made for differences of zonal location, economic development, and degree of European elements in their respective populations.  Mexico and Argentine, having each an area only about one-fourth that of Brazil but a railroad mileage nearly one-fourth greater, have been pushed to this development primarily by a common lack of inland navigation.  Similarly South Africa, stricken with poverty of water communication south of the Zambesi, has constructed 7500 miles of railroads[678] in spite of the youth of the country and the sparsity of its white population.  Similar geographic conditions have forced the mileage of Australian railways up to twice that of South Africa, in a country which is still in the pastoral and agricultural stage of development, and whose most densely populated province Victoria has only fourteen inhabitants to the square mile.  In the almost unpeopled wastes of Trans-Caspia, where two decades ago the camel was the only carrier, the Russian railroad has worked a commercial revolution by stimulating production and affording an outlet for the irrigated districts of the encircling mountains.[679] In our own Trans-Missouri country, where the scanty volume of the streams eliminated all but the Missouri itself as a dependable waterway, even for the canoe travel of the early western trappers, railroads have developed unchecked by the competition of river transportation.[680] With no rival nearer than the Straits of Magellan and the Isthmus of Panama for transportation between the Mississippi and the Pacific coast, they have fixed their own charges on a monopoly basis, and have fought the construction of the Isthmian Canal.

[Sidenote:  Unity of a river system.]

A river system is a system of communication.  It therefore makes a bond of union between the people living among its remoter sources and those settled at its mouth.  Every such river system forms geographically an unbroken whole.  Only where a wild, torrent-filled gorge, like the Brahmaputra’s path through the Himalayas, interrupts communication between the upper and lower course, is human life in the two sections divorced.  But such cases are rare.  Even the River Jhelam, which springs with mad bounds from the lofty Vale of Kashmir through the outer range of the Himalayas down to its junction with the Indus, carries quantities of small logs to be used as railway sleepers; and though it shatters a large per cent. of them, it makes a link between the lumber men of the Kashmir forests and British railroad engineers in the treeless plains of the Indus.[681]

[Sidenote:  The effect of common water supply in arid lands.]