554 GROWTH 



A more complicated series of experiments was made on P. aurelia or 

 P. caudatum and P. bursaria with food supplied by bacteria and yeast. 

 The P. husaria could eat the yeast, but the two other species could not. 

 Varying equilibria of populations could be established, depending on 

 the initial concentrations of the four organisms. In this case the com- 

 petition is in different niches. 



Populations of predators and prey are interesting and have been studied 

 in epidemiology, notably by Ross and Lotka working with the malarial 

 parasite. A simpler case, of less personal interest to man, is the competi- 

 tion of mixed populations of bacteria, Paramecium, and Did'mium nasu- 

 tum. The latter consumes a Paramecium every three hours. In such a 

 mixed population, Gause found that at first both the Paramecium and 

 the Didinium populations grew, but later the didinia ate all of the para- 

 mecia and then promptly starved. With medium with sediment in which 

 some of the paramecia moved about and thus were not available as food 

 for the didinia, the didinia ate the available paramecia and then starved 

 while the remaining paramecia grew. Another experiment utilized Bur- 

 saria truncatella, which preyed on P. bursaria. 



The experiments may be grouped in three classes : ( 1 ) two species in 

 the same ecological niche, competing for the same food; (2) two species 

 in different niches, competing for the same food; or (3) two species, 

 one eating the other. Gause (1935) has given mathematical analyses 

 of the equilibria, depending on the variables involved. Much progress 

 has been made in this phase of biological science, even though it is 

 less than a quarter of a century old, and well-planned experiments or 

 heuristic theoretical analysis may be expected to contribute to an under- 

 standing of the growth of the Protozoa, to ecology, and to historical 

 (evolutionary) biological science. 



Literature Cited 



Adolph, E. F. 1931. The regulation of size. Springfield, III. 235 pp. 

 Allee, W. C. 1934. Recent studies in mass physiology. Biol. Rev., 9: 1-48. 

 Beers, C. D. 1928a. The regulation of dietary insufficiency to vitality in the 

 cilia.te Did/f?iu>/i nasutnm. J. exp. Zool., 51: 121-33. 



1928b. Rhythms in infusoria with special reference to Didinium 



nasutum. J. exp. Zool., 51 : 485-93. 



1928c. Some effects of dietary insufficiency in the ciliate Didinium 



nasutum. Proc. Nat. Acad. Sci., Wash., 14: 132-37. 



