No. 630] HUMAN MORTALITY RATES 29 



bile. The best possible case that could be made out for 

 a biological factor in such deaths would be that contribu- 

 tory carelessness or negligence, which is a factor in some 

 portion of accidental deaths, bespeaks a small but definite 

 organic mental inferiority or weakness, and that, there- 

 fore, accidental deaths should be charged against the nerv- 

 ous system. This, however, is obviously not sound. For 

 in the first place in many accidents there is no factor of 

 contributory negligence in fact, and in the second place in 

 those cases where such negligence can fairly be alleged 

 its degree or significance is undeterminable and in many 

 cases surely slight. 



Senility as a cause of death is not further classifiable 

 on an organological basis. A death really due to old age, 

 in the sense of Metclmikoff, represents, from the point of 

 view of the present discussion, a breaking down or wear- 

 ing out of all the organ systems of the body contem- 

 poraneously. In a strict sense this probably never, or at 

 best extremely rarely, happens. But physicians and reg- 

 istrars of mortality still return a certain number of 

 deaths as due to "senility." Under the circumstances it 

 is not possible to go behind such returns biologically. 



