The most successful of the Tartar princes assumed the military command, to which he was entitled by the superiority either of merit or of power. He was raised to the throne by the acclamations of his equals; and the title of Khan expresses, in the language of the North of Asia, the full extent of the regal dignity. The right of hereditary succession was long confined to the blood of the founder of the monarchy; and at this moment all the Khans, who reign from Crimea to the wall of China, are the lineal descendants of the renowned Zingis.
Edward Gibbon (2015). “History of the Decline and Fall of the Roman Empire V2: the History Focus”, p.354, 谷月社