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.