102 



THE STRUGGLE FOR EXISTENCE 



(6) The data on the growth of the volumes of P. caudatum and P. 

 aurelia in a mixed population are given in Figure 22. The curves of 

 growth of each species in a mixed culture are presented here on the 

 background of control curves corresponding to the free growth of the 

 same species. It is easy to see that the growth of a mixed population 

 consists of two periods: (a) during the first period (till the eighth day), 

 the species grow and compete for the seizing of the still unutilized 

 energy (food resources). But the moment approaches gradually 



P. aurelia 



100 



cr Jn mixed population 



P. caudatum 



Fig. 22. The growth of the "volume" in Paramecium caudatum and Para- 

 mecium aurelia cultivated separately and in the mixed population on the 

 medium of Osterhout. From Gause ('34d). 



when all the utilizable energy is already taken hold of, and the total 

 of the biomasses of the two species tends to reach the maximal pos- 

 sible biomass under given conditions. (This happens on the eighth 

 day ; the total biomass is equal to about 210.) This first period corre- 

 sponds to what we have already observed in yeast cells, (b) After 

 this there can only arise the redistribution of the already seized energy 

 between the two species, i.e. the displacement of one species by another. 

 Figure 22 shows that such a displacement is actually observed in the 

 experiment: the number of P. caudatum gradually diminishes as a 



