BOOKS.

THE EARTH AS MODIFIED BY HUMAN ACTION* SOME three years ago the book now before us was published in America, and also in London, but it has received in this country much less notice than it deserves. The title is a very attractive one, and at first sight gives rise to some wonder that so little has been written on the subject, at least expressly, or in a generalised form. The materials no doubt are voluminous, as a glance at the long list of authorities consulted by the author will show, but they relate to detached portions of the general theme, and few of them are of British origin.

There is curious food for the imagination in picturing what would have been the aspect of our planet if, all other conditions of the present geological era being the same, man had never appeared on its surface, or even if he had appeared only in the shape of the Australian savage, or other apparently unprogressive type of humanity. The thick forest, varied only by the alternate crops of coniferous and broad-leaved deciduous trees, which un- doubtedly, prior to human agency, covered the whole of the dry land except only the frozen regions, the steepest mountain ridges, and such sandy deserts as the Sahara, would have appeared un- broken. No clearings for pasture or cultivation ; no roads, bridges, or glittering network of iron rails ; no spaces covered with habitations, squalid or splendid; no deep mines, no huge factories or arsenals, teeming with the industry of a restless and inventive race ; none of those more august monuments of its power springing from the sense of beauty or the religious instinct; no boats or ships ; no fire, unless volcanic, or from the spontaneous combustion of forests ; many races of living creatures, as the strange moa or the grotesque dodo, might have been still extant, and the myriad herds of animals which we have fostered for own uses, and whose forms we have modified and sometimes ennobled, would have been nowhere. All would have been found going on for ages under the law of natural selec- tion and survival of the fittest, without the dominating influence of spirit and rational will, or of a race of beings capable of hand- ing down the accumulated results of experience, and of modify- ing the aspect of material things by mechanical contrivance. Among the lower animals there is no living creature having even a merely instinctive faculty of construction—unless it be that poor little rodent, the beaver—which is sufficiently large and strong to produce any appreciable change on the earth's surface.

Mr. Marsh is by no means insensible to the larger and more picturesque aspect of his subject, but his aim is, in the main, practical, and his method is scientific, as far as questions so com- plex and uncertain admit of scientific treatment. A large portion of his book is occupied with the consideration of the results of human agency on the aboriginal arboreal clothing which seems to have been so universal over those parts of the earth which are suitable for the habitation of a dense and civilised population.

It is, of course, an inevitable result of the necessity of pasturage and of space for the culture of cereal and root-crops that a very large part of every inhabited country should be cleared of wood.

It is a process naturally regarded as the first step in material progress, and is so closely associated with it in the minds of the

inhabitants of new countries, that trees in the abstract come to be

looked upon almost in the light of a nuisance. This, joined with the enormous consumption of timber in every imaginable exercise of human skill and as fuel, naturally leads to the most reckless expenditure of an article which it requires several generations to replace, which can be easily imported from less advanced regions, and the evil effects of whose destruction are not obvious at the time. In the greater part of the British Islands the absence of lofty and steep mountain ranges, and our insular position which saves us from great extremes of climate, have given us a comparative exemption from the evils of

excessive denudation, although, with the exception of Spain and Portugal, ours is the least-wooded country in Europe, and the

subject has attracted much less interest here than in other States.

Passing over Mr. Marsh's discussion on the advantages of trees for shelter from cold winds, which he shows to be much more important and to extend to a much greater distance from the site of the trees than is generally supposed, and his analysis of the evidence in regard to the general effects of woodland on tempera-

ture, humidity, and rainfall, which is well known to be a depart- ment of investigation full of difficulties and apparent contradic-

• The Earth as Modified by Human Action, By George P. Marsh. London Sampson Low and Go. New York : Smibuer, Armstrong, and Go. 1874.

tions, we would direct the attention of all interested in such matters to his elaborate treatment of the enormous influence exercised by forests on the existence of springs, on the equalisation and flow of water from them, on the volume of rivers and the formation of torrents, and the disastrous effects of denudation in the production of sudden, short-lived, and excessive inundations. This is the one point in reference to the agency of woods on which there is substantial agreement among the scientific. Mr. Marsh thus describes the process

With the extirpation of the forest, all is changed. At one season the earth parts with its warmth by radiation to an open sky, and re- ceives at another an immoderate heat from the unobstructed rays of the sun. Hence the climate becomes excessive, and the soil is alternately parched by the fervour of summer and seared by the rigours of winter. Bleak winds sweep over the surface. Precipitation becomes as irregular as temperature. Melting snows and vernal rains, no longer absorbed byloose and bibulous vegetable mould, rush over a frozen surface, and pour down the valleys seaward, instead of filling a retentive bed of absorbent earth, and storing up moisture to feed perennial springs ; the soil is bared of its covering of leaves, broken and loosened by the plough, de- prived of the fibrous rootlets which held it together, dried and pulver- ised by sun and wind, and at last exhausted by new combinations. The face of the earth is no longer a sponge, but a dust-heap, and the floods which the waters of the sky pour over it hurry swiftly along it slopes, carrying in suspension vast quantities of earthy particles, which in- crease the abrading power and mechanical force of the current, and augmented by the sand and gravel of falling banks, fill the beds of the streams, direct them into now channels, and obstruct their outlets. The rivulets are heated, evaporated, and reduced in their summer cur- rents, but swollen to raging torrents in spring and autumn."

The evidence that the great floods which have from time to time during the last half century been so destructive in Switzer- land, and in many districts of France and Italy, have been mainly caused by the felling of the forests on the high grounds, appears to be overwhelming. In the Department of the Loire, especially, it was universally remarked that the wooded grounds suffered no change, while in the denuded districts the whole soil of cleared and cultivated fields was swept away and the rocks laid bare. The same was seen in the Upper Rhine in 1868. The clearings in the province of the Ardeche have produced the most melancholy results within the last thirty years, one-third of its area having become barren ; and new torrents had, in 1842, destroyed 70,000 acres of good land, an evil which has been going on ever since that time. The denudation of the crests of the Vosges has done infinite berm in Alsace. Many places in Provence, rich and inhabited half a century ago, have become deserts. Thousands of torrents have been formed within the last dozen years on the southern flank of the Piedmontese Alps and in Dauphiny, and grassy slopes have been converted into stony chasms by the cutting of the woods above. In the Depart- ment of the Lower Alps, between 1842 and 1852, 61,000 acres went out of cultivation from this cause. In Italy, the demand for Italian iron during the wars of Napoleon I., the trade with England being cut off, necessitated vast cuttings of wood for fuel, and the effects are felt to this day, especially in the valley of the Po. In fact there is scarcely a country on the continent of Europe in which the reckless destruction of forest has not been admitted, both in popular belief and by the verdict of science, to have been the cause of misery, of the amount of which the majority even of well-informed persons in England have little conception. The evil has been made the subject of minute in- vestigation by innumerable scientific men, and the leading scientific bodies in France, Italy, Germany, and even Russia, and has engaged the attention of Government in several of these countries. Much has been done and is now proceeding for the repair of the mischief, especially in France, under legislative authority, by the operations of Reboisement, or the judicious planting of certain places near the ravines which have been formed. The results of this process are necessarily gradual, but enough of benefit has already accrued to confirm the accepted theory of the origin of the evil. The principle on which the efficiency of forests depends must be evident from the quotation we have given above. We may form some conception of the extent to which the impetus of violent rain is diminished by subdivision and evaporation, when it falls upon wooded in place of open ground by the fact that it has been calculated that the leaves of a moderately large elm present a surface equal to 200,000 square feet. The chief influence of forests, however, evidently depends on the vast spongy mass formed by the innumerable root-fibres ; and in situations where the winters are severe, on the much smaller depth to which the earth is frozen in woodland than in the open, so that melting snow and spring rains flow in full volume over the latter, while they are detained and absorbed in the former. Any- body may test the effects of wood on drainage, by observing the ditches on either aide of a road where the ground on one side is planted and on the other cultivated. Czteris paribus, the water after heavy rain will be seen to linger for some time on the one side, and to disappear rapidly in a sort of flood on the other.

Our space will not allow as to follow Mr. Marsh in his state- ment in regard to the formation of sand and of dunes or sand- hills, or his account of some of the greatest human operations on the earth's surface, in the shape of the cutting of isthmuses and of embanking and draining, such as the embankment of the Mississippi, which at the time he wrote extended to no less than 2,500 miles, and is now probably greater; the draining of the Lake of Haarlem and that of the Lago Celano (the ancient Fucinus), which was effected at the sole expense of the Prince 'forlonia, perhaps the greatest operation ever undertaken by a single man, and which has produced 42,000 acres of as rich land' as any in Italy.

The book, though it is, as we have said, scientific in method, is intended less for the professed physicist than for persons of general intelligence and culture, and to such we sincerely recom- mend it. The style is clear and often graphic, and the work is full of interesting and suggestive information. If we have any- thing to say in the shape of adverse criticism, it is that Mr. Marsh has divided his subject into too many short sections.