Charles Darwin is best known for formulating the theory of evolution by natural selection. The heyday of eugenics came after Charles Darwin’s death, but Darwin helped supply the rationale for eugenics in various passages in his book The Descent of Man, where he warned that “civilized” societies were undercutting natural selection and destroying themselves by saving the sick, helping the poor, vaccinating people against small-pox, and allowing their worst members to breed. Darwin did go on to caution that humans can’t follow the dictates of “hard reason” in such cases without undermining their “sympathy… the noblest part of our nature.” At the same time, he expressed hope that natural selection would in fact continue to eliminate those he considered a drag on society. Scholars continue to debate how much support Charles Darwin lent to Social Darwinism. Charles Darwin’s son Leonard later became the Chairman of the British Eugenics Society for many years, and he spoke at the Second International Congress of Eugenics in New York City in 1921.