Yeah, i know what you mean - i'll never forget the shock i got when after a thorough grounding in german language and culture and thought, from watching Hogan's Heroes, i started going out with a german girl, who just didn't look like or talk like Sargeant Schultz, didn't walk like him, none of that .... completely mind-boggling, they were totally different, right down to the fact that she wasn't on telly
'Es tesis central del presente libro que las distintas razas del mundo tienden a mezclarse cada vez más, hasta formar un nuevo tipo humano ... '
[loose translation, i will replace raza with 'race' but it doesn't mean the same as in english, it's more a combination of heritage and culture, and in JV's usage here, language as the centre of culture] -
'It is the central thesis of this book that the different races of the world tend to continually mix themselves together, forming a new human type ... '