The bone structure of a bat wing, which still strongly shows signs of its origins as a more normal weight-bearing limb, poses an interesting problem:
If bats did, as is theorized by evolution, develop their unique characteristics as a species over a long period of time, there would have to have been some intermediate stages. You couldn't go from foreleg to fully functional wing in one generation. But the idea of survival of the fittest would hold that those new forms had to have some sort of advantage to survive and reproduce.
So what could those in-between proto-bats have done with their new limbs that weren't quite legs and weren't quite wings?