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 664 THE AMERICAN JOURNAL OF SOCIOLOGY

already conquered by the Persians, was taken by the Arabs. By the end of the seventh century all of Byzantine Africa had fallen into their hands ; in the eight, the whole of Spain ; to which was added, from the eighth to the tenth century, the Balearics, Sicily, and Sardinia.

The Lombards made an irruption into Italy in the seventh century. The frontier of the Danube gave way under the pres- sure of the Slavs ; the Croats established themselves in Dalmatia and in Pannonia; the Serbs, in Upper Mcesia, in Dacia, and in Dardania; still other Slavs, in Lower Mcesia, in Thrace, Mace- donia, and Thessaly. Toward the end of the seventh century the Bulgarian Finns founded south of the Danube a powerful king- dom, which dominated the Slavic tribes and extended as far as Rhodope and Albania. From the eighth to the tenth century there was a return of Byzantine power in Asia as well as in Europe; the Bulgarian kingdom was itself annihilated under the combined efforts of Byzantium, the Hungarians, the Russians, etc. At the beginning of the eleventh century the Empire of the East was almost as extensive as at the time of Justinian, but it contained the most extraordinary mixture of populations, differ- ing both in origin and in language. Religion was their only com- mon bond. By military force it united them, while it restrained, oppressed, and held them down. Religion spread even beyond the military frontiers; it bore the influence of Byzantium, by means of the Greek friars and of economic relations, among the Slavs of Moravia, among the Croats, the Serbs, and the Bul- garians. The Khazars were converted in the ninth century. Russia passed to the Orthodox Greek church in the following century.

As throughout western Europe during the Middle Ages, the military structure gathered strength at the center and at the extremities. In the Byzantine Empire, starting with the middle of the seventh century, the old provinces were fused into govern- ments more and more extensive from a territorial point of view, but at the same time more and more centralized in the hands of military commanders. The provinces were called themes a word which denotes at once the territory and the body of troops