The thing that charms me about Darwin is that he's not afraid to look ridiculous (remember when he hung out watching crabs and coconuts?). Today he has a "Mr. Bean Goes To Argentina" style experience:
One day, as I was amusing myself by galloping and whirling the balls [the bolas, a kind of three-balled slingshot -- ed] round my head, by accident the free one struck a bush; and its revolving motion being thus destroyed, it immediately fell to the ground, and, like magic, caught one hind leg of my horse; the other ball was then jerked out of my hand, and the horse fairly secured. Luckily he was an old practised animal, and knew what it meant; otherwise he would probably have kicked till he had thrown himself down. The Gauchos roared with laughter; they cried out that they had seen every sort of animal caught, but had never before seen a man caught by himself.
Darwin would be a hell of a fictional detective, because he seems all English and clumsy and cutting a comic figure, but he's also intensely interested about why things are -- a long passage begins, "The general, and almost entire absence of trees in Banda Oriental is remarkable." This leads him to muse on why the Falklands don't have the same trees as nearby Tierra del Fuego, but, after considering a number of factors, he gives up; this also is admirable in fictional detectives, because it would allow the cops to bring in the wrong guy and use up a big chunk of your novel. I guess all scientists are detectives, really, but Darwin seems more open to telling us how ludicrous he looks; maybe because he has the confidence of the self-funded. And, of course, he nailed the biggest perp of all, the guy behind the Genesis project.