World—Europe, Asia and Africa—yet these form practically one land mass, which in its turn approaches America very nearly at Bering Straits, and, less nearly, in the North Atlantic through the intervention of the British Isles, the Faeroes, Iceland, Greenland, etc. The centre of this land mass lies in Europe, a point not without its importance.

In this great land mass of the northern hemisphere life has reached its highest degree of development, both as regards animal form and as regards human societies. It was in the northern hemisphere that the highest mammals, the placentals, arose. There are many remarkable resemblances between the faunas of Europe, of Asia and of Africa, and a similar, if less marked, resemblance between those of North America on the one hand and of Europe and temperate Asia on the other. On the other hand, the two great land masses which occur in the southern hemisphere, South America and Australia, show very marked differences in their fauna, both from each other and from the northern land masses, and in both cases the fauna has a primitive aspect, which is best marked in Australia.

When we come to consider man, somewhat similar conditions present themselves. The great civilisations developed in the land mass of the Old World, though the waterless desert of the Sahara cut off much of Africa from participation in them. America developed a relatively high civilisation of its own, but as the icefields and ice-pack of the north formed a greater barrier to the migrations of man than to those of the northern animals, this American civilisation was for long cut off from that of the Old World, and when free communication became possible, it went down before that of the eastern world.

We must connect these facts directly with the peculiar distribution of land and water in the northern hemisphere, which made free intercourse possible, alike for the land animals and for man. The importance of this intercourse may be suggested in a few words. When a group of organisms is limited, from whatever cause, to a particular zone of the earth’s surface, the members of the group tend to acquire characters fitting them for this restricted area. But if the area is open, constantly or periodically, to incursions of organisms from adjacent areas, then, with the widening of the environment, and the greater intensity of the struggle for existence, evolution is quickened and new characters appear. The men of the Eurasian continent learnt, on the fierce battle-grounds of that continent, lessons which enabled them to conquer without difficulty the more isolated human groups of the southern hemisphere. The fact that they took south with them the mammals of the north, who also have thriven at the expense of the native forms, shows that the hold of the southern animals upon their habitat was no less precarious than that of man himself.

One other point is worth notice in connection with the distribution of land and water over the surface of the globe. We have seen that the northern hemisphere is the region where organic evolution has been most marked. It is, as it were, a great biological laboratory. On the other hand, in the southern hemisphere, which has fewer land masses to interfere with the circulation of the atmosphere, many physical phenomena occur in a more marked and orderly fashion than to the north. The westerly winds of the south blow with a force and a constancy which makes it impossible to compare them with the more variable westerlies of the north. Even the ocean currents of the south seem to show more constancy than those of the north. If the northern hemisphere is a great biological laboratory, the southern may be described as a physical one, and one of the great interests of the further exploration of the Antarctic is that it will probably cast light upon some important meteorological problems. (See Dr. W. S. Bruce’s volume on Polar Exploration.)

The distribution of land and water, with all its effects on climate and on the distribution of life, is, as we have seen, caused by the main features of the relief of the earth, by the existence of vast depressions in which the water accumulates, and of relative elevations from which it flows. But the minor details of relief, hill and valley, ocean depth and continental shelf, are also important, and exercise a very marked effect upon distribution. They therefore demand in their turn some consideration.

Taking first the prime distinction between land surface and ocean floor, we note that the two differ from one another markedly, alike in their characteristics and in the conditions to which they are exposed. The land is subjected to constantly varying conditions: to the alternation of day and night, and to the changes of the seasons, with corresponding variations in temperature; to the fluctuations of the weather; to running water, and so forth. In the great ocean depths at least, on the other hand, the conditions are remarkably uniform. Neither diurnal nor seasonal changes have here any effect; the temperature seems to fluctuate but little; the water is almost still. This uniformity of physical conditions is reflected in the uniformity of the surface over wide areas. While the land surface shows marked irregularities, the ocean floor has a monotonous character, with more gentle outlines.

In its most general form the characters of the sea bottom may be briefly stated. Round the great land masses there is an area of relatively shallow water, which is sometimes only a few miles wide, and at other times extends outwards for hundreds of miles. This region is the Continental Shelf, and its seaward boundary for convenience’ sake is taken at a depth of 100 fathoms, or 600 feet. Within this zone the influence of the land is still felt, and some of the characters of land surfaces appear. Thus we sometimes find that river valleys are prolonged outwards over the Continental Shelf, giving a markedly irregular appearance to the ocean floor. The British Islands lie upon a Continental Shelf of this kind, and this is one of our reasons for knowing that they are really only a part of the continent of Europe, separated from it by a slight depression.

The Continental Shelf slopes away from the land gently, and is widest where it fringes low continents, and narrowest where mountains approach the coast. Over it is spread the waste of the land, the coarser lying near the shore-line, the finer extending outwards to the steep seaward slope. This rapid slope leads down to the more or less uniform ocean plateau, whose surface is broken by the great ocean abysses, the greatest of which has a depth of about six miles. Relative but not absolute uniformity thus characterises all that part of the ocean floor which lies below about 100 fathoms.

Again, though the ocean floor is doubtless being slowly raised by the deposition upon it of the oceanic oozes, yet it is also true that as compared with the land surface it displays great constancy. While the land surface is constantly changing owing to the varying forces which act upon it, the floor of the ocean can vary but little from age to age, unless it is acted upon by the internal forces of the earth.

Turn now to the land. We note at once the two characters of marked irregularity of surface, and of changeableness. The changeableness is due to the forces of erosion which act upon the surface, and of these forces the most important to the geographer is running water. It is running water, aided by other agents, which carves the land into hill and