Recapitulation theory
Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". Ontogeny is the steps an organism takes in its development before it is born, starting as a single cell and ending as a baby. Phylogeny is the steps a species takes in its development, starting as a very simple organism and evolving into the current organism. "Recapitulates" is a synonym for "reflects" and implies that the ontogeny of an organism is similar to the phylogeny of its species.