THE PROTOVERTEBRATE 29 



whether they were then or ever numerotis enough to 

 set the evolution of the entire vertebrate phylum in a 

 pattern of external armor that was to persist for one 

 hundred and fifty million years. 



The eurypterid theory seems to the writer to be in- 

 adequate by itself to account for the armor of the ostra- 

 coderms. All samples of the fossil record from Silurian 

 and Devonian times, from Spitsbergen to Colorado, sug- 

 gest that some death-dealing enemy, swift, merciless and 

 irresistible, lurked in every comer of the world. This 

 enemy, we believe, was the medium in which the early 

 vertebrates were undergoing evolution: it was an enemy 

 they could not see but one that pursued them every 

 minute of the day and night, one from which there was 

 no escape though they deployed from Spitsbergen to 

 Colorado— the physical-chemical danger inherent in their 

 new envirormient: their fresh-water home. 



Water diJBFuses rapidly through all physiological mem- 

 branes in accordance with diflFerences in osmotic pres- 

 sure—more exactly, the diflFusion pressure of water mole- 

 cules—on either side. Salts lower the diffusion pressure 

 of the water in which they are dissolved and conse- 

 quently water diffuses from any dilute salt solution into 

 a more concentrated one until the diffusion pressure is 

 equalized. When the first migrant from the sea sought 

 to invade fresh water, its blood and tissues were rela- 

 tively rich in salts, a physical-chemical inheritance from 

 its primordial salt-water home. (We may on straight 

 extrapolation assume that at the opening of either Cam- 

 brian or Ordovician time the sea had at least one-half 

 and perhaps five-sixths of its present salinity.) This sa- 

 line heritage could not be wholly cast aside, and when- 

 ever the organism sought to migrate from salt into fresh 

 water, water tended to move into the body by osmosis. 

 Were this influx of water not arrested, the animal would 

 have taken in so much water that it would have met 

 death by dissolution as a gelatinous mass. Those mutant 

 forms that possessed the advantage of waterproof armor 



