BACTERIA IN NATURAL PROCESSES. 95 



of the world would be impossible if bacterial 

 action were checked for any considerable length 

 of time. The life of the globe is, in short, de- 

 pendent upon these micro-organisms. 



BACTERIA AS SCAVENGERS. 



In the first place, we may notice the value of 

 these organisms simply as scavengers, keeping 

 the surface of the earth in the proper condition 

 for the growth of animals and plants. A large 

 tree in the forest dies and falls to the ground. 

 For a while the tree trunk lies there a massive 

 structure, but in the course of months a slow 

 change takes place in it. The bark becomes sof- 

 tened and falls from the wood. The wood also 

 becomes more or less softened ; it is preyed upon 

 then by insect life ; its density decreases more 

 and more, until finally it crumbles into a soft, 

 brownish, powdery mass, and eventually the 

 whole sinks into the soil, is overgrown by mosses 

 and other vegetation, and the tree trunk has dis- 

 appeared from view. In the same way the body 

 of the dead animal undergoes the process of the 

 softening of its tissues by decay. The softer 

 parts of the body rapidly dissipate, and even the 

 bones themselves eventually are covered with the 

 soil and disintegrated, until in time they, too, dis- 

 appear from any visible existence. This whole 

 process is one of decay, and the result is that 

 the solid mass of the body of the tree or of the 

 animal has been decomposed. What has become 

 of it ? The answer holds the secret of Nature's 

 eternal freshness. Part of it has dissipated into 

 the air in the form of gases and water vapour; 

 part of it has changed its composition and has 

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