German immigrants invaded this region in the eighth century. Their language held its own until the fourteenth, after which it is represented only by linguistic islands dotting here and there the sea of Slavs. It is, however, still possible to distinguish settlements of German origin from the old Polish villages. The latter are situated on high ground or well-protected sites. They are generally characterized by the existence of a central open space and the random distribution of houses and lanes. The German villages, on the other hand, are found at the heads of valleys and usually occupy a rectangular site spreading over the two banks of a river. Each habitation has its own land appurtenance extending rearwards towards the valley slopes. The roads follow natural depressions. Taken as a whole, these German villages are admirably molded on the relief of the surface. The western linguistic boundary of Poland extends through the German provinces of Silesia and Posen. Here a gradual replacement of the language by German since the sixteenth century is noticeable. At that time the Oder constituted the dividing line, south of the point of the confluence of the Nissa between Brieg and Oppeln. As late as 1790 the population of Breslau was largely Polish. Today over 75 per cent of the inhabitants of the city and the neighboring towns and villages are Germans. The district north and south constitutes in fact an area of linguistic reclamation. The westernmost extension of Polish occurs in Posen, at the base of the provincial projection into Brandenburg. Around Bomst the percentage of Polish inhabitants is as high as 75 per cent. The line extends northwards to Birnbaum, after which it assumes a northeasterly direction. In spite of this occidental reach, however, the area of Polish speech within German boundaries is broken in numerous places by German enclaves of varying size.[105] In western Prussia, the Poles form compact inclusions in the German mass and attain the Baltic shores, where they occupy the entire western coast of the Gulf of Danzig. From Oliva and Danzig the line extends to Dirschau (Tezew) and crosses the Vistula about six miles below the city. It then strikes east and turns southwards towards Marienwerder (Kwidzyn) and Graudenz. Proceeding due east from here, the boundary passes south of Eylau, the southern territory of the Masurian lakes, and on into Russian territory, until Suwalki is reached. The eastern frontier begins at this point and is prolonged southwards, according to Slav authorities, through Augustow, Bielostok,[106] Surash, Bielsk, Sarnaki, Krsanostaw and Tomaschow.