[Sidenote:  Commerce.]

Commerce, which so largely underlies colonization, is itself a form of historical movement.  It both causes and stimulates great movements of peoples, yet it differs from these fundamentally in its relation to the land.  Commerce traverses the land to reach its destination, but takes account of natural features only as these affect transportation and travel.  It has to do with systems of routes and goals, which it aims to reach as quickly as possible.  It reduces its cortege to essentials; eliminates women and children.  Therefore it surmounts natural barriers which block the advance of other forms of the historical movement.  Merchant caravans are constantly crossing the desert, but not so peoples.  Traders with loaded yaks or ponies push across the Karakorum Mountains by passes where a migrating horde would starve and freeze.  The northern limit of the Mediterranean race in Spain lies sharply defined along the crest of the Pyrenees, whose long unbroken wall forms one of the most pronounced boundaries in Europe;[182] yet traders and smugglers have pushed their way through from time immemorial.  Long after Etruscan merchants had crossed northward over the Alps, Roman expansion and colonization made a detour around the mountains westward into Gaul, with the result that the Germans received Roman civilization not straight from the south, but secondhand through their Gallic neighbors west of the Rhine.

[Sidenote:  Commerce a guide to various movements.]

Commerce, though differing from other historical movements, may give to these direction and destination.  The trader is frequently the herald of soldier and settler.  He becomes their guide, takes them along the trail which he has blazed, and gives them his own definiteness of aim.  The earliest Roman conquest of the Alpine tribes was made for the purpose of opening the passes for traders and abolishing the heavy transit duties imposed by the mountaineers.[183] Fur-traders inaugurated French expansion to the far west of Canada, and the Russian advance into Siberia.  The ancient amber route across Russia from the Baltic to the Euxine probably guided the Goths in their migration from their northern seats to the fertile lands in southern Russia, where they first appear in history as the Ostrogoths.[184] The caravan trade across the Sahara from the Niger to the Mediterranean coast has itself embodied an historical movement, by bringing out enough negro slaves appreciably to modify the ethnic composition of the population in many parts of North Africa.[185] It was this trade which also suggested to Prince Henry of Portugal in 1415, when campaigning in Morocco, the plan of reaching the Guinea Coast by sea and diverting its gold dust and slaves to the port of Lisbon, a movement which resulted in the Portuguese circumnavigation of Africa.[186]