The problem of delimiting Polish national boundaries is complicated on the east and west, as has been stated, by the absence of prominent surface features. On both sides the lines of linguistic parting provide the only practicable demarcation. On the north and south, however, the Baltic and the Carpathians may be utilized advantageously as national frontiers. But the fate of the Polish region is strongly outlined by nature, for the entire basin of the Vistula is a regional unit. Any partitioning of this basin would probably be followed by political conflicts. NOTE ON THE SLAVS In the ninth century the Slavs occupied the eastern plains of Europe between the valleys of the Elbe and the Dnieper. Southward they spread to the northern foothills of the mountains of central Europe. Although subdivided into tribes bearing different names, there existed no essential differences among them as to language or custom. The pagan divinities worshiped in the drainage area of the Vistula were the gods of the inhabitants of the Dnieper valley. Tribal authority was exercised by a chief designated as Kniaz or Voivod throughout these lowlands. Intercourse between the various groups was constant. A vague political union is even discerned by some historians. The Poles and Ruthenians and, to a lesser extent, the Bohemians, are the best modern representatives of these original Slavs. All the eastern Slavs, however, have mixed more or less with Asiatic peoples. Some light is thrown on the European origin of the peoples of Aryan speech by the growth of the Slavs. The Slavs of Europe now form by far the most important ethnic group of that continent. They comprise about 160,000,000 individuals out of a total of 400,000,000 inhabitants of Europe. Two-thirds of this Slavic element consists of Russians (66,000,000 Great Russians, 32,000,000 Little Russians, and about 8,000,000 White Russians).[129] Next to the Russians in numerical importance are the Poles (23,000,000). The Serbo-Croatian group can only muster half the Polish array. The Bohemians follow, 8,000,000 strong, while the Bulgarian group does not quite attain 6,000,000. Smaller groups are the 2,000,000 Slovenes, the 2,000,000 Slovaks and the less important enclave communities of German lands like the Wend in Lusatia.