ABSTRACT
Morphology attempts to describe and order natural forms in terms of their differential similarity, i.e., it is an inherently comparative approach. Still, description is only the first step. According to the constructional morphology triangle (Pl. 1.1) there are three major reasons why two organisms should be similar. (1) Shared ancestry is the chief measuring stick for our standard classification. With regard to species and higher taxa, cladistics provides an elaborate terminology comparable to terms in human genealogy. With regard to organs, we talk about homology. (2) Shared function may cause similarity in originally different structures. Since it is secondary, we call this kind of similarity functional convergence. (3) Shared fabrication in a morphogenetic sense causes similarities of still another kind. They are particularly common in skeletal structures and could be called fabricational analogies.
