STRUGGLE IN NATURAL CONDITIONS 25 



Certain interesting observations have been also recently made by 

 ecological entomologists (Payne, '33, '34) on the host-parasite bal- 

 ance. It is possible to trace the process of the destruction of popula- 

 tion of the moth Ephestia by the hymenopterous parasite Microbracon 

 in the laboratory, and it appears from the observations that a great 

 many factors are important for the process of their interaction, and 

 particularly various relations between "susceptible" stage of host and 

 "effective" stage of parasite. The beginning of the theoretical investi- 

 gation of this case has been given by the interesting papers of Bailey 

 ('31, '33) and Nikolson ('33). For such investigations, however, the 

 populations of unicellular organisms are somewhat more convenient. 



(14) The few examples given above show sufficiently that the 

 processes of the struggle for existence among animals are of extreme 

 importance from a practical point of view. They are sharply out- 

 lined in isolated microcosms and therefore there is nothing surprising 

 if they have attracted the particular attention of the workers in the 

 domain of fishery. Lately the problem of the relations between 

 predatory and non-predatory fish has been discussed by Italian 

 authors (D'Ancona ('26, '27), Marchi ('28, '29), Brunelli ('29)). 

 D'Ancona collected the data of a statistical inspection of the fish- 

 markets in Triest, Venice and Fiume for several years. He claims 

 that the diminished intensity of fishing during the war-period (1915- 

 1920) has caused a comparative increase of the number of predatory 

 fish. He therefore reasons that fishing of normal intensity causes a 

 relative diminution of the number of predatory fish and a compara- 

 tive increase of the non-predatory ones. But his data are not con- 

 vincing and indeed Bodenheimer ('32) has recently shown that such 

 variations in the fish population existed before and after the war. 

 They are apparently not connected with the intensity of fishing but 

 probably are the results of certain changes of the environment. 

 However it may be, the material collected by D'Ancona stimulated 

 the highly interesting mathematical researches on the struggle for 

 existence of Vito Volterra. Although his mathematical theories are 

 not confirmed in any way by D'Ancona's statistical data, the im- 

 portance of Volterra's methods as a new and powerful tool in the 

 analysis of biological populations admits of no doubt. 



(15) In concluding this descriptive chapter of our book let us note 

 the following picture of the struggle for existence in nature. It is 

 only in the domain of botany that these processes are coming to be 



