Why does Roxana switch the babies?

Roxana is a slave who gave birth to a son the same day her master's wife gave birth to his only surviving son. When Percy Driscoll, Roxana's master, is widowed, Roxana becomes the sole caretaker of his young son, Thomas. Many people in town comment how alike the two babies look. Roxana is a light skinned black woman and her son is as blond and fair skinned as young Thomas. When the babies are seven months old, Roxana becomes frightened by the thought that the master might one day sell her son down river. At first Roxana considers killing her child as well as herself. However, Roxana realizes it would be much simpler to switch the babies.