My history books tell me that the advent of darwinism changed racism in Europe. They started to believe that some races were better than others and that some races were doomed to fall because of the natural selection. Did the europeans think that there were any ancient races that went exctinct in the past?
A lot of the trope of "ancient forgotten race"/"lost city" does indeed come from the racial prejudices of colonialist Europeans. When confronted with African architecture like in Ghana or Zimbabwe, they often attributed it to some other (preferably white) race or "ancient Roman legion". A classic literary example is the novel "Mines of king Solomon" where the civilized black people are this thanks to ancient Hebrews...