[Sidenote:  Tendency toward ethnic and cultural unity in a river valley.]

Every river tends to become a common artery feeding all the life of its basin, and gradually obliterating ethnic and cultural differences among the peoples of its valley.  The Nile, with its narrow hem of flood-plain on either bank and barrier sands beyond, has so linked race and history in Egypt and Nubia, that the two countries cannot be separated.  A common highway from mountains to sea, a common frontier of trackless desert have developed here a blended similarity of race, language and culture from the delta to Kordofan.  The Hamitic race seems to have originated in the south and migrated northward down the Nile towards the delta.  Later the whole valley, north and south, received the same Semitic or Arab immigration, which spread from Cairo to the old Sudanese capital of Sennar, while a strain of negro blood has filtered in from the equatorial black belt and followed the current down to the sea.[690] The culture of the valley originated in Lower Egypt, and, with that easy transmissibility which characterizes ideas, it moved upstream into Ethiopia, which never evolved a culture of its own.  Just as noticeable is the political interplay.  The rule of the Pharaohs extended far up the Nile, at times to the Third Cataract at 20 deg.  N.L.; and at one period Ethiopian kings extended their sway over Egypt.  At another, a large body of mutinous Egyptian soldiers abandoned their country and their wives, and emigrated along the one line of slight resistance open to them into Ethiopia, to found there a new state and new families by marriage with native women, thus contributing to the amalgamation of races in the valley.

[Sidenote:  Identity of country with river valley.]

The most pronounced types of the identity of a country with a river valley are found where strongly marked geographical boundaries, like deserts and mountains, emphasize the inner unity of the basins by accentuating their isolation from without.  This is especially the case in high mountain regions; here canton or commune or county coincides with the river valley.  Population hugs the margins of the streams where alone is soil fit for cultivation, and fairly level land suitable for dwellings.  Above are the unoccupied heights, at once barrier and boundary.  In the Alps, Salzburg is approximately identical with the valley of the Salzach, Uri with that of the Reuss, the Valais with the upper Rhone, the Engadine with the upper Inn, Glarus with the Linth, Graubuenden or Grisons with the upper Rhine, Valtellina with the Adda.  So in the great upheaved area of the Himalayas, the state of Kashmir was originally the valley of the upper Jhelam River, while Assam, in its correct delimitation, is the valley of the Brahmaputra between the Himalayan gorge and the swamps of Bengal.[691]