tion, but it is also to some extent concerned with the actual plants and animals that live in 

 each of these. The distribution of these latter is now pretty well known to botanists and zoolo- 

 gists, but that of even the major belts themselves, let alone the innumerable subdivisions of 

 these, and more especially of the zones on the uplands and mountains, is widely ignored. 



There are no less than five ways in which North America has been defined. In all cases its 

 northern, western, and eastern boundaries are clear, precise, and coincident: the dispute con- 

 cerns the southern. None of these suggestions is wholly satisfactory from all points of view. 

 There is, however, another choice which avoids most of the objections to all the others, and this 

 is the one that we have adopted in this book. By this, the southern boundary of North America 

 is placed arbitrarily along the twentieth parallel north. This line cuts across south-central 

 Mexico, near Mexico City, and follows the great range of volcanic peaks that stretch across 

 that country and divide it clearly into two parts. To the north of it, there are no lowland or 

 true tropical growths, though there are outliers of montane tropical and subtropical forests; to 

 the south of it, there are no true deserts. In fact, it lies athwart the Southern Scrub Belt of the 

 northern hemisphere. 



In North America, as thus defined, we have eleven major belts — namely, reading from the 

 North Pole: the Arctic Icefields, the Barren Lands, the Tundra, the Boreal Coniferous Forests, 

 the North Temperate Woodlands closed-canopy forests, the Parklands, the Prairies, the North- 

 ern Scrublands, the Hot Deserts, the Southern Scrublands, and the subtropical Savannahs and 

 Orchardbush. The vast boreal forests may be further subdivided into a northern Spruce- 

 Aspen, a central Pine-Spruce, and a southern Transition of Softwood-Hardwood mixed-forest 

 subbelts. The continent straddles seventy degrees of latitude and is thus some five thousand 

 miles long. It forms a triangle of the same width at its widest, and it has an area of 9,355,000 

 square miles, which is just 16 per cent of the total land surface of the earth and makes it the 

 third-largest continent or land mass. The flags of five nations fly over it — those of Denmark 

 (Greenland), Canada, the United States, Mexico, and France (the islands of Saint-Pierre and 

 Miquelon in the Gulf of St. Lawrence). Its highest point is Mount McKinley in Alaska, which 

 tops 20,320 feet. The greater part of it is clothed in coniferous forests, those of its northern 

 part forming the second-largest continuous forest in the world. 



Our approach to this continent is, as it were, as from the North Pole, across the Arctic 

 snow and icefields to the Barren Lands to the east side, and south into the Spruce-Aspen belt 

 of the boreal forests. From there we turn west, following this same forest across the top of the 

 continent to Alaska; then south to the next belt, and east once more. Arriving at the east 

 coast, we again drop southward into the next belt and once again travel west to the Pacific 

 coast; and then repeat the process going east, then west and east again, until we have reached 

 the Northern Scrub Belt at the southern tip of Florida. From there we go west for the last time, 

 following that belt in its great northward bow to the Canadian border and then south to 

 southern California, from where we pass east into the Hot Deserts. These narrow into the 

 funnel of upland Mexico which is cupped by the mighty Sierras, which are themselves ringed 

 by the Southern Scrublands, the subtropical Savannahs, and the Orchardbush. 



Since the belts are of such great length, I have divided them up into what we call natural 

 provinces. There are twenty-one of these in North America, most of which are already well 

 known in their own right under the names used in this book. These provinces form our chap- 

 ter heads. Each is one of Nature's major oikoi or "houses, " and each is clothed in a unique 

 flora and supports a particular fauna. The boundaries between these provinces are not arbi- 

 trary and are mostly prescribed by very definite and often well-known physiographic features. 

 From the point of view of flora and fauna they should be looked upon primarily as transition 

 belts, but in many cases they represent lines at which distribution of individual species or 

 even groups of plants or animals makes the most abrupt changes. 



NOTE ON THE DELIMITATION OF DESERTS 



There is only one area on the North American continent where the boundary between two 

 major vegetational belts is in question. This is the junction of the Hot Desert (with red soils) 

 and the North Scrub Belt (with gray soils) in the Great Basin (see Chapter 17). The soil types 

 change abruptly along an arc bowing northward across the southern end of this province. 

 However, there is every reason to assume that the southwestern half of the Colorado Plateau 

 is true desert. Also, Death Valley definitely lies in that belt. Thus, by protracting a north- 

 wardly curving line from the east of the former to the western edge of the latter across the 

 Great Basin, we find that the true Hot Desert Belt encompasses almost half of this province. 

 I tend to accept this as the dividing line between the two belts, despite the findings of soil 

 analysis and classification, principally because it conforms with the curves made by all other 

 major belts throughout the western third of this continent. 



