No. 57S] 



CYCLES AND BHYTHMS 



73 



We understand by a " cycle," in the sense with which 

 the term was first employed by Calkins, a more or less 

 periodic alternation of high and low vitality as measured 

 by the division rate. The lowering division rate indi- 

 cates the approach of a period of depression which was 

 interpreted as the equivalent of old age in metazoa, since 

 it indicates a weakening in the chain of vital activities and 

 ends in death unless conjugation or its equivalent is 

 permitted. No one since Maupas, so far as I am aware, 

 has attempted to limit a cycle in terms of definite numbers 

 of generations or definite lengths of time. In 1904 I 

 stated : 



The well-marked cycles, therefore, with periods of depression which 

 demanded stimulation of a decided character, were apparently of six 

 months duration, while intermediate cycles of less importance were about 

 three months long. . . . During the' first three cycles the number of 

 generations was nearly the same (200, 198, and 193, respectively), the 

 last, on the other hand, was much less, the individuals dividing onlv 

 126 times." 



The period of six months, more or less, or 200 + genera- 

 tions were not regarded as measures of the cycle, and it 

 was understood at that time that conjugation or its equiva- 

 lent always inaugurates a new cycle. Woodruff in 1905 

 introduced the term " rhythm" to designate the lesser 

 periodic fluctuations which I had called "intermediate 

 cycles. ' ' Since the entire substance of the much-discussed 

 problem of immortality in infusoria is bound up with this 

 question of the cycle, it is necessary to analyze the so- 

 called rhythms of Woodruff to see how they agree with or 

 differ from the so-called cycles. In Paramecium the 

 cycle consists of the history of a bit of protoplasm in an 

 ex-conjugant and its progeny from which conjugation or 

 its equivalent is excluded, until natural death of the en- 

 tire race ensues. If conjugation or its equivalent occurs 

 the old cycle is abandoned and a new one is started, and 

 there must be as many new cycles as there are times when 

 conjugation or its equivalent takes place. It is imma- 



13 "Studies on the Life History of the Protozoa," IV. Jour. Exp. Zooi., 

 Vol. I, ]904, p. 424. 



