This is a nineteenth century idea put forward by a German zoologist named Ernst Haeckel (1834-1919) that development in childhood exactly parallels evolution. Phylogeny refers to evolution and ontogeny to development across the lifespan. Haeckel believed that every stage in the development of a human embryo exactly parallels the stages of human evolution. Besides having a faulty view of human evolution (e.g., there was no cow stage in human evolution), this theory oversimplified the processes of embryonic development. While Haeckel’s specific theory has been discredited, he was correct in pointing out important parallels between evolution and maturation. Careful understanding of our development across the lifespan does offer some clues as to our evolutionary history.