Haeckel formulated the theory known as the theory of recapitulation or biogenetic law. The theory expresses that "ontogeny recapitulates phylogeny". The development of an organism (ontogeny) expresses all the intermediate forms of its ancestors throughout evolution (phylogeny). For example, the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive stages in the evolution of the animal's remote ancestors (phylogeny).