30 Jlcrriam — Geographic Distribution of Life. 



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

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

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

 the i)eninsula of Lower California entitles it to rank as a minor 

 sulxlivision 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 adjg-cent islands. 



The humid division of the Upper Sonoi-an comprises the area 

 in the eastern United States conunonly known as the Carolinian 

 Fauna. The opossum (Didelphis) here finds its northern limit, 

 as tlo the so-called })ine mouse (subgenus Pltymy>i) and the 

 Georgian bat ( Vespenujo fjeorgicmus). 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 JNIissouri obliquely through North 

 Dakota and into eastern Montana. Another subdivision of the 

 arid Upper Sonoran occupies the greater ])art of the Great Basin 

 between the Rocky Mountains and the High Sierra, reaching 

 northerly from tlie upi)er l)order of the Lower Sonoran to and 

 including the plains of the Columl^ia and Snake Rivers.- Another 

 part of noteworthy extent is a narrow belt encircling the interior 

 basin oT 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 Barl)ara 2>h^iii- The following 

 genera of mammals find their northern limit in the arid Upper 

 Sonoran Zone : Perndipus, Microdijwdops, Perogncdhns, OnycJtomys, 

 Spilogale, Urocyon, B(tssariscus, and Antrozous. 



Interposed lietween the Boreal and Sonoran Regions through- 

 out their numerous- Avindings and interdigitations, is the Neutral 

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

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



* Walter E. Bryant in Zoe, II, No. S, Oct., 1801, 18(). See also his im- 

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

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



fProf Louis Agassiz, in his liigldy iniiKirtant work on Lake Superior, 

 clearly recognized the transition nature (jf this zone, for he says : " The 

 State of Massachusetts, with its long arm stretclied into the ocean east- 

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

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



