COMPETITION FOR COMMON FOOD IN PROTOZOA 113 



peculiar: having attained a certain maximum, the density of popula- 

 tion decreases and remains stationary at a lower level. Direct 

 observation shows that the bacteria at the close of the twenty-four 

 hour intervals between the changes of medium remain partly uncon- 

 sumed, and the limiting factor here is apparently an accumulation 

 of waste products and not an insufficiency of food. The fluctuations 

 in the density of population of the separately growing S. pustulata 

 are probably connected with some complex processes of the influence 

 of metabolic products on growth. As to the mixed populations, the 

 same regularity with which we had to deal previously repeats itself 

 here : one species finally completely displaces another, and the species 

 displaced is always Stylonychia pustulata. 



(3) Let us summarize the data of this chapter. We have studied 

 the competition between two species for a source of energy kept con- 

 tinually at a certain level. This process may be divided into two 

 periods. In the first period the two species compete for the still un- 

 utilized resources of energy. In what proportion this energy will be 

 distributed between the two species is determined by the system of 

 Vito Volterra's differential equations of competition, but the co- 

 efficients of the struggle for existence in these equations change in 

 the course of the growth of the population and are therefore more 

 complicated than in the preceding chapter. In the second period there 

 is but a redistribution of the completely seized energy between the two 

 species, which is again controlled by the differential equations of com- 

 petition. Owing to its advantages, mainly a greater value of the 

 coefficient of multiplication, one of the species in a mixed population 

 drives out the other entirely. 



