COMPETITION FOR COMMON FOOD IN PROTOZOA 103 



result of its being driven out by P. aurelia. As several further experi- 

 ments have shown (see Fig. 24), the process of competition under our 

 conditions has always resulted in one species being entirely displaced 

 by another, in complete agreement with the predictions of the mathe- 

 matical theory. 



If we consider the curves in Figure 22 more in detail, we shall note 

 that they generally are of a rather complicated character. It is 

 interesting to note that P. caudatum in a mixed culture at the begin- 

 ning of the experiment grows even better than separately. This is 

 apparently a consequence of the more nearly optimal relationships 

 between the density of the Paramecia and that of the bacterial food, 

 in accordance with the observations of Johnson ('33). 



in 



(1) Although the situation in our experiments with Osterhout's 

 medium has been considerably simpler than in the case of the "oaten 

 medium," it is still too complicated for a clear understanding of the 

 mechanism of competition. In fact, why has one species been vic- 

 torious over another? In the case of yeast cells we answered that 

 the success of the species during the first stage of competition depends 

 on definite relations between the coefficients of multiplication and 

 the alcohol production, and that it can be exactly predicted with the 

 aid of an equation of the struggle for existence. What will be our 

 answer for the population of Paramecia? 



To investigate this problem we made the conditions of the experi- 

 mentation the next step in the simplification. We endeavored to 

 make a medium with a very small concentration of nutritive bacteria 

 and optimal in its physicochemical properties for Paramecia. Under 

 such conditions the competition for common food between two species 

 of Protozoa has been reduced to its simplest form. 



(2) As Woodruff has shown ('11, '14), the waste products of Para- 

 mecia can depress the multiplication and be specific for a given 

 species. In any case we are very far from an exact knowledge of 

 their role and chemical composition. Therefore first of all we must 

 eliminate the complicating influence of these substances. This 

 problem is the reverse of the one we had to do with in the preceding 

 chapter. There in the experiments with yeast we tried to set up 

 conditions under which the food resources of the medium should be 

 very considerable at the time when the concentration of the waste 



