BIOGENETIC LAW. — The original formulation given by E. Haeckel (v.) in chap. 20 of his 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 briefly repeat the stages which, according to transformist theory, would have been slowly evolution (v.) by the species to which the individual in question belongs.
This assertion 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 on earth earlier and were regarded 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 1628, T. Harvey, in his treatise on the movements of the heart and blood in lower animals, had observed that every animal passes, during its development, through stages corresponding to the different organizations of the animal scale. Two centuries later, Stefano Geoffroy-St-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 advanced by G. Cuvier concerning the existence, from the beginning, 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), and L. Agassiz (1857). They specified the correspondences between embryonic development and the chronological and organizational succession of the various animal groups, without, however, arriving at a clearly evolutionary interpretation. Darwin himself, treating the subject at length in chap. 14 of his work On the Origin of Species, put forward this interpretation with great caution, since he said: “Since the embryonic state of every species and of every group of species partly shows us the structure of their ancient and less modified progenitors, we can readily infer the reason why ancient and extinct forms of life must resemble the embryos of their descendants, that is, our existing species. Agassiz believes that this is a law of nature, but I limit myself to declaring that I hope subsequently to see the truth of this law confirmed” (Canestrini translation, p. 401). The first explicitly to 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 stated the thesis in his work Für Darwin (1864). Two years later E. Haeckel gave this concept a precise formulation, elevated it to the status of the “fundamental biogenetic law” (biogenetische Grundgesetz), and subsequently returned to it extensively, applying it to man in his Anthropogenie (1874, whose 5th edition contains 30 plates and 500 figures) and modifying it later in response to the criticisms soon directed against II. From this point onward, in fact, controversy arose between its supporters and detractors, and works for and against it multiplied, so that it is impossible to list them briefly. Among the supporters were or are H. Schmidt, D. Rosa, and G. Colosi; among the opponents, E. von Baer, L. Vialleton, and W. Garstang; less categorical were C. Emery, M. Caullery, and L. Cuénot. At present, these disputes, like the more general ones concerning evolutionism, have largely subsided and are regarded more as a matter of philosophical speculation than as a subject of investigation directly useful to the progress of experimental research.
Connected by Haeckel with his “theory of the gastrea” (according to which the embryonic gastrula stage, common to almost all animals, would be the repetition of an ancestral stem form called the gastrea) and incorporated into his monistic conception (v. MONISM), the biogenetic law was favorably received by the followers of transformism. Through its merit, embryology entered, as an auxiliary to paleontology and comparative anatomy, into the reconstruction of the genealogical trees of species, providing valuable clues for filling their gaps: thus, the embryonic dentition of whales would attest to their descent from tooth-bearing ancestors, like other cetaceans. Moreover, the biogenetic law readily explained the existence of transitory structures, such as the embryonic branchial arches of mammals, apparently unnecessary for development, by assigning them a purely phyletic significance.
But criticisms of the Haeckelian conception soon arose, and the dispute was particularly violent at the end of the last century and the beginning of the present one, at times degenerating into personal polemics. Now, apart from the accusations of distortion and falsification directed against Haeckel concerning certain of his citations and illustrative proofs, it is generally recognized that the correspondence between ontogenetic stages and supposed phylogenetic stages is often merely superficial and crude.
The principal criticisms are: 1) Although apparently similar, embryonic rudiments can never be exactly compared with fully developed organs: thus, the fetal branchial arches of mammals could not function like those of adult fish, from which they also differ anatomically. Nor can embryonic organic evolution (e.g. that of the heart cavities) be directly equated with the evolution of those same organs in the adults of the animal series. 2) There is heterochrony, that is, a difference in chronological succession, between several embryonic structures and the corresponding structures in the hypothetical phyletic series. Thus, the gastrula-like appearance of sponges, cited by Haeckel as an example of an ancestral definitive structure (archigastrula), follows the 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 found only in these three classes of animals, which appeared on earth relatively late, after fish and amphibians. 3) In the development of the individual there occur stages (such as the nymph of insects), embryonic organs (such as the amnion and allantois of higher vertebrates), and also embryonic stages of definitive organs (such as the vesicular stage of the eye) that have no correspondence in the definitive structures of lower animals, whether extant or, so far as is known and logically inferable, extinct. 4) The biogenetic law is consistent with the linear phylogenetic conception of the animal series, but the validity and characteristics of that series remain to be specified.
These criticisms had already been intimated by von Baer (who was incautiously cited by Haeckel in his own favor and vigorously repudiated by him) in his fourth law of 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 legitimate only between embryos, not between embryos and adults. And even this comparability, already noted in the famous drawings by G. His (1874) of human, pig, and chicken embryos, is debatable: O. Hertwig in fact observed that even the earliest stages of various embryos (blastulae and gastrulae), although superficially similar, are in reality quite different, as is shown by the early appearance of slight differences which, becoming more pronounced later, reveal the existence from the outset of each embryo’s orientation toward the adult type peculiar to its respective species.
Haeckel and his supporters sought to refute and, in part, forestall the criticisms.
Così alla prima e alla terza obiezione lo stesso Haeckel oppose la distinzione fra caratteri embrionali palingenetici (da πάλιν= di nuovo, ripetuto), che sarebbero quelli rispondenti ai caratteri definitivi dei progenitori e quindi in accordo con la legge b., e caratteri cenogenetici (da πάλιν= nuovo), che sarebbero quelli peculiari dell'embrione, adattati alla sua speciale vita e quindi non rispondenti alla legge b.: la manifestazione della legge, rivelata dalla palingenesi, sarebbe perciò più o meno « falsificata » dalla cenogenesi, il cui intervento spiegherebbe (con comodità forse eccessiva!) ogni alterazione nei parallelismi fra embrioni ed adulti. Inoltre l'accelerazione o tachigenesi dei processi embrionali rispetto a quelli filogenetici spiegherebbe l'esistenza di stadi di sviluppo « condensati » e accavallati, come quello di crisalide, che non hanno corrispondenza in stadi definitivi di organismi inferiori. Alla seconda obiezione Rosa ed altri risposero che i confronti non vanno fatti fra interi embrioni e interi adulti, nessuno stadio dei primi corrispondendo mai totalmente ad alcuno dei secondi, ma soltanto fra singoli organi o strutture degli uni e degli altri, indipendentemente dalla loro successione cronologica. Infine all'ultima obiezione, che tocca l'evoluzionismo nelle gravi questioni dell'eredità dei caratteri e della derivazione unica (monofiletismo) o plurima (polifiletismo) dei viventi, si rispose (Rosa) che la b. non sancisce la necessità della derivazione di un dato gruppo animale da un unico progenitore né, tanto meno, pretende che un dato gruppo vivente derivi da un altro pure vivente (ad es. che gli anfibi attuali derivino da pesci attuali), ma dice soltanto che i primi ebbero degli antenati con caratteri comuni ai secondi. Inoltre nell'ontogenesi si erediterebbero non già i caratteri somatici appariscenti dei progenitori, sibbene la loro potenziale capacità di insorgenza, insita fin dal germe nel patrimonio ereditario (idioplasma) della specie; altrimenti non si spiegherebbe la trasmissibilità ai figli di caratteri che i genitori non hanno, ad es., le strutture dell'ape operaia da parte del fuco e della regina.
Ora, a prescindere dall'ulteriore discutibilità delle giustificazioni addotte, è certo che esse limitano siffattamente la portata della legge b. da ridurne assai il primitivo valore, tanto da far concludere al Morgan (Embriologia e Genetica, Torino 1938, p. 183) che « se la teoria della ricapitolazione è una legge, questa legge presenta tante eccezioni che diventa inutile e dannosa ». Ciò non annulla interamente la rilevabilità di parallelismi fra strutture embrionali e strutture definitive; ma attribuire a tali somiglianze, spesso
più apparenti che sostanziali, il significato biogenetico presenta, aggravati, i pericoli insiti nella metodologia trasformista che da fenomeni di similitudine e di successione ricava conclusioni di parentele e di derivazioni. Resta a vedere perché questi parallelismi esistano; alcuni biologi si richiamarono alla legge della correlazione fra le parti di Cuvier, altri fecero appello ai concetti di unità del piano di organizzazione della natura o al principio finalistico della necessità di un dato indirizzo nei processi di sviluppo. Ma tali considerazioni esulanò in gran parte dal campo strettamente sperimentale per entrare in quella filosofico. E di un interesse puramente storico-speculativo è oggi da molti considerata la concezione haeckeliana, alla quale va tuttavia ascritto il merito di aver favorito, soprattutto all'epoca della sua enunciazione, il fervore della ricerca scientifica.
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