In 1878 he published The Aryans: A contribution to historical anthropology.
Based on the physical characteristics attributed to Indo-Europeans (fair hair, blue or light eyes, tallness, slim hips, fine lips, a prominent chin).
Whites will still doggedly point to artifacts from the periods of Greek and Roman rule and say: See, these are the "Real Egyptians".
Of course, there is absolutely no reason why a reasonably intelligent human being, would ever think that the ancient Egyptians were White people, but then again, reason and intelligent thought, have nothing to do with this: This is about a "Need" to believe.
The archaic character of the neighboring Lithuanian language induced him to believe that the Lithuanians were a surviving relic of this oldest Aryan race.