46 Physiology of the Kidney 



as to endow the organism with considerable swimming power, 

 to enable it to move swiftly through the sea, or, as an alterna- 

 tive, to live in a swiftly flowing river. According to one 

 theory, first propounded by Chamberlain' and substantially 

 supported by Barrell,^ the sluggish ancestors of the chordates 

 had already migrated from the sea into the quiet brackish or 

 fresh water lagoons of the Cambrian continents when the 

 Grand Canyon revolution overtook them; the tilting of the 

 land accelerated the motion of the rivers and this accelerated 

 motion fostered the evolution of the dynamic, chordate 

 form. But another theory, offered by Moody,^* has it that 

 the prochordates appeared in fresh water somewhat later, 

 being literally driven into the rivers and lakes in Ordovician 

 time by the attacks of the giant marine cephalopods that had 

 then risen to supremacy in the seas. 



Whichever theory we accept, it is now agreed that it was 

 in the fresh waters of the Paleozoic continents, and not in the 

 sea, that the first chordates, and from them the ostracoderms 

 and early fishes, were evolved. When, in 1930, Professor E. K. 

 Marshall and I reviewed the comparative anatomy of the kid- 

 ney and on the question of the habitat of the early verte- 

 brates followed the fresh-water thesis as set forth by Cham- 

 berlain and Barrel, we were conscious of treading on uncer- 

 tain ground. ^^ But since that time the subject has been care- 

 fully reviewed by Romer and Grove,"^ and in the face of this 

 new evidence the fresh water hypothesis can no longer be de- 

 nied. 



Now, the very matrix of life is water, and the evolution 

 of the kidney is essentially the story of the evolution of the 

 regulation of the water content of the body. Marine inver- 

 tebrates — worms, star-fish, molluscs, etc. — are generally in 

 osmotic equilibrium with the sea, and they therefore face no 



