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. [Sidenote: Isolation and differentiation.]