122 ORGANIC AGENCIES 



Organic Agencies 



The organic agencies are animals and plants, both living and 

 after death. In some respects these agencies tend to counteract 

 the destmctiveness of others, and the protective effects may be 

 taken up first. 



(i) Protective Effects. — The protective effects of organisms 

 are almost entirely those of plants, since animals, on land at least, 

 are not sufficiently abundant to be of any importance in this con- 

 nection. A thick covering of vegetation, especially the elastic, 

 matted roots of grassy turf, protects the soil against the mechani- 

 cal wash of rain. How complete this protection often is, may 

 be seen in the different effects produced by a heavy fall of rain 

 upon a "grass field and on the adjoining ploughed lands, or even on 

 the roads. The roads may be so washed out as to be impassable, 

 while the grass fields have not suffered at all. In certain of the 

 western bad lands, the efficient protection given by grass is very 

 well shown ; where the grass has established itself thickly, the 

 country is gently rolling, but where it is absent, the wild and 

 broken bad lands are developed. 



Vegetation, especially grass, protects loose, light soils from the 

 wind, and often this is the only means by which sand dunes can 

 be held in place and prevented from overwhelming valuable lands. 

 Even the banks of rivers and the seacoast may be efficiently pro- 

 tected by plants. Dense masses of seaweed growing on the rocks 

 form an elastic buffer against the surf, and along low-lying tropical 

 coasts the mangrove trees, with their interlacing aerial roots, so 

 break the force of the waves that they cannot wash away even 

 fine mud. 



The only protection afforded by animals that requires mention 

 is in the case of coral reefs, which, thrown up along or parallel 

 to the coast, shield it from the heaviest surf. 



(2) The Destructive Effects of the organic agencies are decid- 

 edly subordinate to those of the other classes which have so far 

 been considered, but they are not without importance. We have 

 already learned how greatly the chemical activity of rain-water 



