Page:Popular Science Monthly Volume 75.djvu/378

374, like the hemlock, are found growing along the higher land and in cool ravines as far south as the Lower Delaware Valley.

The more familiar trees, however, mingle over a wide area of country—southern New England and the Middle Atlantic district—a transition region that lies between the northern coniferous forest and the broad-leaved, summer-green forest of the great interior valley and south Atlantic slope.

Something besides temperature appears to control the distribution of certain trees. Since the settlement of the country numerous species, the natural habitats of which are far to the south, have been planted and grown successfully in more northern localities. The catalpa, the sourwood and the several species of magnolia are illustrations of this. Just what is the determining factor in preventing such trees from spreading northward (or others from spreading southward) it is difficult to say. Possibly some subtle condition in the balance of nature—some item in the struggle for existence, has helped or hindered the spread of certain trees, excluding some from localities already occupied by others. It is well known that where pines have been cut off certain species of oaks will spring up and occupy the land. The oak seedlings must have been abundantly scattered over the soil for a long period of time. Only a comparatively few species are enabled to hold their own through some superior advantage in adaptation and grow up to form a forest. A vast number must of necessity lie dormant in the soil for centuries. In Denmark, since glacial times, there has evidently been a succession of forests—oak following fir and beech following oak through a period of many thousands of years. The varied relations of the different species of trees to light, heat, moisture and soils, to animals, and to other trees and plants, are involved in slow, deep-seated processes, the expression of which is the forest as we see it. Our point of view, however, is but momentary in the vast events of nature—a fleeting glimpse, merely, of one picture in an endless biograph.

The question of the succession of forests suggests another question—that of the origin of the present Atlantic forest and its relation to other forests in different parts of the world. In a general survey of the forest trees of eastern North America there appears a large number of types which are common also to Europe. With the exception of the bald cypress and the hemlocks all the other coniferous types—pines, spruces, firs and larches—are represented in Europe by closely allied species. This is true also of a number of the deciduous trees—the oak, beech, chestnut, elm, willow, birch, aspen, walnut, ash, maple, plane tree, linden or basswood, and others are represented on both sides of the Atlantic by more or less nearly related forms. This is not surprising when we come to consider that all of the above-named trees are decidedly northern in their distribution, and exist under very similar climatic conditions. This is especially true of the coniferous