Tell me, how does a reptile turn into a bird? How is half a wing helpful? It would not help the half reptile half bird survive so therefore it would would not have any advantage over the other animals. Macroevolution does not make sense.
However, I do believe in Microevolution.
A reptile develops a loose flap of skin between it's arm and it's trunk. At first, the reptiles with this feature are able to use it to "glide" from tree to tree when seeking food or avoiding predators.
Later this flap develps into something more substantial, as the reptiles who have a more developed flap are more successful.
The arm's muscles develop, and fill out the flap, forming a wing, and allowing the reptile to fly longer distances and in a more-controlled fashion.
Eventially, the reptile's scales develop into feathers -- this is the longest portion of the process, but as the longer and later feathered scales are useful in aerial maneuvering, they contribute to the ability of the reptile to survive, especially with similarly adapting predators and prey.
That's the jist of it.
It works. It's evolution. And hell, it even sort of goes together with the free-market ideals I so cherish...
Me and Stephen Jay Gould didn't always see eye to eye, but ultimately, he is right. Species go through periods -- sometimes very long ones -- where they don't change (they might not "need" to), but when the ecosystem changes, so will the gene-pool.