382 SENESCENCE AND REJUVENESCENCE 



Calkins, it is possible at present only to point out certain probable 

 factors concerned. In the first place, the rate and course of indi- 

 vidual senescence or rejuvenescence under a given complex of con- 

 ditions is probably different in different races of Paramecium and 

 other forms, and the rate and course of individual senescence or 

 rejuvenescence in a given race may differ under different conditions. 

 Differences of this kind also appear to some extent in planarians. 

 In Planaria velata the course of the age cycle depends upon the 

 character of the food (see pp. 169-75). With some kinds of food 

 progressive senescence from generation to generation occurs and 

 in a few generations death results, while with others rejuvenes- 

 cence and senescence balance each other in each generation. 

 Doubtless similar relations exist in Paramecium and other ciliates 

 between character of food, rate of senescence in each generation, 

 and degree of rejuvenescence in each reproduction. And it is not 

 at all improbable that various other factors besides nutrition, e.g., 

 many chemical agents, may influence the rate, degree, and course 

 of development, senescence, and rejuvenescence. 



Whether, as Calkins believes, some races of Paramecium and 

 other ciliates are not even potentially capable of conjugation can 

 be determined only by extensive investigation, and then only with 

 a certain degree of probability. It is of course conceivable that 

 in organisms with great capacity for agamic reproduction the 

 capacity to attain gametic maturity may not be realized under 

 ordinary conditions (see chap, x), but as yet we have no adequate 

 basis for maintaining that the potentiality is absent in such cases. 



In the higher animals a definite sequence of events is a funda- 

 mental characteristic of the life cycle, and it seems not wholly 

 logical to maintain that a sequence is entirely absent in the simpler 

 forms. We can scarcely doubt that an individual Paramecium, 

 continuing to live without reproduction and with sufiicient food 

 for maintenance in a constant medium which does not inhibit 

 metabolism, will undergo certain more or less definite changes, and 

 will show a life history. And it seems probable that if these changes 

 proceed sufficiently far without interruption by reproduction or 

 change in external conditions, the individual may attain maturity — 

 the physiological condition in which conjugation occurs — or may 



