DESTRUCTION OF ONE SPECIES BY ANOTHER 



123 



corresponding process of the struggle for existence from all the ele- 

 mentary interactions between two species which we have so far 

 examined. In the case of an elementary interaction between preda- 

 tor and prey in a homogeneous microcosm very similar results were 

 obtained in various analogous experiments (see Table 6, Appendix). 

 In any case the more attention we give to the technique of experi- 

 mentation, the greater will be this similarity. In other terms, in a 

 homogeneous microcosm the process of the struggle for existence in 

 every individual test tube was exactly determined by a certain law, 

 and this could be expressed by more or less complex differential equa- 



O I 2 3 v s~ e 



Days 



Fig. 31. The growth of mixed population consisting of Didinium nasidum 

 and Paramecium caudatum (oat medium with sediment). Numbers of indi- 

 viduals pro 0.5 c.c. 



tions. For every individual microcosm the quantities of the preda- 

 tor and the prey at a certain time t could be exactly predicted with a 

 comparatively small probable error. 



Such a deterministic process disappears entirely when a refuge is 

 introduced into the microcosm, because the struggle for existence is 

 here affected by a multiplicity of causes. If we take a group of 

 microcosms with similar initial conditions the following picture is 

 observed after a certain time: (1) in some of the microcosms in spite 

 of the existence of a refuge all the prey are entirely devoured (they 

 might have accidentally left the refuge, hidden inadequately, etc.). 



