194 GARY N. CALKINS. 



through five such periods, and have been able to rejuvenate them 

 " parthenogenetically " five successive times. I feel that I am 

 now in the position to confirm the early observation of Dujardin 

 that, in Paramcecium at least, there are more or less regular 

 periods of vigor and depression which alternate with each other 

 in fairly regular succession. At each period of depression, the 

 organisms, had they not been stimulated in some way, would 

 have died, thus showing that they were physiologically worn out. 

 The subtitle of this paper is taken from one of the periods of 

 depression when the last survivors of a long line were nearly lost. 



The methods employed in the investigation are practically the 

 same as those used by Maupas. The infusoria are kept in cells 

 in depression slides filled with a food medium of hay-infusion. 

 The original Paramaciitm of the A series was taken from the pond 

 water of Van Cortland Park, New York, and transferred to one 

 of these cells. In two days it had divided three times and of the 

 resulting eight individuals four were isolated in the same way as 

 the first had been, while the remainder were set aside as " stock." 

 The four lines thus started have been maintained throughout and 

 give a better basis for the study of vitality of the race than one 

 line could possibly have given., A second series B was started 

 at the same time with an individual Paramceciwn from Fort Lee, 

 New Jersey, and four lines of this series were likewise carried on 

 with the A series. Individuals are isolated every one or two 

 generations and the records are kept in the form of a table show- 

 ing the number of generations since the last isolation and the 

 total number since the beginning. 



The division-rate is taken as the measure of vitality, for it rep- 

 resents, the rate of metabolism, growth and reproduction. A 

 better index of the general vitality could not be found and 

 while fairly constant' from day to day, its fluctuations mark out 

 clearly the periods of vigor and depression. The variations of 

 the rate are difficult to follow when the daily records alone are 

 considered and the cultures had been under way for ten months 

 before a method was discovered of representing it graphically. 

 When the daily rate of division is plotted the fluctuations of the 

 curve, due partly to varying temperature and the food conditions, 

 are perplexing and it is extremely difficult to compare the con- 



