Another distinct grouping of species in the bosom of this region is dependent on the nature of the soil. It includes the cork-oak, the chestnut, and the maritime pine, which are limited to the siliceous zone, and with them a whole train of plants and shrubs which are found with striking uniformity wherever the mineral composition of the soil is of similar character. So strict an adaptation, so absolute a selection, could not have been the work of a small number of centuries, but they may properly be attributed to causes that existed in a remote past.
To complete our review of the Mediterranean forest-grouping, we should consider, besides the dominant forms, some exceptionally sheltered parts of the regions, and other regions in which intermittent rigors of temperature have spared only the hardiest types. Three elements may be distinguished in the aggregate of about two hundred species included in the forestal vegetation of this region: First, the principal and characteristic element, in which plants with persistent leaves predominate, and which includes among its rarer types species in a declining condition, which are cantoned upon the best-sheltered or most southern points, and are transitional toward tropical types; next, the mountaineer element, to which altitude is favorable; and a third element, to which heat and moisture are agreeable. The last includes plants with deciduous leaves, which, while they can accommodate themselves to a cold climate, are better adapted to southern temperatures, and do not grow spontaneously in the central region; with which corresponds a group of trees—large, not very numerous, and usually monotypal—which deserves attention because of what it has been in the past, and which may be said to represent the southern prolongation of the central group, elbowing into the Mediterranean region. Its members, including the alder, the Eastern witch-elm, the hornbeam, the plane, the liquidambar, the fig, vine, some ashes, lindens, and walnut, are more in contrast with the mass of the Mediterranean plants than with the types equivalent in order that are domiciled farther north. It might be said of them that, while they are found associated with the former plants, they belong naturally to the category of the latter, as they would visibly had not the free extension of these to the southward been arrested. This view is confirmed by the examination of the forestal flora of a corresponding latitude in America. The absence on that continent of any vegetable domain equivalent to that of the Mediterranean disengages that element, and supplies, through the plane, the liquidambar, the persimmon, and American vines and walnuts direct from the ancient world, a parallelism or a repetition of forms which the study of paleontology helps to illustrate.
The principal elements of the Mediterranean group—hollies.