From the uniting power of rivers it follows that they are poor boundaries.  Only mountains and seas divide sharply enough to form scientific frontiers.  Rivers may serve as political lines of demarcation and therefore fix political frontiers; but they can never take the place of natural boundaries.  A migrating or expanding people tend always to occupy both slopes of a river valley.  They run their boundary of race or language across the axis of their river basin, only under exceptional circumstances along the stream itself.  The English-French boundary in the St. Lawrence Valley crosses the river in a broad transitional zone of mingled people and speech in and above the city of Montreal.  The French-German linguistic frontier in Switzerland crosses the upper Rhone Valley just above Sierre, but the whole canton of Valais above the elbow of the river at Martigny shows fundamental ethnic unity, indicated by identity of head form, stature and coloring.[692] Where the Elbe flows through the low plains of North Germany, its whole broad valley is occupied by a pure Teutonic population—­fair, tall, long-headed; a more brunette type occupies its middle course across the uplands of Saxony, and speaks German like the downstream folk; but its upper course, hemmed in by the Erz and Riesen Mountains, shows the short, dark and broad-headed people of the Bohemian basin, speaking the Czech language.[693] On the Danube, too, the same thing is true.  The upper stream is German in language and predominantly Alpine in race stock down to the Austro-Hungarian boundary; from this point to the Drave mouth it is Hungarian; and from the Drave to the Iron Gate it is Serbo-Croatian on both banks.[694] Lines of ethnic demarcation, therefore, cut the Elbe and Danube transversely, not longitudinally. [See map page 223.]

The statements of Caesar and Pliny that the Seine and Marne formed the boundary between the Gauls and Belgians, and the Garonne that between the Gauls and Aquitanians, must be accepted merely as general and preliminary; for exceptions are noted later in the text.  Parisii, for instance, were represented as holding both banks of the Seine and Marne at their confluence, and the Gallic Bituriges were found on the Aquitanian side of the Garonne estuary.

[Sidenote:  Scientific river boundaries.]

Only under peculiar conditions do rivers become effective as ethnic, tribal or political boundaries.  Most often it is some physiographic feature which makes the stream an obstacle to communication, and lends it the character of a scientific boundary.  The division of the Alpine foreland of southern Germany first into tribal and later into political provinces by the Iller, Lech, Inn, and Salzach can be ascribed in part to the tumultuous course of these streams from the mountains to the Danube, which renders them useless for communication.[695] The lower Danube forms a well maintained linguistic boundary between the Bulgarians and Roumanians, except in the northwest