This article traces the personal as well as the intellectual and scientific relationship between Charles Darwin and his younger half-cousin Francis Galton. Although they had been on friendly terms as young men, and Darwin had in some ways been a role model for Galton, the two did not share major scientific interests until after the publication of Darwin's "On the Origin of Species" in 1859. That work precipitated a religious and philosophical crisis in Galton, which he gradually resolved after conceiving and developing the basic ideas of "hereditary genius" and eugenics. More mathematically inclined than Darwin, he subsequently contributed to the Darwinian evolutionary discussion, and to the future science of psychology, by proposing the basic concept of the nature-nurture dichotomy, the conceptual and statistical foundations for behavior genetics, and the idea for intelligence testing.