440 



PROTOPLASM 



brates left the sea for the land, they took with them a blood 

 system which was in equilibrium with the sea water in which 

 it had developed. The following table (from Henderson) 

 compares the relative proportions of salts in the ocean and in 

 vertebrate blood: 



The differences in salt concentration are explained by differ- 

 ences existing between the salt content of the sea when our 

 vertebrate ancestors were marine and the salt content of the sea 

 today. The vertebrates took with them the proportions of salt 

 existing in the sea at the time when they left it. Since then, the 

 sea has changed, but not the blood. 



Whether life started in the sea or on the land we cannot say; 

 but if on land, it soon went into the sea and remained there until 

 the close of the Cambrian period. In early Cambrian, perhaps, 

 the blood stream was established. Our blood is, therefore, 

 presumably as salty as that of the ocean in early Cambrian, and 

 it is one-third as salty as is the sea at present. 



Substitution. — The question whether or not some other 

 element will do as a substitute cannot, as we have seen, hold 

 in certain cases {e.g., barium for calcium). The answer for 

 boron has been given by Brenchley, and it too is No. Among 

 52 elements tested, none proved capable of replacing boron. 

 McHargue says the same for manganese; no other one of the 

 more common elements, including iron, copper, zinc, boron, or 

 arsenic, will replace manganese in the growth of plants. These 

 facts indicate that in nutrition an element plays a certain role 

 because it is that particular element and not merely because itj 



