The role of embryology, genetics and morphology within mid twentieth century evolution theory, is discussed in the context of the growth to dominance of natural selection as the orthodox mechanism of adaptive evolution. The unification of neo-Mendelian heredity and neo-Darwinian selection theory, is descnbed as the core of\ud modern synthetic neo-Darwinism as it emerged in 1930s mathematical population genetics. As selectionism strengthened within synthetic neo-Darwinism, embryological development was excluded from its traditional causal role in adaptive evolution within the "old synthesis" of Haeckelian recapitulation and neo-Lamarckian inheritance. A two-tier embryology was created, as embryology was understood to deal separately with the experimental analysis of ontogenetic development, and the historical descriptive analysis of phylogenetic lineages. Neither tier informed the other, or played any direct causal role in the mechanism of the creation of adaptive evolutionary novelty. That adaptive evolutionary mechanism was entirely the preside of natural selection. However, as the selectionist synthesis hardened in the 1940s, late nineteenth century Darwinists' concerns over the hereditary fixation of highly\ud specific adaptive somatic modifications resurfaced. Consequently, the strategic defence of the synthetic theory against any resurgence of neo-Lamarckian heredity, involved an appeal to the principles of modem synthesis developmentalism; namely, the developmentalist syntheses of Waddington and Schmalhausen. The unforeseen implication of these moves by founding supporters of the synthetic theory, was that the disciplines upon which 1940s developmentalism rested--namely, Western chemical embryology and Soviet evolutionary morphology--did after all playa central\ud and causal role in the mechanism of adaptive evolution. Attempts to characterise the alternative and developmentalist syntheses of Waddington and Schmalhausen as the\ud "missing links" to an otherwise incomplete modem synthesis, are historically evaluated. These attempts are thought to embody either a mistaken understanding of the essential nature of synthetic neo-Darwinism, or an obfuscation of the continuing issue of its synthetic adequacy
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