Then, as my caffeine-deprived neurons groaned to life, a second thing came to mind: despite having no legs, nor any need for legs, dolphins and whales nevertheless have itty bitty legs and pelvic bones buried, useless, under mounds of blubber. These rudimentary legs and pelvises are called vestigial structures; they are relics of the whales' common ancestry with their legged cousins.
The taxi driver seemed intrigued, but, as Heathrow loomed closer, I could see that he was not going to be convinced. Gosh, I thought, if only I had a handy dandy photo of whale pelvises that I could whip out on just such occasions. Surely such a prop would deftly banish those pesky whiffs of creationist stubbornness that tend to linger at the ends of such conversations. If only... If only...
Well, warm up your inkjets, my friends, because today, in a New York Times slide show called "Bred in the Bone", I found THIS:
Meet the striped dolphin, Stenella coeruleoalba. More to the point, squint your eyes and meet the striped dolphin's vestigial pelvis and legs. They are the small but unavoidably present bones you can see in the photo above, floating in space precisely where fully developed pelvis and legs would have been were the dolphin not so well adapted to its liquid environment.