Purely a technicality but Twain was in fact a racist in that he
recognized distinctions between groups/populations of peoples and
accepted both qualitative and quantitative distinctions between them.
He may or may not have come to accept Native Americans as fully human
but for most of his writings they were on a par with African bushmen -
also a massively unfair and racist judgment call on his part.  Twain
was an Orientalist, the accepted sociological theory of the day. It's
unclear if his distinctions were made from a biological perspective or
cultural.  I can't recall anything about the nature/nurture debate in
Twain's writings.
-- 
 There are more things in heaven and earth, Horatio, than are dreamt of
                          in your philosophy.
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