In the words of the physical geographer, the earth's surface consists of the solid crust, or lithosphere, of the mass of water forming the seas and oceans and constituting the hydrosphere, and of that envelope of gas which we call the atmosphere. Considered separately, each of these is the concern of special sciences, and not of the geographer proper. His business it is to take the facts furnished by the meteorologists, the physicists, the geologists, and so forth, and with these facts in hand to proceed to consider the effect of the interaction of earth and water and air in a way which the separate sciences cannot do. We must further note that it is the interactions of these three which make the earth a possible home for life as we know it, and it is these interactions therefore which influence the distribution of life on the surface of the globe. There may have been a period when the crust of the earth was clothed in a uniform sheet of water, just as the globe is now enveloped in a complete covering of air, but at present, as through the long ages of geological time, the lithosphere consists of elevations and hollows, and it is in the hollows that the water accumulates, so that we can distinguish between the dry land and the ocean beds. Both chemically and physically the fluid hydrosphere differs markedly from the solid lithosphere, and it is, above all, the physical differences which are of supreme importance to the geographer. Because of them sea and land respond differently to the stream of solar energy which pours down upon our globe, and it is this different response which is the predominating factor in the production of the different climates, which again determine in its main outlines the distribution of living organisms. This being so, it is clear that it is of great importance to the geographer to know exactly the distribution of land and water over the surface of the earth. As the North Polar regions are still inadequately known, and the South Polar regions hardly known at all, we cannot as yet determine exactly this distribution, but any globe will show that land and ocean are very unequally distributed. The great land masses cluster round the North Pole, while the southern hemisphere consists largely of water. We thus have a land hemisphere and a water one. According to recent calculations the oceans occupy some 72 per cent. of the entire surface of the globe, leaving only 28 per cent. of land. But while in the northern hemisphere there is about one and a half times more water than land, in the southern there is about six times more water, both figures being liable to error, as indicated above, owing to our uncertainty as to the land and water of the Polar zones.