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