The idea of organic evolution is that living things have arisen from pre-existing things and that various species have arisen by a gradual process of change over a very long period of time. Present day species have not always existed in the present form, but have arisen from pre-existing ancestor. Evidences for evolution : evidence from fossils: A fossil is a part or an impression or an animal that lived a very long time ago(up to a millions of years) some fossils are preserved in snow some in sedimentary rocks . Reference round up biology,modern biology, college biology