98 THE STRUGGLE FOR EXISTENCE 



the experiments were made in a moist thermostat at 26°C. with pure 

 lines of infusoria. 



(3) In an experiment of such a type all the properties of the me- 

 dium are brought to a certain invariable "standard state" at the end 

 of every 24 hours. Hence, we acquire the possibility of investigat- 

 ing the following problem : can two species exist together for a long 

 time in such a microcosm, or will one species be displaced by the 

 other entirely? This question has already been investigated theoreti- 

 cally by Haldane ('24), Volterra ('26) and Lotka ('32b). It appears 

 that the properties of the corresponding equation of the struggle for 

 existence are such that if one species has any advantage over the other 

 it will inevitably drive it out completely (Chapter III). It must be 

 noted here that it is very difficult to verify these conclusions under 

 natural conditions. For example, in the case of competition between 



TABLE IX 



Contents of the microcosms in the experiments with Osterhout' s medium 



CONTENTS OF THE MICROCOSM 



(1) Paramecium caudatum separately. 



(2) Stylonychia pustulata separately . 



(3) Paramecium aurelia separately... 



(4) P. caudatum + P. aurelia 



(5) P. caudatum + S. pustulata 



(6) P. aurelia + S. pustulata 



NUMBER OP 

 MICROCOSMS 



4 

 5 

 3 

 3 

 3 

 3 



two species of crayfish (Chapter II) a complete supplanting of one 

 species by another actually takes place. However, there is in nature 

 a great diversity of "niches" with different conditions, and in one 

 niche the first competitor possessing advantages over the second will 

 displace him, but in another niche with different conditions the ad- 

 vantages will belong to the second species which will completely dis- 

 place the first. Therefore side by side in one community, but occupy- 

 ing somewhat different niches, two or more nearly related species 

 (e.g., the community of terns, Chapter II) will continue to live in a 

 certain state of equilibrium. There being but a single niche in the 

 conditions of the experiment it is very easy to investigate the course 

 of the displacement of one species by another. 



(4) Two series of experiments were arranged by us in which the 



