"It was very serendipitous for the vertebrates' conquest of the land that the fishes developed pelvic limbs and an anus about halfway along the body. When the first amphibians crawled out of the water on four legs the tail that trailed uselessly behind them was not destined to become a vestigal nonentity. It was an organ the anatomical simplicity and mechanical flexibility of which would enable their descendants to put it to all manner of uses."