CHAPTER VI 



NUMBERS : DYNAMICS 



' ^ I ""HE causes which check the natural tendency 

 X of each species to increase are most obscure. 

 Look at the most vigorous species ; by as much as 

 it swarms in numbers, by so much will it tend to 

 increase still further. We know not exactly what 

 the checks are even in a single instance. Nor wiU 

 this surprise any one who reflects how ignorant we 

 are on this head, even in regard to mankind, although 

 so incomparably better known than any other animal. 

 This subject of the checks to increase has been ably 

 treated by several authors, and I hope in a future 

 work to discuss it at considerable length, more espe- 

 ciaUj^ in regard to the feral animals of South America.' 

 These words were written by Charles Darwin about 

 seventy-five years ago in TAe Origin of Species. The 

 work on ecology of animals of which he speaks but 

 which he never wrote, would have been of intense 

 interest, since these problems of numbers lie at the 

 very heart of all theories concerned with natural 

 selection and the origin of new varieties and species. 

 What progress has been made during these eighty- 

 five years ? The next impetus to the study of the 

 population problems of animals came from economic 

 biologists. Hjort in 1904 was studying the fluctua- 

 tions in numbers of the cod in European waters. 

 From about 1870 onwards entomologists became 

 involved in a long series of costly and difficult experi- 

 ments in which parasites were used in order to 



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