Czech historian Konstantin Josef Jireček (1918) considered that the population of Duklja was a very mixed population of newly arrived Slavs and older people like Albanians and Romanians. Croatian historian Milan Šufflay (1925-1927) considered that the Vlach-Albanian-Montenegrin symbiosis is seen in the etymology of the names, in Piperi, Moguši, Kuči, and the surnames with suffix -ul (Gradul, Radul, Serbul, Vladul), and toponymical names of mountains, Durmitor and Visitor. Serbian historian Vladislav Škarić (1918) considered that many brotherhood names, like Sarapi, Radomani and others in Montenegro, belonged to migrants from central Albania, while Bukumiri from Bratonožići, Vajmeši from Vasojevići, Ibalji from Herzegovina came from northern Albania. Serbian ethnologist Jovan Cvijić (1922) noted the Slavic assimilation and migration of many groups of Mataruge, Macure, Mugoši, Kriči, Španji, Ćići and other Vlachs who were mentioned as brotherhoods or tribes. He considered that all gornji (upper) tribes lived in the parts of currently Serb tribes in Brda and Old Herzegovina, and that many groups were assimilated into the tribes of Piperi, Kuči, Bratonožići, Bjelopavlići among others, who preserved their old name. American cultural anthropologist Christopher Boehm stated that some clans descended from the pre-Slavic 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.