64 Bacteria in Relation to Country Life 



for their main supply on springs, which, as will be shown 

 later, are comparatively poor in bacteria. 



The determination of the number of bacteria in water 

 may be of distinct advantage in two directions. It has 

 been demonstrated that surface waters are influenced 

 by the character of the soil over which they flow and 

 by the drainage that they receive, thus giving rise to 

 marked variations between one stream and another, 

 even when no actual pollution exists. Yet, in the same 

 stream, and within the limits of seasonal variation, the 

 character and number of bacteria are fairly constant. 

 Hence, any sudden and abnormal increase in the num- 

 ber of organisms in any particular stream should be 

 regarded with suspicion, and as possibly, though not 

 necessarily, due to pollution with sewage. 



The counting of bacteria in water is also of value in 

 measuring the efficiency of filters whose purpose it is 

 to remove the organisms. It is therefore customary to 

 count the number of bacteria in the unfiltered, as well 

 as in' the filtered water, and a measure is secured thereby 

 of the proportion retained in the filter. Still, even here, 

 mere numbers are not considered a sufficient proof of 

 the efficiency of filtration, and attempts are made to 

 determine the proportion of certain kinds of bacteria 

 that are retained during the process. 



Causes affecting the increase or decrease of bacteria 

 in water. — The familiar appearance of certain grasses in 

 the meadow, of certain weeds in the fields, and of cer- 

 tain trees in the forest, is largely the outcome of natural 

 selection. The fact, for instance, that sorrel appears 

 in quantity in acid soils is attributed to its adaptation 



