Varying Vigor in Bacteria 159 



humus- or manure-nitrogen is affected by the character 

 of the soil, by climatic conditions, and by the crop. 

 The character of the soil bacteria is also important. 

 The soil bacteria accomplish their work because of 

 their vast numbers, but the amount of work accom- 

 plished is not necessarily proportionate to their num- 

 bers. The soil bacteriologists have come to realize, 

 more and more, that bacteria, like higher plants, show 

 differences in vigor and in their ability to survive the 

 competition of other species. A cultivated field aban- 

 doned to itself is soon overrun by weeds; a certain 

 number of the latter become more prominent than 

 others, and a characteristic flora is, in time, established. 

 The nature of this flora is determined by soil and climate, 

 and likewise by the adaptation of the predominant 

 species to soil and climatic conditions. 



Something of the same nature holds good in regard 

 to the bacteria of our soils. The natural methods of 

 selection bring about not only a predominance of certain 

 species under given conditions of soil and climate, but 

 also endow these species with varying degrees of vigor 

 which may find expression in the rate of increase or in 

 the ability to form characteristic products. For instance, 

 there are certain species of bacteria capable of fixing 

 atmospheric nitrogen. Two strains of these bacteria 

 may be apparently alike in every particular, one multi- 

 plying as rapidly as the other. The amount of nitrogen 

 fixed by these two may show considerable differences. 

 We find analogous conditions when, by breeding or 

 selection, a strain of wheat, corn or potatoes is developed 

 that yields larger amounts of plant substance. In con- 



