of the Tiber gave her knowledge of the upstream country and directed her conquest of its valley; and the movement thus started gathered momentum as it advanced.  Caesar’s occupation of Gaul meant to his generation simply the command of the roads leading from the Mediterranean to the northern sources of tin and amber, and the establishment of frontier outposts to protect the land boundaries of Italy; this represented a bold policy of inland expansion for that day.  The modern historian sees in that step the momentous advance of history beyond the narrow limits of the Mediterranean basin, and its gradual inclusion of all the Atlantic countries of Europe, through whose maritime enterprise the historical horizon was stretched to include America.  In the same way, mediaeval trade with the Orient, which had familiarized Europe with distant India and Cathay, developed its full historico-geographical importance when it started the maritime discoveries of the fifteenth century.  The expansion of the geographical horizon in 1512 to embrace the earth inaugurated a widespread historical movement, which has resulted in the Europeanization of the world.

[Sidenote:  Civilization and mobility.]

Civilized man is at once more and less mobile than his primitive brother.  Every advance in civilization multiplies and tightens the bonds uniting him with his soil; makes him a sedentary instead of a migratory being.  On the other hand every advance in civilization is attended by the rapid clearing of the forests, by the construction of bridges and interlacing roads, the invention of more effective vehicles for transportation whereby intercourse increases, and the improvement of navigation to the same end.  Civilized man progressively modifies the land which he occupies, removes or reduces obstacles to intercourse, and thereby approximates it to the open plain.  Thus far he facilitates movements.  But while doing this he also places upon the land a dense population, closely attached to the soil, strong to resist incursion, and for economic reasons inhospitable to any marked accession of population from without.  Herein lies the great difference between migration in empty or sparsely inhabited regions, such as predominated when the world was young, and in the densely populated countries of our era.  As the earth grew old and humanity multiplied, peoples themselves became the greatest barriers to any massive migrations, till in certain countries of Europe and Asia the historical movement has been reduced to a continual pressure, resulting in compression of population here, repression there.  Hence, though political boundaries may shift, ethnic boundaries scarcely budge.  The greatest wars of modern Europe have hardly left a trace upon the distribution of its peoples.  Only in the Balkan Peninsula, as the frontiers of the Turkish Empire have been forced back from the Danube, the alien Turks have withdrawn to the shrinking territory of the Sultan and especially to Asia Minor.