Race and genetics
Mongoloidism (the story went) is a defect in which the child of caucasoid parents atavistically expresses the traits of a previous, less advanced stage of human evolution—the mongoloid stage. The mongoloid child, as a throwback to a more primitive type of human, is mentally deficient. The mongoloid child of caucasoid parents, by expressing traits still current among Asians, demonstrates that Europeans are more highly evolved than Asians. The child demonstrates that Europeans have already passed through the mongoloid stage of human evolution that the Asians are still in.
Mongoloidism was discredited when black parents were shown to sometimes have kids who had the "mongoloid" traits. Since the scheme of ranking human races had Asians above (later than) Africans, there was no way to explain how Africans could "atavistically" express Asian traits. According to racial theory, they had never passed through a "mongoloid" phase, as Europeans had.
More generally, the fallacy that evolution is a "ladder" has been replaced the with model that it is a branching tree.
Mongoloidism is now known as Down's syndrome or Down syndrome.
In the US, it looks as though more fetuses with Down's syndrome are aborted than are carried to term.