The Evolution of the Kidney 49 



the salts and proteins of the tissue cells drew water by osmotic 

 pressure so that by degrees the organism tended to pass from 

 excessive hydration to edema and /// extremis to swell to 

 death. We may confidently assert that were the osmotic in- 

 filtration of water not arrested, survival in fresh water would 

 be impossible. The first step towards arresting the infiltra- 

 tion of water would naturally be to insulate the body as far 

 as possible by a waterproof covering. Why not believe that 

 the ever-present armor of the fossilized vertebrates of Silur- 

 ian and Devonian time was a defense against the osmotic in- 

 vasion of fresh water rather than against the claws and tail- 

 spines of the eurypterids? 



In the history of evolution we see repeated instances 

 where some adaptation is carried to absurd and disadvan- 

 tageous overdevelopment, and perhaps an insulation serving 

 primarily to repell fresh water may have been the genesis of 

 spines and tubercles and other armored absurdities as would 

 later serve to ward off strong-jawed, sharp-toothed predators 

 such as had not yet been evolved. It seems that it was from 

 certain of these protuberant spines that the fins were evolved. 

 If we take this path of interpretation, we must conclude that 

 what started out to be merely a waterproof insulation was 

 destined to supply the fishes with fins for swimming, with 

 an armament for battle, and the tetrapods with legs with 

 which to crawl about on land. 



But to invest the body in waterproof armor entailed im- 

 portant changes in internal anatomy as well. The multiple 

 segmental openings of the archaic coelomic tubules had to be 

 obliterated, and these tubules had to be arranged to drain into 

 the one posterior member which still pierced the now 

 armored skin. The evolution of the first archinephric duct 

 may have been fostered by the waterproofing of the body. 



