Here's why:

* Vestigial means something that is reduced in size or function compared to its ancestral form.

* Structure refers to a part of an organism's body.

Examples of vestigial structures include:

* Appendix in humans: Once thought to aid in digestion, it now has little to no known function.

* Whales' pelvic bones: These bones are remnants of their land-dwelling ancestors, but are no longer used for walking.

* Snake's pelvic girdle: Some snakes have rudimentary hip bones, even though they lack legs.

Vestigial structures provide strong evidence for evolution, as they show how organisms change over time and adapt to new environments.