i think the Charles Stuart University site probably has some good stuff on this but...in case it doesnt...
vertebrate forelimbs are homologous structures. in a range of vertebrate you can find the same basic bone structure: radius, ulna, metacarpals and phalanges.
human: reasonably slender ulna, rows of close fitting metacarpals and phalanges with an opposable thumb for movement and fine manipulation
bat: very thin, hollow ulna, fused together one row of metacarpals and long thin phalanges that support the membrane of the wing
crocodile: thick ulna for support and power to propel through water and on land, thick metacarpals fused together for power and stability, phalanges attached so that they can bend at right angles for walking.
horse: 'leg' tapers, walk on one phalange
homologous structures suggest common ancestry. examination of the similarities of bone structure support the idea of evolution in that it is easy to see how the structure has changed to support different lifestyles - such as the human and the crocodile or the whale (whicb has metacarpals and phalanges fused together to form the support for a flipper)