42 THE AMERICAN NATURALIST [Vol. LIV 



least able to meet successfully the vicissitudes of the 

 environment. The ectoderm has changed most in the 

 course of evolution. The process of differentiation which 

 has produced the central nervous system of man had as a 

 concomitant the differentiation of a protective mechan- 

 ism, the skull and vertebral column, which very well keeps 

 the delicate and highly organized central nervous system 

 away from direct contact with the environment. The 

 skin exhibits many differentiations of a highly adaptive 

 nature to resist environmental difficulties. It is then not 

 surprising that the organ systems developed from the 

 ectoderm break down and lead to death less frequently 

 than any other. 



The figures of Tables XII and XIII make it clear that 

 man's greatest enemy is his own endoderm. Evolution- 

 ally speaking, it is a very old-fashioned and out-of-date 

 ancestral relic, which causes him an infinity of trouble. 

 Practically, all public health activities are directed 

 towards overcoming the difficulties which arise because 

 man carries about this antediluvian sort of endoderm. 

 We endeavor to modify the environment, and soften its 

 asperities down to the point where our own inefficient 

 endodermal mechanism can cope with them, by such 

 methods as preventing bacterial contamination of water, 

 food and the like, warming the air we breathe, etc. But 

 our ectoderm requires no such extensive amelioration of 

 the environment. There are at most only a very few if 

 any germs which can gain entrance to the body through 

 the normal, healthy, unbroken skin. We do, to be sure, 

 wear clothes. But it is at least a debatable question 

 whether upon many parts of the earth's surface we 

 should not be better off without them from the point of 

 view of health. 



These tables indicate further in another manner how 

 important are the fundamental embryological factors in 

 determining the mortality of man. Of the three localities 

 compared, England and the United States may fairly be 

 regarded as much more advanced in matters of public 



