From “Betty & Veronica: The Cultural Politics of Hair Colour” by Jeet Heer on Sans Everything:
“… One very common convention of the nineteenth-century novel is the use of two heroines, one dark and one light. The dark one is as a rule passionate, haughty, plain, foreign or Jewish, and in some ways associated with the undesirable or with some kind of forbidden fruit like incest. When the two are involved with the same hero, the plot usually has to get rid of the dark one or make her into a sister if the story is to end happily.”
Images via The Gloss, SFGate, The Film Experience.