Why Bacteria Increase or Decrease 65 



to conditions under which other plants do not thrive. 

 In the struggle for existence, therefore, the sorrel is 

 enabled to crowd out the others when they are not 

 already crowded out by their own inability to grow. 

 Similarly, the addition of lime or gypsum to the soil is 

 followed by the appearance of white clover. This is an 

 indication that conditions have been created favoring 

 the vigorous development of the clover and favoring 

 the suppression by it of other plants not so well adapted 

 to the changed character of the soil. 



Something of the same nature occurs in water. Just 

 as soils of different origin and composition have their 

 own kinds of weeds, and their own proportions of the 

 different weeds, so waters of different origin and com- 

 position have, though in a more limited sense, their 

 own kinds of bacteria, and their own proportions of 

 the different kinds. Just as there are certain grasses 

 and other plants widely distributed where they are not 

 interfered with by man, so there are certain kinds of 

 bacteria almost universally distributed in our lakes and 

 rivers. They include the normal inhabitants of drinking- 

 water, as well as others which are not, strictly speaking, 

 water bacteria, but are so universally distributed in 

 the soil or in the intestinal tracts of birds, mammals 

 and insects as to gain constant access to surface water. 

 There is, however, an important difference between 

 the normal water bacteria and the others just mentioned. 

 The former can readily multiply in ordinary drinking- 

 water, while the latter are strangers there and perish 

 sooner or later. 



It thus becomes interesting to inquire why some 



