XVI 



ANIMATE AND INANIMATE NATURE; 

 PLANTS, ANIMALS, AND MAN IN THEIR 

 RELATION TO ONE ANOTHER 



84. The earth is acted upon by the physical forces of nature, 

 and has been thus influenced ever since it came into existence. 

 Earthquakes, volcanic eruptions, ocean waves and currents, 

 rivers, rains, and winds destroy land in one place and make 

 new land in other places. Compared with the work of these 

 agencies, the influence of plants and animals seems insignifi- 

 cant, although it is of great importance for man. 



Plants increase the fertility of the soil by the decay of 

 their roots, leaves, stalks, and wood. The dark color in the 

 surface soil is due to the finely divided particles of decayed 

 vegetation. In forest regions this black surface soil is 

 sometimes three feet thick, while in the prairie regions it 

 is seldom more than a foot thick. This admixture of 

 decayed vegetation is of the greatest importance to the 

 agriculturist, because very few plants grow in pure sand 

 or in pure clay. The roots of plants penetrate into the 

 crevices of rocks, and, by their growth, widen the cracks ; 

 and the carbonic acid, which is one of the products of the 

 decay of plant and animal tissue, accelerates the rotting 

 of rocks. Rocks which are not below the frost line are 

 much changed by that agency. The moisture, penetrating 

 the fine vertical cracks and accumulating under the horizon- 

 tal layers, expands very powerfully when it freezes, and 



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