After the Civil Wars all the military meaning of the Thames disappears.  Nor is it likely to revive short of a national disaster; but that disaster would at once teach us the strategical meaning of this great highway running through the south of England with its attendant railways, it would re-create the strategical value of the point where the Thames turns northward and where its main railways bifurcate; it would provide in several conceivable cases, as it provided to Charles I. and to William III., the line of approach on London.

* * * * *

So far as we have considered the Thames, first as a line of pre-historic settlements, passing successively into the Roman, the barbaric and the Norman phases of our history; and secondly, as a field on which one can plot out certain strategical points and show how these points created the original importance of the towns which grew about them.

In the next part of these notes I propose to consider the economic or civil development of the Thames above London, and to show how the foundations of its permanent prosperity was laid.  That economic phenomenon has at its roots the action of the Benedictine Order.  It was the great monasteries which bridged the transition between Rome and the Dark Ages throughout North-Western Europe; it was they that recovered land wasted by the barbarian invasions, and that developed heaths and fens which the Empire even in its maturity had never attempted to exploit.

The effect of the barbarian invasions was different in different provinces of the Roman Empire, though roughly speaking it increased in intensity with the distance from Rome.  It is probable that the actual numbers of the barbarian invaders was small even in Britain, as it certainly was in Northern Gaul, but we must not judge of the effect produced upon civilisation by this catastrophe, as though it were a mere question of numbers.  So large a proportion of the population was servile, and so fixed had the imagination of everyone become in the idea that the social order was eternal; so entirely had the army become a professional thing, and probably a thing of routine divorced from the civilian life round it, that at the close of the fourth century a little shock from without was enough to produce a very considerable result.  In Eastern Britain, small as the number of the invaders must necessarily have been, religion itself was almost, if not entirely, destroyed, and the whole fabric of Roman civilisation appears to have dissolved—­with the exception, of course, of such irremovable things as the agricultural system, the elements of municipal life, and the simpler arts.  Even the language very probably changed in the eastern part of the island, and passed from what we may conceive to have been Low Latin in the towns and Celtic dialects in the country-sides, with possibly Teutonic settlements here and there along the eastern shore, to a generally confused mass of Teutonic dialects scattered throughout the eastern and northern half of the island and enclosing but isolated fragments of Celtic speech.