context: His answer was that in many cases animals exist with intermediate structures that are functional. He presented flying squirrels, and flying lemurs as examples of how bats might have evolved from non-flying ancestors. He discussed various simple eyes found in invertebrates, starting with nothing more than an optic nerve coated with pigment, as examples of how the vertebrate eye could have evolved. Darwin concludes: "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case."
question: గబ్బిలాల నుండి ఉద్భవించిందని డార్విన్ భావించిన కొన్ని పక్షి జాతులు ఏమిటి?
answers: flying squirrels, and flying lemurs