THE AMERICAN NATURALIST 



[Vol. LVI 



rejuvenation hypothesis, chiefly represented by Biitschli, 

 Maupas and Richard Hertwig. " It has been found that 

 protoplasmic activity tends gradually to diminish in in- 

 tensity, and that associated with this diminution are 

 certain morphological alterations in the structure and 

 composition of the cell " (Kellicott, p. 209). These modi- 

 fications are known as senescence, the senescent condi- 

 tion of the cell consisting frequently in the relatively 

 large proportioti of cytoplasm as compared with nuclear 

 substance. Conjugation is assumed to restore the senes- 

 cent protoplasm to its original condition of vigor, bring- 

 ing about rejuvenation. It follows from this that pro- 

 toplasmic activity is cyclic and that periods of senescence 

 would lead to death unless fertilization should occur. 



The real evidence for this cyclic character of the life 

 processes of the Protozoans has been furnished by the 

 observations of Maupas and Calkins on Paramecium. 

 But observations of Jennings have shown that in differ- 

 ent forms of Paramecium conjugation and rejuvenation 

 may occur at very different intervals, and Woodruff has 

 been able to prevent cyclic relations by substituting nor- 

 mal conditions for the artificial and more uniform ones 

 of the laboratory. '-By continually altering the char- 

 acter of the food, and by imitating in other ways the 

 naturally variable conditions of pond life, he has been 

 able to continue a single race of Paramecium for over 

 five years " (quoting from Kellicott), during which peri- 

 od more than 3,000 generations were formed by simple 

 fission. It follows from these observations that proto- 

 plasmic activity among the Ciliata may not be cyclic in 

 character under certain conditions, and that when cyclic 

 I)eriods of depression or senescence do occur, the proto- 

 plasm may be restored to a condition of normal vigor, 

 either by physical or chemical stimuli, or by fertilization 

 (Kellicott, p. 212). 



Fertilization is in these cases a form of reaction that 

 takes place when external conditions become too uniform 

 to bring forth the normal vegetative activities, and that 



