A PHYSIOLOGICAL CHARACTER. 145 



strains of the D. pulex stock, but it scarcely affected the two S. 

 exspinosus strains (figures 14a and 17c). ^ 



During August-September 1913 all the D. pulex strains and all 

 the S. exspinosus strains show relatively high reproductive indices. 

 Again, for October-November 1914 all the strains of D. pulex and 

 2 (of 4) strains of S. exspinosus have high reproductive indices. 

 These are particularly marked for the D. pulex strains. For the 

 period immediately following, December-January 1915, most of the 

 strains of both species show considerably lowered though not particu- 

 larly small reproductive indices (figures Ic, 7a, 8a, 11a, 12a, 13a, 14a, 

 and 17c). In October-November 1915 all the 4 strains of S. exspino- 

 sus and 3 of the 4 strains of D. pulex have high reproductive indices. 

 Still other periods of more or less coincident fluctuations in repro- 

 ductive indices may be seen in the curves. 



Mean reproductive indices for all the D. pulex strains and for all 

 the S. exspinosus strains were determined, and it was found that the 

 coincident high and low points in reproductive index as given above 

 were clearly reflected in these means. 



These fluctuations in reproductive indices are in two cases 

 (August-November 1912 and June-July 1913) clearly related to 

 poor food, and perhaps to other unfavorable conditions, and it is 

 believed the other coincident fluctuations are likewise associated with 

 environmental factors. 



Independent Fluctuations in Vigor. 



Vigor is closely related to environmental conditions and many 

 coincident fluctuations in vigor have just been referred to; but be- 

 cause of the necessarily somewhat differential treatment (see page 140) 

 of the two strains of the same line (and of the different lines), inde- 

 pendent fluctuations are much in evidence in the figures showing 

 reproductive index (figures Ic, 3a, 7a, 8a, 11a, 12a, 13a, 14a, and 

 17c). 



For example, in figure 17c, showing reproductive indices for the 

 two strains of Line 757, conspicuous instances in which the changes 

 in reproductive index for the two strains of this line are in reverse 

 directions (as compared with the preceding period) are seen in the 

 February-March 1914, August-September 1916, October-Novem- 

 ber 1916, and February-March 1917 periods. Six other less con- 

 spicuous fluctuations (in reverse directions) are seen in this same 

 figure. Some of these small fluctuations are probably due to the 

 reproductive indices for the two-month period, having been derived 

 from a relatively small amount of data; but this is not beheved to 

 be the case in general and can scarcely be assumed to account for 



' It ha3 been found in other observations that while the physiology of these two species of 

 Cladocera is fundamentally very similar, yet physiological differences not infrequently appear. 

 The lack of a pronounced lowering of the reproductive indices for the two S. exspinosus lines for 

 this period is an illustration of these physiological differences between the two species. 



