Devolution, de-evolution, or backward evolution is the notion that a species can change into a more "primitive" form over time. Devolution presumes that there is a preferred hierarchy of structure and function, and that evolution must mean "progress" to "more advanced" organisms. This may include the idea that some modern species that have lost functions or complexity accordingly must be degenerate forms of their ancestors. It should be noted though, that according to the definition of evolution, what is described as devolution is its self evolution.