In Edgar Allan Poe's "The Fall of the House of Usher," it is implied that the Usher family's illnesses are a result of incest.
At the beginning of the story, the unnamed narrator approaches the titular mansion and reflects on his previous relationship with Roderick Usher—they had been childhood friends. Roderick has asked his friend to come visit because of his physical and mental illness. The narrator reflects on his knowledge of the Usher family and its poor health:
the entire family lay in the direct line of descent, and had always, with very trifling and very temporary variation, so lain. It was this deficiency, I considered, while running over in thought the perfect keeping of the character of the premises with the accredited character of the people, and while speculating upon the possible influence which the one, in the long lapse of centuries, might have exercised upon the other—it was this deficiency, perhaps, of collateral issue, and the consequent undeviating transmission, from...
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