THE MAMMALS 139 



sels SO that blood could be shunted away from the surface 

 when heat needed to be conserved. To keep the body 

 temperature down in hot weather and during physical 

 activity, these same blood vessels had to be dilated and 

 the skin wet with sweat, the evaporation of which would 

 increase heat loss. Thus a major factor in the evolution of 

 the homeothermic state was the increased development 

 of the nervous control of the blood vessels, both in the 

 skin and throughout the internal organs of the body, 

 one of the consequences of which was that the arterial 

 blood pressiure was not only stabilized but set at a 

 relatively higher level than in antecedent forms. Eleva- 

 tion of body temperature also required an increased sup- 

 ply of fuelstuffs for the tissues, increased oxygen con- 

 sumption, and increased production of carbon dioxide 

 and other metabolic products— all of which required an 

 increased rate in the circulation of the blood. But in- 

 creased blood pressure and increased circulation meant 

 an increase in the rate of glomerular filtration, which in 

 turn required increased tubular reabsorption of valuable 

 constituents from the filtrate, and particularly an in- 

 creased capacity to conserve the valuable water in that 

 filtrate by making a superconcentrated urine, a task 

 achieved in a truly significant manner by the mammals 

 for the first time in either vertebrate or invertebrate his- 

 tory. Since the mammals persisted in the urea-excreting 

 habitus, and since urea is a very soluble substance with 

 relatively great osmotic pressure per gram of protein 

 nitrogen, the homeothermic state invited them to go in 

 the direction of a superconcentrating kidney. The birds 

 exhibit this concentrating power to a slight degree, but 

 they were spared the requirement of an osmotically con- 

 centrated urine because of their uric acid-excreting 

 habitus. 



Once this concentrating kidney had been evolved 

 as an adaptation to aridity, it supplemented warm- 

 bloodedness as an adaptation to frigidity, and the mam- 

 mals were enabled to compete, dry spell for dry spell. 



