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.