But pressing need of further boundary revision on the basis of language is felt in the peninsula. Resumption of hostilities in this part of Europe in 1915 was due principally to the moot case of the nationality of the Slavs of Macedonia. Serbs and Bulgars both claim them as their own. In reality the Macedonians are a transition people between the two. They occupy a distinctive area formed by the twin valleys of the Vardar and Struma and surrounded by a mountainous bulwark assuming crescentic shape as it spreads along the Balkan ranges and the mountains of Albania and the Pindus. For centuries this Macedonian plain has constituted the cockpit of a struggle waged for linguistic supremacy on the part of Bulgarians and Serbs. The land had formed part of the domain of each of the two countries in the heyday of their national life. To this fact in part the present duality of claim must be ascribed. The entire northwestern Macedonian highland was under Serbian rule until the fall of 1915. East and south of the mountains Bulgarian speech predominates in districts peopled exclusively by Macedonians. The Greek element is practically entirely absent here; Serbians begin to appear in small numbers; south of Monastir and Okrida offshoots of the Pindus Rumanians are found; but the Macedonian element is present everywhere in overwhelming majority. [Illustration: FIG. 53--Sketch map of the western Balkans. The dotted area represents the northern area of Greek language. Black dots show Rumanian settlements of the Pindus mountains and adjoining regions.] Physically Macedonia is the region of the basins of the Vardar and Struma. Under Turkish rule it was divided into the vilayets of Monastir, Uskub and Salonica. The area is isolated from the rest of the peninsula by a practically continuous line of mountains, which, starting with the Pindus, Grammos and Albanian ranges on the west, extend through the Shar, Suhagora, Osogov and Rilo uplifts on the north and connect on the east with the Rhodope massif. Macedonia is thus well defined on the surface. Within these natural boundaries, it may be divided into an elevated region extending over its northwestern portion and a lowland spreading thence to Ægean waters. The Bistritza valley forms a convenient feature to mark the beginning of the modern Hellenic area.