ART OF BOONOMiC DEVELOPMENT 9 density of population that will be supported, and whence its immigration is to be arranged. 8. Natural Trade Routes.--Merchants having to convey goods from place to place naturally choose the cheapest route. In former times this meant by water where it was available. By land it meant seeking the most conveniently accessible point on an accus- tomed line of travel'--a caravan route or great through road. Such mutes or roads became great channels of trade, because they had been chosen with a view to avoiding natural obstacles and risks from highway robbers and daceits. The chief natural obstacles are mountains and rivers, marshes and thick belts of forest. Trade routes have tended always to converge on places where an obstacle is most easily passed, as for example on passes across mountain r?nges, on fords of rivers, or on the head of a lake, estuary or marsh. These, and the transhiprecur 'stations from land to water- carriage, have from time immemorial been converging points of trade and have often grown into towns of considerable importance. The building of railways has-in mostr countries greatly altered the trade routes, particularly the minor ones-- not so mdch the great routes. The latter have often been followed by the railways &ither on account of existing big towns, or because great natural obstacles affec? a railway as much, or more than a road. Thus railways u?ilize passes over mountain ranges, and broad valleys ?hrough hilly country, ?ns? a.s much as roads; and easy bridging points on grea? rivers are often ?he same, or close to ?he old fords, or ferries,, On the o?her hand it is ?rue ?ha? communications are less and less eon?rolled b? physical features, as population increases. Very eos?ly railway works can ?hen be profitably under- taken, or are required for strategic purposes, as, for example, the three great Trans-Alpine railways, and
