the South Alley Burrow had an absolutely greater 
surface area. These differences may be considered 
as differences in what may be called “metabolic 
rate”. Through the first 16-months history of 
these two burrows, observations were made on the 
time of opening of new entrances into the burrow 
and the time at which dirt was excavated from 
each of these passages. These data provide 
another index of the difference in character of 
the growth or metabolism of these two burrows 
(figs. 25 and 26). Two differences are apparent. 
First, by April 1948 26 passages had been con- 
structed into the North Alley Burrow, whereas 
only 18 had been constructed into the South 
Alley Burrow. This is an indication of a greater 
underground construction by the members of the 
North Alley Burrow. Second, by the end of 
1947 excavation from the initial 10 passages into 
the North Alley Burrow had ceased, whereas 6 
of the initial 10 passages into the South Alley 
Burrow were utilized for the excavation of dirt 
for several more months. Where excavations 
cease, the adjoining tunnels deteriorate through 
the accumulation of dirt falling from the roof and 
from rotting nesting material and stored food. 
Thus, early in the history of the North Alley 
Burrow there is evidence of the deterioration of 
its physical structure which was associated with 
the more poorly integrated social structure of the 
rats inhabiting it in comparison with that of the 
rats inhabiting the more stable physical structure 
of the South Alley Burrow. 
Statements regarding sociology find their sup- 
port on pp. 136-246, where this topic is considered 
in detail. 
B. A Stable Nonsenescing Burrow. Regardless of 
the social structure of the group of rats inhabiting 
a burrow system there is usually some evidence 
of the deterioration or senescence of its physical 
structure. One burrow system was encountered 
in Baltimore which gave evidence of becoming 
stabilized in size without the accompaniment of 
deterioration in physical structure. It was located 
on the north embankment of a railway “cut” 
directly opposite the passenger loading platform 
of the Pennsylvania Railroad Station. A 35° 
20 
