THE EVOLUTION OF DEATH 63 



abolism becomes slower, but certainly one cannot therefore 

 assert that every lessening of the metabolism implies a becom- 

 ing old. According to the sum total of our knowledge we 

 must regard organization as the cause of function. This is 

 the only interpretation which a physiologist may admit. 

 When therefore an organization is so altered that the metabo- 

 lism diminishes, this diminution has to be considered a conse- 

 quence and not a cause. Metabohsm, however, is influenced 

 by many factors, as every practicing physician experiences 

 daily. If we accept Child's opinion we are led logically to 

 the conclusion that each one of us may become alternately 

 young and old according as his metabolism increases or 

 diminishes. We should have to say, for example, that a 

 man who performs strenuous and muscular work was reju- 

 venated, while on the contrary one carrying on mental work, 

 during which the metabohsm is less, might become older. It 

 seems to me clear that we cannot interpret the diminution of 

 the metabohsm as a characteristic of age in the sense of Cal- 

 kins. In other words, we cannot view the depression in proto- 

 zoa as senescence. Thus we reach the conclusion that natural 

 death, so far as we know at present, does not occur in unicellu- 

 lar organisms, and as a consequence of this we mention the 

 corollary that natura,l death first appeared in the world as the 

 higher multicellular plants an-d animals were evolved. 



We pass now to the examination of senescence in the 

 higher animals, a theme which has claimed my active interest 

 for many years. If we consider the phenomena as they are 

 known to us all, we recognize at once that a diminution of the 

 rapidity of growth is characteristic of age, and thus we are 

 induced to investigate growth. Obviously we must determine 

 how the rapidity of growth' alters with advancing age. For 

 such an investigation it is important to exclude the influence 



