The third cause of survival was probably the excellence of the original construction, though here we must hesitate a little because we cannot but note that the Great North Road to York, which was quite as important and which was twin to the north-western road, has suffered very grievous modification indeed. But there can be no doubt that the construction of the Watling Street was very thorough, and that this expenditure of economic effort preserved it through the Dark Ages as much as anything did. Oddly enough, what is in most cases the strongest motive of all for the preservation of a road was here entirely absent, and that is what I have called the “potential” between the two terminals. When there is a long and continued motive for joining up two terminal points the Road has a cause of survival superior to any other. There was, and remains to this day, an extremely strong “potential” of this kind between the ports serving the Channel straits, with their nucleus at Canterbury, and the economic capital inland at London. It therefore, as the Roman road between the one terminal and the other, remained permanent throughout the centuries, with the exception of the deflection towards the Thames which grew up in the Dark Ages to serve the landing places at Gravesend. But such a “potential” is entirely lacking for the north-western road communication--so far as we know--to go between London and Chester. The trade with Ireland ceased almost during the early Dark Ages. The north-western road led nowhere. If it was preserved, therefore, as it _has_ been preserved, it must have been due to other causes which escape us. There it runs, however, still almost uninterruptedly used, from the Marble Arch in London to Oakengates in Shropshire, and in places still acts as part of the main artery leading from south-east to north-west. v (2) STANE STREET. The Stane Street (which I must be excused for quoting so continuously as I know it in great detail) is, I think, the leading example of a road still remaining for the most part and clearly showing how the later systems were built up upon a Roman backbone.