CHAPTER IV 



THE KIDNEY 



The fact that the theater of evolution of the early verte- 

 brates was in fresh rather than salt water had momen- 

 tous consequences. In acquiring a heavy exoskeleton of 

 waterproof armor, the protovertebrate was transformed 

 from a dynamic, free-swimming form to a sluggish crea- 

 ture that perforce kept to the bottom of the rivers and 

 lakes where it groveled in the mud for food. However, 

 it came about that in time certain movable spines on 

 the armor of the ostracoderms evolved into the fins of 

 the Silurian and Devonian fishes, and from the pectoral 

 and pelvic fins there evolved the four legs of the tetra- 

 pods, permitting them to engage in amphibious life and 

 to establish the great empires of the reptiles, birds, and 

 mammals. Again, the armor of the ostracoderms re- 

 quired a complete reconstruction of the head and the 

 conversion of the primitive gill-arches into jaws; and 

 without jaws and the ostracoderm plates that persisted 

 as teeth, the jaw-bearing vertebrates would scarcely 

 have attained their predatory supremacy over the in- 

 vertebrates. Without the predatory power of jaws and 

 teeth and the possibility of swift and accurate pursuit 

 of prey there would have been no evolution of the 

 distance-sense-organs of smell, sight, and hearing, of 

 elaborate muscular co-ordination, of prevision of how 

 to get from here to there and the possible consequences 

 of going from here to there— in short, there would have 



