The Evolution of the Kidney 57 



to reveal to us the exact time at which the uremic habitus, 

 as an adaptation to salt water, was acquired. 



Returning now from the fishes to the main evolutionary- 

 tree: during the coal-ages the low-lying lands were heavily 

 clothed in tropical and subtropical vegetation. There was a 

 high rainfall, the air was humid, the world was a swampy- 

 paradise inhabited by spiders, scorpions, centipedes and snails, 

 and lorded over by Amphibia that lived half in water and 

 half on land. But on the whole life was as stagnant as the 

 swamps in which it lived. It was too comfortable, and in 

 comfort the living organism comes to rest, its evolution stops 

 or regression begins. 



The moist paradise of the coal ages lasted until the Per- 

 mian; then in the great Appalachian Revolution a majestic 

 range of mountains, 3 to 4 miles high, was corrugated in the 

 region that now lies between Newfoundland and Alabama. 

 The Southern Hemisphere passed into a severe glacial period, 

 and in the Northern Hemisphere the warm moist climate of 

 the Carboniferous was replaced by aridity and seasonal chilli- 

 ness. The cycads, equisetums, clubmosses and tree ferns of 

 the coal measures were exterminated; all the great families 

 of the marine elasmobranchs were destroyed along with most 

 of the marine and fresh water teleosts; and the stagnant Am- 

 phibia changed slowly towards more terrestrial forms. It was 

 the sheer pressure of world-wide Permian dessication that 

 fostered the evolution of the reptiles which were driven in 

 extremis to living permanently on land. These new reptiles 

 had tough hides and relatively long legs with which to crawl 

 from one water hole to another; the e^g, for the first time in 

 vertebrate history, was encased in a waterproof shell and con- 

 tained within it the allantoic sac to receive the waste products 



