The selective breeding of humans as "alphas", "betas", etc. may have been inspired by the eugenics movement[1], which had prominent supporters in the U.K. at the time (e.g., Winston Churchill).[2] Except that in the novel, instead of just getting rid of "inferior" people, they actively bred them into a servant caste. (After Hitler gave eugenics a bad name, it became much less respectable.)
[1] https://en.wikipedia.org/wiki/Eugenics
[2] https://en.wikipedia.org/wiki/Eugenics#Supporters_and_critic...
Maybe opium ? It was more loosely used before the 1926 law that created stronger controls.
I've always assumed (perhaps wrongly) that culture was very different then and have been always been curious as to how he came to his prediction.