The progress of Germanization eastwards to the Mittelmark, between Elbe and Oder, and the Neu-Mark farther east, towards but not up to the Vistula, must not be interpreted as meaning that these regions were still purely Slavonic in the days of the Elbe frontier. Probably German was already in use here and there, but Germanization and the spread of the Church involved agricultural development and closer settlement, and so changed the character of the country. It was done under German leadership, and brought in German settlers. In the early Middle Ages there was a recession of Slavonic on the Prussian plain, and now only one fully Slavonic region in the German area remains. This is the Wendish area, and the Wends are supposed to have spread down from the hill slopes on the northern frontier of Bohemia. The present area of the Wends is astride the upper Spree, and especially around Kottbus, and the district is called Lusatia, but in the Middle Ages they spread farther north. It is largely Germanized in speech at the present time, but preserves old features and Slav names. Slav place-names are as characteristic on the German plain east of the Elbe as are Celtic names in the non-Celtic speaking part of Scotland, and illustrate a change of language of a people already having some settled organization. This contrasts with the rarity of survival of Celtic town names on the English plain. The changes of language areas since the Middle Ages have probably been quite small. The zone west of the Vistula has remained a zone of admixture of German and Polish-speaking people, whence the 'Polish corridor to Danzig' of the recent treaties, linguistically and traditionally justifiable and yet admitted to be dangerous politically. The relation of the Teutonic languages of Jutland to one another in origin is beyond our scope, but the dialect of the western side of the base of Jutland has remained fairly distinct {41} as 'Frisian', and has attempted a literary development in the last century. The new treaty boundary between Germany and Denmark gives what was North Slesvig to Denmark, and is intended to run along a line in the linguistic frontier-zone. The Low German speech of the Rhine mouths, with the long isolation of the people of that region and their concentration on the work of fighting the sea, developed into Dutch. We thus have Dutch, Frisian, Low German, High German or Alemannic, and naturally a Middle German developing between the two latter along the zone where the southern hills grade into the northern plain.