The recapitulation theory by Ernst Haeckel

Ernst Haeckel (born February 16, 1834, † August 9, 1919)

The recapitulation theory, also known under the terms biogenetic principle or Biogenetic Basic Law, is a theory established by Ernst Haeckel, the relationship between ontogenesis and phylogenesis. The biologist published his conjectures in 1866 in 'General Morphology. II: General Evolution of Organisms. '.
The core hypothesis of the biogenetic principle is the following sentence: "The ontogenesis recapitulates phylogenesis. "To understand this sentence, first clarify the terms:
ontogenesis = Individual development of a living being. In humans, this means the period from fertilization of the egg to the adult.
phylogenesis = Tribal history of the species. In the narrower sense referred only to the tribal history of more closely related species. For example, The development from rodent-sized mammals, via monkeys to humans.
Thus, the theory of recapitulation assumes that the development of an organism (ontogenesis, in this case only the period between fertilization and birth is meant) repeats (recapitulates) that development of one's own phylogeny in a very short time. The theory is based on the observation that embryos of different species in early developmental stadium are almost identical (see figure).

In the 19th century Haeckel achieved great prominence with his theory. Influenced by Charles Darwin, Haeckel provided with the Biogenetic Basic Law the apparent proof, the development of complex life forms from simpler life forms. However, the recapitulation theory has been refuted in its core because in the embryonic phase no complete repetition of the phylogenetic history takes place, at least not in a form that would justify a designation such as 'Basic Law' / 'Basic Law'.
Nevertheless, the recapitulation theory also contains a 'true core' that supports evolutionary theory from an embryonic perspective:
1. Embryos of different species look very similar to each other.
2. Embryos of animal species that have previously separated in geological terms look more different (e.g., fish and human) than those that are relatively more closely related (e.g., pig and human).
3. Embryos develop traits they no longer have at birth (e.g., gill arch, tail and lanugo hair).