At some future period, not very distant as measured by centuries, the civilised races of man will almost certainly exterminate, and replace, the savage races throughout the world.

Descent of Man, 1898, Darwin

An excellent lesson in inheritance is found in the very family so responsible for our understanding of it. Erasmus Darwin, a lively author and notable polymath, lived on through his grandchild, Charles Darwin, who eloquently defined Natural Selection. His other grandchild, Francis Galton, was an explorer and a pioneer in geography, anthropology, meteorology, and social science, eventually fathering Eugenics.

Thanks to their work, we can illustrate that greatness and undesirableness may run in a family. Be it criminality, homosexuality, or alcoholism, degeneracy stains several generations if allowed to. You may see one or two degens do well, but they are not special and would've been better were they healthy. Knowing this all, we have the right and duty to ensure the better-born will inherit the Earth.