Enlightenment thinking encouraged Americans to make sense of their country and society, to discover the ordering principle that might explain the diversity they saw around them. One obvious, and troubling, example of diversity was skin color. The system of biological classification developed by Swedish scientist Carolus Linnaeus, used to name and categorize all living things by genus and species, was generally accepted by American scientists, but it posed difficult questions. While some believed that all human beings were of the same species, as Linnaeus suggested, others maintained that blacks and whites were of two different species (with the obvious implication that whites were superior). It was well known that the mating of animals of different species produced sterile offspring. The fact that no such reproductive problem resulted from the mixture of Caucasian and Negro parentage led some scientists to the conclusion that blacks and whites...