Most of the larger American mammals are not restricted to any one faunal zone. The bison, now greatly reduced in numbers, for merly roamed over nearly the entire region between the Appa lachian and the Rocky Mountains. The black bear and beaver were also widely distributed. The Virginia deer still ranges from Maine to the Gulf States and from the Atlantic coast to the Rocky Mountains. The grizzly bear, cougar, coyote, prairie-dog and antelope are still found in several of the Western States, and the grey wolf persists from northern Michigan westward.
Flora.—The Alpine flora, found in the United States only above the limit of trees, consists principally of small plants which bloom brilliantly for a short season. The flora of the Hudsonian and the Canadian zone consists largely of white and black spruce, tamarack, canoe-birch, balsam-poplar, balsam-fir, aspen and grey pine. In the Alleghanian Transition zone the chestnut, walnut, oaks and hickories of the South are interspersed among the beech, birch, hemlock and sugar maple of the North. In the western arid Transition zone the flora consists largely of the true sage-brush (q.v.), but some tracts are covered with forests of yellow or bull pine. The Pacific coast Transition zone is noted for its forests of giant conifers, principally Douglas fir, Sitka spruce, Pacific cedar and western hemlock. Here, too, mosses and ferns grow in pro fusion, and the salal, thimbleberry, salmon-berry and devil's-club (Oplopanax horridum) are characteristic shrubs.
In the Carolinian zone the tulip tree, sycamore, sweet gum, rose magnolia, short-leaf pine and sassafras find their northernmost limit. Sage-brush is common to both the western arid Transition zone and the Upper Sonoran zone, but in suitable soils of the lat ter several greasewoods are characteristic species, and on the mountain slopes are some nut pines and junipers. The Austroripa rian zone has the long-leaf and loblolly pines, magnolia and live oak on the uplands, and the bald cypress, tupelo and cane in the swamps. The Lower Sonoran zone is noted for its numerous cactuses, some of which grow to the height of trees; the mesquite, creosote bush, acacias, yuccas and agaves are also common. The Tropical belt of southern Florida has the royal palm, coconut palm, banana, manchineel and mangrove ; the Lower Sonoran belt in the lower valley of the Colorado has giant cactuses, desert acacias and palo-verdes.
Almost all of the United States east of the 98th meridian is naturally a forest region, and forests cover the greater part of the Rocky Mountains, the Cascades, the Sierra Nevada and the Coast Range, but throughout the belt of plains, basins and deserts west of the Rocky Mountains and on the Great Plains east of the Rocky Mountains the prevailing type of vegetation ranges from bunch grass to sage-brush and cactuses.
The eastern forest region differs from the densely forested re gion of the Pacific Coast Transition zone in that it is essentially a region of deciduous or hardwood forests, while the latter is essen tially one of coniferous trees ; it differs also from the forested region of the Rocky Mountains in that the latter is essentially a region of coniferous trees which occupy only a part of its area.
The United States is rich in the variety of its native forest trees, some of which, as the species of sequoia (q.v.), are the most mas sive known. C. S. Sargent (Manual of North American Trees, 2nd ed., 1922) describes 717 species, together with numerous varieties; G. B. Sudworth (1927, see Bibliography below) gives the names and geographical distribution of 1,177 species and varieties. Of these, 182 are of economic value, either because of the timber and other useful products which they yield or by rea son of their importance in forestry. This forest flora not only includes numerous coniferous soft woods and deciduous hard woods but it also embraces various subtropical trees.
Besides the native flowering plants, estimated to comprise from 12,000 to 15,00o species, many hundred species, introduced from other regions—chiefly Europe, Asia and tropical America—have become naturalized. A large proportion of these are common annual weeds of fields, pastures and roadsides. In some districts these naturalized "aliens" comprise 5o% or more of the total plant population. W. L. Jepson in his Manual of the Flowering Plants of California (1925) enumerates 4,019 species, of which are native to the State (1,416 species endemic) and 292 (7%) are naturalized from other regions. In the north-eastern United States from Minnesota to Kansas and eastward the pro portion of introduced species is much greater, Gray's New Manual Geographical Growth of the Nation.—The achievement of independence found the people of the United States owning the entire country from the Gulf of Mexico to the Great Lakes, excepting Florida, as far west as the Mississippi river; but the actual settlements were, with a few minor exceptions, confined to a strip of territory along the Atlantic shore. The depth of settle ment, from the coast inland, varied greatly, ranging from what would be involved in the mere occupation of the shore for fishing purposes to a body of agricultural occupation extending back to the base of the great Atlantic chain, and averaging some 25o miles. Westward, beyond the general line of continuous settlement were four extensions of population through as many gaps in the Appa lachian barrier, constituting the four main paths along which migration westward first took place : the Mohawk valley in New York; the upper Potomac—upper Ohio river connections; the Appalachian valley through south-western Virginia into Tennes see; and, around the southern base of the Appalachian system. Four outlying groups beyond the mountains—one about Pitts burgh, one on the Great Kanawha river in West Virginia, one in northern Kentucky and the last along the Cumberland river in Tennessee—constituted perhaps a twentieth part of the total population of the nation. Finally, there were in 1790 about a score of small trading and military posts, mainly of French origin, scattered over the then almost unbroken wilderness of the upper Mississippi valley and region of the Great Lakes.