280 ORGANIC EVOLUTION CONSIDERED 



free nitrogen is, however, so great that it will, no 

 doubt, last for millions of years. 



If the supply of this element had been very small, 

 its compounds would long since have been washed 

 into the oceans, thus rendering the growth of plants, 

 and, consequently, animal life on the dry land, impos- 

 sible. 



Besides, if the quantity of nitrogen in the air were 

 greatly decreased, thus leaving the atmosphere mostly 

 oxygen, the destructive effect of conflagrations would 

 be so great that cities could not exist, and the perils 

 to property and to human life would be greatly mul- 

 tiplied. 



If, the quantity of nitrogen in the air were largely 

 increased, then the oxygen of the air would be too 

 much diluted to serve, in the best way, the purposes 

 of breathing and combustion. 



Phosphorus is a fifth element that is necessary to 

 the organic world. It is found in greatest abundance 

 in the seeds of plants and the bones of animals. It 

 exists in small quantities, as phosphates, in all fertile 

 soils. In the free state it is extremely poisonous, 

 and its preponderance, as an element, in the structure 

 of the earth would destroy all life, either directly, as a 

 poison, or indirectly, by uniting with all the oxygen 

 of the air. 



The metal calcium is a necessary part of bones, and 

 of the shells of animals. There is nothing, so far as 

 we know, that could take the place of phosphate of 

 calcium in the growth of bones, or of carbonate of 

 calcium in shells and corals. 



In addition to the above, iron, chlorine, sulphur, 

 sodium and other elements are important and prob- 

 ably necessary in most plants and animals. If any 

 one of the various metals of the alkalies had greatly 

 preponderated in the composition of the earth, then 



