The Adriatic Sea alone provides the Serbo-Croatian peoples with a definite boundary. The line of the Drave on the north once formed a frontier for the twin group. In modern times a number of Croatian settlements have pushed forward along the Raab valley toward Slovak territory. Here they stand in danger of becoming lost in the midst of the Hungarian population, as were the Serbian settlements which in the seventeenth century were scattered as far as Budapest. On the southwest, the boundary of the Serbian linguistic area presents many obstacles to accurate delimitation. There was a time when all northern Albania was part of the Serbian empire. In the eleventh century the Serbian kingdom, established in the Lake Scutari district, comprised Albanian populations within its boundaries. Immediately before the Turkish conquest Serbian language and customs had advanced as far south as Epirus. The coming of the Asiatics caused profound changes in the distribution of Balkan populations through the conversions to Mohammedanism by which it was attended. Many Serbians who had penetrated to the south and a large number of Albanians became followers of the Prophet. Their descendants became “Turks” and as such endured the vicissitudes which marked the decline of Ottoman power. The Serbian element lost its individuality in the midst of Albanians. A record of its former advance in northern Albania subsists in the Serbian villages which are scattered in the region. Small areas of Serbian are found at Zumberak and Mariondol on the southern slope of the Uskok mountains in Croatia near the Carniola boundary. These Serbian groups occur on the border line separating Croats from Slovenes. They were founded by refugees from the south and east who had settled in the military confines of the Empire previous to 1871. In 1900 the population of Mariondol consisted of a few hundred inhabitants, many of whom have since emigrated to the United States. In Zumberak for the same year the number of inhabitants was estimated at 11,842, of whom 7,151 were “uniats” and 4,691 Catholics.[184] The conversion of these Serbians from Greek orthodoxy was accomplished in the eighteenth century.