The eugenics movement was started by Darwin’s cousin, the polymath Francis Galton (1822-1911). He reasoned, after reading Darwin’s book On the Origin of Species, that human society thwarts natural selection by allowing the less desirable varieties to survive. Eugenics (from the Greek meaning “good birth”) aimed to improve the quality of the human race by selective breeding, just as animal breeders selectively breed to enhance desirable characteristics. Galton didn’t prescribe that coercion be used to further his aims, assuming that a widespread knowledge of the principles of eugenics would automatically favour “sensible” breeding patterns.
Eugenics was enthusiastically taken up in the US and Europe. However, it gradually degenerated into notions of racial superiority and racial hygiene. The concept of racial hygiene was vigorously promoted in German biomedical circles. It was enthusiastically adopted by the Nazis who used it to “scientifically” rationalise their racist ideology. The whole thing ended up in the horror of the concentration camp gas chambers.
Eugenics was an awful chapter in the history of science and a permanent reminder of how careful we must be when we interpret social organisation in evolutionary terms.
The back of the late Michael Crichton's book State of Fear, has a wonderful exposition of how science, when used outside of scientific practice by non-scientists can go horribly wrong. Now then, repeat after me: Darwin does not equal Hitler.