corner of Bulgaria, where the hill country between the Timok River and the Danube has enticed a small group of Roumanians across to the southern side.  From this point down the stream, a long stretch of low marshy bank on the northern side, offering village sites only at the few places where the loess terrace of Roumania comes close to the river, exposed to overflows, strewn with swamps and lakes, and generally unfit for settlement, has made the Danube an effective barrier.[696] Similarly, the broad, sluggish Shannon River, which spreads out to lake breadth at close intervals in its course across the boggy central plain of Ireland, has from the earliest 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 the people dwelling on the opposite banks.