30 Merriam Geographic Distribution of Life. 



me now, excepting that of islands, than is embraced in the region 

 above denned," * but he omits to name the forms by which it is 

 characterized. It is evident, however, that the peculiar fauna of 

 the peninsula of Lower California entitles it to rank as a minor 

 subdivision of the Lower Sonoran Zone. It is in effect an insu- 

 lar fauna of recent origin, bearing the same relation to that of 

 the mainland as do several of the adjacent islands. 



The humid division of the Upper Sonoran comprises the area 

 in the eastern United States commonly known as the Carolinian 

 Fauna. The opossum (Z)/Vr// >///.>) here finds its northern limit, 

 as do the so-called pine mouse (subgenus Pitymys) and the 

 Georgian bat (Vesperugo georgianus). Before reaching the 100th 

 meridian this area gradually loses its moisture and spreads out 

 over the Great Plains as the arid or true Upper Sonoran, reach- 

 ing an altitude of about 4,000 feet along the east foot of the 

 Rocky Mountains in the latitude of Colorado, and sending a 

 tongue northward along the Missouri obliquely through North 

 Dakota and into eastern Montana. Another subdivision of the 

 arid Upper Sonoran occupies the greater part of the Great Basin 

 between the Rocky Mountains and the High Sierra, reaching 

 northerly from the upper border of the Lower Sonoran to and 

 including the plains of the Columbia and Snake Rivers. Another 

 part of noteworthy extent is a narrow belt encircling the interior 

 basin of California the valley of the Sacramento and San 

 Joaquin rivers and a branch of the same along the coast be- 

 tween Monterey and the Santa Barbara plain. The following 

 genera of mammals find their northern limit in the arid Upper 

 Sonoran Zone : Perodipus, Microdipodops, Perogndthus, Onychomya, 

 Spilogale, Urocyon, Bassariscus, and Antrozous. 



Interposed between the Boreal and Sonoran Regions through- 

 out their numerous windings and interdigitations, is the Neutral 

 or Transition Zone. The humid division of this zone, known 

 as the Alleghanian Fauna,f covers the greater part of New 



* Walter E. Bryant in Zoo, II, No. 3, Oct., 1891, 180. See also his im- 

 portant ' Catalogue of the Birds of Lower California,' Proc. Calif. Acad. 

 Sci., 2d ser., II, 1889, 237-320. 



t Prof. Louis Agassiz, in his highly important work on Lake Superior, 

 clearly recognized the transition nature of this /one, for lie says : " The 

 State of Massachusetts, with its long arm stretched into the ocean oast- 

 ward, or rather the region extending westward under the same parallel 

 through the State of New York, forms a natural limit between the vcgcta- 



