424 Gary N. Calkins. 



The early curves appeared to indicate a periodicity of three- 

 month intervals, and this v^as taken to be the time of the usual 

 life cycle in culture of Paramcecium candatum; this conclusion w^as 

 based partly upon my ow^n results and partly on those of Jou- 

 kowsky and of Simpson, both of whom found that cultures of this 

 infusorian died out after three months of treatment. It w^as 

 found, however, and it may be seen from the now completed 

 curve of the A series (see Diagram I) that trimonthly periods of 

 depression were not fatal and that recovery occurred without 

 purposeful stimulation. Thus in the first apparent depression 

 (May, 1901,) the recovery was thought to be due to the stimula- 

 tion by jolting on a railroad trip of six hours; another in March, 

 1902, was considered due to a slight rise in temperature. These 

 periods of depression differ markedly from those of August and 

 December, 1901, and of June, 1902, when the individuals con- 

 tinued to die at a high rate, notwithstanding repeated jolting 

 experiments, increase in temperature, and the like, and the race 

 was saved only by change to a special diet after numerous 

 attempts and failures with foods of different kinds. The well- 

 marked cycles, therefore, with periods of depression which de- 

 manded stimulation of a decided character, were approximately 

 of six months' duration, while intermediate cycles of less impor- 

 tance were about three months long. The first of the six-month 

 cycles ran from February i, i90i,to August i, 1901, (see Diagram 

 I); the second from August 15 to January i, 1902; the third from 

 January i to July i, and the last from July to December 19, 1902. 

 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 only 126 times. 



The stimulation which resulted in the renewal of vitality after 

 the periods of depression in August and December, 1901, was due 

 to the change from hay infusion diet to beef extract for a 

 limited period (see Studies I and III). The same change failed to 

 work in the July, 1902, period of depression, and after the race 

 had become reduced to only six individuals, a successful sub- 

 stitute for the beef extract was found in the extracts of pancreas 

 and brain (see Studies III). Recovery, however, was not so 



