When all we knew of Europe, and especially of our own islands, belonged to the comparatively short time during which they have been known to history, we had in truth much to wonder at in the political changes these countries were seen to have undergone; and our imaginations could be busy with the contrast between the unchanged features of our lands and seas and the ever-varying character of those who dwelt upon or passed over them. It is interesting to think that on such a river bank or on such a shore Cæsar or Charlemagne have actually stood, and that perhaps the grass or flowers or shells under their feet looked just the same as they do now, that the waves beat upon the strand in the same cadence, or the water flowed by with the same trickling sound. But when we open the pages of geology, we have unrolled before us a history of the earth itself, extending over periods compared with which the longest epoch of what is commonly called history seems scarcely more than a day, and of mutations in the face of nature so grand and awful that as we reflect upon them, forgetting for an instant the enormous periods required to bring these changes about, they sound like the fantastic visions of some seer, telling in allegorical language the history of the creation and destruction of the world. Of such changes, not the greatest, but the most interesting to the question we have at present in hand, were those vicissitudes of climate which followed upon the time when the formation of the crust of the earth had been practically completed. We learn of a time when, instead of the temperate climate which now favours our country, these islands, with the whole of the north of Europe, were wrapped in one impenetrable sheet of ice. The tops of our mountains, as well as of those of Scandinavia and the north of continental Europe, bear marks of the scraping of this enormous glacier, which must have risen to a height of two or three thousand feet. Not a single green thing, therefore, might be seen between our latitudes and the pole, while the ice-sheet, passing along the floor of the North Sea, united these islands with Scandinavia and spread far out into the deep waters of the Atlantic. For thousands of years such a state of things endured, but at last it slowly passed away. As century followed century the glacier began to decrease in size. From being colder than that of any explored portion of our hemisphere, the climate of northern Europe began to amend, until at last a little land became visible, which was covered first with lichens, then with thicker moss, and then with grass; then shrubs began to grow, and they expanded into trees and the trees into forests, while still the ice-sheet went on decreasing, until now the glaciers remained only in the hills. Animals returned from warmer climates to visit our shores. The birds and beasts and fishes of the land and sea were not much different from those which now inhabit there; the species were