work was in experimental embryology; some of it was done in collaboration with Huxley, who would go on to be one of the leading figures of the modern synthesis. The Elements of experimental embryology, written with Huxley, was the best summary of the field at that time (1934). In Embryos and Ancestors (1930) de Beer stressed the importance of heterochrony, and especially paedomorphosis in evolution. According to his theories, paedomorphosis (the retention of juvenile features in the adult form) is more important in evolution than gerontomorphosis, since juvenile tissues are relatively undifferentiated and capable of further evolution, whereas highly specialised tissues are less able to change. He also conceived the idea of clandestine evolution, which helped to explain the sudden changes in the fossil record which were apparently at odds with Darwin's gradualist theory of evolution. If a novelty were to evolve gradually in an animal's juvenile form, then its development would not appear in the fossil record at all, but if the species were then to undergo neoteny (a form of paedomorphosis in which sexual maturity is reached while in an otherwise juvenile form), then the feature would appear suddenly in the fossil record, despite having evolved gradually. De Beer worked on paleornithology and general evolutionary theory, and was largely responsible for elucidating the concept of mosaic evolution, as illustrated by his review of Archaeopteryx in 1954. De Beer's also reviewed Haeckel's concept of heterochrony, with particular emphasis on its role in avian evolution, especially that of the ratites, in 1956. Dedicated to the popularisation of science, he received the Kalinga Prize from UNESCO. De Beer was the first to propose the Col de la Traversette as the likely site where Hannibal had crossed the Alps with his elephants. His thesis received support in 2016 when Mahaney et al. reported that sediments had been identified at the pass that had been churned up by "the constant movement of thousands of animals and humans" and dated them to the time of Hannibal's invasion. De Beer and the modern synthesis The conventional view had been that developmental biology had little influence on the modern synthesis, but the following assessment suggests otherwise, at least as far as de Beer is concerned: Publications Comparative, Embryology And Evolution Of Chordate Animals (1922) Growth – 1924 An introduction to experimental embryology – 1926 The comparative anatomy, histology and development of the pituitary body – 1926