NUMBERS : DYNAMICS 67 



and makes certain assumptions about the dependence 

 of reproductive rates upon the abundance of food. 

 Nicholson, on the other hand, bases his theory upon 

 considerations of the chances of predators (or para- 

 sitoids) searching for and finding their prey (or host) 

 in different densities of population. Here again 

 certain assumptions are made about the ecological 

 reactions of animals while searching for food. Future 

 work alone can decide whether the assumptions 

 made in these mathematical studies are borne out 

 by ecological investigations. There seems no reason 

 to doubt that the purely mathematical side of the 

 theories is sound. At present we can only say that 

 there exists a very important possibiHty, suggested 

 by these workers, that even if the physical con- 

 ditions of the environment were completely constant, 

 or at any rate varied with exact regularity, we should 

 find in populations of wild animals oscillations that 

 were independent of any regular oscillations in the 

 environment. In fact, just as the human social and 

 economic system has certain properties (depending 

 upon the monetary sj^stem and the reactions and 

 biological relationships of human beings organized 

 in a certain complex but definite manner) that lead 

 to the development of trade cycles or epidemics, so 

 animal communities with their peculiar mode of 

 organization so difi'erent from plants, but somewhat 

 analogous to human communities, are subject to 

 internal oscillations of population that result from 

 and are set up from these inter-relations and the 

 great powers of increase of animals. 



It seems then likely, or at least possible, that 

 fluctuations in numbers in animal communities may 

 be brought about in two different ways : first by 

 external disturbances in the environment, and 

 secondly by internal oscillations peculiar to the 

 community itself. But in both types of fluctuations 

 the organization of animal inter-relations plays a 

 dominant part. Nicholson has studied the probable 



