Every tetrapod limb shares the same bones — humerus, radius, ulna, carpals, metacarpals, phalanges. Only the proportions differ. Same developmental program, different parameters. These four forms are reachable from each other through single-gene mutations.
The Kirschner principle: When a bone gets longer, nerves and muscles don't need separate mutations — they find their way through exploratory developmental processes. Here, mutating trunk genes reshapes the whole limb coherently. Mutating bud genes changes only the digits. budGrad is the Sonic Hedgehog gradient — it makes the thumb shorter than the pinky, or (in the bat) the wing-fingers longer than the thumb-claw.