THE ORIGIN OF FOREST-GROUPINGS.
��predecessors. " The forest " may be defined as an association of trees freely grouped over a space ; or, as the vegetable kingdom delivered to its own forces and meeting conditions favorable to its becoming master of the soil and spreading its wealth over it. The " virgin " forest is the forest into which man has penetrated only in passing, or upon which he has never laid hand to attack or modify it. It is peculiarly the forest of hot countries, or of the intertropical zone, where everything concurs to stimulate luxuri- ance of the vegetable kingdom. Even in temperate climates, whose pretensions of this kind are modest, we have only to trans- port ourselves into some region where the native forest yet exists in all its primitive grandeur, to perceive at once the might and majesty of the vegetable kingdom thus abandoned to itself, and having uncontested possession of the territory. Forestal asso- ciations interpret the influence of the climate to which they are adapted. They change in aspect and composition according to the latitude, and present characteristic diversities combined in a deter- mined and successive order as we advance from the neighborhood of the polar circle toward the south. In the review which we can make of them, they constantly present a double aspect, as thej^ may be considered in themselves, or as with reference to their re- lations with the past, and their bonds of kindred with anterior vegetations. But, previous to placing ourselves at the latter point of view, we should glance at existing plants to determine the feat- ures of the order which now presides over their distribution.
The domain of the forest extends beyond the polar circle in Europe and Siberia, where it reaches and even passes a little above the seventieth degree. In America it retreats from that region about Labrador and Hudson Bay, the polar circle being hardly indented in the interval between Mackenzie River and Bering Strait. But in this domain, as in those that succeed it, an essen- tial distinction should be made between the resinous forests, con- sisting almost exclusively of conifers, and those that are composed of "foliage-trees." In the north, the forests of resinous trees extend over great spaces. In central or more southerly regions, these forests prefer the mountainous masses. Besides the coni- fers, many foliage-trees — among them the birches, alders, aspens, willows, and mountain-ash — penetrate within the polar circle, and constitute a part of the arctic forests. South of the sixtieth degree in Europe, and of a lower latitude in America, there ex- tends a richer assemblage of varieties, but insensibly connected with the preceding one. The birch, oak, elm, maples, ashes, and limes are its characteristic trees, while the foliage-trees and coni- fers of the preceding group are not excluded from it. The latter show a tendency to graduate themselves on the slopes as they ascend them, much as, in going from southern to northern coun-