Some adaptations, on the other hand, become useless. These adaptations are vestigial: remaining but functionless. Whales and dolphins have vestigial leg bones, the remains of an adaptation (legs) that their ancestors used to walk. Why did the author include this selection?

This is because vestigial parts are usually homologous to most of the structures that are functional in other species of organisms. E. g ... the Vestigial legs which are non functional in whale and Dolphins, are useful to other organisms. Hence, if these organisms possessed these structures, they should share certain characteristics with each other. Therefore these organisms can be said to evolved from one another. Thus their vestigial organism should be included by the author as evidence of evolution.