Balfour, Garstang and de Beer: The First Century of Evolutionary Embryology1

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Abstract
Evolution has been integrated with embryology during two great periods: the latter half of the 19th C as evolutionary morphology/embryology, and the latter third of the 20th C as evolutionary developmental biology. My mandate was to use the contributions of three embryologists/morphologists: Francis (Frank) Balfour (1851–1882), Walter Garstang (1868–1949) and Gavin de Beer (1899–1972) to discuss the foundations of evolutionary embryology in the UK from 1870 (when “every aspiring zoologist was an embryologist, and the one topic of professional conversation was evolution,” Bateson, 1922, p. 56), through the 1920s (“ontogeny does not recapitulate phylogeny, it creates it,” Garstang, 1922, p. 81) to the 1970s (“homology of phenotypes does not imply similarity in genotypes,” de Beer, 1971, p. 15). Evolutionary embryology was driven by a comparative embryological approach that sought homology of adult structures in germ layers and ancestry in embryos, and sought to differentiate larval adaptations from retained ancestral characters. An initial emphasis on a phylogenetic mechanism (recapitulation) slowly gave way to more mechanistic approaches that included heterochrony and the integration of embryology with physiological genetics. Germ layers, homology, larval evolution, larval origins of the vertebrates, paedomorphosis and heterochrony underpinned the origins of evolutionary embryology, and so I discuss each of these topics.