III. Geographic conditions influence the economic and social development of a people by the abundance, paucity, or general character of the natural resources, by the local ease or difficulty of securing the necessaries of life, and by the possibility of industry and commerce afforded by the environment. From the standpoint of production and exchange, these influences are primarily the subject matter of economic and commercial geography; but since they also permeate national life, determine or modify its social structure, condemn it to the dwarfing effects of national poverty, or open to it the cultural and political possibilities resident in national wealth, they are legitimate material also for anthropo-geography. [Sidenote: Size of the social group.] They are especially significant because they determine the size of the social group. This must be forever small in areas of limited resources or of limited extent, as in the little islands of the world and the yet smaller oases. The desert of Chinese Turkestan supports, in certain detached spots of river-born fertility, populations like the 60,000 of Kashgar, and from this size groups all the way down to the single families which Younghusband found living by a mere trickle of a stream flowing down the southern slope of the Tian Shan. Small islands, according to their size, fertility, and command of trade, may harbor a sparse and scant population, like the five hundred souls struggling for an ill-fed existence on the barren Westman Isles of Iceland; or a compact, teeming, yet absolutely small social group, like that crowding Malta or the Bermudas. Whether sparsely or compactly distributed, such groups suffer the limitations inherent in their small size. They are forever excluded from the historical significance attaching to the large, continuously distributed populations of fertile continental lands. [Sidenote: Effect upon movements of peoples.] IV. The next class belongs exclusively to the domain of geography, because it embraces the influence of the features of the earth's surface in directing the movements and ultimate distribution of mankind. It includes the effect of natural barriers, like mountains, deserts, swamps, and seas, in obstructing or deflecting the course of migrating people and in giving direction to national expansion; it considers the tendency of river valleys and treeless plains to facilitate such movements, the power of rivers, lakes, bays and oceans either to block the path or open a highway, according as navigation is in a primitive or advanced stage; and finally the influence of all these natural features in determining the territory which a people is likely to occupy, and the boundaries which shall separate from their neighbors. [Sidenote: River routes.]