122 THE STRUGGLE FOR EXISTENCE 



Such a situation has recency been analyzed by Lotka ('32a). We 

 might be told in this case that in experimenting with a homogeneous 

 microcosm without refuges we have sharply disturbed the process of 

 elementary interaction of two species. Instead of investigating "in 

 a pure form" the properties of the differential equation of the struggle 

 for existence we obtain a thoroughly unnatural phenomenon, and 

 all the conclusions concerning the absence of innate oscillations in 

 numbers will be entirely unconvincing. But for our case this is not 

 true. We have already mentioned that Dininium does not actively 

 hunt for Paramecia but simply seizes everything that comes in its 

 way. In its turn Paramecium fights with the predator by throwing 

 out trichocysts and developing an intense rapidity of motion, but 

 never hiding in this connection in the refuge of our type. In this man- 

 ner, we have actually isolated and studied "in a pure form" the ele- 

 mentary phenomenon of interaction between the prey and the preda- 

 tors in a homogeneous microcosm. The refuge in our experiment 

 presents a peculiar "semipermeable membrane," separating off a part 

 of the microcosm into which Paramecium can penetrate owing to its 

 taxis, in general quite independently of any pursuit of the predator, 

 and which is impenetrable for Didinium. 



When the microcosm contains a refuge the following picture can 

 be observed (see Fig. 31): if Paramecium and Didinium are simul- 

 taneously introduced into the microcosm, the number of predators 

 increases somewhat and they devour a certain number of Paramecia, 

 but a considerable amount of the prey is in the refuge and the preda- 

 tors cannot attain them. Finally the predators die out entirely owing 

 to the lack of food, and then in the microcosm begins an intense mul- 

 tiplication of the Paramecia (no encystment of Didinium has been 

 observed in our experiments) . We must make here a technical note : 

 the microcosm under observation ought not to be shaken in any way, 

 as any shock might easily destroy the refuge and cause the Paramecia 

 to fall out. On the whole it may be noted that when there appears a 

 refuge in a microcosm, a certain threshold quantity of the prey cannot 

 be destroyed by the predators. The elementary process of predator- 

 prey interaction goes on to the very end, but the presence of a certain 

 number of undestroyed prey in the refuge creates the possibility of 

 the microcosm becoming later populated by the prey alone. 



(2) Having in the experiment with the refuge made the microcosm 

 a heterogeneous one, we have acquired an essential difference of the 



