In the panbiogeographic evolutionary synthesis, orthogenesis is referred to as a process which involves an inherent tendency to vary in certain directions, as ancestors vicariate or break apart into descendants. Orthogenesis has origins in the studies of form, but is given a biogeographic perspective in panbiogeography. The panbiogeographic use of orthogenesis is compatible with the origin of the term as presented by Haacke and Eimer. Misinterpretations of orthogenesis describing it as mystical, teleological and linear are invalid. The orthogenetic aspect of evolution was recognized by Darwin as "laws of growth," but was neglected in favor of natural selection. Although an internal component to evolution is recognized by contemporary biologists, it is often considered to be secondary to natural selection. Where recognition is given to an internal tendency for evolution to proceed without requiring the action of natural selection, terms such as "constraints," "bias," and "potential" may imply orthogenesis.