METHODS OF CULTIVATION. 159 



outset. It is evident that if one species multiplies 

 more promptly and rapidly than the others which 

 are associated with it, it will soon be present in 

 excess in a culture-fluid inoculated with the com- 

 mingled species, and that by using this stock to 

 start a second culture before other forms have 

 time to multiply, repeating the operation if neces- 

 sary through a series of cultures, we shall at last 

 exclude all except the single species which has 

 taken precedence by virtue of its rapid multipli- 

 cation. 



In the same way we may take advantage of 

 conditions relating to the composition of the cul- 

 ture medium, and to the temperature at which it 

 is maintained after inoculation with impure stock. 

 When the conditions are most favorable for the 

 development of a particular species, it is evident 

 that this will take precedence over others with 

 which it is associated. And it may happen that 

 conditions extremely favorable for one are entirely 

 unsuited for other species which, accordingly, do 

 not multiply at all. 



We have examples of this in the experiments 

 which have been made upon living animals, which 

 may be considered culture-experiments, in which 

 the blood of the animal serves as a culture-fluid, 

 and in which the temperature maintained is neces- 

 sarily that of the species used in the experiment. 

 Thus in the form of septicaemia in the mouse, 

 which has been studied by Koch, a drop of putrid 

 blood " containing bacteria of the most diverse 



