122 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



of nothing that can intervene to inject new potential. 

 For the human clock cannot be wound anew, not so 

 much as by a single turn of the key. In short, all 

 that man can do to secure the full potential for which 

 his organization fits him is to protect his slowly 

 declining machinery from the innumerable insults 

 that threaten it. 



The surprising thing is not that the life of the 

 human animal is so short, but that it is so long. If 

 we compare the longevity of man with that of 

 animals, we find it to be superior in all but a few 

 cases, such as the elephant and parrot. For this 

 relatively long life, there is a clear explanation in 

 the possession of an elaborate system of defenses. 

 With this system effectively operative, long life is 

 possible ; with it impaired, there must come disease 

 or death, or both. 



