14 



THE AMERICAN NATURALIST [Vol. LIV 



as given in official reports of vital statistics, under the 

 code known as the International Classification, into 

 another classification upon a biological basis. The basis 

 taken is organological, each " cause of death" is charged 

 against that organ or organ system, the functional break- 

 down of which is fundamentally responsible for the death. 

 It is found that from 85 to 90 per cent, of all statistically 

 recognized causes of death can be subsumed under such a 

 biological classification. It is found when this is done 

 that the order of significance of the different organ sys- 

 tems in responsibility for human mortality is in general 

 that of the following list, the arrangement being in de- 

 scending order : 



1. Respiratory system. 



2. Alimentary tract and associated organs. 



3. Circulatory system and blood. 



4. Nervous system and sense organs. 



5. Kidneys and related excretory organs. 



6. Primary and secondary sex organs. 



7. Skeletal and muscular system. 



8. Skin. 



9. Endocrinal system. 



The arrangement differs slightly for different coun- 

 tries. If the further step is taken of referring the dif- 

 ferent organs and organ systems to the primary germ 

 layers from which they embryologically developed, it is 

 found that the death rates chargeable to organs of (a) 

 ectodermic, (b) mesodermic and (c) endodermic origin 

 stand to each other somewhere between the ratios of 1 to 

 2.3 to 4.4 and 1 to 4.4 to 7. The evolutionary and public 

 health significance of these results is discussed at some 

 length. 



