In a restricted sense physical Macedonia may be defined as the southerly extension of the Serbian mountain belt whose drainage leads to the Ægean. Thus it consists first of a mountain belt extending between the upper valleys of the Black Drin and Struma. To this zone must also be added a hill country which forms its continuation to the Ægean Sea. The Vardar valley is entirely within this area and divides it into equal east and west sections. The northern boundary of the area is found at the central watershed north of Uskub. Four important basins lie within these boundaries. The Tetovo basin, west of Uskub, lies close to the watershed. Southward the Monastir and Strumitza basins occupy approximately homologous positions with respect to the Vardar cut. The twin basin of Serres and Drama extends over the southeastern portion of the country. These basins have been the only important centers of Macedonian populations. The Macedonian highland is peopled by shepherds and woodcutters. The lowlanders are husbandmen. All are generally bilingual, speaking either Greek and Bulgarian or Bulgarian and Serbian. A knowledge of Turkish usually prevails among all classes. Occupation generally affords a reliable national clue. As a rule the Macedonians, and by this term we shall hereafter denote the Bulgarian-speaking element of Macedonia, are tillers of the soil. The Greeks are traders and control a large share of the commerce of the entire region. Land is held by the Macedonians or the former ruling Turkish gentry. It is worked however by the Macedonians. The inhabitants of Macedonia may be divided into four groups according to their vernaculars. The number of individuals in each group is estimated as follows:[193] Bulgarians 1,172,136 or 81.5% of the total Christian population. Greeks 190,047 “ 13.22 “ “ “ “ “ Rumanians 63,895 “ 4.44 “ “ “ “ “ Albanians[194] 12,006 “ 0.84 “ “ “ “ “