As is indicated on the map just referred to, the forests of North America in a general way form a broad belt, for the most part within the influence of winds from the ocean, which surrounds a large area of treeless plains and plateaus in the west-central portion of the interior continental basin, but is broken and rendered irregular in its southwestern part of the treeless valley of the Great Basin region. The irregular circular belt of tree-covered land is closed at the south by the forest on the lowlands of Mexico and Central America. This vast forest belt, extending from the Atlantic to the Pacific, and from Panama to northern Canada, presents great variations even in its larger features, and, for convenience, and also with the aim of expressing in a rough way natural relationships, needs to be subdivided for purpose of study. The basis for such a subdivision has already been suggested, as the forests, like all other divisions of the life of the continent, are an expression of climatic conditions--that is, the boundaries of the botanical and zoological provinces should agree with those of the climatic provinces. On this basis we have the tropical forest, which covers the more humid portions of the east and west margins as well as all of the southern portion of Mexico, together with nearly all of Central America and the West Indies, and includes the southern extremity of Florida. Within the tropical forest, however, there are high mountains on which trees with the general characteristic of those more northern floras find a congenial habitat. The two austral and the transition provinces are to a great extent clothed with diversified forests, which are naturally divided into two portions: an eastern division, embracing the Atlantic and Gulf border of the United States, together with the eastern part of the Mississippi Valley and the Great Lakes region; and a western division, in which is included the lands bordering the Pacific from near Mount St. Elias southward to the vicinity of San Francisco, and also several irregular branches or detached island-like areas on the Pacific mountains, in the United States and Mexico. The former of these divisions may, in a general way, be termed the Atlantic, and the latter the Pacific forest. Separating them is the treeless west-central portion of the Continental basin. Both the Atlantic and Pacific forests merge at the north with the boreal forest, which extends diagonally across the continent from Newfoundland to Alaska. Peninsula-like and island-like areas occupied by the boreal forest occur in the sea-like expanse of the transition and austral provinces, on both the Atlantic and Pacific mountains.