The relative importance of seaborne and overland routes in different time periods determined the historic role of ships and caravans in the long-distance trade of Eurasia. It is axiomatic that the technology of shipping and the art of navigation must attain a certain level of development before men are able to sail safely and carry goods across the open sea. There can be little doubt that from the tenth century AD the bulk of the commodities exchanged at the transcontinental level went by sea rather than by land. This is not to say that road transport was negligible or unimportant. Few port-towns of the Indian Ocean produced export goods in any quantity. Most articles had to be brought from a distance to the places of shipment. The close relationship between ships and the organisation of road transport in pre-modern Asia arose from the geographical distribution of the main producing areas and the location of urban centres acting as commercial emporia. The movement of goods by land, whether carried by pack animals or in wheeled vehicles, was complicated. There were too many factors which decided its economic success, and not all of them were under the control of merchants and transporters. The supply and price of pack animals — camels, horses, donkeys, and oxen — they knew and accounted for in advance; but it was not possible to predict with certainty the policy of local rulers and chiefs, who were in a position to hinder the movement of the herds if it did not suit their political and financial interests.