62 Physiology of the Kidney 



stresses ahead of the other. Let us suppose that the proto- 

 mammalian forms got off to warm- blooded ness first, in adap- 

 tation to frigidity, rather than to uric acid excretion, in adap- 

 tation to aridity. The progressive evolution of warm- 

 bloodedness entailed a marked increase in the circulation of 

 the blood, which in turn entailed a corresponding increase 

 in arterial blood pressure; this increased blood pressure re- 

 sulted in an increased rate of filtration through the glomeruli, 

 and this entailed an increased need for conserving water by 

 reabsorbing it from the tubules. Rapid elevation of body 

 temperature, it would seem, would foster increased reabsorp- 

 tion in the tubules by accentuating the very need for it. It 

 is plausible, therefore, that the thin, intermediate segment of 

 the mammalian tubule with its accentuated capacity for re- 

 absorbing water was simply a sequel of the evolution of the 

 warm-blooded state, which evolutionary step may have been 

 taken before the habitus of uric acid excretion had become 

 fixed in the general reptilian stock. Once the intermediate 

 segment of the mammalian kidney had been evolved as an 

 adaptation to frigidity, it served as an adaptation to aridity 

 as well, for the enhancement of water conservation which 

 it effected enabled the mammals to compete, dry spell for dry 

 spell, with the more sluggish reptilian forms. Into whatever 

 dry spot the reptiles could radiate, the mammals could follow 

 them, and when the desert night descended and forced the 

 cold-blooded reptiles into sleep, the warm-blooded mammals 

 remained active and alert. But more important, perhaps, was 

 the change in temperature that marked the Laramide revolu- 

 tion; it may have been the inability of the reptiles to endure 

 this period of refrigeration and dessication that led to their 

 almost total extinction,'^ while the furry warm-blooded 

 mammals, equipped to meet both vicissitudes, could carry on. 



