At this point commence those enormous alterations in the surface level and climate of this part of the world which produced such extraordinary results, and during which man made his first appearance in Britain. At the very commencement of the Pleistocene era the temperature in Britain was lowered to such an extent as to produce a sudden disappearance of the semi-tropical fauna and flora: the land had reached the high elevation of 500 feet above the present level, joining Scotland and Scandinavia, and there had appeared in the North Sea large blocks of ice, which rapidly increased in size and quantity, and continually pushed farther south, until at length, after a long lapse of time, the whole of Northern Europe, Asia, and America as far as the latitude of about 45° became like a huge ice-house, the Arctic cold driving all life before it to a more southern latitude, those forms which had lived in Britain during Meiocene and Pleiocene times being the first to disappear on the earliest sign of the approaching cold, and the Arctic flora and fauna which took their place being afterwards compelled also to move southward, owing to the intense severity of the glaciation. When this state of things had lasted a very considerable time the climate became milder, the melting ice deposited its boulder-clay, and the high continent commenced to sink again to its former level, during which gradual submergence the climate became still warmer, until it at length reached a more than temperate mildness, at one time being almost tropical. Still the land continued to sink, and this submergence lasted until the British part of the great continent had become a large archipelago of small islands, the surface of the land being upwards of one thousand feet below the present level. It has been calculated that such a submergence would require at the least 88,000 years to be completed; so that a general idea may be formed of the enormous periods of time occupied by these glacial and inter-glacial epochs. While the British archipelago existed, another change of climate took place, resulting in another glacial period, but probably not of such intensity as the previous one. At this period the upper boulder clay was deposited in the sea, to be afterwards upheaved above the sea level in Yorkshire and other places. After a long continuance of this glaciation the land commenced to rise again and the climate to improve, until, after a period of about 136,000 years (according to careful computation), there was produced another continental condition, the ground reaching about 600 feet higher than now, and the climate becoming temperate once more. England, Ireland, Scotland, Scandinavia, Denmark, the Netherlands, France, and Spain once again formed a mighty continent, the climate of which was cold-temperate, becoming milder year by year, and the elevation of which was gradually declining, as it has continued to do until the present time, the British islands slowly