Marcus was succeeded by his son Commodus, whom he had named Caesar in 166 and with whom he had jointly ruled since 177. Biological sons of the emperor, if there were any, were considered heirs; however, it was only the second time that a "non-adoptive" son had succeeded his father, the only other having been a century earlier when Vespasian was succeeded by his son Titus. Historians have criticized the succession to Commodus, citing Commodus's erratic behaviour and lack of political and military acumen. At the end of his history of Marcus's reign, Cassius Dio wrote an encomium to the emperor, and described the transition to Commodus in his own lifetime with sorrow: