Why is evolutions primary mechanism mutation ?

The problem with using the example of language evolution is that linguists can trace exactly how, for example, French and Spanish both evolved from classical Latin. Even though French and Spanish are quite different, their gradual evolution from Latin has been worked out in detail.
People can make the comparison, but it's a false analogy. Nature is infinitely more complex than languages. The evolution of, say, a bat's wing from a functional forelimb of a non-flying mammal through gradual transitions is not only empirically absent in the fossil record, it is conceptually impossible to visualize. If you doubt this, I'd love to see I diagram of five of six bat precursors, from a non-flying mammal to a bat precursor who has achieved the ability to glide. With each precursor, explain to me how the gradually elongated phylanges are going to have a selective advantage with each stage, ensuring preferential survival of the newer species. At some point the animal is going to have to sacrifice its forelimb in favor of flight, prior to the achievement of its ability to even glide. I want to see what such an animal looks like.