EVOLUTION AND MORTALITY 



2-75 



of their mortality considerably tends to 

 increase one's confidence that this classi- 

 fication of mortality has some real biologi- 

 cal significance. 



This confidence is further increased if 

 table z is studied in detail. Let us con- 

 sider first Class II, the mortality attribu- 

 table to breakdown of the respiratory 



tiles. When we come to man this pro- 

 gression is broken. Respiratory mortality 

 is here lower. The thought at once sug- 

 gests itself that here is to be observed a 

 result of man's intelligent control of his 

 environment through housing, sanitation, 

 and hygiene. It is at least conceivable 

 that man's real biological position here 



60 



^Reptiles 



\% Mammals 



1 Man-SSo Paulo 



i Man-England and Wales 



uJLl 



■\ 



K1 nrfrB_ 



in m. z 3ze m m tk 



Sex. Md. Skel. Alim. Herv. Skin. Endo. 



Organ Classes 



Fig. i. The Mortality, per Hundred Deaths prom All Causes, Distributed among Nine Organ Systems, 

 in Reptiles, Birds, Mammals, and Man 



system. It is seen that this mortality 

 steadily and regularly increases propor- 

 tionately as we pass from reptiles, through 

 birds, to mammals. There appears to be 

 a clear evolutionary progression here. 

 The respiratory system of mammals ap- 

 pears to be less effectively adapted for 

 coping with the environmental stresses 

 and strains put upon it than it is in rep- 



should be with a respiratory mortality as 

 high or higher than that of the lower 

 mammals. But because he has learned 

 in some degree how to protect himself 

 from respiratory infections, and to treat 

 them so that they do not so often lead on 

 to death, his actual observed respiratory 

 mortality falls somewhat below that of 

 the lower vertebrate orders. 



