The various invaders found their task easier from the hostility which existed between the various groups. Racial animosity was fostered by inducements held out by the newcomers to one group to join them in attacking another. These troubles destroyed the work of assimilation which had been going on for centuries. Communities now of Greeks, now of Slavs, were driven from the localities they had occupied for long periods, and the constant movement left the Balkan peninsula with its various races intermingled in strange confusion. To adopt chemical nomenclature, hundreds of villages were mechanically mixed with those of other races but never chemically combined. There were Slav villages in the neighbourhood of Athens itself, Albanians in Macedonia: Greeks, Serbians, and Bulgarians largely replaced the Latin race of that province, which in the times of the Crusades was known as Wallachia Proper. Language and race had taken the place of subjection to the empire as a bond of union, and as the Turks gradually pressed forward their advances into the interior, literally from every side, they found the conquest of these isolated and generally hostile communities greatly facilitated by the disunion existing among them. Throughout Macedonia, Thessaly, Epirus, and Greece the boundaries were changed oftener even than allegiance, and though the Greek element predominated in the south and along the coast as far as Salonica and around the coasts of the Aegean and the Marmora, other communities were interspersed among them in great numbers. The subjugation of the Macedonian Serbs and the South Bulgarians can be roughly stated as having been accomplished at the battle on the Maritza. The defeat of the Serbians and Bulgarians was a harder task. But Serbia and Bulgaria were the two portions of the Balkan peninsula where the people were almost all of the same race and could organise themselves for defence. No such organisation was possible south of their territory. [Sidenote: System of Turkish conquests.] The second cause which had contributed to the diminution of the empire and of its population was the system of Turkish conquests. Large numbers of the Christian population were killed; larger numbers were driven away to wander houseless and homeless and either to die of starvation or find their way into the towns. Conquest of a territory or capture of a city, forcible expulsion of the inhabitants or massacre of most of them and occupation of the captured places followed each other with wearisome regularity. The military occupation was that of nomads who replaced agriculturists. Everywhere the cattle of the Christians were raided. Arable lands became the wasteful sheep-walks of nomad Turks.[169] [Sidenote: Black Death.]