 By 1823, Geoffroy St. Hilaire had extended his theory of the “unity of type” to the claim that even the invertebrates shared a common plan with the vertebrates, and by 1825 he had embraced a limited version of transformism. This led him into direct opposition to the claims of his one-time friend and colleague, Georges Cuvier, holder of the chair of Comparative Anatomy. Cuvier’s researches in comparative anatomy and paleontology led him to conclude that animals were formed on a series of four distinct and autonomous body plans (embranchments) that may display some unity of type within the embranchments, but not between these plans. This was the mainstay of Cuvier’s opposition to transformism, and it served as a powerful source of opposition to transformism into the Darwinian era.