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> but their underside is white. This doesn't make sense
I believe the default lowest energy consuming colour state of fur is white. Changing colour from white requires building complex chemical compounds which require more energy. If there is a part of the animal which is never seen by its prey it makes sense not to add colour to that part, thus conserving a bit of energy.
Evolutionary biologists probably could site a dozen other reasons why a white underside is advantageous to a cat. For one example, if you are seeing its white underbelly, the cat is not intending to hunt you as food. Nice signal, what?
> the ancestors of cats must have at one point been capable of flight
Maybe. But, then they evolved into cats.
Of course, if evolution was so damned important to a successful species then way haven't cats evolved into accommodating neko-girls by now?