The Evolution of the Kidney 53 



later paragraph. The more advanced of the fishes, however, 

 in order to survive in the stagnant waters of the continents, 

 took to swallowing air and thus invented lungs and prepared 

 the way for the evolution of the terrestrial vertebrates. 



At the close of the Devonian the earth suffered its third 

 major upheaval in vertebrate history; the periodic dry spells 

 of the Devonian were replaced by protracted and widespread 

 dessication and many of the air-breathing fishes followed the 

 example of the Silurian elasmobranchs and abandoned fresh 

 water for refuge in the sea where they founded the Paleozoic- 

 Mesozoic dynasties of marine teleosts. But certain of the 

 fresh water fishes, the Crossopterygians, learned to use their 

 fins for feet with which to crawl from one pool to another, 

 and thus founded the Carboniferous and Pennsylvanian Am- 

 phibia which needed to return to the water pools only occa- 

 sionally to drink and to lay their eggs. 



For a moment let us consider what must have happened 

 to the bony fishes that took up life in the sea in the Carbonif- 

 erous. Actually, none of those Mesozoic forms survives to- 

 day, all the recent marine teleosts having been evolved since 

 the opening of the Cenozoic era, but the physiology of these 

 recent forms is adequate to illustrate the difficulties of chang- 

 ing one's habitat from fresh to salt water. 



With the migration from fresh to salt water the osmotic 

 relations between organism and environment are reversed; 

 the body tissues are less concentrated than the sea, and unless 

 the composition of these tissues is completely overhauled, 

 they must tend constantly to suffer osmotic dehydration and 

 ultimate dessication and collapse. The marine bony fishes 

 face not a perpetual excess of water, like their fresh water 

 ancestors, but a perpetual deficit of it. In theory they could 

 maintain the accustomed proportion of salt and water in the 



