ontogeny v. phylogeny

Ontogeny is the biological development of an individual organism from an embryo to its adult form. Phylogeny is the evolutionary history of an entire species or higher taxonomic group. In 1866, Ernst Haeckel, a German zoologist, formulated a theory of recapitulation. He conjectured that the stages in the embryonic development of an individual corresponded to the adult stages of prior species in its evolutionary history. For example, a human embryo with gill slits was said to represent a fishlike ancestor. Haeckel’s law is usually presented in a concise form as, ‘ontogeny recapitulates phylogeny’. This theory has since been discredited.