One of the classic arguments on behalf of the theory that various taxa have evolved one from another is what biologists call homologous structures. Homologous structures are structures which are similar, have a similar origin in the embryo but which serve different functions in different animals. A good example is the pectoral fin of a fish, the foreleg of a horse, the arm of a human and the wing of a bird.
In each of these cases the skeletal structure is almost identical, but the appearance and function of the structure is quite different in each organism. This is assumed to be a strong evidence that these structures have evolved either from each other or from a common ancestor.
There are, however, problems with this idea.
This clever 8 minute video explains what homology is and why using homology as evidence for common ancestry is actually an instance of circular reasoning. It also does a good job of explaining why, despite its persistence in many high school and college textbooks, biologists are giving up on homologous structures as an argument for Darwinian evolution.
Thanks to Evolution News for putting us on to the video.