34> Life and Death, Heredity and Evolution 



ing the regular series of fissions. The matter requires actual 

 cytological study before we can know whether or not any- 

 thing of the sort occurs in Glaucoma. 



In Paramecium, Woodruff and Erdmann (1914) found 

 that while the replacement is going on, and for a little before 

 and after, the rate of reproduction is slower than at other 

 times. Does this mean a waning of vitality, which is cor- 

 rected by the replacement? Jollos (1916) found thajt under 

 favorable and uniform conditions, the slowing of the fission 

 rate is limited sharply to the period in which the processes 

 of replacement are occurring. This slowing of cell division 

 would therefore appear to be merely a natural result of the 

 fact that while the complicated changes of endomixis are 

 occurring, the organism does not so readily undergo at 

 the same time the involved processes of fission. It is there- 

 fore not clear that there is any indication of loss of vigor, 

 of senile changes, setting in before each replacement occurs. 

 The process seems to be carried on, like many others in 

 nature, with such a "margin of safety" that there is no 

 indication of exhaustion before it occurs ; it takes place be- 

 fore there is a pressing need for it. 



Thus it appears that in these organisms nature has em- 

 ployed the method of keeping on hand a reserve stock of a 

 material essential to life ; by replacing at intervals the worn 

 out material with this reserve, the animals are kept in a state 

 of perpetual vigor ; not, as individuals, growing old or dying 

 a natural death. Nevertheless, a wearing out process, such 

 as might be called getting old, does occur in the structures 

 employed in the active functions of life, and these must be 

 replaced after a time of service. So far as the conditions 

 in these organisms are typical, deterioration and death do 

 appear to be a consequence of full and active life ; life carries 

 within itself the seeds of death. It is not mating with an- 



