s 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]

Every staple place and trading station is a center of geographical
information; it therefore gives an impulse to expansion by widening the
geographical horizon. The Lewis and Clark Expedition found the Mandan
villages at the northern bend of the Missouri River the center of a
trade which extended west to the Pacific, through the agency of the Crow
and Paunch Indians of the upper Yellowstone, and far north to the
Assiniboine and Saskatchewan Rivers. Here in conversation with British
and French fur-traders of the Northwest Company's posts, they secured
information about the western country they were to explore.[187]
Similarly the trade of the early Jesuit missions at La Pointe near the
west end of Lake Superior annually drew the Indians from a wide circle
sweeping from Green Bay and the Fox River in the south, across the
Mississippi around to the Lake of the Woods and far north of Lake
Superior.[188] Here Marquette f