The variety of explanations of living forms and structures.

Over the last decades the image of vertebrate morphology has become at some places chaotic and even opportunistic, which seems to be due to the lack of appraisal or recognition of the leading concepts of the field. This paper analyses the changes in morphology, and shows that these changes have enriched research aims and that the discipline has a clear methodological and philosophical coherence. The situation in morphology is analysed from a semi-historical and review-like point of view to see what aspect is aimed at for explanation, and what types of explanation are used. A preliminary conclusion is reached that the animal form still has to be explained, and that the types of explanation for living form and structure can be divided into three main categories: (1) a category of typological explanation: (2) a category of functional explanation, including a specific constructional explanation; and (3) a category of causal explanation. Each of these categories has a static and a dynamic aspect. The dynamic aspect includes an explanation connected to life time (the system dynamic and morphogenetic explanation) and an explanation connected to evolutionary time (the historical explanation). These categories are used for structure at the molecular up to and including the organismic level of organization. The main challenge for the future is to integrate these different categories of form-explanation and determine their relative dominance when a theory of biology, that is of biological form and activity, is developed.