Illyrian population of present-day Montenegro, while a majority of the clans were of Slavic origin, either from earliest Serb settlers, or from later Serb immigrants who came to the area from neighbouring regions during Ottoman rule over the Balkans. Croatian linguist Petar Skok (1918–1919) while researching 15th-century Ragusan documents, noted that in the territory of Žabljak and Stolac there existed a specific domain called donji Vlasi or Vlachi inferiores ("lower Vlachs"), and that thus somewhere there had to exist a gornji Vlasi ("upper Vlachs"), not mentioned in documents; he believed it could have been located in today's Katunska nahija in Montenegro, to which probably referred catunos/catuni Cernagore from 1435. Jovan Erdeljanović spoke of the amalgamation of Serbs (Slavs) and Vlachs, and noted that in the older phase of forming of Dinaric tribes, the Serb and Serbicized native brotherhoods united into a tribal unit under one name. Jovan Cvijić noted the uncertainty whether the term "Vlach" in medieval Serbia and other parts was always used for genuine Vlach, or also Serb shepherds, since the term gradually developed a secondary meaning of "shepherd", regardless of ethnicity. According to Croatian-Albanian historian Zef Mirdita (2009), some Serbian scholars like D. Đurđev (1951) often totally rejected Vlach ethnic uniqueness, considered them only a social category and proclaimed them as Serbs or Slavs, against the fact that the Vlachs were always mentioned as, according to Mirdita, a genuine ethnicity in medieval records until the 16th century. Franz Babinger (1951) also opposed Đurđev's point of view. Montenegrin ethnologist Petar Šobajić stated that the first Slavic settlers in the area of Zeta mixed with local Romanized Illyrian natives and Slavicized them, though accepting the natives' tribal names (Španji, Mataguži, Mataruge, Malonšići, Macure, Bukumiri, Kriči). Later Serb settlers entered into conflicts with these early mixed tribes, which eventually resulted in the latters' annihilation, and new stronger tribes were formed. Serbian historian Ivan Božić pointed out that Slavicization wasn't completed in the 15th century, and that contemporaries made clear distinction between Slavs and those who were Vlachs/Morlachs or included Vlach admixture, and also traced Vlach contribution in Montenegro. Serbian historian Sima Ćirković (1968–1973) noted that throughout the entire medieval age Vlach was used as an ethnic term along Greek, Albanian, Bulgarian, Latin, Saxon, and although almost linguistically Slavicized and culturally adapted to the environment, their division from other Slavs in documents show their different origin, economic activities and status, and