What does Lee illustrate with Jem's explanation of Mr. Dolphus Raymond's behavior?

Jem explains that he married a black woman and that he has "mixed" children. Jem says that these children are "sad" because they don't feel accepted by black people or by white people - though they can be accepted in the North. They see one of the mixed children and Scout thinks he looks black. She asks Jem how to determine whether someone is "mixed" or not and Jem says that you can't tell by looking, you have to know their history.

Mr. Raymonds previous (white) fiance comminted suicide when she learned of his colored mistress, which he later married, and had mixed children with. His children were discriminated by all because they were half white and didn't fit in with colored folks, and they were half black so the didn't fit in with white folks