Primitive peoples carry a drag upon their migrations in their restricted geographical outlook; ignorance robs them of definite goals. The evolution of the historical movement is accelerated by every expansion of the geographical horizon. It progresses most rapidly where the knowledge of outlying or remote lands travels fastest, as along rivers and thalassic coasts. Rome's location as toll-gate keeper 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.]