598 
STATISTICS: R. PEARL 
of organs arising from, the endoderm in their embryological development 
while but from 8 to 13% can be regarded as a result of breakdown of 
organ systems arising from the ectoderm. The remaining 30 to 35% of 
the mortahty results from failure of mesodermic organs. Taking a 
general view of comparative anatomy and embryology it is evident that 
in the evolutionary history through which man and the higher vertebrates 
have passed it is the ectoderm which has been most widely differentiated 
from its primitive condition, to the vaHdity of which statement the 
central nervous system furnishes the most potent evidence. The 
endoderm has been least differentiated in the process of evolution, while 
the mesoderm occupies an intermediate position in this respect. 
The results of this study add one more link to the already strong chain 
of evidence which indicates the highly important part played by innate 
constitutional biological factors as contrasted with environmental 
factors in the determination of the observed rates of human mortality. 
Here we have grouped human mortality into broad classes which rest 
upon a strictly biological basis. When this is done it is found that the 
proportionate subdivision of the mortality is strikingly similar in such 
widely dissimilar environments as the United States, England, and 
Southern Brazil. It is inconceivable that such congruent results would 
appear if the environment were the predominant factor in human mortal- 
ity. This conclusion does not overlook the fact that in some diseases 
the environment, in a broad sense, is unquestionably the factor of 
greatest importance. Nor does it imply that every effort should not 
be used to measure in every case the precise relative influence of consti- 
tution or heredity as compared with environment in the natural history 
of particular diseases. This constitutes one of the most pressing and 
difficult problems of medical science. 
^ A complete account of this investigation will appear shortly in the American Naturalist, 
That paper must be consulted for detailed discussion of the intricate pathological and 
«mbryological points involved in the statistical treatment of the data in this investigation. 
