THE SMALL-MAMMAL PROBLEM 743 
progress, finding congenial surroundings in the cellars and 
drains of the towns, and along the banks of the rivers and 
canals. As described above (p. 583), the coming of A. nor- 
vegicus to Britain led to the almost complete extinction of 
R. rvattus as an inland inhabitant of this country during the 
eighteenth and nineteenth centuries. Quite recently, however, 
R. rattus has been recovering its lost ground—a fact of import- 
ance in connection with the argument now being presented. 
When &. xorvegicus arrived in Britain, in the early part 
of the eighteenth century, it found &. vattus in complete 
possession. At that date rat-proofing was not attempted, and 
the rat population in the towns was kept in control by the 
ratcatcher. So far as human intervention was concerned, both 
species were upon terms of equality. In these conditions, 
favoured by its own constitution and character, 2. norvegicus 
triumphed and #. vattus was defeated. In the later part of 
the Victorian period it was difficult to procure any example 
of the latter species away from the vicinity of the docks. 
Towards the end of the nineteenth century, and in later years 
down to 1915, great changes took place in many of our Cities. 
Many of the older houses were demolished, and upon their 
sites were erected buildings of stone, characterised by a lavish 
use of cement in their foundations, as well as by the possession 
of perfect sanitary appliances and well-guarded drains. These 
buildings, particularly the most modern examples, are for the 
most part proof against rats seeking entrance through base- 
ments, and therefore they remain free from infestation by 
R. norvegicus. But FR. rattus is primitively an arboreal species; 
and telephone wires and cables now extend from building to 
building, bridging the streets. The chief waterside colonies 
of R. rvattus to be found in our ports are situated for the 
most part in lofty, solidly-built warehouses. These rats 
issue from the upper floors of the dockside premises, and 
pass along the telephone wires to the roofs of other buildings. 
Roof kitchens are among the improvements of the modern 
structures, and FR. vattus soon invades them, effecting an 
entrance from the roof through skylight, window, or ventilator. 
In this way new colonies are formed at increasing distances 
from the docks, and in many towns, as in London, the species 
