126 
ZOOLOGICAL GEOGRAPHY. 
[part iri. 
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 100°th. 
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 Bocky 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 G-rande 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 
