QUOTE : a principle that more recently has become known as exaptation. Darwin explained that some features could develop to serve a different purpose initially, then be adapted for a new purpose later. A small, feathery flap, for instance, too small for flight, could be used to keep a creature warm in cold weather. A bird frequently tucks its head under a wing when sleeping or when cold; what if this was the original purpose of the structure that eventually became a wing? Natural selection could favor these natural capes and select for larger and more thermally efficient variations. If the creature happened to live in trees, eventually this newly shaped limb could prove useful for jumping and gliding, at which point natural selection would begin to emphasize variations that aided flight. Bird evolution would then be well on its way.