The Bacteria in Natural Waters. 



We have dwelt thus at length upon the general relation 

 between bacteria and organic decomposition because in 

 this relation will be found the master key to the distri- 

 bution of bacteria in water as well as in other natural 

 habitats. It is trUe that certain peculiar forms may at 

 times multiply in fairly pure waters; but in general large 

 numbers of bacteria are foxmd only in connection with 

 the organic matter upon which they feed. Such organic 



Fig. 1. 



matter is particularly abundant in the surface layer of the 

 soil. Here, therefore, the bacteria are most numerous; 

 and in other media their numbers vary according to the 

 extent of contact with the living earth. 



Natural waters group themselves from a bacteriological 

 standpoint in four well-marked classes, according to their 



