BIOGENETIC LAW. - The original formulation given by E. Haeckel (v.) in ch. 20 of the work Generelle Morphologie der Organismen (1866), 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 development of the individual would repeat, in brief, the stages that, according to the transformist theory, would have been slowly passed through in the course of evolution (v.) by the species to which the individual in question belongs.
This assertion rests on the observation of structural similarities between certain stages of the development of higher organisms (e.g. mammalian embryos) and the definitive stages of lower organisms (e.g. reptiles, amphibians and fishes) that appeared on earth earlier and are regarded by transformism as their progenitors. Haeckel went so far as to affirm that "phylogeny is the mechanical cause of ontogeny" (Studien zur Gastrea-Theorie, 1877).
Already T. Harvey in his treatise on the movements of the heart and blood in lower animals (1628) had observed that every animal passes, in the course of its development, 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 and attributed it to that "unity of the plan of organization" which he maintained in opposition to the thesis upheld by G. Cuvier concerning the existence from the very origin of four distinct animal groups. Similar observations were made by J. Meckel (1811), E. R. de Serres (1842, who likened human organogenesis to a transitory comparative anatomy), L. Agassiz (1857), who specified the correspondences between embryonic development and the chronological and organizational succession of the various animal groups, without however arriving at a clear evolutionistic interpretation. Darwin himself, dealing extensively with the subject in ch. 14 of his work On the Origin of Species, put forward such an interpretation with great caution, for he said: "As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants, namely of our existing species. Agassiz believes this to be a law of nature; but I am bound to confess that I only hope to see the truth of this law hereafter confirmed" (trans. Canestrini, p. 401). The first to declare explicitly that ontogeny is to be considered as a recapitulation of phylogeny was Fritz Müller, who was led to this by studies on 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 this concept a precise formulation, raised it to the value of a "fundamental biogenetic law" (biogenetische Grundgesetz) and returned to it extensively afterwards, applying it to man in his Anthropogenie (1874, whose 5th ed. is rich with 30 plates and 500 figures) and modifying it also in consequence of the criticisms that were soon directed at him. From this moment indeed the controversy flared up between supporters and detractors and the works for and against multiplied to such an extent that it is impossible to list them briefly. Favorable were or are, among others, H. Schmidt, D. Rosa, G. Colosi; contrary E. von Baer, L. Vialleton, W. Garstang; less categorical C. Emery, M. Caullery, L. Cuénot. At present the disputes, like the more general ones on evolutionism, are somewhat dormant and are considered more a matter of philosophical speculation than a subject of investigation directly useful to the progress of experimental inquiry.
Linked by Haeckel with his "gastrea theory" (according to which the embryonic stage of gastrula, common to almost all animals, would be the repetition of an ancestral stem-form called gastrea) and framed within his monistic conception (v. MONISMO), the biogenetic law was received favorably by the followers of transformism. For through its merit embryology entered as an auxiliary of paleontology and comparative anatomy in the reconstruction of the genealogical trees of species, providing precious clues to fill their gaps: thus the embryonic dentition of whales would attest their derivation from progenitors provided with teeth, like other cetaceans. Moreover the biogenetic law easily explained the existence of transitory structures, such as the embryonic branchial arches of mammals, apparently not necessary for development, attributing to them a purely phyletic significance.
But against the Haeckelian conception criticisms soon arose and the dispute was particularly violent at the end of the last century and the beginning of the present one, sometimes degenerating into personal polemic. Now, apart from the accusations of misrepresentation and mystification directed at Haeckel concerning certain of his citations and illustrative proofs, it is by now generally recognized that the correspondence between ontogenetic stages and supposed phylogenetic stages is often only superficial and crude.
The principal criticisms are: 1) Although apparently similar, embryonic rudiments can never be exactly compared to definitive organs: thus the fetal branchial arches of mammals could not function like those of adult fishes, from which they also differ in anatomy. And the embryonic organic evolution (e.g., that of the cavities of the heart) is not directly assimilable to the evolution of the same organs in the adults of the animal series. 2) There exists heterochrony, that is, diversity of chronological succession, between several embryonic structures and the same structures in the hypothetical phyletic series. Thus the gastreal appearance of sponges adduced by Haeckel as an example of an ancestral definitive structure (archigastrula) is subsequent to the larval stages, whereas in embryos the gastrula stage is among the earliest; likewise the amnion appears embryologically very early in reptiles, birds and mammals, but is present only in these three classes of animals, which appeared on earth relatively late, after fishes and amphibians. 3) In the development of the individual there are found stages (such as the nymph of insects) or embryonic organs (such as the amnion and allantois of higher vertebrates) or again embryonic stages of definitive organs (such as that of the eye vesicle) that have no correspondence in definitive structures of lower animals, whether extant or, so far as is known and appears logically inferable, extinct. 4) The biogenetic law accords with the linear phylogenetic conception of an animal series, but the consistency and characteristics of this remain to be specified.
Such criticisms had already been adumbrated by von Baer (incautiously cited by Haeckel in his own favor and vigorously contradicted by him) in his fourth law on embryonic development, according to which the embryo of one animal never resembles the adult of another animal, but possibly only its embryo. Comparisons would therefore be permissible only between embryos, not between these and adults. And even such comparability, already noted in the famous drawings of G. His (1874) of embryos of man, pig and chicken, is debatable: indeed O. Hertwig noted that already the first stages of various embryos (blastulae and gastrulae), even if superficially similar, are in reality quite different, as attests the early appearance of slight differences which, becoming more pronounced later, reveal the existence from the very origin of the orientation of each toward the adult type peculiar to the respective species.
Haeckel and his supporters sought to refute and, in part, to forestall the criticisms.
Thus to the first and third objections Haeckel himself opposed the distinction between palingenetic embryonic characters (from πάλιν = again, repeated), which would be those corresponding to the definitive characters of the ancestors and therefore in agreement with the b. law, and cenogenetic characters (from πάλιν = new), which would be those peculiar to the embryo, adapted to its special life and therefore not corresponding to the b. law: the manifestation of the law, revealed by palingenesis, would therefore be more or less "falsified" by cenogenesis, whose intervention would explain (with perhaps excessive convenience!) every alteration in the parallelisms 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 that of the chrysalis, which have no correspondence in the definitive stages of lower organisms. To the second objection Rosa and others replied that comparisons should not be made between whole embryos and whole adults, no stage of the former ever corresponding totally to any of the latter, but only between individual organs or structures of the one and the other, independently of their chronological succession. Finally, to the last objection, which touches on evolutionism in the grave questions of the inheritance of characters and of the single (monophyletism) or multiple (polyphyletism) derivation of living beings, it was replied (Rosa) that the b. does not sanction the necessity of the derivation of a given animal group from a single ancestor nor, still less, claims that a given living group derives from another also living (e.g. that present-day amphibians derive from present-day fishes), but says only that the former had ancestors with characters common to the latter. Moreover, in ontogenesis what would be inherited is not the conspicuous somatic characters of the ancestors, but rather their potential capacity for arising, inherent from the germ onward in the hereditary patrimony (idioplasm) of the species; otherwise one could not explain the transmissibility to the offspring of characters that the parents do not have, e.g. the structures of the worker bee on the part of the drone and the queen.
Now, apart from the further questionability of the justifications adduced, it is certain that they limit the scope of the b. law so greatly as to reduce considerably its original value, to the point of leading Morgan (Embriologia e Genetica, Turin 1938, p. 183) to conclude 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 nullify the detectability of parallelisms between embryonic structures and definitive structures; but to attribute to such similarities, often more apparent than substantial, the biogenetic meaning presents, in aggravated form, the dangers inherent in the transformist methodology that from phenomena of similarity and succession draws conclusions of kinship and derivation. It remains to be seen why these parallelisms exist; some biologists appealed to Cuvier's law of the correlation between parts, others appealed to the concepts of the unity of the plan of organization of nature or to the finalistic principle of the necessity of a given direction in the processes of development. But such considerations lay largely outside the strictly experimental field, entering that of philosophy. And by many the Haeckelian conception is today considered of purely historical-speculative interest, to which, however, the merit must be ascribed of having favored, especially at the time of its enunciation, the fervor of scientific research.
Bibliography
J. F. Meckel, Entwurf einer Darstellung der zwischen dem Embryozustande der höheren Tiere und dem permanenten der niederen stattfindenden Parallele (Meckel: Beiträge zur vergleich. Anat., 2), Lipsia 1811; Et. Geoffroy-St-Hilaire, Principes de philosophie zoologique, Parigi 1830; E. R. A. de Serres, Principes d'organogénie, ivi 1842; Milne Edwards, Considérations 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'Etienne G. St-Hilaire, Parigi 1847; H. Milne Edwards, Leçons sur la physiologie et l'anatomie comparée de l'homme et des animaux, I, ivi 1857; F. Müller, Für Darwin, Lipsia 1864; E. Haeckel, Generelle Morphologie der Organismen, Berlino 1866; L. Agassia, De l'espèce et de la classification en zoologie, trad. F. Vogeli, Parigi 1869, capp. 26 e 27; E. Ray Lankester, On the primitive cell-lovers of the embryo and the basis of genealogical classification of animals, in Ann. Magaz. of Nat. History, 1873; E. Haeckel, Anthropologie, Lipsia 1874; K. E. von Baer, Studien aus dem Gebiete der Naturwissenschaft, Pietroburgo 1876; C. Darwin, Sull'origine delle specie per elezione naturale, trad. G. Canestrini, Torino 1875, pp. 302-401; E. Haeckel, Studien zur Gastroes-Theorie, Jena 1877; A. Sedgwick, On the Law of Development commonly known as Von Baer's Law; and on the Significance of Ancestral Radiments in Embryonic Development, in Quarterly Journal of mir. Science, 36 (1894), p. 35; F. Keibel, Das biogenetische Grundgesetz und die Coenogenese, in Ergebnisse der Anat. und Entwicklung, di Meckel e Bonnet, 7 (1898); E. Schmidt, La legge biogenetica di Haeckel ed i suoi avversari (Conferenze ed opuscoli popolari, 5), Odenkirchen 1902 (cit. in E. Haeckel, Le meraviglie della vita, trad. D. Rosa, Torino 1906, p. 241); O. Hertwig, Handbuch der vergleichenden und experimentellen Entwicklungslehre der Wirbeltiere, Jena 1906; L. Vialleton, Un problème de l'Evolution. La théorie de la récapitulation des formes ancestrales au cours du développement embryonnaire (loi biogénétique fondamentale) de Haeckel, Parigi 1908 (ampia bibl.); E. Rabaud, Les phénomènes embryonnaires et la phylogénèse, 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, nn. 165, 166 (1925); H. Karny, Die Methoden der phylogenetischen Forschung, in E. Abderhalden, Handbuch der biologischen Arbeitsmethoden, IX, Berlino e Vienna 1925, pp. 211-500; L. Vialleton, Morphologie et Transformisme, in L. Cuénot, R. Dalbiez, E. Gagnebin, W. Thompson, L. Vialleton, Le transformisme, Parigi 1927; D. Rosa, Il valore della legge b., in Scientia, 45 (1928), pp. 317-26; id., L'origine des êtres vivants, Parigi 1929, p. 150; J. Needham, The biochemical aspects of the recapitulation theory, in Biological Reviews, 5 (1930); M. Caullery, Le problème de l'évolution, Parigi 1931, pp. 115-26; D. Rosa, s.v., in Enc. Ital., VII, pp. 42-43; L. Cuénot, La génèse des espèces animales, Parigi 1932, pp. 55-63; G. Brunelli, Le teorie sull'origine et l'evoluzione della vita, Bologna 1933, p. 152; L. Nigrit, L'evoluzione, Roma 1935, pp. 213-35; G. R. de Beer, Embryology and Evolution, Oxford 1936; P. Enriquez, Il problema della vita, Bologna 1937; T. H. Morgan, Embryologia e Genetica, Torino 1938, pp. 189-84; G. R. de Beer, Embryology and Taxonomy, in J. Huxley, New Systematics, Oxford 1940, pp. 385-91; W. Zimmermann, Die Methoden der Phylogenetik, in G. Heberer, Die Evolution der Organismen, Jena 1943, p. 43 (cf. anche pp. 261 e 611); E. Guyenot, L'origine des espèces, Parigi 1944, pp. 46-49; G. Colosi, La dottrina dell'evoluzione, Firenze 1945, pp. 92-117; V. Marcozzi, La vita e l'uomo, Milano 1946, pp. 185-189; id., Evoluzione o creazione?, ivi 1948, pp. 40-47.
Sergio Beer