 135), and it is believed to have been equally due to environ- 

 mental influences where the precise character of the environmental 

 influences was not recognized. 



Coincident fluctuations in reproductive indices are likewise seen 

 between the different lines of the same species and also between lines 

 of D. pulex and S. exspinosus. 



With the D. pulex lines there is a remarkable agreement in the 

 occurrence of low reproductive indices for the August-November 

 1912 period (figures lc, 3a, 7a, and 8a). Line 740 (S. exspinosus) 

 likewise shows extremely low points for this period (figure 14a). 

 There are some early data for Line 757 (figure 17c), another line of S. 

 exspinosus, culture of which was begun during the month of Novem- 

 ber 1912, which show very low reproductive indices. This is a striking 

 fact, as showing that whether the lines had been in the laboratory for 

 nearly a year (as in the case of the Daphnia pulex lines), for 4 months 

 (Line 740), or had just been introduced into the laboratory (Line 

 757), low reproductive indices occurred. As has been stated before 

 (page 136), this was a period of particularly poor food conditions and 

 it involved the loss of much laboratory stock. 



During June-July 1913, 7 of 8 strains of D. pulex (figures lc, 3a, 

 7a, and 8a) show lowered reproductive indices. This was again a 

 period of not very successful feeding and of considerable loss of 



