was a tyrant with the character which always makes tyranny repulsive, and this derived in part from his own paranoia. Since he had no son, and thus no obvious heir, he was constantly in danger of being overthrown. Thus, the unresolved issue of succession again proved to be deadly, and in September 96, Domitian was murdered. From Nerva to the abolition of the Principate Nerva, who was elected by the Senate, reversed some of the abuses committed by Domitian, such as the practice of prosecuting individuals for disrespecting the emperor. In 97, Nerva adopted M. Ulpius Trajanus, who was serving as governor of Upper Germany at the time. When Nerva died in January 98, Trajan succeeded him without opposition. Trajan went further than even Nerva had in restoring the image of a free republic. He refused to preside over capital trials against senators, observed the precedent of freedom of speech during senate meetings, and was away from Rome for such extended periods that the senate even regained some independent legislative abilities. In addition, he showed respect for the republican magisterial offices by only holding the Consulship four times during his nineteen-year reign, in contrast to the ten Consulships Domitian had held during his fifteen-year reign. Hadrian succeeded Trajan as emperor. By far, his most important constitutional alteration was his creation of a bureaucratic apparatus, which included a fixed gradation of clearly defined offices, and a corresponding order of promotion. Many of the functions that had been outsourced in the past, such as tax collection, were now to be performed by the state. Hadrian adopted Antoninus Pius, made him his heir, and died shortly thereafter. No real changes to the constitution were made during the reign of Antoninus Pius. He made Marcus Aurelius his heir in 161, and died shortly thereafter. The most significant constitutional development that occurred during the reign of Marcus Aurelius was the revival of the republican principle of collegiality, as he made his brother, L. Aelius, his co-emperor. Marcus Aurelius ruled the western half of the empire, while his brother ruled the eastern half of the empire. In 169, Aelius died, and in 176, Marcus Aurelius made his son, L. Aurelius Commodus, his new co-emperor. This arrangement was revived more than a century later, when the emperor Diocletian established the Tetrarchy. In 180, Marcus Aurelius died, and Commodus became emperor. Commodus' tyranny revived the worst memories of