When Philadelphia naturalist Samuel George Morton died in 1851, no one cut off his head, boiled away its flesh, and added his grinning skull to a collection of crania. It would have been strange, but perhaps fitting, had Morton's skull wound up in a collector's cabinet, for Morton himself had collected hundreds of skulls over the course of a long career. Friends, diplomats, doctors, soldiers, and fellow naturalists sent him skulls they gathered from...