NATURAL DEATH, PUBLIC HEALTH 225 



of the body along the evolutionary pathway appears to be 

 an important factor in determining the time when the 

 organ systems in which they are chiefly involved shall 

 break down. Those organ systems that have evolved 

 farthest away from original primitive conditions are 

 the soundest and most resistant, and wear the longest 

 under the strain of functioning. So then, the second 

 large result is that it is the way potentially immortal 

 cells are put together in mutually dependent organ sys- 

 tems that immediately determines the time relations of 

 the life span. 



But it was possible to penetrate more deeply into the 

 problem than this by finding that the duration of life is 

 an inherited character of an individual, passed on from 

 parent to off spring, just as is eye color or hair color, and 

 with a relatively high degree of precision. This has 

 been proved in a variety of ways, first directly for man 

 (Pearson) and for a lower animal, Drosophila, (Hyde, 

 Pearl) by measuring the degree of hereditary transmis- 

 sion of duration of life, and indirectly by showing that 

 the death rate was selective (Pearson, Snow, Bell, Ploetz) 

 and had been, since nearly the beginning of recorded his- 

 tory, at least. It is heredity which determines the way 

 the organism is put together the organization of the 

 parts. And it is when parts break down and the organ- 

 ization is upset that death comes. So the third large re- 

 sult is that heredity is the primary and fundamental 

 determiner of the length of the span of life. 



Finally, it is possible to say probably, though not as 

 yet definitely because the necessary mass of experimen- 

 tal evidence is still lacking, but will, I believe, be shortly 

 provided, that environmental circumstances play their 



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