If the factor is not one of geographical location, but a natural barrier, such as a mountain system or a desert, its effect is just as persistent.  The upheaved mass of the Carpathians served to divide the westward moving tide of the Slavs into two streams, diverting one into the maritime plain of northern Germany and Poland, the other into the channel of the Danube Valley which guided them to the Adriatic and the foot of the Alps.  This same range checked the westward advance of the mounted Tartar hordes.  The Alps long retarded Roman expansion into central Europe, just as they delayed and obstructed the southward advance of the northern barbarians.  Only through the partial breaches in the wall known as passes did the Alps admit small, divided bodies of the invaders, like the Cimbri and Teutons, who arrived, therefore, with weakened power and at intervals, so that the Roman forces had time to gather their strength between successive attacks, and thus prolonged the life of the declining empire.  So in the Middle Ages, the Alpine barrier facilitated the resistance of Italy to the German emperors, trying to enforce their claim upon this ancient seat of the Holy Roman Empire.

It was by river-worn valleys leading to passes in the ridge that Etruscan trader, Roman legion, barbarian horde, and German army crossed the Alpine ranges.  To-day well-made highways and railroads converge upon these valley paths and summit portals, and going is easier; but the Alps still collect their toll, now in added tons of coal consumed by engines and in higher freight rates, instead of the ancient imposts of physical exhaustion paid by pack animal and heavily accoutred soldier.  Formerly these mountains barred the weak and timid; to-day they bar the poor, and forbid transit to all merchandise of large bulk and small value which can not pay the heavy transportation charges.  Similarly, the wide barrier of the Rockies, prior to the opening of the first overland railroad, excluded all but strong-limbed and strong-hearted pioneers from the fertile valleys of California and Oregon, just as it excludes coal and iron even from the Colorado mines, and checks the free movement of laborers to the fields and factories of California, thereby tightening the grip of the labor unions upon Pacific coast industries.

[Sidenote:  Persistent effect of nature-made highways.]

As the surface of the earth presents obstacles, so it offers channels for the easy movement of humanity, grooves whose direction determines the destination of aimless, unplanned migrations, and whose termini become, therefore, regions of historical importance.  Along these nature-made highways history repeats itself.  The maritime plain of Palestine has been an established route of commerce and war from the time of Sennacherib to Napoleon.[1] The Danube Valley has admitted to central Europe a long list of barbarian invaders, covering the period from Attila the Hun to the Turkish besiegers of Vienna in 1683.  The history of the Danube Valley has been one of warring throngs, of shifting political frontiers, and unassimilated races; but as the river is a great natural highway, every neighboring state wants to front upon it and strives to secure it as a boundary.