Also called "recapitulation theory." This is the scientific theory that "ontogeny recapitulates phylogeny," which means that one can trace the evolutionary development of a species by studying the development of an individual embryo or young of that species. The theory is still used, especially in paleontology, but has been found to not be strictly true when applied to problems in biology.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.