THE SMALL-MAMMAL PROBLEM 745 



Station, habits, and food, no doubt, are regulated for 

 each species to a considerable degree by the constitution of 

 the species ; but often, changing circumstances betray the 

 fact that the familiar and apparent limitations bounding the 

 range of individual choice are not those imposed upon a species 

 by any inelasticity of its own constitution, but are such as 

 result from extrinsic influences such as the competitive presence 

 of other creatures. Exterminate a species from a given area, 

 and many different claimants for the vacant place rapidly appear ; 

 no one of those claimants, perhaps, will be able to fill the 

 whole vacancy, but between them all it is soon filled. In each 

 case the driving power comes from within; "increase and 

 multiply" carries "colonize or perish" as its corollary; few 

 individuals die voluntarily. 



It is, of course, necessary to-day to wage an active war 

 against both species of Rattus and against the House Mouse, 

 not only throughout Britain, but aboard the shipping in our 

 ports. The numbers of these rodents living in our midst 

 are far too great, putting the public health in peril and 

 occasioning serious economic loss ; they must, therefore, be 

 reduced. But mere extermination of these species is not, and 

 cannot be, a solution of the small-mammal problem. The 

 mere killing of a great number of individuals relieves the 

 survivors from much active competition for food and space, 

 and ensures a more rapid rate of breeding. The killing of 

 an old male rat at once provides food and space for three 

 or four immature rats that otherwise would have perished. 

 Extermination of R. norvegicus must inevitably result in better 

 chances for R. rattus in the struggle for existence. Exter- 

 mination of both species of rat must enormously increase the 

 space and food available for Mus musculus. The elimination 

 of these alien parasitic Muridae must inevitably bring us into 

 conflict with our native members of the family. That the 

 latter, hitherto, have not to any large extent invaded our 

 houses and towns is due solely to the fact that the alien 

 species are already in possession, and have been, so far, strong 

 enough to keep the native forms outside in the cold. The 

 externtination of all Muridae, native and alien, from Britain 

 would merely disturb the Balance of Nature in favour of other 



