arn 
mediate vicinity of some extensive artificial shelter, such as a com- 
post heap, straw stack, or the like. Undoubtedly there are individ- 
uals leading a wild life, but they cannot beabundant. There have been 
instances of rats’ becoming soabundant in the fields as to be adecided 
pest. For example, in the summer of 1903 immense numbers appeared 
in several counties of western Illinois, especially in Mercer and Rock 
Island counties, and were a veritable plague during that and the 
following year.* I have never heard of a similar visitation in this 
county. It is pretty well established, however, that the appearance 
of vast numbers in a locality where they have not been known to be 
excessively abundant before is due to continued conditions favor- 
able to the increase of the species, and is therefore liable to occur in 
any locality where the species is fairly abundant. 
Rats are exceedingly fertile. They breed three to five times a 
year, and a litter varies in numbers from six to twenty. The period 
of gestation is twenty-one days. A female rat can breed when less 
than three months old.” The young are brought forth in a very 
undeveloped condition, being hairless, with eyes closed and the 
outer ears glued down over the orifice. 
The rate of reproduction varies with the climate and the food 
supply, a moderate temperature and an abundance of food increasing 
both the number of litters a year and their size. Extensive investi- 
gations made in connection with the United States Biological Sur- 
vey indicate that the average for well-fed individuals in this latitude 
can not be less than ten to a litter. At this rate, supposing there 
were only three litters a year, if a single pair and their progeny 
should breed uninterruptedly for three years, and no deaths occur, 
the total number resulting would be something over 20,000,000 
individuals. 
There is no mammal in the state against which so much that 1s 
injurious can be proven, and for which so little of good can be 
claimed; and the whole economic problem so far as the rat is con- 
cerned, reduces itself to determining how to exterminate it most 
rapidly and satisfactorily. Since, in spite of their great fertility, 
rats are not increasing rapidly, it is evident that there must be many 
agents naturally tending to their destruction, for very few of them, 
comparatively, are killed by man under ordinary circumstances. 
* Lantz, D. E. ‘‘ The Brown Rat in the United States.’ Bull. No. 33, Biol. 
pucy. Uo: Dept: Agr., p. 17. 
