In the first part of this study it was suggested that it is possible to distinguish certain forms or patterns of expansion which recur constantly. We have noted that rivers served as effective boundaries only during a very early stage of economic and technical development and soon became routes of communication instead of barriers. We have noted, too, that expansion tends to follow the line of least resistance. It is not surprising, then, to find, almost as soon as states secure access to a river, a strong tendency to expand up and down the river valley. Movement downstream is the natural flow to the sea, and movement upstream is necessary for purposes of defense, since whoever controls the upper valley has a distinct strategic advantage. Control of the upper valley is necessary also in order to control and regulate the water supply. The next step is to push the frontier to the watershed on the other side of the river, since there, if anywhere, is to be found the first natural geographic and economic limit to expansion. With this movement up and down stream and across the stream to the watershed, we have the evolution of the river valley as an economic unit. The unifying influence of the river is responsible for the fact that practically all early civilizations grew up in river basins–Mesopotamia on the Tigris and Euphrates, Egypt on the Nile, the Punjab on the Indus, and China along the Hoang-ho.