is an archaic term from evolutionary biology referring to evolutionary directions that were considered "retrogressive
." It was a term used in contrast to anagenesis
, which in present usage denotes the evolution of a single population into a new form without branching lines of descent. Cladogenesis
is the term used for branching lines of descent, i.e., when the evolutionary origin of a new form is not accompanied by the disappearance of the ancestral form.