Darwin's natural selection explained the fit between a structure and its function.  But creationists also used the same examples as evidence of a divine creator.  Darwin said the bird's wing was adapted for flying while his opponents said the wing was designed for flying.  Darwin challenged the creationists with vestigial structures.  Darwin felt if you could show a structure has no purpose then the argument for design fails.  Darwin asked, if wings were designed for flight, then why do flightless birds have wings?

Whales evolved from land mammals whose descendants returned to the water.  Fossil whales have been found in Egypt and Pakistan with functional hind limbs showing the vestigial bones near the pelvis of modern whales were homologous with functional limbs.  Modern whales possess in their pelvic region the remnants of these hind limbs.

Distant relative of the whale, Ambluocetus note the fully functional hind legs (46-48 million years before present)

More recent relative of the whale - Basilosaurus note the greatly reduced hind legs (37-40 million years before present)

Above is the pelvic region of a modern Right whale.  Note the vestigial remains of what was once the hindlimbs.