This distribution is of great importance in connection with certain theories as to the actual plan of the earth, but this is a difficult subject which need not concern us here. It is discussed in Prof. J. W. Gregory's volume on _The Making of the Earth_. More interesting is the effect which the arrangement of land and water has had upon that part of the life of the earth which was evolved in late geological time. Though the geographer for convenience' sake recognizes three separate continents in the Old 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.