 Both workers, and others such as Dobzhansky and Wright, explicitly intended to bring biology up to the philosophical standard of the physical sciences, making it firmly based in mathematical modelling, its predictions confirmed by experiment.Natural selection, once considered hopelessly unverifiable speculation about history, was becoming predictable, measurable, and testable.The traditional view is that developmental biology played little part in the modern synthesis, but in his 1930 book "Embryos and Ancestors", the evolutionary embryologist Gavin de Beer anticipated evolutionary developmental biology by showing that evolution could occur by heterochrony, such as in the retention of juvenile features in the adult.