While differences in production determine the directions of trade, it is physical features chiefly that determine the precise routes to be taken; they also determine the lines along which population moves. The point at which a country can most easily be entered may decide what nationality shall possess it. Ease of communication between one part and another by waterways, river valleys, passes through the mountains, or a continuous area with population-supporting resources in all parts, tends to make a homogeneous people. In this way physical features tend to determine the size of a state.

In Asia they have always had great empires; in Europe these could never subsist. Asia has larger plains; it is cut out into much more extensive divisions by mountains and seas. . . .

In Europe the natural division forms many nations of a moderate extent. ... - Montesquieu, op. cit., Book XVTI.

Environments are of two fundamental types in respect of their power to maintain society; those that are so poorly endowed with resources that they can maintain and attract only relatively small numbers of inhabitants, and those that, being richly endowed, support large populations of the native born, and tend to draw a large immigration from elsewhere. Each of these types of environment, in turn, presents two well-marked subdivisions: the isolated, or difficult of access or of egress; and the accessible, a land of ports and open ways, through which the currents of population may easily flow. - American Economic Association, Publications, Third Series, Vol. 5, p. 154 (398), Giddings, "A Theory of Social Causation."

Barriers like mountains, sandy deserts, or wide seas protect a country from external foes and give it a chance to develop a distinctive culture. If these barriers hem a people in as well, they make for isolation and ultimate stagnation; but if they permit free communication with the outside world so that the country can export its distinctive products and import those of other countries, the social life will constitute a part of the world development - how large a part depending on the extent of the country's resources and the genius of the people. Before the advent of modern means of travel, India was the great example of a country so isolated as to be stagnant. In modern times China has been the "hermit kingdom," though it, too, has lately been brought into the main current of the world's affairs by improved means of communication. The student of history can easily name examples of countries with barriers sufficient to protect but not to isolate, also of countries without even protective barriers.