NUMBERS : STATISTICS 55 



limits which frequently come into force before space 

 is filled or food used up, are the attacks of predators 

 and parasites, the latter leading to outbreaks of 

 disease, and to violent fluctuations in the population. 



It is found that every species has a certain range 

 of densities which may be called its optimum range. 

 The range is greater than that found in human 

 populations because animals are mostly shorter- 

 lived and fluctuate more rapidly in response to dis- 

 turbing factors m the environment. Usually a species 

 does not remain at the optimum density but oscillates 

 about this region. In using the term optimum we 

 must be careful to distinguish between optimum 

 conditions which may be physiologically ot ecologic- 

 ally suitable for the species and the optimum density 

 of its population. The optimum density is the 

 highest density at which a species can remain without 

 bringing into action automatic checks such as epidemics 

 or starvation which will bring down its numbers to a 

 level dangerously near extinction. For we are prob- 

 ably justified in assuming that a species which can 

 maintain a consistently high average of numbers for 

 a great many j^ears has on the whole a better chance 

 of evolving quickly than if its density is always very 

 Jew. For, under conditions of ecologically stable 

 high density, mutations must be more frequent ; and 

 also the chances of local extinction through external 

 catastrophes are less than with low densities. In 

 actual practice few species maintain constant popu- 

 lations at all, so that we do not usually find more 

 than an approximation to the theoretically optimum 

 range. 



Probably owing to the interaction of physical 

 adaptations and animal inter-relations (which do not 

 always coincide in their incidence, i.e. are not always 

 in the same habitat), many species do not five under 

 the optimum conditions physiologically. Thompson 

 (1929) has suggested that insects only become pests 

 (i.e. very abundant) when all the conditions happen 



