Galton also reviewed accounts contrasting the taciturn reserve of American Indians with the talkative impulsivity of Africans. He noted that these temperamental differences persisted regardless of climate (from the frozen north through the equator), religion, language, or political system (whether self-ruled or governed by the Spanish, Portuguese, English, or French). Anticipating later work on transracial adoption, Galton pointed out that the majority of individuals adhered to their racial type, even if they were raised by white settlers.

Based on his readings and his personal experiences of exploring Southwest Africa, Galton concluded that the average mental ability of Africans was low, whether they were observed in Africa or in the Americas. In Descent, Darwin acknowledged Galton's work and also accepted the importance of the brain-size differences reported between Africans and Europeans by Paul Broca and other nineteenth- century scientists.

Modern studies confirm Darwin and Galton. The races do differ in average brain size and intelligence. The racial gradient in average intelligence and brain size increases from Africans to Europeans to East Asians.

Although Darwinians emerged victorious among the educated classes in their nineteenth-century battles against Biblical theology, subsequently they lost that ground to egalitarians, Marxists, cultural-relativists, and postmodernists. From Herbert Spencer (1851) to the world depressions of the late 1920s and 1930s, while the political right was ascendant, the political left came to believe, perhaps correctly, that "survival of the fittest" was incompatible with social equality.