134 THE FITNESS OF THE ENVIRONMENT 



Once perhaps the atmosphere of the earth 

 consisted chiefly of water and carbon dioxide; 

 but cooling has caused the condensation of 

 most of the water, and geological processes, 

 more recently aided by the action of vegetation 

 with coal and peat formation, have removed 

 nearly all of the carbon dioxide. The latter 

 transformations have resulted in the substitu- 

 tion of oxygen in the atmosphere. However, 

 the interior of the earth continues to deliver 

 through volcanoes large amounts of carbon 

 dioxide, and thus the original source of atmos- 

 pheric carbon dioxide persists as a diminish- 

 ing supply. 



To-day carbon dioxide makes up only a 

 little more than 0.03 per cent by volume of 

 the whole atmosphere, approximately 4.6 

 kilograms per square meter of the earth's 

 surface, or about 2,300,000,000,000 metric 



Water is, therefore, three fourths, carbon dioxide one fifth, 

 of the total, and all other substances amount to but 2 to 3 

 per cent. 



In like manner the materials ingested by an ordinary green 

 plant are proportioned, water making up more than nine 

 tenths, and carbon dioxide amounting to fully five times the 

 sum of all other substances combined. 



Needless to say, a large part of the water which enters 

 and leaves plants and animals has had no real share in their 

 organization. It is merely the bearer of dissolved substances 

 or the means, through evaporation, of lowering the tempera- 

 ture of their bodies. 



