60 THE ECOLOGY OF ANIMALS 



control introduced insect pests of agriculture and 

 forestry. Medical entomology also added its quota 

 of ideas about the inter-relations of animal numbers, 

 as in the studies of Ross (1911) upon the malaria- 

 carrying mosquito. More recently the transmission 

 of diseases such as bubonic plague, tropical typhus, 

 Rocky Mountain spotted fever, and tularaemia from 

 mammals to man directly or by insect carriers, has 

 focussed attention on the same kind of questions. A 

 great deal of work upon economic problems is con- 

 fined to special, almost watertight, compartments. 

 Such subjects as plagues of field-mice, the carriage of 

 plague by rats and other rodents, malaria and mos- 

 quitoes, the control of the European corn- borer moth 

 in America or the sugar-cane leaf-hopper in Hawaii 

 or gypsy moth in America, have huge Hteratures of 

 their own. Ecologists are therefore faced with a 

 scanty but fairly well-ordered literature on animal 

 population studies by naturalists and ..ecologists, 

 and by a vast and iU- co-ordinated and speciahzed 

 literature of economic biology in which, however, 

 many of the ideas and facts of use to the develop- 

 ment of ecological theories may be found. Economic 

 biologists are not uniformly conscious of the trends 

 of animal ecology, but it should be stated at once 

 that three of the most progressive contributions to 

 the theories of animal numbers have arisen from 

 economic investigations, in one instance on marine 

 fisheries (Volterra,^ 1926), and in the others on 

 problems of insect pests (Thompson, 1924, etc., and 

 Nicholson, 1933). Economic investigations will 

 always supply a very large part of the facts from 

 which ecological generahzations can be made, since 

 their material is often collected on such a huge scale, 

 as for instance in connexion with the North Sea 

 fisheries, the experimental liberation of insect para- 

 sites, the international study of locusts, the routine 

 examination of rats for plague at sea-ports, or the 

 ^ Based on the work of Ancona. 



