A PHYSIOLOGICAL CHARACTER. 143 



some workers to "depression periods" in the Cladocera stock. This 

 explanation is rejected for the following reasons: 



1. These periods of local drops or rises in individual reaction- 

 time curves are extremely irregular in their occurrence and do not 

 affect all the strains or the two strains of the same line equally. 



2. There is no coincidence between changes in reaction-time and 

 reproductive index and no demonstrable relation between reaction- 

 time and the vigor of the stock, as measured by the reproductive 

 index. 



3. Cases such as Lines 714, 794, and 795, in which these non- 

 genetic differences appeared immediately upon the beginning of 

 selection (figures 7c, lie, and 12c), can scarcely be attributed to 

 depression periods, inasmuch as the two strains of each of these 

 lines had just been derived from the same progenitor. 



4. Males or ephippial females were not found in this stock at 

 any time during the selection experiments, either in periods of 

 unusually low or high reproductive indices or unusually slight or 

 great reactiveness; yet if males or ephippial females occurred in any 

 appreciable numbers (particularly the latter) they would have been 

 detected. 



5. Depression periods as such are not seen in the stock bred in 

 the laboratory. 



The writer believes that these fluctuations are merely reflections 

 of relatively local environmental influences upon the individual 

 strains concerned. 



6. Environmental Conditions as Affecting Vigor of Stock. 



Obviously vigor is closely related to environmental factors. As 

 to the Cladocera reared in this laboratory, there is reason to think 

 that their vigor, with most of the species at any rate, is completely 

 controlled by environmental influences and is not affected by internal 

 factors, such as the effect of long-continued parthenogenetic repro- 

 duction. 



Reliability of the Reproductive Index. 



The reproductive index is a safe criterion to use as a measure of 

 general vigor and, it is believed, of general muscular activity as well. 

 During times of lowered reproductive activity (appearing in decreased 

 numbers in first broods and in increased age of mothers when first 

 broods are produced), such as frequently affected several or all of our 

 strains of the same species at the same time, there was high mortality, 

 more sterility, more cases of failure to produce a second brood, and 

 frequently, with the Daphnia species, a noticeable decrease in general 

 muscular activity so far as swimming movements were concerned. 1 



1 It seems surprising that this is not reflected in increased reaction-time means, but (as is 

 shown elsewhere) there is extremely little evidence of a relation between vigor and reaction-time. 

 During part of the time, when the stock was low in reproductive vigor, selections were omitted. 



