In his book "Embryos and evolution" (1930) Gavin de Beer stressed the importance of heterochrony, and especially paedomorphosis, in evolution. Changes to the genes which control timing in development can produce quite striking changes in the adult. de Beer 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 gradual 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. Then, if the species were to undergo neoteny, the feature would appear suddenly in the fossil record, despite having evolved gradually. Stephen Jay Gould commented on this book: