Theory, due to E. Haeckel (1834–1919), asserting that ontogeny (the development of the individual) recapitulates or reflects the phylogeny (the evolutionary history of the group). The theory as such has been rejected as not of general applicability, von Baer’s ‘biogenetic law’ being sufficient explanation for the observations on which it was based. However, either hypermorphosis or retardation of sexual maturity can result in individual cases of recapitulation.