Now, in reality, there cannot be crops, or successions of crops, without the most thorough agriculture; and when we look more closely into nature we discover a system of husbandry of the most surprising kind. Nature does all things unobtrusively; and it is only now that we are beginning to see the magnitude of these secret agricultural operations by which she does already all that man would wish to imitate, and to which his most scientific methods are but clumsy approximations. In this great system of natural husbandry nature uses agencies, implements, and tools of many kinds. There is the disintegrating frost, that great natural harrow, which bursts asunder the clods by the expansion during freezing of the moisture imprisoned in their pores. There is the communistic wind which scatters broadcast over the fields the finer soil in clouds of summer dust. There is the rain which washes the humus into the hollows, and scrapes bare the rocks for further denudation. There is the air which, with its carbonic acid and oxygen, dissolves and decomposes the stubborn hills, and manufactures out of them the softest soils of the valley. And there are the humic acids, generated through decay, which filter through the ground and manure and enrich the new-made soils. But this is not all, nor is this enough; to prepare a surface film, however rich, and to manure the soil beneath, will secure one crop, but not a succession of crops. There must be a mixture and transference of these layers, and a continued mixture and transference, kept up from age to age. The lower layer of soil, exhausted with bringing forth, must be transferred to the top for change of air, and there it must lie for a long time, increasing its substance, and recruiting its strength among the invigorating elements. The upper film, restored, disintegrated, saturated with fertility and strength, must next be slowly lowered down again to where the rootlets are lying in wait for it, deep in the under soil. Now how is this last change brought about? Man turns up the crust with the plough, throwing up the exhausted earth, down the refreshed soil, with infinite toil and patience. And nature does it by natural ploughmen who, with equal industry, are busy all over the world reversing the earth's crust, turning it over and over from year to year, only much more slowly and much more thoroughly, spadeful by spadeful, foot by foot, and even grain by grain. Before Adam delved the Garden of Eden these natural agriculturists were at work, millions and millions of them in every part of the globe, at different seasons and in different ways, tilling the world's fields.