EVOLUTION TRACED BIOCHEMICALLY 



49 



part has gone to form the Hmestone deposits. The sodium 

 and magnesium have always been increasing in the ocean, 

 and the latter relatively more rapidly than the sodium. 

 This would account for the ratio of the two, lOo: 12. i, in the 

 ocean water of today, as contrasted with the ratio of 100:0.8 

 in the blood plasma of mammals. 



On the concentration of salts in the plasma of those 

 vertebrates which have had a marine habitat since their 

 origin, the gradually increasing concentration of the salts 

 in the sea has had a marked effect. This is seen in the elasmo- 

 branchs (sharks, dog fishes and rays) which have been 

 marine since their origin in the Ordovician or early Silurian. 

 The concentration in the plasma of the sand shark, Car- 

 charias littoralis, was found to be 1.938 per cent, slightly 

 more than twice that in the blood plasma of mammals. 

 The ratios of the elements to each other are, however, 

 but slightly changed, particularly in regard to magnesium, 

 the concentration of which is relatively far below that in sea 

 water. 



In invertebrates with a closed circulation which have 

 been marine as long as the elasmobranchs have, the blood 

 plasma is practically sea water. This is the case in the horse- 

 shoe crab (Limulus), which has always been marine since 

 the close of the Cambrian. In the blood of this the concen- 

 tration of the salts found was 2.98 per cent, whereas in the 

 ocean water of the habitat of the horseshoe crabs whose 

 blood was analyzed, it ranged from 2.9 to 3.12 per cent. 

 The sea water thus controls the composition of the plasma 

 in Limulus. It does so also in the case of the lobster, Homarus, 

 descended from a fresh water form of the Cretaceous. 

 The concentration of the salts in its plasma was found 

 to be 2.852 per cent, practically the same as that of its 

 habitat, but the ratios are not the same, as may be seen : 



