The Oikoumene.
Humanity's area of distribution and historical movement call the Oikoumene. It forms a girdle around the earth between the two polar regions, and embraces the Tropics, the Temperate Zones, and a part of the North Frigid, in all, five-sixths of the earth's surface. This area of distribution is unusually large. Few other living species so nearly permeate the whole vital area, and many of these have reached their wide expansion only in the company of man. Only about 49,000,000 square miles (125,000,000 square kilometers) of the Oikoumene is land and therefore constitutes properly the habitat of man. But just as we cannot understand a nation from the study of its own country alone, but must take into consideration the wider area of its spreading activities, so we cannot understand mankind without including in his world not only his habitat but also the vastly larger sphere of his activities, which is almost identical with the earth itself. The most progressive peoples to-day find their scientific, economic, religious and political interests embracing the earth.
Unity of the human species in the relation to the earth.
Mankind has in common with all other forms of life the tendency toward expansion. The more adaptable and mobile an organism is, the wider the distribution which it attains and the greater the rapidity with which it displaces its weaker kin. In the most favored cases it embraces the whole vital area of the earth, leaving no space free for the development of diversity of forms, and itself showing everywhere only superficial distinctions. Mankind has achieved such wide distribution. Before his persistent intrusions and his mobility, the earth has no longer any really segregated districts where a strongly divergent type of the man animal might develop. Hence mankind shows only superficial distinctions of hair, color, head-form and stature between its different groups. It has got beyond the point of forming species, and is restricted to the slighter variations of races. Even these are few in comparison with the area of the earth's surface, and their list tends to decrease. The Guanches and Tasmanians have vanished, the Australians are on the road to extinction; and when they shall have disappeared, there will be one variety the less in humanity. So the process of assimilation advances, here by the simple elimination of weaker divergent types of men, there by amalgamation and absorption into the stock of the stronger.
This unity of the human species has been achieved in spite of the fact that, owing to the three-fold predominance of the water surface of the globe, the land surface appears as detached fragments which rise as islands from the surrounding ocean. Among these fragments we have every gradation in size, from the continuous continental mass of Eurasia-Africa with its 31,000,000 square miles, the Americas with 15,000,000, Australia with nearly 3,000,000, Madagascar with 230,000, and New Zealand with 104,000, down to Guam with its 199 square miles, Ascension with 58, Tristan da Cunha with 45, and the rocky Islet of Helgoland with its scant 150 acres. All these down to the smallest constitute separate vital districts.
Isolation and differentiation.
Small, naturally defined areas, whether their boundaries are drawn by mountains, sea, or by both, always harbor small but markedly individual peoples, as also peculiar or endemic animal forms, whose differentiation varies with the degree of isolation. Such peoples can be found over and over again in islands, peninsulas, confined mountain valleys, or desert-rimmed oases. The cause lies in the barriers to expansion and to accessions of population from without which confront such peoples on every side. Broad, uniform continental areas, on the other hand, where nature has erected no such obstacles are the habitats of wide-spread peoples, monotonous in type. The long stretch of coastal lowlands encircling the Arctic Ocean and running back into the wide plains of North America and Eurasia show a remarkable uniformity of animal and plant forms[298] and a striking similarity of race through the Lapps, the Samoyedes of northern Russia, the various Mongolian tribes of Arctic Siberia to Bering Strait, and the Eskimo, that curiously transitional race, formerly classified as Mongolian and more recently as a divergent Indian stock; for the Eskimos are similar to the Siberians in stature, features, coloring, mode of life, in everything but head-form, though even the cephalic indices approach on the opposite shores of Bering Sea.[299] Where geography draws no dividing line, ethnology finds it difficult to do so. Where the continental land-masses converge is found similarity or even identity of race, easy gradations from one type to another; where they diverge most widely in the peninsular extremities of South America, South Africa and Australia, they show the greatest dissimilarity in their native races, and a corresponding diversity in their animal life.[300] Geographical proximity combined with accessibility results in similarity of human and animal occupants, while a corresponding dissimilarity is the attendant of remoteness or of segregation. Therefore, despite the distribution of mankind over the total habitable area of the earth, his penetration into its detached regions and hidden corners has maintained such variations as still exist in the human family.