During this period of a century and a half, the quiet and prosperity enjoyed by the provincial Britons led to a corresponding advance in wealth and civilisation, and Britain became rapidly one of the most valuable provinces of the Empire. Instead of being estimated, as Appian represents it in the second century, as of so little value that the part of the island possessed by the Romans was a mere encumbrance to them, it is now described by Eumenius, in the end of the third century, as a possession whose loss to the Empire under Carausius was severely felt. ‘So productive,’ says he, ‘is it in fruit, and so fertile in pastures, so rich in metals and valuable for its contributions to the treasury, surrounded on all sides with abundance of harbours, and an immense line of coast.’[87] The cultivation of grain, and the amount of its produce, had so greatly increased, that it had become of importance as an exporting country; and during the reign of Julian it had formed his great resource, from whence he drew a large supply of corn during the great scarcity on the Continent. [Sidenote: Division of Roman Britain into four provinces.] A change had likewise taken place in its government. By the arrangement introduced by Diocletian, and confirmed and established by Constantine, the Roman Empire was divided into four portions, to correspond with the two Emperors and two Cæsars. Each of these dioceses, as they were called, was placed under a great officer termed the prætorian prefect. The diocese of the west consisted of Gaul, Spain, and Britain, and the latter country was governed by a vicarius or vicar. Roman Britain, which from the time of Severus had consisted of two provinces, termed Upper and Lower Britain, was now divided into four provinces,—Maxima Cæsariensis, Flavia, Britannia Prima, and Britannia Secunda,[88] the two former or new provinces being apparently named after his father, who had been Cæsar, and was the founder of the Flavian family. In the absence of any direct indication of the position of these provinces, the natural inference certainly is, that each of the former provinces had been divided into two; and that, while Upper Britain now consisted of Britannia Secunda and Flavia, Lower Britain was represented by Britannia Prima and Maxima Cæsariensis. Each of these provinces had its governor, either a consul or a president. The troops were under the command of the ‘Dux Britanniarum’ and the ‘Comes tractus maritimi.’ Under the former were the troops stationed north of a line drawn from the Humber to the Mersey, following the course of the river Don, and on the Roman wall between the Solway and the Tyne; and those under the latter along the maritime tract, exposed to the incursions of the Franks and Saxons, extending from the Wash to Portsmouth. The former appears, therefore, to have been the military leader in the two northern provinces, while the functions of the latter were exercised within the two southern.