744 THE SMALL-MAMMAL PROBLEM 
is regaining ground lost in the preceding two centuries. Here 
we have an excellent example of the working of the Balance 
of Nature even in the heart of a great city. So long as 
R. norvegicus and FR. rvattus compete on level terms, in a 
temperate country, the former must win; but if the former 
be denied access to a building which remains open in some 
way, and attractive to #. vattus, the latter will enter and 
thrive in its security from competition. 
There was a time when Britain possessed no member of 
the genus Rad¢tus. At that date the House Mouse, which 
arrived from the East possibly with the Neolithic or Bronze 
Age people, was in full possession of the dwellings. On one 
view of the evidence, the fact that the House Mouse has 
developed special insular forms, like those of St Kilda and the 
Faroes, might be cited as proof that J/as musculus had already 
made a conquest of human households and baggage at the 
dates when the first wanderers landed on those remote islands. 
Be that as it may, there is no reason to doubt that before 
the arrival of the Black Rat, the House Mouse filled all the 
accommodation available for parasitic Muridz in Britain, and 
if rats had not arrived in Britain to claim their present large 
share of the existing accommodation, all, in so far as it is 
suitable to JZus musculus, would now be filled by House Mice. 
The presence of a rat population keeps the numbers of mice 
in strict and proportionate control; what the House Mouse 
population of the country can be at any given moment is 
limited by the size of the rat population among other things. 
There are still some countries not yet colonised by A. vattus 
or &. norvegicus, and where the House Mouse is unknown. 
Yet in these countries the small-mammal problem is felt just 
as acutely as in the centres of European civilization. Native 
Muride swarm in the houses of Central Africa, and during 
recent years elaborate, costly, but fruitless attempts have been 
made to exterminate these pests there. In those parts of America 
where the exotic J/uvine have not yet obtained a footing, 
the native Crvicetine play the parts of house mice and house 
rats. Nor need we go so far afield; many of our own country 
houses are infested by Afodemus, more rarely by Lvotomys, 
and even on occasion by the exclusive Arvicola amphibius. 
