66 THE ECOLOGY OF ANIMALS 



year cycle in temperature and rainfall in European 

 weather discovered by Briickner (1890). 



A great many fluctuations are caused by factors 

 primarily external to the life of the animal com- 

 munity. Examples of such factors are periodic 

 frosts, hot or cold summers, destruction of food 

 supphes through drought acting on vegetation, and 

 so on. The simplest hypothesis to account for the 

 widespread instability of animal populations is there- 

 fore that external disturbances in the weather, in 

 vegetation, interference by man or by the arrival of 

 new animals (whether by natural dispersal or the 

 carriage of man) set up trains of internal disturbances, 

 which upset the equilibrium of animal communities 

 and make the populations of many species fluctuate. 

 There is no doubt that this sort of thing does happen 

 to a very great extent. The world contains broad 

 differences of habitat, but does not supply constant 

 environmental conditions, nor even regular cyclical 

 conditions. The varied shape of the continents and 

 oceans of the world, the variable amount of solar 

 energy coming in as sunlight and heat, and the end- 

 less complicated results of these variations upon 

 weather, the irregular mass-production of snow and 

 ice which in turn produce further variations in 

 weather, make a world far different from the thermo- 

 stats of ecological laboratories. 



There is, however, a very interesting and important 

 recent group of theories, based in a large degree upon 

 mathematical treatment of certain ecological assump- 

 tions. The main thesis of these theories is that the 

 structure or organization of an animal community is 

 such that it will in itself give rise to natural rhyth- 

 mical fluctuations in numbers of the animals. The 

 development of these theories is due to the work of 

 Lotka (1920), Volterra (1926), Bailey (1931), and 

 Nicholson (1933). They have approached the ques- 

 tion by somewhat different methods. Thus, Volterra 

 deals with cases of a predatory animal and its prey 



