Hint: Ernst Haeckel's 1866 postulated biogenetic law, also known as 'Recapitulation Theory' that ontogeny recapitulates phylogeny, i.e. the development of the animal embryo and young, tracks the species' evolutionary development. The theory was influential and much-popularized earlier, but in elucidating either evolution or embryonic development, it was of little significance.
Complete answer: Ontogeny Repeats Phylogeny also known as Recapitulation Theory notes that its ancestral characters are replicated by an organism's developmental background. Eg - Tadpole's frog larva has tails and breathes with gills, showing the evolution of amphibians from fishes. Initially, the human embryo has 2 chambered hearts like fish, then 3 chambered hearts like amphibians, then 3 1/2 like Reptiles, and finally 4 chambered hearts, it shows our evolution from fishes, amphibians, reptiles. Ernst Haeckel coined the term' Ontogeny recapitulates phylogeny'. It notes that evolutionary history and all the intermediate forms of its ancestors are represented by the growth of an organism (ontogeny) (phylogeny). Recapitulation means the development of an embryo following the organism's evolutionary past. This is focused on a comparative study of the embryos of associated animal classes.
Note: The statement made by Haeckel was not completely absurd. The earliest stages of individual development frequently vary greatly, but there is a point at which all vertebrate embryos bear a general resemblance to each other, the zootype or phylotypic stage. This is the stage shown in Haeckel's famous figure's first row (with exaggerated similarity across species). "For example, "gill slits" are formed by all vertebrate embryos. These characteristics are not gills, but the cracks between a series of blobs or fingerlike tissue protrusions in the embryo's neck area.