Evolution Modifies Existing Structures Incrementally

Complex adaptations like a bat's wing could not evolve de novo; evolution must modify what already exists incrementally, each step conferring useful advantage—e.g., a bat wing is a hand with membrane left unculled by the embryonic sculpting that normally removes inter-finger webbing, and 'half a wing' is useful because any increase in air-presenting surface lets arboreal animals leap farther.