The picture which these changes present to the imagination is one of the most extraordinary in all geological history. We have been familiar with the idea of worlds drowned in water, and the primeval incandescent earth shows us the possibility of our globe being melted with fervent heat; but here we have a world apparently frozen out destroyed by cold, or doubly destroyed by ice and water. Let us endeavour to realise this revolution, as it may have occurred in any of the temperate regions of the Northern Hemisphere, thickly peopled with the magnificent animals that had come down from the grand old Miocene time. Gradually the warm and equable temperature gives place to cold winters and chilly wet summers. The more tender animals die out, and the less hardy plants begin to be winter-killed, or to fail to perfect their fruits. As the forests are thus decimated, other and hardier species replace those which disappear. The animals which have had to confine themselves to sheltered spots, or which have perished through cold or want of food, are replaced by others migrating from the mountains, or from colder regions. Some, perhaps, in the course of generations, become dwarfed in stature, and covered with more shaggy fur. Permanent snow at length appears upon the hill-tops, and glaciers plough their way downward, devastating the forests, encroaching on the fertile plains, and at length reaching the heads of the bays and fiords. While snow and ice are thus encroaching from above, the land is subsiding, and the sea is advancing upon it, while great icebergs drifting on the coasts still further reduce the temperature. Torrents and avalanches from the hills carry mud and gravel over the plains. Peat bogs accumulate in the hollows. Glaciers heap up confused masses of moraine, and the advancing sea piles up stones and shingle to be imbedded in mud on its further advance, while boreal marine animals invade the now submerged plains. At length the ice and water meet everywhere, or leave only a few green strips where hardy Arctic plants still survive, and a few well-clad animals manage to protract their existence. Perhaps even these are overwhelmed, and the curtain of the Glacial winter falls over the fair scenery of the Pliocene. In every locality thus invaded by an apparently perpetual winter, some species of laud animals must have perished. Others may have migrated to more genial climes, others under depauperated and hardy varietal forms may have continued successfully to struggle for existence. The general result must have been greatly to diminish the nobler forms of life, and to encourage only those fitted for the most rigorous climates and least productive soils.