THE PEST OF RATS 5 
itself in all the warmer parts of the world. It 
is still prevalent in our South Atlantic States, 
in the West Indies and in South America. The 
tame white rats sold as pets are mostly of this 
and the black species. 
In habits these three rats are similar, with 
the important exception that the black rat and 
the roof-rat (which some zodlogists consider 
merely varieties of one species), do not bur- 
row under foundations, etc., as does the brown 
rat. On the other hand they are far more 
agile and addicted to climbing,—a decided ad- 
vantage in the tropics where a large part of 
their food is obtained from trees, in whose 
branches they frequently lodge their nests; and 
are less able to withstand cold than the brown 
rat, which survives arctic winters in whaling 
ships, apparently without distress. They are 
also less prolific, having only ten mammz to 
the brown rat’s twelve, and bearing on the 
average only about five young at a birth to the 
other’s eight. This difference in prolificacy 
alone would account for the great dominance 
of the brown rat, at least in North America; 
and it is to that species—the rat, par excellence 
