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.

[Sidenote:  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.

[Sidenote:  Monotonous race type of small area.]