In dismissing the question of climate, however, we should note that a very serious problem remains unsolved. As far as present evidence goes we seem to be free to hold that the ice-ages which have at long intervals invaded the chronicle of the earth were due to rises of the land. Upheaval is the one constant and clearly recognisable feature associated with, or preceding, ice-ages. We saw this in the case of the Cambrian, Permian, Eocene, and Pleistocene periods of cold, and may add that there are traces of a rise of mountains before the glaciation of which we find traces in the middle of the Archaean Era. There are problems still to be solved in connection with each of these very important ages, but in the rise of the land and consequent thinning of the atmosphere we seem to have a general clue to their occurrence. Apart from these special periods of cold, however, we have seen that there has been, in recent geological times, a progressive cooling of the earth, which we have not explained. Winter seems now to be a permanent feature of the earth's life, and polar caps are another recent, and apparently permanent, acquisition. I find no plausible reason assigned for this. The suggestion that the disk of the sun is appreciably smaller since Tertiary days is absurd; and the idea that the earth has only recently ceased to allow its internal heat to leak through the crust is hardly more plausible. The cause remains to be discovered. We turn now to consider the effect of the great Ice-Age, and the relation of man to it. The Permian revolution, to which the Pleistocene Ice-Age comes nearest in importance, wrought such devastation that the overwhelming majority of living things perished. Do we find a similar destruction of life, and selection of higher types, after the Pleistocene perturbation? In particular, had it any appreciable effect upon the human species? A full description of the effect of the great Ice-Age would occupy a volume. The modern landscape in Europe and North America was very largely carved and modelled by the ice-sheet and the floods that ensued upon its melting. Hills were rounded, valleys carved, lakes formed, gravels and soils distributed, as we find them to-day. In its vegetal aspect, also, as we saw, the modern landscape was determined by the Pleistocene revolution. A great scythe slowly passed over the land. When the ice and snow had ended, and the trees and flowers, crowded in the southern area, slowly spread once more over the virgin soil, it was only the temperate species that could pass the zone guarded by the Alps and the Pyrenees. On the Alps themselves the Pleistocene population still lingers, their successful adaptation to the cold now preventing them from descending to the plains.