On the occasion of the 200th anniversary of the birthday of Johann Friedrich Meckel the Younger (1781-1833) it is appropriate to examine his contributions to the field of clinical genetics. Special emphasis is given to his laws of "diversity" and "reduction." These deal respectively with the evolutionary and developmental differences between organisms and the reasons for similarities in development of parts of an organism and the parts of different organisms. Meckel is an important pioneer of modern clinical teratology because he did not restrict his studies to normal development, but rather concentrated on its aberrations in his reflections on the history of development.

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Under close scrutiny the perogative of self-intelligibility thus attributed to present time is found to be based upon a set of strange postulates. In the first place, it supposes that, within a generation or two, human affairs have undergone a change which is not merely rapid, but total, so that no institution of long standing, no traditional form of conduct, could have escaped the revolutions of the laboratory and the factory. It overlooks the force of inertia peculiar to so many social creations. -Marc Bloch R e Historiun s CraftThe gracious and proper practice of acknowledging important contributions made by predecessors in a given field of endeavor endows the present with the gift of historical perspective. Yet a certain reluctance to trace the transmutation of seminal ideas and to acknowledge the works of their intellectual ancestors characterize workers in many fields of science.Nowhere is this more evident than in the field of embryology whose intellectual history rivals that of physics, chemistry, and mathematics. The rich intellectual tradition of embryology as it developed in the Western world includes powerful thinkers like Aristotle, William Harvey, Carl Ernst von Baer, Johannes Miiller, Wilhelm ROUX, Theodor Boveri, and Hans Spemann, to name only a few. Their working methods, centering upon a mechanistic and morphological approach, have given way to physiological, biochemical, endocrine, and molecular-biological analyses. The abandonment of the older methodology did create a gap in the historical picture of embryology and it did incur an heuristic penalty.So many other younger workers in the biomedical sciences were trained in an atmosphere of presentism that they lost the ability to make important connections between embryological and anatomical-physiological developments. Rich irony in-