Poland’s easterly expansion with its prolonged and finally disastrous conflicts with Russia began after the battle of Grunwald in 1410. Although the Poles then inflicted a decisive defeat on the Teutonic Knights, the western provinces they had lost could not be regained. In the eastern field the basin of the Dnieper merged without abrupt transition into that of the Vistula, just as the basin of the Oder on the west formed the western continuation of the Baltic plain. Four centuries of struggle with Russia ensued until the Muscovite Empire absorbed the greater portion of Poland. The German element is slowly spreading eastward throughout the eastern provinces of Prussia which once formed part of the kingdom of Poland. Emigration of Poles to central and western Germany partly accounts for the German gain. From the larger cities of eastern Germany and more especially from Posen, Bromberg and Danzig, a steady stream of emigrants make their way towards the industrial centers of the west, where they find higher wages and generally improved economic conditions. The German government favors this expatriation of its Slav subjects. None of the vexations to which the Poles are subjected by government officials on their native plains are tolerated in the occidental provinces of the Empire. The result is that notable colonies of Poles have sprung up in the vicinity of industrial centers like Düsseldorf or Arnsberg, in the Munster district and the Rhine provinces. From a racial standpoint, these Poles are practically indistinguishable from Teutonic types. Their presence in Rhenish Prussia and Westphalia is no menace to German unity. They are easily assimilated; the second generation, speaking only German, forgets its antecedents and becomes submerged in the mass of the native population. Slav settlements are particularly numerous and dense along the Rhine-Herne canal between Duisburg and Dortmund.[110] They abound in the coal-producing Emscher valley, where their inhabitants form one-fifth of the population. The Polish settlers favor the flatlands and occupy them in preference to hilly regions. They do not confine their work to mining, but provide labor for the industrial plants clustered around the coal-fields. In the beginning of 1911 the number of Polish miners in the 19 mining districts of the “circle” of Dortmund exceeded that of any other nationality.