888 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
The following genera of mammals find their northern limit in the arid 
Upper Sonoran zone: Perodipus, Microdipodops, Perognothus, Onycho- 
mys, Spilogale, Urocyon, Bassariscus, and Antrozous. 
Interposed between the Boreal and Sonoran regions throughout their 
numerous windings and interdigitations, is the Neutral or Transition 
zone. The humid division of this zone, known as the Alleghanian 
fauna,* covers the greater part of New England (except Maine and 
the mountains of Vermont and New Hampshire) and extends westerly 
over the greater part of New York, southern Ontario, and Pennsyl- 
vania, and sends an arm south along the Alleghanies all the way across 
the Virginias, Carolinas, and eastern Tennessee, to northern Georgia 
and Alabama. In the Great Lake region this zone continues westerly 
across southern Michigan and Wisconsin, and then curves northward 
over the prairie region of Minnesota, covering the greater parts of 
North Dakota, Manitoba, and the plains of the Saskatchewan; thence 
bending abruptly south, it crosses eastern Montana and Wyoming, 
including parts of western South Dakota and Nebraska, and forms a 
belt along the eastern base of the Rocky Mountains in Colorado and 
northern New Mexico, here as elsewhere occupying the interval between 
the Upper Sonoran and boreal zones. 
In Wyoming the transition zone passes broadly over the well-known 
low divide of the Rocky Mountains, which affords the route of the 
Union Pacific Railway, and is directly continuous with the same zone 
in parts of Colorado, Utah, and Idaho, skirting the boreal boundaries 
of the Great Basin all the way around the plains of the Columbia, 
sending an arm northward over the dry interior of British Columbia, 
descending along the eastern base of the Cascade Range and the High 
Sierra to the southern extremity of the latter, and occupying the sum. 
mits of the coast ranges in California and of many of the desert ranges 
of the Great Basin. 
The transition zone, as its name indicates, is a zone of overlapping 
of boreal and Sonoran types. Many boreal genera and species here 
reach the extreme southern limits of their distribution, and many 
Sonoran genera and species their northern limits. Buta single mam- 
malian genus (Synaptomys) is restricted to the transition zone, and 
future research may show it to inhabit the boreal region also. 
* Prof. Louis Agassiz, in his highly important work on Lake Superior, clearly reec- 
ognized the transition nature of this zone, for he says: ‘ The State of Massachusetts, 
withits long arm stretched into the ocean eastward, or rather the region extending 
westward under the same parallel through the State of New York, forms a natural 
limit between the vegetation of the warm temperate zone and that of the cold tem- 
perate zone. - - - Not only is this also the northern limit of the culture of fruit 
trees, but this zone is equally remarkable for the great variety of elegant shrubs 
which occur particularly on its northern borders, where we find so great a variety 
of species belonging to the genera Celastrus, Cratewgus, Ribes, Cornus, Hamamelis, 
Vaccinium, Kalmia, Rhodora, Azalea, Rhododendron, Andromeda, Clethra, Vibur- 
num, Cephalanthus, Prinos, Direa, Celtis, etc,” (Lake Superior, 1850, 182-183. ) 
