Leo Buss expounds a general theory of development through a simple hierarchical extension of the synthetic theory of evolution. He perceives innovations in development to have evolved in ancestral organisms where the germ line was not closed to genetic variation arising during the course of ontogeny. Variants that favor both the proliferation of the cell lineage and the organism harboring them were sequentially incorporated in an increasingly sophisticated epigenetic program. In contrast, variants that favor the replication of the cell lineage at the expense of the individual were eliminated and ultimately favored the fixation of variants that limited the production and/or expression of subsequent variation, creating a stable developmental system. The author traces the origin of the modern preoccupation with the individual as a unit of selection to its historical foundations in the works of August Weismann, exposing defects in the translation of the nineteenth-century germ-soma doctrine into modern evolutionary language. Recognizing that the germ-soma barrier is a derived evolutionary state, Professor Buss illustrates how patterns in embryonic cleavage, gastrulation, mosaicism, induction, and competence arise as a consequence of the often conflicting evolutionary interests of cells and individuals. Building on this foundation, he argues that evolution of controls over the heritability of cell lineages have come to fix developmental programs and establish heterochrony as the principal vehicle of evolutionary change. Finally, Professor Buss argues that the perspective applied to development can be generalized and applied to other transitions between units of selection that have occurred in the history of life. Leo W. Buss is Associate Professor of Biology at Yale University.

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