NUMBERS : DYNAMICS 71 



and the physiological explanations advanced by 

 Hcape have as yet only limited evidence in their 

 support. The means by which animals find their 

 way about are also little understood, and informaiion 

 on this point seems essential for classifying their 

 modes of migration. 



In certain cases at any rate, no distinction can be 

 made between migration, emigration, and nomadism. 

 Thomson (1929) has shown that the woodcock in the 

 British Isles partly undertakes regular seasonal 

 migrations, partly emigrates in all directions, and 

 partly remams resident with local wandering move- 

 ments. We are here concerned with the effect of 

 migration on the density of population. The drifting 

 movements of mixed flocks of small birds searching 

 for food in an English wood or South American forest 

 are good examples of the manner in which the 

 searching movements of animals smooth out local 

 differences in density of numbers. There is evidence 

 that m nearly all animals such searching movements 

 are of great importance in damping down the violence 

 of fluctuations in numbers. It is probable that with- 

 out migration the continued existence of an animal 

 community would be impossible at its present degree 

 of complexity. Movements introduce a certain 

 elasticity in local densities of numbers, which is to 

 say that the psychological reactions of animals form 

 one of the chief problems of animal population. 

 Nicholson (1933) has worked out a theory containing 

 76 conclusions about numbers, based almost entirely 

 on considerations of the results of searching, i.e. of 

 migratory movements. 



The spread of animals, in so far as it is not accom- 

 plished by passive dispersal, is due both to large- 

 scale emigrations, such as tliose of the lemming, and 

 to the accumulated effects of small local wanderings. 

 It is probable that some at least of the large-scale 

 migrations are due to real over-population, i.e. to 

 ' pressure of numbers '. But the studies of Middle ton 



