z 7 8 



THE QUARTERLY REVIEW OF BIOLOGY 



rather to the rate of the evolutionary 

 changes in mortality, than to their sense 

 or direction. 



THE PRIMITIVE GERM LAYERS AND 

 MORTALITY 



One obvious advantage of the organo- 

 logical classification of mortality is that it 

 permits the final grouping of deaths into 

 three large classes, based upon the assign- 

 ment of the different organs of the body 

 to the primary germ layers from which 

 they developed embryologically. There 

 are, of course, difficulties in making as 

 detailed and precise a distribution of the 

 mortality of the lower animals to the 



TABLE 4 



The distribution among the three primary germ layers of 

 each 100 organologically classified deaths 



GERM LAYER 



REP- 

 TILES 



BIRDS 



MAM- 

 MALS 



MAN 

 (SAO 

 PATJLO 

 1317) 



MAN 



(ENG- 

 LAND 

 AND 

 WALES, 

 I9 I 4 ) 



Ectoderm 



Mesoderm 



Endoderm 



0.7 



7-7 

 91.6 



O.I 

 II. 6 



88.3 



1.0 



*3-5 



85.5 



7-2- 



2.8.0 

 64.8 



II. O 



3S- 2 - 

 53-8 



Totals 



100. 



IOO. O 



100.0 



100.0 



100.0 



primary germ layers as can be done with 

 human mortality. But probably the final 

 intrinsic errors are not really greater, 

 because the greater accuracy of the diag- 

 noses based on autopsies in the lower 

 animals, as compared with the general 

 vital statistics of human beings based 

 merely upon the physicians ' opinion as to 

 the cause of death, probably offsets the 

 other deficiencies in the material. In 

 any case I think the results presented in 

 table 4 may be regarded as at least an 

 approximation to the true facts. 



The orderly character of the data in 

 table 4 is striking. Proceeding up the 

 evolutionary scale from the extreme left 



hand column of the table (Reptiles) to the 

 extreme right hand column (Man in 

 England) the proportionate mortality 

 from breakdown of organs developing 

 from the ectoderm, and from the meso- 

 derm, increases, while the proportionate 

 mortality associated with organs which 

 develop from the endoderm diminishes. 



These figures are shown graphically in 

 figure 3 . 



DISCUSSION 



In the initial discussion of the organo- 

 logical classification of mortality the 

 following statement was made (10, pp. 

 168 and 169): 



' 'Taking a general view of comparative anatomy 

 and embryology it is evident that in the evolutionary 

 history through which man and the higher verte- 

 brates have passed it is the ectoderm which has been 

 most widely differentiated from its primitive con- 

 dition, to the vitality of which statement the central 

 nervous system furnishes the most eloquent evidence. 

 The endoderm has been least differentiated in the 

 process of evolution, while the mesoderm occupies 

 an intermediate position in this respect. An elab- 

 orate array of evidence might be presented on these 

 points, but to do so would be supererogation. It 

 would amount simply to repeating any standard 

 treatise on the comparative anatomy of the 

 vertebrates. 



"From the present point of view we see that the 

 germ layer, the endoderm, which has evolved or 

 become differentiated least in the process of evolution 

 is least able to meet successfully the vicissitudes of the 

 environment. The ectoderm has changed most in 

 the course of evolution. The process of differentia- 

 tion which has produced the central nervous system 

 of man had as a concomitant the differentiation of a 

 protective mechanism, the skull and vertebral column, 

 which very well keeps the delicate and highly organ- 

 ized 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." 



These inferences were drawn from an 

 examination of the evidence from human 



