The Evolution of the Kidney 41 



acidity of the urine, for the conservation of the alkaH reserve 

 of the blood, and perhaps for the chemical synthesis of am- 

 monia, hippuric acid, and other compounds which are manu- 

 factured locally by the renal tissue. In the present stage of 

 our knowledge it would be dangerous to be dogmatic about 

 details, and in any case it is not my intention to discuss the 

 finer points of renal function. We are concerned here only 

 with the general pattern of structure and function in this 

 nephric unit, and with the inquiry. How did our kidney 

 come to have the architecture that it does? In pursuit of this 

 inquiry we must digress from the structvxre of the kidney to 

 the general evolutionary history of the vertebrates, which 

 history can appropriately be prefaced by a brief discussion of 

 the structure of the earth. 



According to the geologist the continents upon which we 

 live are but irregular slabs of granite some 15 to 40 miles 

 thick, floating like isolated islands upon a bed of basalt, the 

 rock which makes up the oceanic floor. Under this bed of 

 basalt, which is only some 700 miles thick, is a zone of semi- 

 fluid magnum extending to a total depth of about 1800 miles. 

 Innermost is a core of iron, some 4000 miles in diameter, 

 which is raised far above incandescent heat ( 6,000 °C.) by 

 the enormous pressure existing at the center of the earth. It 

 is now generally agreed by the geologist and the astronomer 

 that the earth was separated from the sun about 2000 million 

 years ago through disruption of the parent body by a passing 

 star, but the daughter planet remained molten and homo- 

 geneous for only a short time, quickly acquiring its present 

 stratified structure as it cooled and crystallized. 



The continents float above the average level of the earth's 

 crust because their granite is lighter than the basaltic bed 

 upon which they rest; as their exposed masses weather down 



