The assumption of the title of emperor was destined to make the German rulers a great deal of trouble. It constantly led them into futile efforts to maintain a supremacy over Italy, which lay without their natural boundaries. Then the circumstances under which Charlemagne was crowned made it possible for the popes to claim, later, that it was they who had transferred the imperial power from the old eastern line of emperors to the Carolingian house, and that this was a proof of their right to dispose of the crown as they pleased. The difficulties which arose necessitated many a weary journey to Rome for the emperors, and many unworthy conflicts between the temporal and spiritual heads of Christendom. [Sidenote: Charlemagne's system of government.] 33. The task of governing his vast and heterogeneous dominions taxed even the highly gifted and untiring Charlemagne; it quite exceeded the capacity of his successors. The same difficulties continued to exist that had confronted Charles Martel and Pippin,--above all a scanty royal revenue and over-powerful officials who were prone to neglect the interests and commands of their sovereign. Charlemagne's distinguished statesmanship is nowhere so clearly seen as in his measures for extending his control to the very confines of his realms. [Sidenote: Charlemagne's farms.] His income, like that of all mediæval rulers, came chiefly from his royal estates, as there was no system of general taxation such as had existed under the Roman Empire. He consequently took the greatest care that his numerous plantations should be well cultivated and that not even a turnip or an egg which was due him should be withheld. An elaborate set of regulations for his farms is preserved, which sheds much light upon the times.[47] [Sidenote: Origin of titles of nobility.] The officials upon whom the Frankish kings were forced to rely chiefly were the counts, the "hand and voice of the king" wherever he could not be in person. They were to maintain order, see that justice was done in their district, and raise troops when the king needed them. On the frontier were the counts of the march, or margraves (marquises), already mentioned. These titles, together with that of duke, still exist as titles of nobility in Europe, although they are no longer associated with governmental duties except where their holders have the right to sit in the upper house of parliament. [Sidenote: The _missi dominici_.] To keep the counts in order, Charlemagne appointed royal commissioners (the _missi dominici_), whom he dispatched to all parts of his realm to investigate and report to him how things were going in the districts assigned to them. They were sent in pairs, a bishop and a layman, so that they might act as a check on one another. Their circuits were changed each year so that they should have no chance to enter into conspiracy with the counts whom it was their special business to watch.[48]