302 THE CHEMISTRY OF THE PRIMITIVE OCEAN [aPP. I 



modified sea water. We still find animals, coelenterates, of this kind in 

 the sea. Then the circulatory system became shut off from the outside 

 world and formed a closed system of canals. When this closure took 

 place the composition of the blood, as regards the relative proportions 

 of the inorganic substances in it, was the same as that of the 

 surrounding sea water. Now these evolutionary changes, first that of 

 the multicellular animal, then the acquisition of an open, and then a 

 closed blood vascular system, took a very long time and during this 

 interval the composition of the sea was continually changing. 



The reader should remember how very strong is the influence of 

 heredity, and how slowly, in spite of evolutionary changes, the more 

 fundamental of the characters of living organisms change. Thus 

 protoplasm is a mixture of immensely complicated substances. Each 

 of the proteids contained in it is a molecule-complex composed of a 

 great number of " building stones," each of which is itself a complex 

 molecule. It is almost infinitely variable in composition, as variable 

 probably as the species of organisms. Yet in spite of this variability 

 we find it everywhere essentially the same in general characters and 

 reactions, though a number of different types of structure are conceivable. 

 If the protoplasm of both animal and vegetable cells is the same, 

 then it is probable (arguing in the approved Darwinian manner) that 

 both animal and vegetable protoplasm is similar, in general composition, 

 structure and reaction to the protoplasm of those ancient organisms 

 from which both kingdoms originated. Again, the highly complex 

 process of nuclear division is similar in both animal and vegetable 

 cells, and this argues in favour of the continuity of structure and 

 reactions of protoplasm all down through the ages. 



W^hy again should vertebrates, molluscs and Crustacea have skele- 

 tons composed of carbonate and phosphate of lime instead of (say) 

 siliceous or clayey skeletons, or perhaps iron ones ? Silicon and 

 aluminium are more abundant in the earth's crust than calcium, and 

 one can easily conceive of bones or shells in which the inorganic 

 matrix is silica or some alumina compound, or some oxide or carbonate 

 of iron. Hosts of organisms have siliceous skeletons (though none 

 have a skeleton in which the earthy basis is aluminium or iron). 

 Obviously the skeletons of the ^ertebrata contain lime as their 

 inorganic basis because at the time when the " provertebrata " lived in 

 the sea the water of the latter contained a large proportion of calcium 

 salts in solution. For long ages these organisms secreted lime from 

 the sea and so a " lime habit " of metabolism became established, and 



