What conclusions can we draw about racial categories from the story Ben's attempts to learn a trade in Chapter XXXVIII (pp. 151-152)?

From Harriet's words in Chapter XXXVIII, we can infer that learning or supporting oneself through trade was vitually impossible for a black man. Benny, a mullato, was initially accepted and well liked, It wasn't until after the others found out his mother was black that he was ostracized and verbally abused. At a loss for a vocation, Ben took to the high seas.

We arrived in New York safely, and I hastened to Boston to look after my children. I found Ellen well, and improving at her school; but Benny was not there to welcome me. He had been left at a good place to learn a trade, and for several months every thing worked well. He was liked by the master, and was a favorite with his fellow apprentices; but one day they accidentally discovered a fact they had never before suspected—that he was colored! This at once transformed him into a different being. Some of the apprentices were Americans, others American-born Irish; and it was offensive to their dignity to have a "nigger" among them, after they had been told that he was a "nigger." They began by treating him with silent scorn, and finding that he returned the same, they resorted to insults and abuse. He was too spirited a boy to stand that, and he went off. Being desirous to do something to support himself, and having no one to advise him, he shipped for a whaling voyage.