CHAPTER VIII 

 LAND AND SEA COMPARED 



IN ORDER properly to understand the conditions 

 under which life in the sea exists it is necessary 

 first of all to compare them closely with condi- 

 tions on the land in order to bring out the corre- 

 sponding features and to emphasize the contrasts. 



We all have a pretty good idea of conditions on the 

 land. On land life exists at the bottom of a sea of air 

 which entirely surrounds the earth. The thickness 

 of this blanket of air about the earth is such that 

 each square inch of the earth's surface supports a 

 weight of nearly fifteen pounds of air. So we say that 

 at the surface of the earth there is an air pressure of 

 approximately fifteen pounds to each square inch. 



Air is very light, much lighter than the living 

 substance — protoplasm — of which the bodies of the 

 plants and of the animals are composed. Conse- 

 quently all land living things, both plants and ani- 

 mals, must live on or just beneath the surface of the 

 ground at the bottom of the sea of air. 



But in spite of its lightness as compared with flesh, 

 air is fairly heavy. Therefore it has considerable 

 supporting power. Besides, it is constantly in mo- 

 tion in one direction or another, and this constant 

 motion gives it transporting power. 



The transporting power of air is of very great im- 

 portance both to the plants and to the animals. Very 



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