The Evolution of the Kidney 6 1 



upon the stage as though they had long been waiting impa- 

 tiently behind the scenes. 



Where these mammals had been throughout the long and 

 fantastic period of the Mesozoic is still a mystery. The oldest 

 known mammalian fossils date from the late Triassic or early 

 Jurassic periods, and these were already advanced and spec- 

 ialized creatures; no remnants of a stock which could have 

 been ancestral even to the Cretaceous forms have been dis- 

 covered. ^° However, it must be believed that truly mammal- 

 ian types were in existence in the early Triassic, and probably 

 even in the Permian, while the reptiles themselves were still 

 in a relatively primitive stage. Certain Triassic reptiles, the 

 cynodonts, resembled the mammals in such features as the 

 posterior jaw elements, the teeth, and the structure of the 

 shoulder girdle, and they stood with their limbs well under 

 the body, and it may be supposed that the cynodont reptiles 

 and the mammals were evolved out of a common Permian 

 stock. It need not be supposed, however, that this common 

 ancestral stock was warm-blooded, but neither must it be 

 supposed that it had acquired the reptiUan habit of excreting 

 uric acid; rather it may have been a semi-aquatic type that 

 degraded its protein nitrogen to urea, as we may suppose 

 was the case in the Pennsylvanian Amphibia. 



Proceeding from this premise, it is to be noted that there 

 were two environmental stresses operating in Permian time: 

 intense aridity and intense frigidity. The Permian was one of 

 the greatest ice ages of all time. Frigidity — the cold nights 

 of the desert and the long, cold, seasonal winters — placed a 

 high premium upon the ability to be continuously active, 

 even as aridity placed a premium upon the ability to travel 

 overland from one water hole to another. A nascent, evolv- 

 ing stock might possibly have adapted itself to one of these 



