Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.In common parlance, "devolution", "de-evolution", or backward evolution is the notion that a species can change into a more "primitive" form. It is associated with the idea that evolution is supposed to make species more advanced, and that some modern species have lost functions or complexity and seem to be degenerate forms of their ancestors. This view is rejected by modern evolutionary theory, in which adaptation arises from natural selection of forms best suited to the environment, and so can lead to loss of features when these features are costly to maintain. Thus for cave dwelling animals, the loss of eyes arises because it is an advantage, not degeneracy. In pre-evolutionary ideas of essentialism following on from Plato's Theory of Forms, species were seen as pure unchanging types. Essentialism is rejected when genetic variation within kind is understood. The concept of created kinds can lead creationists to argue that genetic mutations are "devolution" away from the created type. Evolutionists prefer population genetics and what Ernst Mayr calls "population thinking" in defining species.