The term evolution means an orderly development and is applied to living things. The theory of evolution says that plants and animals have changed through generation after generation and still are changing today. Since this change has been going on for ages, all things that now live on earth are much-changed descendants of others that lived thousands and even millions of years ago. The theory of evolution describes changes and tries to explain them. It shows how these changes followed one another, and how they produced new kinds of living things that were far different from their ancestors. Most of these new types were more complex and efficient than earlier ones. Belief in evolution is based on several different kinds of evidence. They are variation and change, fossils, comparative anatomy, and geographic distribution. Variation and Change Many variation pass from one generation to the next without producing any visible change. Variations that make a plant resist certain diseases or that help it survive under unusual conditions, are used by breeders to develop useful types of plant and domestic animals. Such new types are examples of evolution on a small and very special scale. Fossils Fossils are the remains or traces of things that lived ages ago. They are preserved in rock layers called strata which lie one upon another. When fossils are collected form one layer they reveal variation like those shown by plant and animal today. When fossils from other strata are studied in turn, those variation often form series which record the stages by which new groups developed while old ones disappeared. Comparative Anatomy When different structures of bodies of plants and animals are compared, relationships and changes can be noted. Additionally, the origin of structures which now are small and useless can be explained by this process. For example, the human appendix is alll that is left of an intestinal organ which has reached its greatest size and value in plant-eating animals such as kangaroos and rabbits. Geographic Distribution Much evidence of evolution comes from plants and animals that live on
far from continents. For example, the Galapagos Islands have 26 kinds of land birds, all resembling species found in western . Twenty-three of these species seem to have changed since they reached the island for the Galapagos birds are distinct species. They apparently developed there because of changes that took place after their ancestors drifted from the mainland of South America. It also means that all of them must be related. Although general facts and conclusions about evolution seem clear, the subject still presents many unsolved problems.