ABSTRACT
Homology is a natural kind concept, 1 , 2 but one that has been notoriously elusive to pin down. There has been, and continues to be, sustained debate over the nature of correspondence (e.g. whether homology is an all-or-nothing relation or whether it admits of degrees) and the units of comparison (e.g. whole organisms, traits as morphological outcomes, behavioural activities, biochemical mechanisms, developmental processes, or certain properties of traits).
