In Latin American countries with black and Amer-Indian populations, Darwin’s evolutionary theory was quickly mobilized for theorizing racial differences, while in Spain, attention was focused on class differentiation, explained by a series of Darwinian, social Darwinist, and eugenic hypotheses. The wide variety of approaches to evolutionary and social theory in countries whose culture was very similar illuminates those issues thought to be of particular significance for national identity, whether political, ethnic, or racial. This text provides general readers and academics with a broad perspective on Darwin’s impact in the Spanish- and Portuguese-speaking worlds.