Our vegetable physiologists, Mr. Emerson among the number, are simply unfortunate in their use of terms--those expressing even the commonest operations of nature. In their genesis of plants and trees they need to adhere a little more closely to the genesis of induction, and use language in harmony with the phenomenal facts and characteristics which they are called upon to explain. But Mr. Emerson was not alone at fault in this almost universal slip of the scientific pen. He quotes from a letter of Mr. P. Sanderson, of East Whately, Mass., in which the writer says: "There is an instance on my farm of spruce and hackmatack being succeeded by a spontaneous growth of maple wood;" and he adds that "instances are also mentioned by him (Mr. Sanderson) of beech and maple succeeding oaks; oaks following pines, and the reverse; hemlock succeeded by white birch in cold places, and by hard maple in warm ones; beech succeeded by maple, elm, etc; and, in fact, the occurrence was so common that surprise was expressed at the asking of the question."
These several alternations in timber growths, effectually vouched for by Mr. Emerson, occurring "spontaneously" as stated, can hardly be accounted for on any other theory than the presence of "germs" and "favoring conditions," such as we have named in connection with the Bible genesis. They might possibly be explained on the theory of "scattered seeds," if the several growths had made their appearance gradually, and not "spontaneously," as stated. The misfortune with Mr. Emerson, as well as with his several "reliable correspondents," was, that his facts are too meagrely imparted, in the necessary details, to draw any satisfactory conclusions from them--such as the nearness or distance of surrounding trees of the same species, and the possible chances of their seeds taking lodgment in the soil from which they grew. But, fortunately, there are facts, and those abundantly substantiated, which entirely negative the presence of seeds in the soils where these "spontaneous growths" are said to have appeared. In some instances, they cover large tracts of land, at distances of thirty, forty, fifty, and even hundreds of miles, from any native forest from which seed could have been derived.
Dr. Dwight, in the second volume of his "Travels," mentions visiting a town in Vermont (Panton, near Vergennes), in which a piece of land that had been once cultivated, but was afterwards permitted to lie waste, "yielded a thick and vigorous growth of hickory, where there was not a single hickory tree in any original forest within fifty miles of the place." Of this piece of land he says: "The native growth here was white pine, of which I did not see a single stem in the whole grove of hickory." He is greatly puzzled to account for this isolated growth of hickory, but readily concludes that "the fruit was too heavy to be carried fifty miles by birds; besides" he adds, "it is not eaten by any bird indigenous to Vermont." And even if the birds had carried the nuts thither, not one of them could have been planted there unless the nut-eating bird had been caught and destroyed on the spot, and the nut released from its crop. This might account for the appearance of a single tree, but not for a "whole grove of hickory;" and the squirrels certainly could not have been provident enough to plant any considerable grove in this particular locality, and nowhere else within fifty miles of it. The winds could not have borne them that distance without dropping a single nut by the way, and there is only one supposition left, which is that indicated in the Bible genesis.
While Dr. Dwight emphatically rejects the "transportation theory," he imagined he had solved the difficulty in his suggestion "that the cultivation of the land had brought up the seeds of a former forest, within the limits of vegetation, and given them an opportunity to vegetate." But the utter absurdity of this theory may be demonstrated by any one inside of two years, by placing hickory nuts, in different soils, at a depth to which an ordinary plough-point would reach in cultivation; and then, at the end of the second year, examining those that did not germinate the first year. The commonest observer of a hickory forest knows that if the fallen nuts do not germinate the first year, their vitality is utterly and hopelessly gone. It makes no difference whether you leave the nuts on the ground where they fall, or place them one inch or twenty inches beneath the soil, the result will be the same. At the end of two years, you can pulverize them between thumb and finger almost as easily as so much dried loam. The idea of deriving a new forest from such nuts, is hardly less absurd than that of emptying the Egyptian catacombs of their old mummy-cases, in the expectation of seeing a race of Theban kings stalking the earth as before the foundations of either Carthage or Rome were laid.
Dr. Dwight was a very close and accurate observer of nature, and suffered few of even the minor points of detail to escape him. In the same work, as well as in the same connection, he gives an account of another forest, which he supposes sprang spontaneously from "the seeds of an ancient vegetation." He says: "A field about five miles from Northampton (Mass.), on an eminence called 'Rail Hill,' was cultivated about a century ago (circiter 1720). The native growth here, and in all the surrounding region, was wholly oak, chestnut, etc. As the field belonged to my grandfather, I had the best opportunity of learning its history. It contained about five acres, in the form of an irregular parallelogram. As the savages rendered the cultivation dangerous, it was given up. On this ground there sprang up a grove of white pines, covering the field and retaining its figure exactly. So far as I remember, there was not in it a single oak or chestnut tree;" and he adds, "there was not a single pine whose seeds were, or, probably, had for ages been, sufficiently near to have been planted on this spot." He supposes, however, that the "seeds" (pine cone chits) had lain dormant for ages before cultivation brought them up "within the limits of vegetation."
As early as 1807, Judge Peters, of Philadelphia, became satisfied that all that elevated region around the head waters of the Delaware, Alleghany, and Genesee Rivers, then covered with heavy growths of hemlock, or with forests of beech and sugar-maple, was originally an oak forest, probably covering most of that entire region. And Mr. John Adlum, of Havre de Grace, Md., who originally surveyed the lands south of the great bend of the Susquehanna, between that river and the Delaware, conceived the same idea as early as 1788. The section surveyed by him was chiefly covered with beech and sugar-maple; in fact, it was in what was called, at the time, "the beech and sugar-maple country." He drew his inferences from the fact that he found, here and there, at irregular intervals, red and white oaks growing to an enormous size, none being less than sixteen feet, and many measuring twenty-two feet or more, in circumference five feet above the ground. He says that "the hemlock in this region seems to have succeeded the oak, while the beech and maple no doubt succeeded the hemlock." This last inference would seem to have been made from the fact that clumps of large hemlock trees were, at that time, still growing at intervals among the larger deciduous trees.
Indeed, there is no better established fact in vegetable physiology than that of these alternations of forest growths. They sometimes come on gradually, but, in a majority of instances, they make their appearance at once on the cutting off of old forests, in the tracks of tornadoes, or where fire has devastated extensive regions of timber. From the facts which have been gathered, it is difficult to determine any regular order of alternation, except that oaks and other deciduous trees succeed the different varieties of pine and other evergreen growths, and, perhaps, vice versa. In Dr. Hough's report upon American Forestry, he makes a brief summary of the order of these alternations in different sections of the country, on the authority of persons apparently more or less well-informed on the subject, but by no means accurate observers. He says that in the region about Green Bay, Wis., overrun by the fires of 1871, "dense growths of poplars and birches have sprung up, and are growing rapidly;" but he omits the most important fact of all, in his failure to state the previous growths of timber, or whether there were any neighboring growths of poplar along the track of the burnt district from which seed might have been derived.
Here are some of his more important statements:--
"At Clarksville, Ga., oak and hickory lands, when cleared, invariably grew up with pine. This is true of that region of country generally."
"At Aiken, S.C., the long-leaf pine is succeeded by oaks and other deciduous trees, and vice versa."