A morphogenetic approach to the origin and basic organization of the tetrapod limb

Two complementary approaches underlie the study of evolutionary morphology—one a direct result of the Darwinian revolution, the other with roots that can be traced back to pre-Darwinian times. The former focuses on the issue of diversity and the origin of adaptation. Morphological change is depicted as a chronological progression of various combinations of adaptations and preadaptations (e.g., Bock, 1977). To define a morphological adaptation one has to focus on the relationships between form and function, i.e., on the selective pressures that fuel evolutionary change. This approach is dominated by an attempt to characterize the organism-environment interrelationship via the concept of natural selection as the driving force for morphological diversification. Conversely, the second approach, adopted in this work, seeks the structural and morphogenetic unity within diverse forms irrespective of the functional or adaptive differences between them. Essentially typological, its method goes back to the ideas of the German Naturphilosophie and French transcendental schools of morphology (Russell, 1916), and it addresses the ordered diversity of forms from the perspective of internal organization.