What is the logical fallacy in "Désirée's Baby" by Kate Chopin?

A logical fallacy is an error in reasoning. When someone adopts a position, based on a bad piece of evidence , they commit a fallacy. Kate Chopin allows her characters to make errors in judgment in most of her stories. None are more evident in her story “Desiree’s Baby.”

Racism, prejudice, and unknown ancestry---the events of the story filter through these thematic ideas.  The third person narrator gives the details of the story without emotion and lets the reader come to his own conclusions. Primarily,  the story belongs to Desiree.

The story’s setting is Louisiana prior to the Civil War.   The Valmondes, an upper class couple, find and adopt a little, two-year old girl who was left by her family.  The Valmondes loved her as if she had been their own child. Desiree grew into a beautiful, kind, and gentle girl.

When Desiree was about twenty, Armand Aubigny, a young aristocrat, sees her one day, and instantly falls in love with her.  Monsieur Valmonde believes that he must share Desiree’s background with Armand. Armand told her father that her ancestry did not matter. Soon they are married.

Madame Valmonde comes to visit Desiree about a month after she has given birth to Armand’s son.  Desiree’s mother delights in seeing her daughter and her first grandchild.  When Madame picks up the baby, she takes him to the light. Without Desiree noticing,  her mother has seen something that worries her.  Madame inquires about Armand and the baby.  Desiree says that he is an adoring father. 

About three months later, the baby lies in bed fanned by a mulatto child [bi-racial].  Desiree sees something that she had not seen before. The baby has the same features of the mixed race child. 

She looked from her child to the boy who stood beside him, and back again... "Ah!" It was a cry that she could not help. She stayed motionless, with gaze  upon her child, and her face the picture of fright”

When Armand enters the bedroom, Desiree asks him what does he think is wrong with the baby.  Armand coldly answers that the baby is not white, and neither is Desiree. Sickened, Desiree writes her mother asking her what she should do.  Madame Valmonde tells her to bring the child and come home where she is loved.

Everyone thinks that Desiree has black ancestry since no one knows for sure from where she comes.  It seems a logical conclusion.  Even her mother accepts this as the truth.

After Armand tells Desiree to leave and take the baby, she walks off into the fields by the swamp.  The reader does not know what happens to her or her baby.

Armand, in his anger, tries to eradicate all signs of Desiree and the baby by burning everything that she has touched. He finds a letter which sets the record straight.  The letter is from his mother to his father:

‘But above all,’ she wrote, ‘night and day, I thank the good God for having so arranged our lives that our dear Armand will never know that his mother, who adores him, belongs to the race that is cursed with the brand of slavery.’

Armand threw away his family.  His reasoning said that Desiree’s background was unknown, so she must be the problem.  His reasoning was flawed when he discovers that he was the one with the black heritage.  Jumping to a seemingly logical conclusion, Armand lost the beautiful girl that he loved so much and his bi-racial child because of his foolish racial prejudice and arrogance.

How does Kate Chopin use logical fallacies to create conflict in "Désirée's Baby"? What are some examples?

The major logical fallacy of "Désirée's Baby" relates to assumptions made about Désirée's ancestry. From the start, Désirée's origins are shrouded in mystery. No one knows who her parents are, including Désirée herself. When she has a child with her husband, the wealthy Armand, it becomes clear that the child is of mixed race. Armand assumes that because Désirée's background is a mystery that she must be partially or wholly Black. Ashamed of his child and his wife, he sends the two of them out of his life to salvage his social reputation.

At the end of the story, both the reader and Armand learn that it is actually Armand who is biracial. He finds a letter that reveals his mother "belongs to the race that is cursed with the brand of slavery." Armand thought he knew everything there was to know about himself because of his family name, high status, and wealth. He never suspected either of his parents might have secrets of their own, unknown to him. Therefore, his attempt at logical reasoning was both arrogant and severely flawed.