The only feasible plan for circumscribing the ravages of the northern enemies of the empire which presented itself, was the establishment of powerful colonies of tribes hostile to the Avars and their eastern Slavonian allies, in the deserted provinces of Dalmatia and Illyricum. To accomplish this object Heraclius induced the Serbs, or western Slavonians, who occupied the country about the Carpathian Mountains and who had successfully opposed the extension of the Avar empire in that direction, to abandon their ancient seats, and move down to the south into the provinces between the Adriatic and the Danube. The Roman and Greek population of these provinces had been driven towards the sea coast by the continual incursions of the northern tribes, and the desolate plains of the interior had been occupied by a few Slavonian subjects and vassals of the Avars. The most important of the western Slavonian tribes who moved southward at the invitation of Heraclius were the Servians and Croatians, who settled in the countries still peopled by their descendants. Their original settlements were formed in consequence of friendly arrangements, and, doubtless, under the sanction of an express treaty; for the Slavonian people of Illyricum and Dalmatia long regarded themselves as bound to pay a certain degree of territorial allegiance to the Eastern Empire. [Illustration: A SARACEN] The measures of Heraclius were carried into execution with skill and vigour. From the borders of Istria to the territory of Dyrrhachium, the whole country was occupied by a variety of tribes of Servian or western Slavonic origin, hostile to the Avars. These colonies, unlike the earlier invaders of the empire, were composed of agricultural communities; and to the facility which this circumstance afforded them of adopting into their political system any remnant of the old Slavonic population of their conquests, it seems just to attribute the permanency and prosperity of their settlements. Unlike the military races of Goths, Huns, and Avars, who had preceded them, the Servian nations increased and flourished in the lands which they had colonised; and by the absorption of every relic of the ancient population, they formed political communities and independent states, which offered a firm barrier to the Avars and other hostile nations.