58 GBOGKAPHICAL DISTKIBUTION. 



south, the Neotropical ; and the same can probably be said of the 

 extremity of the peninsula of Florida. With these limitations the 

 Holarctic in the "Western Hemisphere embraces the whole of the 

 United States, and all the region stretching thence northward 

 towards and into the Arctic Sea. In the Eastern Hemisphere the 

 southern boundary may in a general way be said to be the moun- 

 tain complex which, as the Pyrenees, Alps, Balkans, and Caucasus, 

 traverses the south of Europe from the Bay of Biscay to the Cas- 

 pian, the northern line of Persia and Afghanistan, the Hima- 

 laya Mountains, and the Nanling range in China, which forms the 

 southern water-shed to the Yangtse-Kiang. These various boun- 

 daries are principally of a physical nature, and of such a char- 

 acter as to be insurmountable to most animals. 



No other region can compare with the Holarctic in the mani- 

 fold variety of its physical characteristics. Every form of terres- 

 trial configuration, or condition of soil or climate, that may be rep- 

 resented in any other region, is also represented here, and on an 

 imposing scale. From the ice-bound fields of the far north to 

 the burning desert wastes of Turkestan on the south, and from the 

 deep forest-grown lowlands to mountain summits soaring thou- 

 sands of feet above the level of perpetual snow, we pass through 

 all those various gradations of climate which respectively charac- 

 terise the Frigid, Temperate, and Torrid zones. Densely covered 

 forest tracts, supporting, as in the north, a sombre growth of pine 

 and other coniferous trees, or, as in the south, a vegetation of 

 almost tropical luxuriance, alternate with broadly open grass or 

 pasture lands {tundras of Siberia, American prairies and plains), 

 which in some cases support over enormous areas only a very scanty 

 vegetation, and in others display a profuse variety of vegetable 

 productions. It is in this region that, in addition to a most boun- 

 tiful development of desert tracts, we meet with the most elevated 

 table-land (the Central-Asian), and, at the same time, with the 

 greatest expanse of lowland on the surface of the globe, the great 

 plain of Siberia and Northeastern Europe. 



For convenience of treatment, and to facilitate comparison with 

 other zoogeographical publications, the Old and New World divi- 

 sions of the Holarctic region will be considered separately. 



The Old World or Eurasiatic Division (Paleearctic region [in 

 part] of most authors). — The southern boundaries of this region 



