118 



THE STRUGGLE FOR EXISTENCE 



sediment). This figure shows the decrease in the number of Para- 

 mecia as well as the simultaneous increase in number and in volume 

 of the population of Didinium. (We did not continue these curves 

 beyond the point where Didinium attained its maximal volume.) It 

 is evident that the Paramecia are devoured to the very end. As it is 

 necessary that the nutritive medium should contain a sufficient 

 quantity of bacteria in order to have an intense multiplication of 

 Paramecia, we arranged also experiments in the test tubes on a daily 

 changed Osterhout's medium containing Bacillus pyocyaneus (see 

 Chapter V). In Figure 30 are given the results of such an experi- 

 ment which has led up, as before, to the complete disappearance of 

 both Paramecium and Didinium. Thus we see that in a homogeneous 



P. caudatum 



D. nasutum 

 P >-. 



Days 



Fig. 28. The elementary interaction between Didinium nasutum and Para- 

 mecium caudatum (oat medium without sediment). Numbers of individuals 

 pro 0.5 c.c. From Gause ('35a). 



nutritive medium under constant external conditions the system 

 Paramecium-Didinium has no innate periodic oscillations in numbers. 

 In other words, the food chain: bacteria — > Paramecium — » Didinium 

 placed in a limited microcosm, with the concentration of the first link 

 of the chain kept artificially at a definite level, changes in such a direc- 

 tion that the two latter components disappear entirely and the food re- 

 sources of the first component of the chain remain without being utilized 

 by any one. 



We have yet to point out that the study of the properties of the 

 predator-prey relations must be carried out under conditions favor- 

 able for the multiplication of both prey and predator. In our case, 

 there should be an abundance of bacteria for the multiplication of 



