50 HUMAN BIOLOGY 



The sea water has dominated ahnost completely the 

 inorganic composition of the blood plasma in Limulus, less 

 so in Homarus, and very much less so in Carcharias, although 

 it has increased therein the Na:Mg ratio, which, however, 

 is only about one-fourth that in the blood of Limulus. 



The circulatory fluid of the eovertebrate must have been, 

 therefore, sea water of the time, and the concentration and 

 proportion of the salts in it then obtaining were maintained 

 in the blood of the vertebrates which exchanged the marine 

 for a land habitat in the early Carboniferous, and from 

 which later mammals developed. The blood plasma of the 

 latter is, on its inorganic side, then, but the sea water of the 

 early Cambrian, when the ratios of the elements and 

 the concentration of these were different from what they 

 are now. 



The maintenance of these through hundreds of millions of 

 years is undoubtedly a function of the vertebrate kidney. 

 There is in invertebrates no organ or organs having this 

 function, for the coxal glands of the lobster, which are 

 excretory, maintain the ancient ratios of the inorganic 

 elements in its blood plasma but do not control the concen- 

 tration of these, and in consequence the salts of its blood 

 are as concentrated as those of its habitat. In the whales, 

 the Cetacea, on the other hand, which have had a marine 

 habitat almost as long as the lobster (since the early Eocene), 

 the concentrations of the salts and the ratios of the elements 

 therein are the same as in the blood plasma of the horse 

 and pig, which, with the Cetacea, were derived, it is held by 

 some paleontologists, from a mammal form of the Triassic. 



In the long ages the kidney has ever thus performed 

 functions with a constancy and regularity which are unri- 

 valled in the world of life, except by those of the cell nucleus 

 which is, of course, of vastly more remote origin. This 

 constancy contrasts with the variations in functions which 

 other organs in vertebrates have undergone. It has made 

 the Vertebrata, with all their range of development, possible, 

 and without it there could be no change of habitat from 

 sea to land or fresh water and back again to sea, for with 

 each such change there would be a variation in the inorganic 

 composition of the internal medium, an impossible handicap 



