48 Physiology of the Kidney 



to tail in apparently impregnable armor which took the form 

 of bony plates, scutes or scales. Any sample of the vertebrate 

 population of Silurian-Devonian times from Pennsylvania to 

 Spitzbergen suggests that some death-dealing enemy, swift, 

 merciless and irresistible, lurked in every corner of the world. 



Why all this heavy armor? As Romer^*^ has pointed out, 

 the only visible enemies of the ostracoderms and early fishes 

 were the eurypterids that shared with them the continental 

 waterways. Admittedly some of these eurypterids were 

 much larger than the ostracoderms and fishes, and possessed 

 strong claws, but they were primarily sluggish mud crawlers 

 and unless they struck with their pointed tails, as does their 

 enfeebled descendent, LimuluSy or injected poison, as do their 

 offspring, the scorpions, their fearsomeness may have been 

 more apparent than real. The thesis that the armor of the 

 early vertebrates served primarily to protect them from pre- 

 dacious enemies is perhaps open to question. May I offer an 

 alternative suggestion: these vertebrates had an enemy which 

 they could not see, but one which pursued them every minute 

 of the day and night, and one from which there was no 

 escape though they fled from Pennsylvania to Spitzbergen — 

 a physical-chemical danger inherent in their new environ- 

 ment. When the first migrant from the sea took up residence 

 in fresh water, its blood and tissues, bearing the physical- 

 chemical imprint of its marine home, were rich in salts; for 

 we may on straight extrapolation assume that at the opening 

 of either Cambrian or Ordovician time the sea had one-half 

 or better of its present salinity. This saline heritage might be 

 in part erased, but it could not wholly be cast aside without 

 re-organizing every nerve and muscle cell. The evolution of 

 a regulated internal environment, if it had not yet begun, was 

 imperatively imminent. For in the new fresh-water habitat 



