278 ORGANIC EVOLUTION CONSIDERED 



composed of calcium combined with carbon and 

 oxygen. Plants are composed mostly of carbon, 

 hydrogen and oxygen, with a small per cent of nitro- 

 ■gen, together with the ingredients of the ash. 



Carbon, hydrogen, oxygen and nitrogen are pre- 

 eminently the most important elements in the organic 

 world. If any one of these elements had been left 

 out of the earth, or if it had been bound up in cer- 

 tain compounds in the earth's crust, so that it could 

 not be used by living beings, then life could not exist. 



Considering them in detail: Oxygen constitutes 

 more than one-fifth of the volume and nearly one- 

 fourth the' weight of the air, in which it is mixed, 

 but not chemically combined, with nitrogen. It com- 

 prises eight-ninths of the weight of water, and from 

 forty to fifty per cent of the solid crust of the earth. 

 It is by far the most abundant element in the surface 

 of the earth ; and when we consider the fact that it 

 combines with each of the seventy elements .except 

 one, we at once see the necessity for its great abun- 

 dance. If there had been only enough to aid in form- 

 ing the solid crust of the earth, then water and the 

 free oxygen of the air would not exist, and life would 

 be impossible. 



If to the oxygen entering into the solid part of the 

 earth there had been added the enormous quantity 

 that is found in water, and no more, then the free 

 oxygen of the air would be absent, and animal life 

 could not exist, for all animals require free oxygen. 



Hydrogen is another element that is necessary in 

 the structure of every living thing. Its absence would 

 mean the absence of life. It constitutes one-ninth of 

 the weight of water, and it forms a part of certain 

 other compounds that serve as food for plants. If it 

 existed in only small quantity, as is the case with 

 most elements, then it would all be locked up in com- 



