Читать книгу The Earth as Modified by Human Action - George P. Marsh - Страница 34

The quantitative value of animated life, as a geological agency, seems to be inversely as the volume of the individual organism; for nature supplies by numbers what is wanting in the bulk of the animal out of whose remains or structures she forms strata covering whole provinces, and builds up from the depths of the sea large islands, if not continents. There are, it is true, near the mouths of the great Siberian rivers which empty themselves into the Polar Sea, drift islands composed, in an incredibly large proportion, of the bones and tusks of elephants, mastodons, and other huge pachyderms, and many extensive caves in various parts of the world are half filled with the skeletons of quadrupeds, sometimes lying loose in the earth, sometimes cemented together into an osseous breccia by a calcareous deposit or other binding material. These remains of large animals, though found in comparatively late formations, generally belong to extinct species, and their modern congeners or representatives do not exist in sufficient numbers to be of sensible importance in geology or in geography by the mere mass of their skeletons. [Footnote: Could the bones and other relics of the domestic quadrupeds destroyed by disease or slaughtered for human use in civilized countries be collected into large deposits, as obscure causes have gathered together those of extinct animals, they would soon form aggregations which might almost be called mountains. There were in the United States, in 1870, as we shall see hereafter, nearly one hundred millions of horses, black cattle, sheep, and swine. There are great numbers of all the same animals in the British American Provinces and in Mexico, and there are large herds of wild horses on the plains, and of tamed among the independent Indian tribes of North America. It would perhaps not be extravagant to suppose that all these cattle may amount to two thirds as many as those of the United States, and thus we have in North America a total of 160,000,000 domestic quadrupeds belonging to species introduced by European colonization, besides dogs, cats, and other four-footed household pets and pests, also of foreign origin.

If we allow half a solid foot to the skeleton and other slowly destructible parts of each animal, the remains of these herds would form a cubical mass measuring not much short of four hundred and fifty feet to the side, or a pyramid equal in dimensions to that of Cheops, and as the average life of these animals does not exceed six or seven years, the accumulations of their bones, horns, hoofs, and other durable remains would amount to at least fifteen times as great a volume in a single century. It is true that the actual mass of solid matter, left by the decay of dead domestic quadrupeds and permanently added to the crust of the earth, is not so great as this calculation makes it. The greatest proportion of the soft parts of domestic animals, and even of the bones, is soon decomposed, through direct consumption by man and other carnivora, industrial use, and employment as manure, and enters into new combinations in which its animal origin is scarcely traceable; there is, nevertheless, a large annual residuum, which, like decayed vegetable matter, becomes a part of the superficial mould; and in any event, brute life immensely changes the form and character of the superficial strata, if it does not sensibly augment the quantity of the matter composing them. The remains of man, too, add to the earthy coating that covers the face of the globe. The human bodies deposited in the catacombs during the long, long ages of Egyptian history, would perhaps build as large a pile as one generation of the quadrupeds of the United States. In the barbarous days of old Moslem warfare, the conquerors erected large pyramids of human skulls. The soil of cemeteries in the great cities of Europe has sometimes been raised several feet by the deposit of the dead during a few generations. In the East, Turks and Christians alike bury bodies but a cople of feet beneath the sculptures of the ignoble poor, and of those whose monuments time or accident has removed, are opened again and again to receive fresh occupants. Hence the ground in Oriental cemeteries is pervaded with relics of humanity, of not wholly composed of them; and an examination of the soil of the lower part of the Petit Champ des Morts, at Pera, by the naked eye alone, shows the observer that it consists almost exclusively of the comminuted bones of his fellow-man.] But the vegetable products found with them, and, in rare cases, in the stomachs of some of them, are those of yet extant plants; and besides this evidence, the discovery of works of human art, deposited in juxtaposition with fossil bones, and evidently at the same time and by the same agency which buried these latter—not to speak of human bones found in the same strata—proves that the animals whose former existence they testify were contemporaneous with man, and possibly even extirpated by him. [Footnote: The bones of mammoths and mastodons, in many instances, appear to have been grazed or cut by flint arrow-heads or other stone weapons, and the bones of animals now extinct are often wrought into arms and utensils, or split to extract the marrow. These accounts have often been discredited, because it has been assumed that the extinction of these animals was more ancient than the existence of man. Recent discoveries render it certain that this conclusion has been too hastily adopted.

On page 143 of the Antiquity of Man, Lyell remarks that man "no doubt played his part in hastening the era of the extincion" of the large pachyderms and beasts of prey; but, as contemporaneous species of other animals, which man cannot be supposed to have extirpated, have also become extinct, he argues that the disappearance of the quadrupeds in question cannot be ascribed to human action alone.

On this point it may be observed that, as we cannot know what precise physical conditions were necessary to the existence of a given extinct organism, we cannot say how far such conditions may have been modified by the action of man, and he may therefore have influenced the life of such organisms in ways, and to an extent, of which we can form no just idea.] I do not propose to enter upon the thorny question, whether the existing races of man are genealogically connected with these ancient types of humanity, and I advert to these facts only for the sake of the suggestion, that man, in his earliest known stages of existence, was probably a destructive power upon the earth, though perhaps not so emphatically as his present representatives. The larger wild animals are not now numerous enough in any one region to form extensive deposits by their remains; but they have, nevertheless, a certain geographical importance. If the myriads of large browsing and grazing quadrupeds which wander over the plains of Southern Africa—and the slaughter of which by thousands is the source of a ferocious pleasure and a brutal triumph to professedly civilized hunters—if the herds of the American bison, which are numbered by hundreds of thousands, do not produce visible changes in the forms of terrestrial surface, they have at least an immense influence on the growth and distribution of vegetable life, and, of course, indirectly upon all the physical conditions of soil and climate between which and vegetation a mutual interdependence exists. In the preceding chapter I referred to the agency of the beaver in the formation of bogs as producing sensible geographical effects.

I am disposed to think that more bogs in the Northern States owe their origin to beavers than to accidental obstructions of rivulets by wind-fallen or naturally decayed trees; for there are few swamps in those States, at the outlets of which we may not, by careful search, find the remains of a beaver dam. The beaver sometimes inhabits natural lakelets and even large rivers like the Upper Mississippi, when the current is not too rapid, but he prefers to owe his pond to his own ingenuity and toil. The reservoir once constructed, its inhabitants rapidly multiply so long as the trees, and the harvests of pond lilies and other aquatic plants on which this quadruped feeds in winter, suffice for the supply of the growing population. But the extension of the water causes the death of the neighboring trees, and the annual growth of those which could be reached by canals and floated to the pond soon becomes insufficient for the wants of the community, and the beaver metropolis now sends out expeditions of discovery and colonization. The pond gradually fills up, by the operation of the same causes as when it owes its existence to an accidental obstruction, and when, at last, the original settlement is converted into a bog by the usual processes of vegetable life, the remaining inhabitants abandon it and build on some virgin brooklet a new city of the waters. [Footnote: I find confirmation of my own observations on this point (published in 1863) in the North-West Passage by Land of Milton and Cheadle, London, 1865. These travellers observed "a long chain of marshes formed by the damming up of a stream which had now ceased to exist," Chap. X. In Chap. XII, they state that "nearly every stream between the Pembina and the Athabasca—except the large river McLeod—appeared to have been destroyed by the agency of the beaver," and they question whether the vast extent of swampy ground in that region "has not been brought to this condition by the work of beavers who have thus destroyed, by their own labor, the streams necessary to their own existence."

But even here nature provides a remedy, for when the process of "consolidation" referred to in treating of bogs in the first chapter shall have been completed, and the forest re-established upon the marshes, the water now diffused through them will be collected in the lower or more yielding portions, cut new channels for their flow, become running brooks, and thus restore the ancient aspect of the surface.

The authors add the curious observation that the beavers of the present day seem to be a degenerate race, as they neither fell huge trees not construct great dams, while their progenitors cut down trees two feet in diameter and dammed up rivers a hundred feet in width. The change in the habits of the beaver is probably due to the diminution of their numbers since the introduction of fire-arms, and to the tact that their hydraulic operations are more frequently interrupted by the encroachments of man. In the valley of the Yellowstone, which has but lately been much visited by the white man, Hayden saw stumps of trees thirty inches in diameter which had been cut down by beavers. —Geological Survey of Wyoming, p. 135.

The American beaver closely resembles his European congener, and I believe most naturalists now regard them as identical. A difference of speceies has been inferred from a difference in their modes of life, the European animal being solitary and not a builder, the American gregarious and constructive. But late careful researches in Germany have shown the former existence of numerous beaver dams in that country, though the animal, having becaome rare to form colonies, has of course ceased to attempt works which require the co-operation of numerous individuals.—Schleiden, Fur Baum und Wald Leipzig, 1870, p. 68.

On the question of identity and on all others relating to this interesting animal, see L.H. Morgan's important monograph, The American Beaver and his Works, Philadelphia, 1868. Among the many new facts observed by this investigator is the construction of canals by the beaver to float trunks and branches of trees to his ponds. These canals are sometimes 600 to 700 feet long, with a width of two or three feet and a depth of one to one and a half.]