By Robbert Striekwold
Theories of recapitulation arose in the late 18th but came to full bloom in the early 19th century. The common feature of such theories was the idea of a strong relationship between embryonic and species development. Such views were popular especially among the German Naturphilosophen, whose worldviews proved highly conductive to recapitulationist thinking. Species development in this context often implied not historical transformation but a kind of static progression, in a unified and morally meaningful universe. Elaborate and sometimes radical systems arose, leading one Naturphilosoph to ask the rhetorical question, “What are the lower animals but a series of human abortions?”
The question was asked by Lorenz Oken (1779-1851), a German naturalist with a holistic and progressivist understanding of the universe. According to him, all developmental processes proceeded sequentially through four stages representing Earth, Water, Air, and Fire. This included human history, where these elements were mapped onto various kinds of cultural achievements, with the fiery activities of science and war at the pinnacle. In the case of embryonic development, however, the four elements translated into the processes of nutrition, digestion, respiration, and motion, respectively.
These processes are represented by various organs, all present in the human body. All other animals contain only a subset of these organs, rendering them into imperfect humans. Embryonic development then becomes a matter of progression from an undifferentiated beginning, through a sequential addition of organs, to the ultimate outcome: man. The development of other species stops at a certain point lower on the scale, and they are thus in progressive terms similar to an aborted human fetus. Therefore, Oken writes, the “whole animal kingdom is none other than the representation of the several activities or organs of man; naught else than man disintegrated”.  The development of a human embryo moves through the entire animal kingdom, recapitulating its scale of increasing perfection.
The Meckel-Serres Law
Oken’s theory of recapitulation was not evolutionary, however. The hierarchy of species was represented ideally, not historically. As many other Naturphilosophen, he believed that the entire animal kingdom was arranged in a predetermined sequence, in which every species had its place at a certain level of perfection. Gottfried Reinhold Treviranus (1776-1837), on the contrary, amassed huge amounts of fossil evidence to argue for progressive species development. He believed in a strong parallel between the developmental stages of embryos and the evolutionary stages of species.
But the famous German anatomist Johann Friedrich Meckel (1781-1833) provided the perhaps most sophisticated and synthetic system among the early recapitulationists. According to him, the careful study of embryos could be used to uncover the evolutionary history of species. His principle of recapitulation, in which higher species would progressively develop away from an initial animal Urtype, would later be termed the “Meckel-Serres law”. Like the German Naturphilosophen, the French physiologist Étienne Reynaud Augustin Serres (1786-1868) believed in progressive sequences of development both in embryos and species, and set out to prove that “lower animals are, for certain of their parts, permanent embryos of higher classes.”  In order to do so, he very successfully applied the theory of recapitulation to the field of teratology, i.e., the study of embryonic malformations – an approach also endorsed and practised by Meckel.
Teratology and Recapitulation
The idea was quite simple. If during development the human embryo goes through stages representing lower animals, then malformed embryos may in fact be stuck in some earlier stage. These he called “monsters by defect”, which arose because they had too little formative force in their development. Men with undescended testicles, for example, were similar in that respect to adult fish. In one particularly bizarre example, Serres studied a human embryo lacking a head, which he identified as being in a mollusc stadium. Molluscs lack heads, and Serres believed he could point to other similarities of this embryo to adult molluscs, such as pockets under the skin he took to be involved in respiration. He gave various other examples of such developmental arrests, at one point echoing Oken in identifying living invertebrates as malformed human embryos. More generally, and again echoing Oken, he held that the “entire animal kingdom can, in some measure, be considered ideally as a single animal which, in the course of formation and metamorphosis of its diverse organisms, stops in its development here earlier and there later.” 
Some embryos, however, did not have too few parts but too many: these Serres called “monsters by excess”. They had too much formative force, but could not transcend their type. Therefore, rather than sprouting entirely new organs, they would simply duplicate already existing ones, as in the growth of extra fingers or toes. Meckel used similar categories for embryonic defects, and identified a clear “excess of generative energy” in the case of conjoined twins, where not just one but a great number of organs had been duplicated.
The Most Important of Questions
Teratology proved to be a very popular and successful field in the early 19th century. This lent a lot of prestige to the principle of recapitulation, which had helped to put it on this footing. The theory of embryonic recapitulation thus became highly influential, but it was certainly not without its powerful critics. In part III, I will discuss some of them, but I will focus on what some regard as one of the most important, others as one of the most annoying questions in the history of biology: what did Charles Darwin have to say about all this.
Robbert J Striekwold is a M.Sc. student in the History and Philosophy of Science programme at Utrecht University, specializing in the history and philosophy of evolutionary theory. He is writing his thesis on conceptual issues in modern evolutionary developmental biology.
 Lorenz Oken, paraphrased in Gould, S. J., 1977, Ontogeny and Phylogeny (Cambridge, MA: Harvard University Press). I would very much recommend this book for further reading.
 Oken, L., 1847, Elements of Physiophilosophy, trans. A. Tulk (London: Ray Society), p. 19.
 Russell, E. S., 1916, Form and Function. A Contribution to the History of Animal Morphology (London: J. Murray).
 Serres, E. R. A., 1824, Explication du système nerveux des animaux invertébrés. Ann. Sci. Nat., p. 247.
 (A) Headless foetus studied by Serres. (B) Pockets on the back of the foetus which Serres believed to be involved in respiration through the skin. From Serres, 1860, Principes d’Embryogénie, de Zoogénie et de Teratogénie.
 Serres, E. R. A., 1860, Principes d’Embryogénie, de Zoogénie et de Teratogénie. Mém. Acad. Sci., vol. 25: p. 834.
 Meckel, J. F., 1815, De Duplicitate Monstrosa Commentarius (Halae et Berolini: e Librariis Orphanotrophei).