xvi EDITOR'S INTRODUCTION 



foremost an ecologist : and I look forward to the time when 

 all the present ad hoc branches of applied biology will be 

 unified in relation to laboratories of pure and applied ecology. 

 I will give but one example of the value of ecological 

 knowledge and the ecological outlook in these matters. It is 

 a familiar fact that serious plagues of mice, rats, and other 

 rodents occur from time to time in various parts of the world, 

 often causing a great deal of material damage. At the moment 

 that I write these lines, the newspapers record a rodent plague 

 in California so serious that all crops are in danger over a 

 considerable section of the State. Readers of Mr. Elton's 

 book will discover that these violent outbreaks are but special 

 cases of a regular phenomenon of periodicity in numbers, which 

 is perfectly normal for many of the smaller mammals. The 

 animals, favoured by climatic conditions, embark on reproduc- 

 tion above the mean, outrun the constable of their enemies, 

 become extremely abundant, are attacked by an epidemic, 

 and suddenly become reduced again to numbers far below the 

 mean. When such a number-maximum is so accentuated as 

 to become a plague, remedial measures are called for locally, 

 and large sums of money may be spent. Eventually the normal 

 epidemic breaks out and the plague abates. The organisers 

 of the anti-rodent campaign claim the disappearance of the 

 pest as a victory for their methods. In reality, however, it 

 appears that this disappearance is always due to natural causes, 

 namely, the outbreak of some epidemic ; and that the killing 

 off of the animals by man has either had no effect upon the 

 natural course of events, or has delayed the crisis with the 

 inevitable effect of maintaining the plague for a longer period 

 than would otherwise have been the case 1 In the latter event, 

 it would actually have been a better counter-measure to do 

 nothing at all than to spend time and money in fruitless killing. 

 If remedial measures are to be desired, they must be of some 

 special sort. Either they must encourage the development 

 of the epidemic, as by introducing infection among the wild 

 population of the pest species ; or they must aim at reducing 

 reproduction, as in the Rodier anti-rat campaign, where after 

 trapping, only females are killed and all males liberated once 



