In this excerpt from james henry hammond's speech "cotton is king," which piece of text uses a fallacy to justify the existence of the institution of slavery?

in all social systems there must be a class to do the menial duties, to perform the drudgery of life. that is, a class requiring but a low order of intellect and but little skill. its requisites are vigor, docility, fidelity. a.) [such a class you must have, or you would not have that other class which leads progress, civilization, and refinement.] it constitutes the very mud-sill of society and of political government; and you might as well attempt to build a house in the air, as to build either the one or the other, except on this mud-sill. fortunately for the south, she found a race adapted to that purpose to her hand. a race inferior to her own, but eminently qualified in temper, in vigor, in docility, in capacity to stand the climate, to answer all her purposes. b.) [we use them for our purpose, and call them slaves.] we found them slaves by the common "consent of mankind," which, according to cicero, "lex naturae est." c.) [the highest proof of what is nature's law. we are old-fashioned at the south yet; slave is a word discarded now by "ears polite; " d.) [i will not characterize that class at the north by that term; but you have it; it is there; it is everywhere; it is eternal.]