542 LAND ANIMALS 



Cultivated fields, like the original steppe, are the haunt of small 

 and moderate-sized rodents. In Europe and North America the fields 

 support a numerous population of Arvicolidae, sometimes, by reason of 

 rapid multiplication, in enormous numbers. The field mouse (Microtus 

 arvalis) and its larger relative [M. terrestris) are the European repre- 

 sentatives; no less than 57 species and subspecies of Microtus have 

 been described in North America. In Europe, Microtus is supplemented 

 by the hamster, Cricetus cricetus, a true steppe form, which is still 

 constantly extending its range to the west, and by the gopher (Citellus) 

 which is beginning to enter Germany from the east. In North America, 

 the pocket gophers (Geomys) play a considerable role in the fauna of 

 the cultivated land, and the ground squirrels, Citellus, afford excellent 

 examples of this adaptability; C. tridecemlineatus is completely nat- 

 uralized in the fields of Illinois and Wisconsin. The gray ground squir- 

 rel (C. franklini) has recently extended its range eastward as far as 

 Indiana, but in the Chicago area it is still confined to the vicinity of 

 the railways, living on the grain lost from the cars. The grain-hauling 

 railroads have thus been effective highways for its eastward spread. 

 In eastern North America the cottontail rabbit and the woodchuck 

 are much more abundant in cultivated regions than under natural 

 conditions. These are forest margin forms, and in general such animals, 

 especially burrowing types, thrive under the conditions produced by 

 man in areas that were formerly forested. On the other hand Wood' 1 

 says of an Illinois field in crop: 



"Apparently nothing but a veritable desert could be more unfavora- 

 ble for mammalian life than these well tilled fields. ... In large corn 

 fields of eighty to one hundred and sixty acres, when the corn was 

 about one foot in height and was being repeatedly cultivated and left 

 almost absolutely free from weeds, I repeatedly set traps near the 

 center of the field, at every tenth hill along the rows. . . . The average 

 result of one night's setting was a white-footed mouse in one trap 

 in ten. Very rarely a specimen of short -tailed shrew was taken. If 

 these traps were set near the edge of the field the proportion of traps 

 containing animals was increased." Even so, two varieties of white- 

 footed mice, a shrew, two spermophiles, the pocket gopher, and the 

 house mouse live in such situations on the outskirts of Chicago. When 

 the corn is shocked up in the field in autumn, about 90% of the traps 

 set about the shocks contain animals. 



Controlled forests.— Regulated forests are the civilized analogues 

 of the natural forest. As yet these are but little developed in the United 

 States; in Germany they are particularly extensive. Forestry as a de- 

 partment of agriculture endeavors to produce the greatest possible in- 



