Author of the Origin of Species (1859) and the Descent of Man (1871), Charles Darwin (1809–1882) famously challenged the popular belief that every species had been separately and immediately created by divine fiat. His theory of evolution by natural selection was based on what he considered an empirical fact: the presence of variation among members of every species. Darwin's powerful argument was that, in competition for limited resources, those variants having characteristics that favored them in their struggle would tend to be preserved and produce more offspring than those less advantaged. Over many generations the gradual accumulation of advantageous variations would lead to the emergence of a new species markedly different from its progenitor. Applied to humankind the argument was particularly contentious for the continuity it affirmed between animals and humans, and because the idea of species transformation was...