Phenotypic changes over geological time must result from alterations in the morphogenetic mechanisms associated with organogenesis. Recent advances in our understanding of such mechanisms suggest that there are certain patterns of metabolic activity and organelle synthesis which are present in many different cell lineages at different times during embryonic development, and that this ubiquity of genomic potential is available for expression at any time. Microevolutionary sequences of morphological change probably result from modulations of quantitative aspects of organogenesis, e.g., rates of cell proliferation, or cell density. It is also possible that certain macroevolutionary steps (“neomorphs”) may result from qualitative changes in the developmental programs, and such events underly “ultimate refinement” of morphological adaptation. The selection pressures involved in these different sequences of morphological change “see” embryonic processes in different ways. This thesis is illustrated with reference to the evolutionary history of the tetrapod limb.