Some animals and plants in our world are very common, like houseflies, cats or daisies. They are not in danger of dying out. Other species are very rare. Sometimes only a few of them are left. Such species may disappear forever.
Ever since life on earth began, animals and plants have died out and new ones have appeared. Dinosaurs, for example, roamed the planet during the earth’s middle ages. 65 million years ago, they disappeared forever.