The Five Good Emperors ruled Rome in the 2nd Century. The first was Nerva, chosen to be emperor because of his reputation for fairness and wisdom. He adopted his successor, Trajan, who in turn adopted Hadrian, who adopted Antoninus Pius, who adopted Marcus Aurelius. This allowed capable men to be designated as heir and be properly trained for the position.
This trend ended when Marcus Aurelius had a male heir, the incompetent Commodus, survive to adulthood.