Some species of stoneflies fly easily and often, others a little, others not at all. but one nonflying species , Allocapnia vivipara, shows a provocative intermediate adaptation: It sails.

Imagine the scene: A cool wind blows across the surface of the pond. A. vivipara proceeds about its business only to find a frog about to make a meal of it. A. vivipara raises its tiny not-quite-wings and catches the wind. All of a sudden, A. vivipara glides across the surface of the pond beyond the reach of that darting tongue, propelled by its sails.

A. vivipara encapsulates in its anatomy and behavior one plausible account of getting off the ground (or in this case off the pond) by increments. Early ancestors of winter stoneflies had small winglike appendages that were not strong enough to support their weight while aloft. Evidence suggests that these appendages were too small and flimsy for flight. However, sometimes the membranes caught the wind and allowed the stonefly to escape from predators or find mates, an incremental survival advantage. Thus selection for stonefly sails began. Over many generations, the wings-to-be became larger and stronger. Eventually, some species of stonefly could lift away from the water altogether, another kind of survival advantage. The development of true flying wings began.

Mother nature doesn't think in the human sense of the word. The "thinking" of biological evolution is a kind of blind search process, one that we'll examine deeply toward the end of this book. But if mother nature did think, what kind of thinking would she be doing about A. vivipara? Interestingly, engineering has a name for this kind of thinking: repurposing. Appendages developed originally for sailng got repurposed for flying. Repurposing is one of mother nature's favorite ways of making breakthroughs - fins to legs, forelimbs to wings, scales to feathers. And repurposing is certainly a conspicuous mechanism in technological development, as Stone Age knives become scrapers, spears became arrows, wheels became waterwheels, fireworks became guns, oscilloscopes became television sets.