ilizations have depended upon their relations to area. Therefore
problems of area, such as the expansion of a small territory, the
economic and political mastery of a large one, dominate all history.

[Sidenote: 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.

[Sidenote: 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