Chapter VI 

 THE DESTRUCTION OF ONE SPECIES BY ANOTHER 



(1) In the two preceding chapters our attention has been concen- 

 trated on the indirect competition, and we have to turn now to an 

 entirely new group of phenomena of the struggle for existence, that 

 of one species being directly devoured by another. The experimental 

 investigation of just this case is particularly interesting in connection 

 with the mathematical theory of the struggle for existence developed 

 on broad lines by Vito Volterra. Mathematical investigations have 

 shown that the process of interaction between the predator and the 

 prey leads to periodic oscillations in numbers of both species, and 

 all this of course ought to be verified under carefully controlled labora- 

 tory conditions. At the same time we approach closely in this chap- 

 ter to the fundamental problems of modern experimental epidemi- 

 ology, which have been recently discussed from a wide viewpoint by 

 Greenwood in his Herter lectures of 1931. The epidemiologists feel 

 that the spread of microbial infection presents a particular case of 

 the struggle for existence between the bacteria and the organisms 

 they attack, and that the entire problem must pass from the strictly 

 medical to the general biological field. 



(2) As the material for investigation we have taken two infusoria 

 of which one, Didinium nasutum, devours the other, Paramecium 

 caudatum (Fig. 27). Here, therefore, exists the following food chain: 

 bacteria — * Paramecium — > Didinium. This case presents a consider- 

 able interest from a purely biological viewpoint, and it has more 

 than once been studied in detail (Mast ('09), Reukauf ('30), and 

 others). The amount of food required by Didinium is very great 

 and, as Mast has shown, it demands a fresh Paramecium every three 

 hours. Observation of the hunting of Didinium after the Paramecia 

 has shown that Didinium attacks all the objects coming into contact 

 with its seizing organ, and the collision with suitable food is simply 

 due to chance (Calcins '33). Putting it into the words of Jennings 

 ('15) Didinium simply "proves all things and holds fast to that which 

 is good." 



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