132 



THE STRUGGLE FOR EXISTENCE 



Let us now turn our attention to the graph for the relaxation inter- 

 action. Suppose we introduce a definite amount of the predator, iV 2 , 

 at different densities of the prey (Ni). Then, before the critical 

 threshold of the latter is reached (N°, Fig. 34), an epidemic of Didi- 

 nium cannot start and the curves return on the ordinate. After the 



60 



.S. 70 



t^ io 



^ 



SO 



SO 



<0 

 5 V* 



% 3 * 



/V 2 (predator) 



20(30) 



O 10 20 ZO VO SO tO 



Individuals (Mi)-* 



Fig. 35. Diagram illustrating the transformation of the usual time-curves 

 (1) into relative graphs of interaction (2) for the "classical" Lotka-Volterra 

 fluctuation in numbers. 



critical threshold is reached there appears a relaxation which leads to 

 the destruction of the prey — the curves cross the abscissa. 



(2) This little amount of theory enables us to formulate our prob- 

 lem thus: How do the biological adaptations consisting of a very 

 active consumption of Paramecium by Didinium disturb the condi- 



theoretical case of the classical Volterra's oscillation in the usual form. If we 

 note the values of Ni and JV2 at different moments of time, and then plot Ni 

 against the corresponding N2, we shall obtain the closed curve reproduced 

 below. 



