RIVER OF LIFE 39 



animals low in the scale of evolution are simply aggregates of 

 cells, bathed on all sides by sea water, which is at the same 

 time a supplier of food and a source of oxygen. As larger and 

 more elaborate bodies were evolved, the internal body cells 

 became shut off from direct access to salt water, but they 

 nonetheless still required life-giving oxygen and food. This 

 vital need was provided by the evolution of circulating body 

 fluids which carried food and oxygen from cells in contact with 

 the outside world to those buried deeper in the body. These 

 fluids — and blood is one of them — were in effect an internal 

 sea. Since such fluids as blood have taken over the functions 

 of sea water it is not surprising that they closely resemble it in 

 composition. 



During the long course of evolution in the early seas, sev- 

 eral groups of marine animals have independently been able 

 to adapt themselves first of all to life in fresh water, and later, 

 in a fewer cases, to life in air. These adaptations to new sur- 

 roundings were remarkable achievements, since before they 

 could occur it was not only necessary that the animal should 

 be able to carry its own sea water with it, in the form of the 

 bloodstream, but also that it should have a system of chemical 

 control which would keep the salts at a constant strength and 

 so prevent a fatal upset of the body chemistry. When the early 

 ancestors of land animals left the sea, first for fresh water and 

 later for the land, they thus carried with them a permanent 

 sample of the ocean. 



Although the salts of sodium, calcium, potassium, and mag- 

 nesium in the blood of human beings and of most vertebrates 

 are in the same ratio as in the modern ocean, they are in about 

 half the concentration. This difference, paradoxically enough, 

 is in perfect accordance with our thesis. Many believe that the 

 salts of the ocean may have originated as a solution of materials 

 derived from the erosion of the land and brought into the sea 

 by rivers. Heated by the sun and constantly blown over by 



