Thus, the normal circulation is notably complete in woodlands, and is notably deranged by deforestation; when the trees are felled and not replaced by other cover transpiration ceases, the air dries so that seeds and seedlings may wither, and the soil-water level lowers; the duff is desiccated and wind-blown, leaving the previously porous soil to harden and bake; and as storms arise the raindrops are no longer dashed into spray by twigs and foliage and conveyed gently through litter and natural mulch into a friable soil of enormous capacity for feeding springs and brooks, but beat still harder the indurated ground—and then gather in surface rivulets and rills running swiftly down the slopes, eroding and gullying the soil on the way, clogging valleys with the debris, and rushing as turbid torrents into the sea with little benefit and large injury on their way. The Forest Service was created largely to counteract reckless deforestation and maintain the timber supply in certain sections of the country; yet it has grown into administration of 170,000,000 acres of woodland, while its highest duty has come to be that of acquiring and diffusing definite knowledge and directing specific effort toward control over the powers of nature in such manner as to protect the water supply and regulate that balance of industries connected with woods and waters required for the common prosperity. The investigations extend into vegetal physiology and the vital mechanism of the individual seed or slip or tree no less than into the collective action and relations of the forest considered as a unit, and pass over into both natural and artificial production. The aim is increased efficiency of both individual trees and forests; the end is higher national efficiency; the means, progressive control over natural powers through definite knowledge and purposive application.
Over millions of acres of grass-lands and former woodlands, the native flora is wholly or partly replaced by crop plants, and it is the manifest destiny of all available lands to be consecrated wholly to production or inhabitation or other human uses. In prehistoric times men began to subsist on the products of the soil, and through unwitting selection improved wheat and rye and barley and rice in the old world and corn and beans in the new; and during the historical period the improvement of the useful types and the replacement of Useless or noxious types were continued under the guidance of increasing knowledge. Since the Department of Agriculture was created—and largely through its agency—the sum of human knowledge relating to crop plants and their efficiency in this country has more than doubled; and now the utility of plants is traced directly to individual vitality—to the specific factors of cell function and reaction to stimuli and hereditary tendency combining with capacity for transpiration to determine rate and limit of growth—which is itself measurably susceptible of modification. In this way the Bureau of Plant Industry is steadily
