THE PROBLEM AND METHODS OF INVESTIGATION 87 



We are accustomed, and experience justifies the custom, to 

 measure age in man and the higher vertebrates by the time elapsed 

 since birth. We say that the individual is a certain number of 

 years old, and from the age in years we can reach fairly definite 

 conclusions as to physiological condition, i.e., physiological age. 

 In many of the lower forms, however, senescence does not neces- 

 sarily proceed at an approximately definite rate. In such organisms 

 the time elapsed since the beginning of development does not afford 

 any measure of the physiological age attained, for, as the following 

 chapters will show, the organism has not necessarily continued to 

 grow old during all of that time. Thus it is possible that among 

 members of the same brood, beginning development at the same 

 time, some may attain a much greater physiological age in a given 

 length of time than others. In short, we cannot measure age in 

 all organisms in terms of time. 



And, finally, we may attempt to modify the processes of senes- 

 cence and rejuvenescence and so to gain further insight into their 

 nature. The influence of external conditions and of quantity 

 and quality of nutrition may be determined. We may expect to 

 find that factors which influence the fundamental metabolic pro- 

 cesses or the structural substratum will affect the course or char- 

 acter of senescence and rejuvenescence in one way or another if 

 their action continues for a sufficiently long time. In many of 

 the lower forms reproduction may be induced experimentally by 

 the isolation of pieces of the body, which undergo a reorganization 

 into complete new individuals. These experimental reproductions, 

 wherever they can be induced to occur, affect the course of senes- 

 cence and as a matter of fact bring about a greater or less degree 

 of rejuvenescence. The problem is then accessible to analytic 

 investigation in the lower forms, and the results of such investiga- 

 tion afford a firm foundation for the interpretation of the phe- 

 nomena of senescence and rejuvenescence in the higher organisms, 

 where they are less accessible to experimental methods. 



REFERENCES 



Alexander, F. G., and Cserna, S. 



1913. "Einfluss der Narkose auf den Gaswechsel des Gehims," Biochem. 

 Zeitschr., LIII. 



