The Road has the same canalizing effect where it overcomes an obstacle such as a broad river, or a mountain chain, or a belt of dense woodland. For instance, the fertile lowland fringe of South Wales and the corresponding fertile land to the east of the Severn were connected, when primitive methods alone could be used, by the bridge at Gloucester, high up the river. The lower reaches were too much for the earlier engineers, especially in the face of such a tide as runs on them. As a result the whole of that line of communications remained for 2000 years highly deflected, and only quite recently has there been some attempt at the more natural line by the piercing of the Severn tunnel. This effect of the Road in canalizing human effort is specially marked in the case of armies. The saying “an army is tied to the road” is a truth which historians should always keep in mind. There have been great cavalry raids in history--not often of permanent effect--which marched on a broad front, almost free of roads, and dependent only upon a sufficiency of forage. They have come from the grazing grounds of Asia, as a rule, and swept over the plains of Eastern Europe; but the organized and disciplined forces which have moulded history have always of necessity followed the Road. An army is not an island. It is an organism connected by a stalk with its base and dependent on this stalk for its feeding and equipment, its passing back of its prisoners and its wounded, and all its life. All these depend upon the Road. There are even cases in history--more numerous than one might imagine--where the first creation of the Road has been due to military action _alone_. I believe that the United States show examples of this, especially along the border between the northern and southern states east of the Mississippi. Certainly Europe shows them in striking fashion: it was a military necessity which made the great roads linking up the stations on the Rhine with the towns of Gaul and the rest of the Empire; it was a military necessity which made the regular roads over the Alpine passes. You can hardly say that there was a _commercial_ necessity for the great trunk road which struck the Rhine at Cologne, and which there later created the first bridge across the river. The country beyond was barbarous, and though a large number of Roman merchants penetrated it and a corresponding amount of trade was done, the main necessity for Cologne was a military necessity. Military necessity which drove the great road from the heart of Northern France to this isolated point and so opened up the wild wooded region in between. iv