Creationists like to challenge the theory of evolution by asking how it can be that things can evolve incrementally since in its early stages the new feature seems to lack its final functionality. They pose questions like “What is the use of half an eye or half a wing?” Of course, scientists have long explained this. They have shown how the eye could have evolved by tiny changes and in fact even right now almost the full spectrum of differential eye development can be seen in existing species.
They have also pointed out that it is a mistake to assume that the final functionality of a feature was the only functionality all along, and that features may have had other functions in the early stages and only later became adapted to its final use.
Carl Zimmer had a nice article earlier this year in National Geographic about the evidence that feathers might have evolved for a different purpose long before flight occurred. More recently, he reports on new research results that add to our knowledge of what purpose those non-flying feathers in primitive wing forms might have served.