It began in England. Out of Darwinism. Once people had to accept that the Ancient Greek philosophies of “the ideal form” couldn’t exist. (Because of evolution, everything is always evolving) But you could use Darwin and Mendel and genetics to engineer the biology you preferred.
Nazis got a lot of their ideas from the US, including lebensraum from Manifest Destiny.
Also the popularity of eugenics began in the US. You even read about it in The Great Gatsby.
It began in England. Out of Darwinism. Once people had to accept that the Ancient Greek philosophies of “the ideal form” couldn’t exist. (Because of evolution, everything is always evolving) But you could use Darwin and Mendel and genetics to engineer the biology you preferred.