COMPETITION FOR COMMON FOOD IN PROTOZOA 



95 



owing to an increase in the density of food the absolute values of the 

 maximal population in both species have considerably increased. 

 The character of growth of the mixed population now essentially 

 differs from the former one : Paramecium strongly influences Stylony- 

 chia, while Stylonychia has almost no influence upon Paramecium. 

 We simply have here an "alchemical stage" of investigation, and the 

 absence of an exact control of the conditions of the medium creates 

 the impression of a complete arbitrariness of the results of our experi- 

 ments. Unfortunately many protozoological researches are still in 

 this stage, and the idea is very widespread that "Protozoa are not 





♦oe 



*oo- 



J.myttlus 



Jn mixed population 



P. caudatum 



Separately *-& sls ~~~ o f 

 Jn mixed population 



V J - 6 7 



Days 



Fig. 19. The growth in number of individuals of Paramecium caudatum and 

 Stylonychia mytilus cultivated separately and in the mixed population. Me- 

 dium contains wild bacteria. 



entirely satisfactory for the study of populations as they require 

 bacteria for food, and it is very difficult to measure accurately and to 

 analyze the relations between protozoan population and the bacterial 

 population." 



In order to draw any reliable conclusions as to the quantitative 

 laws of the struggle for existence in Protozoa we must begin by elabo- 

 rating the technique of cultivation, keeping in mind the following ex- 

 cellent words of Raymond Pearl: "When the biologist exercises some- 

 thing approaching the same precision and infinitely painstaking care, 

 over all the most trivial details of a biological experiment that the 



