After the first 11 Roman emperors, which consisted of the Julio-Claudian and Flavian Dynasties, between which was a chaotic year with 4 emperors, came a time when Rome was ruled by 5 good, non-dynastic emperors in a row. At the start of the first chapter of his Decline and Fall of the Roman Empire
, Roman historian Edward Gibbon writes about the times,
"The image of a free constitution was preserved with decent reverence: the Roman senate appeared to possess the sovereign authority, and devolved on the emperors all the executive powers of government. During a happy period of more than fourscore years, the public administration was conducted by the virtue and abilities of Nerva, Trajan, Hadrian, and the two Antonines."
Each emperor adopted his successor. The final emperor was Marcus Aurelius whose natural son followed him and broke the string of Roman luck.