Yet although the history of the Empire furnishes an unparalleled series of successful revolutions, it must not be supposed that the importance of the Senate’s formally transmitting the succession was ever questioned or obscured. The Senate’s authority was rendered stable by the many peaceful instances of dynastic succession; it was rendered creditable by such a stand as that made against the tyrant Maximin; it was kept alive by the fact that when, in the days of the “thirty tyrants,” the Empire was breaking up, Italy was still the only formal centre of a world power; it was bound up with the magic name of Rome, and even in the third century was welcomed with relief by an army sick of its own lawless violence.[1680] But whether we lay more stress on the _de facto_ or the _de jure_ element in the act of election, we must admit that the elective principle was not the sole determinant in the transmission of the Principate. It was crossed by two others, both of which were typically Roman. These were the principles of nomination and of hereditary succession. Nomination took the form of designation by some significant act. One of the most significant modes in which the Princeps could point to his choice of a successor was to invest an individual with an approximation to those powers which were of the essence of the Principate, and thus to make him in a sense a colleague in the Empire (_collega_, _consors imperii_). The powers chosen were the _proconsulare imperium_, the _tribunicia potestas_, or both. It was thus that Augustus at different times designated Agrippa and Tiberius for the throne,[1681] that Tiberius pointed to Germanicus and Drusus as his destined successors, that Nerva nominated Trajan, Trajan Pius, and Pius Marcus Aurelius.[1682] Although such a position is described as one of colleagueship in the imperial power, yet it did not confer, as regards the _imperium_, the most characteristic rights of the Principate. The colleague did not possess joint command over the praetorian guard or the fleet, nor joint administration over all the Caesarian provinces,[1683] unless these rights were conferred by special mandate, as they were on Tiberius during the closing years of Augustus’ life;[1684] nor had the colleague, although in possession of an independent _imperium_, any right to triumph, except by the will of the Princeps,[1685] for his victory had been due to legions which had taken the _sacramentum_ to another. The name _imperator_ was not borne by this assistant to the throne unless it was specially conferred, as it was by Vespasian on Titus and by Hadrian on Antoninus Pius.[1686] It is uncertain whether the possessor of the _tribunicia potestas_ and of the _proconsulare imperium_ in its lower form had to have these powers reconferred on his accession to the throne. In the case of the _imperium_, since it fell short of that required for the imperial position, reconferment is probable. But yet the possession