DESTRUCTION OF ONE SPECIES BY ANOTHER 



115 



All the experiments described further on were made with pure 

 lines of Didinium ("summer line") and Paramecium. In most of 

 the experiments the nutritive medium was the oaten decoction, "with 

 sediment" or "without sediment," described in the preceding chapter. 

 Attempts were also made to cultivate these infusoria on a synthetic 

 medium with an exactly controlled number of bacteria for the Para- 

 mecia, but here we encountered great difficulties in connection with 

 differences in the optimal physicochemical conditions for our lines 

 of Paramecium and Didinium. The introduction of a phosphate 

 buffer and the increase of the alkalinity of the medium above pH = 



Fig. 27. Didinium nasutum devouring Paramecium caudatum 



6.8-7.0 has invariably favored the growth of Paramecium, but hin- 

 dered that of Didinium. Satisfactory results have been obtained on 

 Osterhout's medium, but here also Didinium has grown worse than 

 on the oaten medium. Therefore, absolute values of growth under 

 different conditions can not be compared with one another though all 

 the fundamental laws of the struggle for existence remained the same. 

 The experiments were made in a moist thermostat at a temperature 

 of 26°C. 



(3) Let us first of all analyze the process of interaction between 

 the predator and the prey from a qualitative point of view. It is well 

 known that under natural conditions periodic oscillations in the num- 

 bers of both take place but in connection with the complexity of the 



