> Aren't all animals in the process of evolving?
Nope. All *species* are. *You* will never evolve. Your offspring will. Maybe.
> Shouldn't each animal be its own species?
Give it a shot. Next time you nail your missus, turn yourself over to the authorities for an immediate stoning.
What is a species?
In other words, when you and I speak of a "cat," how do we know that we're speaking of the same thing?