126 ZOOLOGICAL GEOGRAPHY. [PART m. 



fauna, which, when the cold period arrived, would descend to 

 the lowlands, and people the country to the east, west, and 

 south, with similar forms of life. 



The first, and most important sub-division we can make, 

 consists of the Eastern United States, extending across the 

 Mississippi and the more fertile prairies, to about the 100th. 

 meridian of west longitude, where the arid and almost desert 

 country commences. Southwards, the boundary bends towards 

 the coast, near the line of the Brazos or Colorado rivers. To 

 the north the limits are undefined ; but as a considerable number 

 of species and genera occur in the United States but not in 

 Canada, it will be convenient to draw the line somewhere near 

 the boundary of the two countries, except that the district 

 between lakes Huron and Ontario, and probably Nova Scotia, 

 may be included in the present sub-region. As far west as 

 the Mississippi, this was originally a vast forest country ; and it 

 is still well wooded, and clothed with a varied and luxuriant 

 vegetation. 



The next, or Central sub-region, consists of the dry, elevated, 

 and often arid district of the Rocky Mountains, with its great 

 plateaus, and the barren plains of its eastern slope ; extending 

 northwards to near the commencement of the great forests north 

 of the Saskatchewan, and southward to the Rio Grande del Norte, 

 the Gulf of California, and to Cape St. Lucas, as shown on our 

 maps. This sub-region is of an essentially desert character, 

 although the higher valleys of the Rocky Mountains are often, 

 well wooded, and in these are found some northern and some 

 western types. 



The third,or Californian sub-region,is small, but very luxuriant, 

 occupying the comparatively narrow strip of country between the 

 Sierra Nevada and the Pacific. To the north it may include Van- 

 couver's Island and the southern part of British Columbia, while 

 to the south it extends to the head of the Gulf of California. 



The fourth division, comprises the remainder of North America ; 

 and is a country of pine forests, and of barren wastes towards 

 the Arctic Ocean. It has fewer peculiar species to characterise 

 it than any other, but it possesses several characteristic arctic 



