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 Ra¢tus 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 2. xovvegzcus must inevitably result in better 
chances for FR. vattws in the struggle for existence. Exter- 
mination of both species of rat must enormously increase the 
space and food available for MW/us musculus. The elimination 
of these alien parasitic Muridz 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 
extermination of all Muride, native and alien, from Britain 
would merely disturb the Balance of Nature in favour of other 
