As to the aboriginal man of America, I know of no respectable evidence on which the opinion of his inferiority of genius has been founded, but that of Don Ulloa... Don Ulloa's testimony is of the most respectable. He wrote of what he saw, but he saw the Indian of South America only, and that, after he had passed through ten generations of slavery. It is very unfair, from this sample, to judge of the natural genius of this race of men; and after supposing that Don Ulloa had not sufficiently calculated the allowance which should be made for this circumstance, we do him no injury in considering the picture he draws of the present Indians of South America, as no picture of what their ancestors were, 300 years ago.

(TJ to the Marquis de Chastellux, June 7, 1785.)
http://lcweb2.loc.gov/cgi-bin/ampage?collId=rbc3&fileName=rbc0001_2007jeffca...