t times proved a sufficient barrier to divide the
plain into two portions, Connaught and Meath,[697] contrasted in history,
in speech and to some extent even in race elements.[698] A different
cause gave the Thames its unique role among the larger English rivers as
a boundary between counties from source to mouth. London's fortified
position at the head of the Thames estuary closed this stream as a line
of invasion to the early Saxons, and forced them to make detours to the
north and south of the river, which therefore became a tribal
boundary.[699]

Where navigation is peculiarly backward, a river may present a barrier.
An instructive instance is afforded by the River Yo, which flows
eastward through northern Bornu into Lake Chad, and serves at once as
boundary and protection to the agricultural tribes of the Kanuri
against the depredations of the Tibbu robbers living in the Sahara or
the northern grassland. But during the dry season from April to August,
when the trickling stream is sucked up by the thirsty land and thirstier
air, the Tibbu horsemen sweep down on the unprotected Kanuri and retreat
with their booty across the vanished barrier. The primitive navigation
by reed or brushwood rafts, practiced in this almost streamless
district, affords no means of retreat for mounted robbers; so the
raiding season opens with the fall of the river.[700]

[Sidenote: Rivers as political boundaries.]

For political boundaries, which are often adopted with little reference
to race distribution, rivers serve fairly well. They are convenient
lines of demarcation and strategic lines of defense, as is proved by the
military history of the Rhine, Danube, Ebro, Po, and countless other
streams. On the lower Zambesi Livingstone found the territories of the
lesser chiefs defined by the rivulets draining into the main river. The
leader of the Makololo formally adopted the Zambesi as his political and
military frontier, though his people spread and settled beyond the
river.[701] Long established political frontiers may become ethnic
boundaries, more or less distinct, because of protracted political
exclusion. To the Romans, the Danube and Rhine as a northeastern
frontier had the value chiefly of established lines in an imperfectly
explored wilderness, and of strategic positions for the defense of an
oft assailed border; but the long maintenance of this political frontier
resulted in the partial segregation and hence differentiation of t