74 THE ECOLOGY OF ANIMALS 



therefore results in the destruction of as manj^ 

 eggs as are being produced ; the final density varies 

 with the conditions of each experiment (e.g. tempera- 

 ture). In an analogous manner the number of Arctic 

 terns nesting on a certain island in Spitsbergen was 

 hmited by the reactions of the birds, which fight 

 usually for a nesting territory of a particular size 

 (though apparently the territory has little or no 

 adaptive significance) (Summerhayes and Elton, 

 1928). In nature, animals live neither under con- 

 stant physical conditions nor as isolated units. The 

 result is that the populations fluctuate in a very 

 compUcated manner. The chief factors affecting the 

 fluctuations are. the type of inter-relations with other 

 species, the optimum conditions physiologically for 

 reproduction and existence, and the length of Hfe. 

 Migration is of great importance in smoothing out 

 local differences in density and allowing of adjust- 

 ment by the species to changing conditions. 



Fluctuations in numbers have an important bear- 

 ing upon evolution theories. Elton (1924, 1930) has 

 suggested that they might allow non-adaptive (i.e. 

 neither useful nor harmful) characters to spread in a 

 population, and that it may be in this way that the 

 apparently non-adaptive differences between closely 

 allied species are established. Ford (1931) believes 

 that no gene can exist in an animal without having 

 some kind of physiological influence upon viability, 

 and therefore that this explanation is not vaHd. 

 Haldane (1932) has analysed the matter mathematic- 

 ally and concludes tentatively that genotypic varia- 

 tions may spread through fluctuations in the popula- 

 tion, but at such a slow rate as to be negligible in 

 the evolution of species. A partial bridge between 

 these divergent points of view is the theory of Ford 

 (1931) that useless or even normally non-viable 

 characters may become established l3y spreading 

 during the expansion phase of a population when 

 competition is less severe, and may at the same 



