only fords or ferries.

Nevertheless, it is true of the Thames, as of nearly every other
division in Europe, that it was much more of a boundary at the end of
the Roman Empire, and is more of a strict boundary to-day, than it was
during the Dark Ages, and presumably also before the Claudian
invasion. Thus we may conjecture with a fair accuracy that in the last
great ordering of boundaries within the Roman Empire, which was the
work of Diocletian, and so much of which still survives in our
European politics to-day (for instance, the boundary of Normandy), the
Thames formed the division between Southern and Midland Britain. It is
equally certain that it did _not_ form any exact division between
Wessex and Mercia.

The estuary has, of course, always formed a division, and in the
barbarian period it separated the higher civilisation of Kent from
that of the East Saxons, who were possibly of a different race, and
certainly of a different culture. But the Thames above London Bridge
was not a true boundary until the civilisation of England began to
form, towards the close of the Dark Ages. It is perpetually crossed
and recrossed by contending armies, and the first result of a success
is to cause the conqueror to annex a belt from the farther bank to his
own territories.

It is further remarkable that the one great definite boundary of the
Dark Ages in England--that which was established for a few years by
Alfred between his kingdom and the territory of the Danish
invaders--abandons the Thames above bridges altogether, and uses it as
a limitation in its estuarial part only, up to the mouth of the Lea.