But further, the heats of summer and the frosts of winter aid this slow alteration. In the extremes of heat and of cold, the soil contracts more than the roots of the grasses do; and similar though less striking differences take place during the changes of temperature experienced in our climate in a single day. When the rain falls on the parched field, or when a thaw comes on, the earth expands, while the roots of the grasses remain nearly fixed; hence the soil rises up among the leaves, mixes with the vegetable matter, and thus assists in the slow accumulation of a rich vegetable mould. The reader has witnessed in winter how, on a field or a by-way side, the earth rises above the stones, and appears inclined to cover them; he may even have seen in a deserted and undisturbed highway, the stones gradually sinking and disappearing altogether, when the repetition of this alternate contraction and expansion of the soil for a succession of winters has increased in a great degree the effects which follow from a single accession of frosty weather. So it is in the fields. And if a person skilled in the soils of a given district can make a guess at the time when a given field was laid down to grass, by the depth at which the stones are found beneath the surface, it is because this loosening and expansion of the soil, while the stones remain fixed, tends to throw the latter down by an almost imperceptible quantity every year that passes. Such movements as these act in opening up the surface-soil, in mixing it with the decaying vegetable matter, and in allowing the slow action of the rains gradually to give its earthy portion a lighter character. But with these, among other causes, conspire also the action of living animals. Few persons have followed the plough without occasionally observing the vast quantities of earthworms with which some fields seem to be filled. On a close shaven lawn many have noticed the frequent little heaps of earth which these worms during the night have thrown out upon the grass. These and other minute animals are continually at work, especially beneath an undisturbed and grassy sward—and they nightly bring up from a considerable depth, and discharge on the surface, their burden of fine fertilizing loamy earth. Each of these burdens is an actual gain to the rich surface soil, and who can doubt that in the lapse of years, the unseen and unappreciated labours of these insect tribes must both materially improve its quality and increase its depth?