How's this for a coincidence? Charles Darwin and Abraham Lincoln were born in the same year, on the same day: Feb. 12, 1809. As historical facts go, it amounts to little more than a footnote. Still, while it's just a coincidence, it's a coincidence that's guaranteed to make you do a double take the first time you run across it. Everybody knows Darwin and Lincoln were near-mythic figures in the 19th century. But who ever thinks of them in tandem? Who puts the theory of evolution and the Civil War in the same sentence? Why would you, unless you're writing your dissertation on epochal events in the 19th century? But instinctively, we want to say that they belong together. It's not just because they were both great men, and not because they happen to be exact coevals. Rather, it's because the scientist and the politician each touched off a revolution that changed the world....