One example of preadaptation is in dinosaurs, which evolved feathers with the function of thermo-insulation and display long before they were used for flight by early birds. Sweat glands in mammals were later transformed into mammary glands. Another example is the long journey of the mammalian ear ossicles, which started in the gill covers of ancient fish, then became part of the lower jaw of reptiles, and then became part of the inner ear of mammals. Another example is the wings of penguins. Once used for flying, they are now used for 'flying' under water.