BIOGENETIC LAW. — The original formulation given by E. Haeckel (v.) in chapter 20 of his work *Generelle Morphologie der Organismen* (1866), the 41st ontogenetic thesis, states that “ontogeny is the brief and rapid recapitulation of phylogeny, determined by the physiological functions of heredity and adaptation”; that is, the successive stages of an individual’s development would briefly repeat the stages that, according to transformist theory, were slowly traversed during the evolution of the species to which the individual belongs.
This formulation rests on the observation of structural similarities between certain stages in the development of higher organisms (e.g., mammalian embryos) and the definitive stages of lower organisms (e.g., reptiles, amphibians, and fish) that appeared earlier on Earth and are considered by transformism as their ancestors. Haeckel went so far as to assert that “phylogeny is the mechanical cause of ontogeny” (*Studien zur Gastrea-Theorie*, 1877).
As early as T. Harvey, in his treatise on the movements of the heart and blood in lower animals (1628), had observed that every animal, in the course of its development, passes through stages corresponding to the different organizations of the animal scale. Two centuries later, Étienne Geoffroy-Saint-Hilaire noted the parallelism between the adult forms of lower animals and the embryonic forms of higher animals, attributing it to that “unity of the plan of organization” which he upheld in contrast to Cuvier’s thesis of the original existence of four distinct animal groups. Similar observations were made by J. Meckel (1811), E. R. de Serres (1842—who likened human organogenesis to a transient comparative anatomy), and L. Agassiz (1857), who specified correspondences between embryonic development and the chronological and organizational succession of various animal groups, without, however, arriving at a clear evolutionary interpretation. Even Darwin, who dealt extensively with the subject in chapter 14 of *On the Origin of Species*, cautiously advanced this interpretation, saying: “As the embryonic state of each species and each group of species reveals to us in part the structure of their less modified ancient progenitors, we can easily understand why ancient and extinct forms of life should resemble the embryos of their descendants, that is, of our existing species. Agassiz believes this to be a law of nature, but I merely declare that I hope to see the truth of this law confirmed in the future” (trans. Canestrini, p. 401). The first to explicitly declare that ontogeny should be regarded as a recapitulation of phylogeny was Fritz Müller, who was led to this conclusion by studies in the embryology and comparative anatomy of crustaceans and echinoderms, and enunciated this thesis in his work *Für Darwin* (1864). Two years later, E. Haeckel gave the concept a precise formulation, raised it to the status of a “fundamental biogenetic law” (*biogenetische Grundgesetz*), and later expanded upon it, applying it to man in his *Anthropogenie* (1874; 5th ed. rich in 30 plates and 500 figures), and modifying it in response to the criticisms soon directed against II.
From this point onward, a controversy flared between supporters and detractors, and pro and con works multiplied to such an extent that it is impossible to summarize them briefly. Among those favorable were or are H. Schmidt, D. Rosa, and G. Colosi; among the opponents, E. von Baer, L. Vialleton, and W. Garstang; while C. Emery, M. Caullery, and L. Cuénot took a more cautious stance. At present, the disputes, like those on evolutionism in general, have largely subsided and are considered more a matter of philosophical speculation than a subject of investigation directly useful to experimental progress.
Linked by Haeckel to his “gastraea theory” (according to which the embryonic stage of gastrula, common to almost all animals, would be the repetition of an ancestral stem-form called gastraea) and framed within his monistic conception (v. MONISM), the biogenetic law was favorably received by the followers of transformism. Thanks to it, embryology entered as an auxiliary to paleontology and comparative anatomy in reconstructing the genealogical trees of species, providing valuable clues to fill their gaps: thus, the embryonic dentition of whales attests to their descent from toothed ancestors, as in other cetaceans. Moreover, the biogenetic law easily explained the existence of transient structures, such as the embryonic branchial arches of mammals, seemingly unnecessary for development, by attributing to them a purely phylogenetic significance.
However, objections to the Haeckelian conception soon arose, and the dispute was particularly heated at the end of the last century and the beginning of the present one, sometimes degenerating into personal polemics. Now, apart from the accusations of distortion and falsification leveled at Haeckel regarding certain of his citations and illustrative proofs, it is now generally recognized that the correspondence between ontogenetic and supposed phylogenetic stages is often merely superficial and crude.
The main objections are: 1) Although seemingly similar, embryonic rudiments can never be exactly compared to definitive organs: thus, the fetal branchial arches of mammals could not function as those of adult fish, from which they also differ anatomically. And the embryonic development of organs (e.g., that of the heart chambers) cannot be directly assimilated to the evolution of the same organs in the adults of the animal series. 2) There exists heterochrony, i.e., a difference in chronological sequence, between several embryonic structures and the same structures in the hypothetical phylogenetic series. Thus, the gastrean aspect of sponges, cited by Haeckel as an example of a definitive ancestral structure (archigastrula), occurs after larval stages, whereas in embryos the gastrula stage is among the earliest; similarly, the amnion appears embryologically very early in reptiles, birds, and mammals, but is present only in these three classes of animals, which appeared relatively late on Earth, after fish and amphibians. 3) In the development of the individual, stages (such as the pupa of insects) or embryonic organs (such as the amnion and allantois of higher vertebrates) or even embryonic stages of definitive organs (such as the vesicle stage of the eye) are found that have no correspondence in definitive structures of lower animals, either extant or, as far as is known and logically appears, extinct. 4) The biogenetic law accords with the linear phylogenetic conception of the animal series, but the consistency and characteristics of this series remain to be clarified.
Such objections had already been foreshadowed by von Baer (unwisely cited by Haeckel in his favor and vigorously refuted by him) in his fourth law of embryonic development, according to which the embryo of an animal never resembles the adult of another animal, but only its embryo. Comparisons would therefore be permissible only between embryos, not between these and adults. And even this comparability, already noted in G. His’s famous drawings (1874) of embryos of man, pig, and chicken, is questionable: indeed, O. Hertwig pointed out that even the earliest stages of various embryos (blastulae and gastrulae), although superficially similar, are in reality quite different, as evidenced by the early appearance of slight differences that, intensifying later, reveal the existence from the outset of a direction peculiar to each toward its adult type.
Haeckel and his supporters sought to refute and, in part, preempt these criticisms.
Thus, Haeckel himself countered the first and third objections by drawing a distinction between palingenetic embryonic characters (from πάλλυ = again, repeated), which would correspond to the definitive characters of ancestors and thus align with the biogenetic law, and cenogenetic characters (from καλύφει = new), which would be peculiar to the embryo, adapted to its special life and therefore not in accordance with the biogenetic law. The manifestation of the law, revealed by palingenesis, would thus be more or less "falsified" by cenogenesis, whose intervention would explain (perhaps with excessive convenience!) every alteration in the parallels between embryos and adults. Furthermore, the acceleration or tachygenesis of embryonic processes relative to phylogenetic ones would explain the existence of "condensed" and overlapping developmental stages, such as the chrysalis stage, which have no correspondence in definitive stages of lower organisms.
To the second objection, Rosa and others responded that comparisons should not be made between entire embryos and entire adults, since no stage of the former ever fully corresponds to any of the latter, but only between individual organs or structures of one and the other, regardless of their chronological sequence.
Finally, to the last objection—which touches on evolutionism in the grave issues of the heredity of characters and the single (monophyletism) or multiple (polyphyletism) derivation of living beings—Rosa replied that the biogenetic law does not affirm the necessity of a given animal group deriving from a single ancestor, nor does it claim that a given living group derives from another living group (for example, that present-day amphibians derive from present-day fish), but merely states that the former had ancestors with characters in common with the latter. Moreover, in ontogeny, it is not the conspicuous somatic characters of ancestors that are inherited, but rather their potential capacity for emergence, inherent from the germ in the hereditary patrimony (idioplasm) of the species. Otherwise, it would not be possible to explain the transmissibility to offspring of characters that parents do not possess, such as the structures of the worker bee from the drone and the queen.
Now, setting aside the further debatable nature of the justifications offered, it is certain that they so restrict the scope of the biogenetic law as to greatly diminish its original value, to the point that Morgan (Embriologia e Genetica, Turin 1938, p. 183) concluded that "if the theory of recapitulation is a law, this law presents so many exceptions that it becomes useless and harmful." This does not entirely negate the relevance of parallels between embryonic and definitive structures; but attributing to such similarities—often more apparent than real—the biogenetic significance presents, in an aggravated form, the dangers inherent in transformist methodology, which derives conclusions of kinship and derivation from phenomena of similarity and succession. It remains to be seen why these parallels exist; some biologists appealed to Cuvier’s law of correlation between parts, others invoked the concepts of the unity of the plan of organization in nature or the finalistic principle of the necessity of a given direction in developmental processes. But such considerations largely transcend the strictly experimental field to enter the philosophical realm.
Today, many consider the Haeckelian conception to be of purely historical and specialized interest, though it must be credited with having fostered, especially at the time of its enunciation, the fervor of scientific research.
J. F. Meckel, Entwurf einer Darstellung der zwischen den Embryozustande der höheren Tiere und dem permanenten der niederen stattfindenden Parallele (Meckels Beiträge zur vergleichenden Anat., 2), Leipzig 1811; E. Geoffroy-St-Hilaire, Principes de philosophie zoologique, Paris 1830; E. R. A. de Serres, Principes d’organogénie, ibid. 1842; Milne Edwards, Considerations sur quelques principes relatifs à la classification naturelle des animaux, in Ann. Sc. Nat., 3 (1844), p. 65; Is. Geoffroy-Saint-Hilaire, Vie, travaux et doctrines scientifiques d’Étienne G. St-Hilaire, Paris 1847; H. Milne Edwards, Leçons sur la physiologie et l’anatomie comparée de l’homme et des animaux, I, ibid. 1857; F. Müller, Für Darwin, Leipzig 1864; E. Haeckel, Generelle Morphologie der Organismen, Berlin 1866; L. Agassiz, De l’espèce et de la classification en zoologie, trans. F. Vogeli, Paris 1869, chaps. 26 and 27; E. Ray Lankester, On the primitive cell-layers of the embryo and the basis of genealogical classification of animals, in Ann. Magaz. of Nat. History, 1873; E. Haeckel, Anthropogenie, Leipzig 1874; K. E. von Baer, Studien aus dem Gebiete der Naturwissenschaften, St. Petersburg 1876; C. Darwin, Sull’origine delle specie per elezione naturale, trans. G. Canestrini, Turin 1875, pp. 392–401; E. Haeckel, Studien zur Cestrea-Theorie, Jena 1877; A. Sedgwick, On the Law of Development commonly known as Von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development, in Quarterly Journal of Micros. Science, 36 (1894), p. 35; F. Keibel, Das biogenetische Grundgesetz und die Coenogenese, in Ergebnisse der Anat. und Entwickelung, by Merkel and Bonnet, 7 (1898); E. Schmidt, La legge biogenetica di Haeckel ed i suoi avversari (Conferenze ed opuscoli popolari, 3), Odenkirchen 1902 (cited in E. Haeckel, Le meraviglie della vita, trans. D. Rosa, Turin 1906, p. 343); O. Hertwig, Handbuch der vergleichenden und experimentellen Entwicklungslehre der Wirbeltiere, Jena 1906; L. Vialleton, Un problème de l’évolution. La théorie de la récapitulation des formes ancestrales au cours du développement embryonnaire (loi biogénétique fondamentale) de Haeckel, Paris 1908 (extensive bibliography); E. Rabaud, Les phénomènes embryonnaires et la physiologie, in Scientia, 32 (1915), pp. 270–89; L. Vialleton, A propos de la loi biogénétique, ibid., 33 (1916), pp. 101–14; W. Garstang, The Theory of Recapitulation: a Critical Restatement of the Biogenetic Law, in Journal of the Linnean Society (Zool.), 35 (1922), p. 81; G. K. Noble, An outline of the Relation of Ontogeny to Phylogeny within the Amphibia, in Amer. Mus. Novitates, nos. 165, 166 (1925); H. Karny, Die Methoden der phylogenetischen Forschung, in E. Abderhalden, Handbuch der biologischen Arbeitsmethoden, IX, Berlin and Vienna 1925, pp. 211–500; L. Vialleton, Morphologie et Transformisme, in L. Cuénot, D. Dalibez, E. Gagnebin, W. Thompson, L. Vialleton, Le transformisme, Paris 1927; D. Rosa, Il valore della legge b., in Scientia, 45 (1928), pp. 317–26; id., L’origine des êtres vivants, Paris 1929, p. 150; J. Needham, The biochemical aspects of the recapitulation theory, in Biological Review, 5 (1930); M. Caullery, Le problème de l’évolution, Paris 1931, pp. 115–26; D. Rosa, s.v., in Enc. Ital., VII, pp. 42–43; L. Cuénot, La genèse des espèces animales, Paris 1932, pp. 55–63; G. Brunelli, Le teorie sull’origine e l’evoluzione della vita, Bologna 1933, p. 152; L. Nigris, L’évolution, Rome 1935, pp. 213–35; G. R. de Beer, Embryology and Evolution, Oxford 1936; P. Enriquez, Il problema della vita, Bologna 1937; T. H. Morgan, Embriologia e Genetica, Turin 1938, pp. 169–84; G. R. de Beer, Embryology and Taxonomy, in J. Huxley, New Systematics, Oxford 1940, pp. 365–93; W. Zimmermann, Die Methoden der Phylogenetik, in G. Heberer, Die Evolution der Organismen, Jena 1943, p. 43 (cf. also
pp. 261 and 611); E. Guyenot, L'origine des espèces, Paris 1944, pp. 46-49; G. Colosi, La dottrina dell'evoluzione, Florence 1945, pp. 92-117; V. MAROZIA, La vita e l'uomo, Milan 1946, pp. 183-189; id., Evoluzione o creazione?, ibid. 1948, pp. 40-47. Sergio Beer