  Darwin’s theoretical task was, by the time he opened his species notebooks, tolerably clear: the only process that could produce the systematic patterns in the fossil record and the otherwise strange biogeographic distribution of species he now understood so widely and deeply was a process of slow, gradual transformation of species. He needed to come up with a natural, causal theory that would account for such transformations, and every element of that theory had to identify ‘causes now in operation’, causes that could be investigated empirically. The problem, and the methodological constraints, had been advocated by his geological hero, and now close friend, Charles Lyell; and they had been defended philosophically by his philosophical hero, Sir John Herschel. 