It is a truism of biology that evolution is conservative. When something evolves, it must build upon what nature has already produced. New features may be added, and old features may undergo some alteration, but most things remain the same. It is for this reason that the wings of bats, the hands of human beings, and the fins of whales look astonishingly alike in their skeletal form. They even have the same number of bones. Evolution laid down the cornerstones for basic physiology long ago.
