3OO L. L. WOODRUFF. 



viz., "A rhythm is a minor periodic rise and fall of the fission 

 rate, due to some unknown factor in cell metabolism, from which 

 recovery is autonomous." The rhythms are more evident when 

 a more constant environment is maintained, as I have shown in 

 a study of the effect of a particularly stable environment on 

 Gastrostyla steinii, during the months of July, August and Sep- 

 tember (cf. Fig. 5). 



Gregory ('09) has plotted the curve of a Stylonychia culture 

 for five-day periods from the data of Popoff ('07), which shows 

 that the first four of the so-called "deep depression" periods 

 emphasized by Popoff resolve themselves into " normal rhythms 

 from which recovery is autonomous " Gregory also points out 

 in her own 548 generation culture of Tillina magna that "the 

 curve which represents the general vitality of the protoplasm 

 shows the normal rhythmic fluctuations observed by Woodruff." 



I have previously interpreted as rhythms the tri-monthly de- 

 pressions in vitality, which Calkins and the earlier workers on 

 Paramecinm have noted, and the results obtained from my cul- 

 ture of Paramecium seem to indicate that the semi-annual cycles 

 of Calkins are also actually rhythms, recovery from which was 

 not autonomous under the conditions of a constant environment. 

 The general occurrence of rhythms in the life history of infusoria 

 is established, I believe, but to what they are due is still awaiting 

 discovery. 



Gregory has emphasized the point that " Enough considera- 

 tion has not been taken of the fact that not only does each indi- 

 vidual vary in its degree of sensitiveness at different periods in 

 the life history, as suggested by Towle and shown by the rhythms 

 of Woodruff, but each individual of the same species as well as 

 of different species has its own peculiar protoplasmic reactions. 

 Woodruff himself has failed to consider this fact in his last paper 

 on the effects of a varied environment on Paramecium. . . . He 

 cannot logically compare his results with those of Calkins for he 

 is not dealing with the same protoplasm. . . ." In 1905 I 

 wrote : " My cultures lead me to believe, with Simpson, that 

 the personal equation, if I may use that term, of the individual 

 selected to start a culture has the most influence in determining 

 the number of generations attained. . . . Calkins' discovery of 



