_How we may get Valid Results by Studying only a Part of the World._--It is entirely possible to study by themselves the activities of such a part of the world, and we will therefore draw a line of demarcation about the countries which constitute the economic center of it, and thus include an area within which economic causes produce speedy effects. Each part of this area quickly responds to influences that originate in any other part. If the steel mills in America make radical improvements in their machinery, this change should, in the absence of a strong monopoly, affect the price of rails in England, Germany, etc. Within the central region wages and interest tend toward uniformity, though, as we have seen, they do not attain it. Across the boundary which separates this center from the outer zone, economic influences act in a more feeble way and are unable to bring rates of wages and interest even to an approximate equality. Western Europe, America, and whatever regions are in very close connection with them, we treat as a society, with the remainder of the world as its environment. This center trades with the environing region, sends some capital and labor thither, and draws some of each thence to the home countries. Willingly or otherwise, it instructs the people of the outer region in modern methods of industry, and thus causes what we may regard as a slow annexation of a part of the outer zone to the economic center and a modification of the character of industries at home and abroad. The principal movement of labor is in an inward direction, and from our point of view it is immigration not into one country merely but into all economic society. The predominant movement of capital has been outward. _Mode of Studying Interchanges between Center and Environing Zone._--All these movements have to be recognized in a study of the economic life of the central society. How, for example, is commerce with undeveloped regions to be regarded if we have the center only in view? It is simply one of two possible ways of getting goods. The people of the center can make a commodity that they use, or they can make something to send into the outlying countries in exchange for it. In the latter case they acquire it indirectly rather than directly, but they acquire it by their own industry in the one case as well as in the other.