Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Penguins do not use their wings, known as flippers, to fly in the air. However, they do use them to move in the water. The theory of evolution suggests that penguins evolved to use their wings for a different purpose. A whale's pelvic bones, which were once attached to legs, are also vestigial structures. Whales are descended from land-dwelling ancestors that had legs. Answer: vestigial
What do you call structures like a whale's pelvic bones, which were once attached to legs, or wings that cannot be used for flight?