505 
When one considers how small a proportion of the animals pres- 
ent would be likely to find traps thus set across a field, one is forced 
to conclude that the number actually present at all times may be 
considerable, even in the middle of the best-cultivated fields. If 
traps are set along fences between such fields, at every post, the 
number containing animals will rise to one third or even one half of 
the traps. The average number of birds found in the corn fields 
of this belt was nearly one to the acre, and the number of mam- 
mals present, even in the middle of the larger fields, can hardly be 
less, while for the whole belt 1t must be considerably greater. Dur- 
ing spring and early summer, as we have said, white-footed mice 
constitute the great bulk of the mammalian life in the center of these 
large fields. Near the edge the mole, Scalopus aquaticus, may be 
present, sometimes in considerable numbers, but it seldoms pene- 
trates more than 50 yards into the field. Shrews, voles, and gophers 
are present along the fences and in adjoining fields not too recently 
disturbed, and make short incursions into the corn fields at that 
time of year. But in fall, if the grain or corn be shocked and allowed 
to stand a fortnight or so, traps set by the shocks show quite differ- 
ent conditions. The following may serve as a rather extreme illus- 
tration. In a corn field on the university farm where the white- 
footed mouse had been taken early in the year (1907), after the corn 
had been cut and shocked for some time fifty traps were set over 
night, one by each of as many consecutive shocks. The next morn- 
ing thirty-seven of these traps contained specimens—one of them a 
single house-mouse, Mus musculus. In 1908, thirty-one traps were 
set in the same field under similar conditions except that the corn 
had not been shocked so long, and only ten specimens were taken, 
nine of which were house-mice and one was a white-foot. At first 
the conclusion was drawn that the house-mice had entirely driven 
out the prairie-mice. However, when traps were set in an adjoin- 
ing part of the same field from which the shocks had been removed, 
the usual number of white-footed mice was taken, with the addition 
of one specimen of the house-mouse. Evidently the house-mice 
invaded the field after the corn was cut, and the prairie-mice were 
either driven from the shelter of the shocks or disdained it. Proba- 
bly the former is the truth, for I have often taken them by recently 
cut shocks of corn and grain. 
