CHAPTER IX. 



THE DISTRIBUTION OF MICRO-ORGANISMS AND 

 THEIR RELATION TO SPECIAL HABITATS. 



General Distribution. Micro-organisms are very generally 

 distributed over the surface of the earth and in its waters, and 

 are carried about as dust in the air. They flourish abundantly 

 in the digestive canals of animals and on their body surfaces. 

 Wherever there is organic matter, the dead remains of animal 

 and plant life, there are micro-organisms in abundance living 

 upon the dead material and, if the temperature and moisture be 

 suitable, transforming it into simpler chemical substances. In 

 the soil, bacteria, yeasts, molds and protozoa are fairly numerous, 

 especially in fertile soils near the surface. Their number rapidly 

 diminishes in the deeper layers, and at a depth of six to twelve 

 feet they are very scarce or entirely absent. The surface waters 

 of the earth contain large numbers of bacteria and protozoa, 

 especially numerous where organic matter is abundant. The air 

 contains considerable numbers of molds and bacteria suspended 

 as dust. The deep layers of the soil and water below impervious 

 rock strata are free from micro-organisms. The surfaces of snow- 

 covered mountains and of the frozen polar regions of the earth, as 

 well as the atmosphere in these regions, are practically free from 

 microbes. The atmosphere over large bodies of water during 

 calm weather, the air in damp cellars, in sewers and in undisturbed 

 rooms is germ-free, because the suspended dust particles settle 

 out and do not escape again into the air unless swept up by air 

 currents, which must be rather violent to remove them from 

 moist surfaces. 



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