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 R. rattus 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 R. rattus, the latter will enter and 

 thrive in its security from competition. 



There was a time when Britain possessed no member of 

 the genus Rattus. 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 Mus 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 Muridae 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 Mus 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 R. rattus 

 or R. 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 

 Muridae swarm in the houses of Central Africa, and during 

 recent years elaborate, costly, but fruitless attempts have been 

 made to exterminate these pests there. I n those parts of America 

 where the exotic Murince have not yet obtained a footing, 

 the native Cricetince 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 Apodemus, more rarely by Evotomys, 

 and even on occasion by the exclusive Arvicola amphibius. 



