Some insomniacs do endless sequences of sums in their heads, while more traditional conservatives rely on counting sheep—or sheep in elephants’ clothing. An instinctive Machiavellian even as a child, and dimly conscious of the reality of power, I preferred to count rulers. In elementary school I learned the American presidents, and in high school I worked on English kings—“I know the kings of England, and I quote the fights historical, from Marathon to Waterloo in order categorical”—which I alternated with whole acts, taking all the parts, from Gilbert and Sullivan. It goes without saying which is my favorite episode of The Simpsons.
Sometime in the 1970’s I switched to Roman emperors from Augustus to Augustulus. Marcus Aurelius, Commodus, Pertinax, Didius Julianus, Septimius Severus. With nothing else to do at 3 a.m., I sometimes begin to see a pattern in the succession of tough soldiers, distinguished bureaucrats, and princes born to the purple. The perennial problem of the Roman Empire was succession. They never came up with a system that could reliably generate competent successors. The most effective method was adoption: Nerva adopted Trajan, who adopted Hadrian, who adopted Antoninus Pius, who adopted Marcus Aurelius, who, unluckily, produced a son and heir, the terrible Commodus. Some...