It has been asserted by writers, that when the grade of Quinteroon is arrived at, all trace of black blood is lost, and that they cannot be distinguished from the whites. Now if this be true, most of the Mulattoes must cease to breed before they arrive at this point of mixture; for though I have passed most of my life in places where the two races have been mingling for many generations, I have rarely if ever met an individual tainted with black blood, in who I could not detect it without difficulty. These higher grades should be extremely common if the chain was not broken by death and sterility. How else can the fact be accounted for?
--Josiah Clark Nott, "The Mulatto a Hybrid--probable extermination of the two races if the Whites and Blacks are allowed to intermarry", American Journal of the Medical Sciences, v. 6, July 1843, p. 255
Apparently Nott couldn't fathom how he might meet a 'black' person who "cannot be distinguished from the whites", because every black person he ever met he could tell was black!
He hadn't caught on to the complete lack of logic in that statement two and a half years later, when he repeated it pretty much verbatim in the Southern Quarterly Review.