60 Physiology of the Kidney 



ever taking a drink of fresh water, why have they not lost 

 their glomeruH? Why, on the contrary, have the glomeruH 

 reached their fullest development in the order Mammalia? 



Let us review briefly what is known about early mam- 

 malian evolution. Through all the Mesozoic the mammals 

 remained in the background and let the reptiles have the 

 stage. During the dessication of the Permian these thick- 

 skinned animals, their legs ever growing longer, began to 

 crawl on their bellies all over the world, and to establish their 

 reputation for grotesquerie. In the Triassic, which was, like 

 the Permian, a period of aridity but one lacking marked 

 seasonal extremes of heat and cold and generally warm 

 enough to permit the luxuriant growth of ferns, tree ferns 

 and equisetums, reptilian peculiarities began to reach ex- 

 tremes. The more advanced took to walking on their hind 

 legs and strutted about like the lords of the universe. In the 

 Jurassic the climate reverted to subtropical humidity, and 

 the reptilian paradise was but slightly disturbed by the dia- 

 strophic movement that raised the Sierra Nevadas and 

 ushered in the Cretaceous. Here reptilian evolution culmi- 

 nated, on the one hand, in the great dinosaurs, the most mag- 

 nificent creatures and probably the dumbest per kilogram of 

 body weight that the earth has ever seen, and, on the other 

 hand, in the flying reptiles whose jaws were still filled with 

 teeth and whose front feet were still tipped with claws. Then, 

 at the end of the Cretaceous, when the Rocky Mountains 

 and the Andes were slowly rising, the curtain is rung down 

 on this Mesozoic scene with a suddenness that is almost dra- 

 matic. The dinosaurs dissappeared, the birds lost their teeth 

 and shaped their forelimbs into delicate wings, and a host of 

 new actors, in the form of the Cenozoic mammals, rushed 



