STUDIES ON LIFE-HISTORY OF PROTOZOA. 2O5 



absent, and that the division -rate would maintain a fairly uniform 

 level. The chemical composition of a pond is constantly chang- 

 ing ; every rainfall washes new salts into the water ; every wind 

 assists in turning the water over, and with these changes, it is 

 probable that the organisms get new supplies of the substances 

 necessary for their continued activities. When other things fail, 

 the organisms may resort to conjugation, but we may con- 

 sistently doubt whether this method of recuperation for Protozoa 

 is as widespread in nature as hitherto supposed. 



One other analogy is suggested by these experiments, viz., 

 the similarity of these periods of depression, and the subsequent 

 proliferation, to tumorous growths of various kinds in the higher 

 animals. The race of Paramcecium may be compared with the 

 tissues composing the body of a Metazoon. When the cells com- 

 posing these tissues approach the age-limit of the race, they divide 

 less and less often ; finally they come to a halt, so far as division 

 is concerned, and senile degeneration begins, which ends in 

 death. If, in a given tissue, after it has passed the period of 

 active division, some stimulant be given at a local area, thus 

 inducing a renewal of the dividing energy and proliferation of 

 the cells of that area, the result, from a pathological point ot 

 view, would be a tumor ; from a biological point of view, it 

 might well be the rejuvenescence of the cells, as in the case of 

 Paramcccinm. As in the latter case, a number of agents might 

 be responsible for calling out the increased activity, as, for example, 

 a blow, a parasite, an enzyme, a changed condition of the imme- 

 diate environment, etc. Any one of these might "rejuvenate" 

 the tissue-cells in the same way that a chemical induces a new 



cycle of generations of Paramccciuin. 

 MARINE BIOLOGICAL LABORATORY, 

 WOODS HOLI., August 6, 1902. 



