PERIODICITIES IN ORGANISMS. 331 



attack on this problem I have been primarily concerned with these 

 simple organisms. 



It has long been known that in the higher animals and man the 

 most characteristic physiological feature of the process of senescence 

 is a progressive decrease in the rate of metabolism, or of certain 

 fundamental metabolic reactions from birth and undoubtedly from 

 still earher stages onward. This is accompanied, as Minot and 

 others have shown, by a decrease in the rate and final cessation of 

 growth. 



In my own experiments on the lower animals I have found that 

 a similar decrease in metabolic rate and rate of growth occurs dur- 

 ing the life history. A very simple method has made it possible to 

 use the metabolic condition to a very large degree as a criterion of 

 physiological age. This method depends on the fact that the sus- 

 ceptibility to many, if not all, agents which kill by decreasing 

 metabolism in one way or another, varies with the general metabolic 

 rate. In concentrations of such agents which kill in the course of a 

 few hours the individual with the higher metabolic rate is the more 

 susceptible and dies sooner than the individual with lower rate. 

 This method has been checked and controlled by various others in 

 many ways and can be used very widely for comparing the metabolic 

 condition of different individuals among the lower organisms. 



I have found that the general metabolic rate in the simple ani- 

 mals, particularly in certain of the planarian worms which have 

 been the chief experimental material, decreases from a very early 

 stage of development on through life, as growth and development 

 proceed. A progressive decrease in rate of growth also indicates 

 that these animals undergo senescence. 



These planarians undergo a process of fission in nature in which 

 the posterior body region separates ofl: and becomes a new individual, 

 but we do not need to wait for the occurrence of such fission, for 

 we can induce the development of new individuals by cutting the 

 animals into pieces. Each piece develops a head at its anterior 

 end, a tail at its posterior end, and undergoes more or less dedif- 

 ferentiation and internal reorganization and usually develops into 

 a new whole with the characteristic structure of the species, but 



