78 THE FITNESS OF THE ENVIRONMENT 



Such properties are our present concern. 

 Doubtless if it were not for the enormous 

 quantity of water which exists upon our 

 planet, all its physical properties would be of 

 little avail to bring about its universal im- 

 portance in nature. This, however, as has 

 been above explained, appears to be neither 

 an accidental nor an uncommon phenomenon. 

 Of the total extent of the earth's surface 

 the oceans make up about three fourths, and 

 they contain an amount of water sufficient, 

 if the earth were a perfect sphere, to cover 

 the whole area to a depth of between two and 

 three miles. This corresponds to about 0.2 per 

 cent of the volume of the globe. The occur- 

 rence of water is, moreover, not less important 

 and hardly less general upon the land. In 

 addition to lakes and streams, water is al- 

 most everywhere present in large quantities in 

 the soil, retained there mainly by capillary 

 action, and often at greater depths. The 

 atmosphere also contains an abundance of 

 water as aqueous vapor and as clouds. Now 

 the very occurrence of water upon the earth, 

 and especially its permanent presence, is due 

 in no small degree to its chemical stability 

 in the existing physical and chemical con- 

 ditions. This stability is of great moment in 

 the various inorganic and organic processes 



