In biology, and particularly in morphology, various types of explanation are found,e.g. causal, teleological, historical, etc.In this article an attempt has been made to analyse the relations between the various explanations to strive for an encompassing explanatory theory.The general structure of the explanatory theories appeared to be very similar, but the terms defining the phenomena and the types of the relations within the theories differ. To obtain a unifying theory it is necessary to develop methods to connect or transform the (...) different elements.An important aspect is that the definition of selected phenomena depends on the explanation pursued. (shrink)