If a large tract of land is cleared for culture, and, after some years it is neglected, it is not long before forest trees spring up all over it. This is a well known fact, especially in the Southern States. It is not that the seeds are already in the earth, or they would have sprouted and been destroyed by cultural operations; but the seeds are carried there by various outside agencies. Pine seeds for instance are blown some distance by wind or on to the feathers or backs of animals, and are dropped often at long distances from the parent tree. Heavier seeds with acorns and nuts are carried by birds or animals as food, and a few escape eating, and then grow, and in numberless ways get a chance to grow a long distance from the original tree. In a few years, according to their kind, these again produce seed and form new centres of distribution, till, in say one hundred and fifty years forests may appear as many miles away from an original forest centre. If however anything happens to keep the trees from growing high enough to mature seed, such as the browsing of animals, or prairie fires, extension from a centre could not go on. "We thus see how there are circumstances which sometimes favor the extension, and sometimes restrict the forest area.

These views have been recently narrated by the writer of this in papers on the " origin of grassy prairies," and "on the timber line of high mountains," but some recent inquiries make a repetition necessary.