The first appearance of legitimation by subsequent marriage is in commentaries on Gratian's Decretum in the 12th c., citing Justinian's law, and then explicitly through two papal decrees of Alexander III, dated 1172 and 1180, both later incorporated into canon law (in the Decretals, 4.17.6 and 4.17.1). The former states: "tanta est vis matrimonii, ut, qui antea sunt geniti, post contractum matrimonium legitimi habeantur" (such is the power of matrimony that those who are born before matrimony are held to be legitimate after it).
Canon law was following Roman law but extending it to some degree: Roman law only allowed legitimation of the children of concubines, whereas canon law allowed any children, as long as both parents were free to marry at the time of said children's birth (children of such unions are called naturales). Also, Roman law required a written document and consent of the children, which canon law did not. But canon law did not allow the legitimation by subsequent marriage of adulterous children or children of incest (only a papal rescript could). The same decree of Alexander III continues: "si autem vir, vivente uxore sua, aliam cognoverit, et ex ea prolem susceperit; licet post mortem uxoris eandem duxerit; nihilominus spurius erit filus, et ab hereditate repellendus; praesertim si in mortem uxoris prioris alteruter eorum aliquid fuerit machinatus." The Pope's power to legitimate is unlimited.
Canon law generally governed marriages during the Middle Ages. But, starting with the 16th c., its applicability to civil law was increasingly questioned even in Catholic countries. Increasingly, marriage was seen as a civil contract whose validity is determined, or can be restricted, by secular law (see a more extended discussion). And, of course, Protestant countries did not necessarily follow Catholic canon law.
Legitimation by subsequent marriages continues in modern canon law. Canon 1139, in book 4, part 1, title 7, chapter 8, states: "Illegitimate children are legitimated by the subsequent marriage of their parents, whether valid or putative, or by a rescript of the Holy See."
See also the entry on Legitimation in the 1910 Catholic Encyclopaedia.
As said above, legitimation by subsequent marriage is not so much a feature of canon law as it is of Roman law. Judicial systems influenced by Roman law (as was canon law) incorporated many of its features. This is true particularly of legal systems in Italy, Germany, France, and Scotland. The case of Germany is discussed in greater detail next. The trend seems to be now toward the abolition of the concept of legitimacy, as has already happened in Germany (see below) and may soon happen in Scotland.
In France, the relevant text is article 331 of the Code Civil. The original article in the Code of 1803 excepted the children of incest and adultery, but those restrictions have been removed. Note, however, that before 1803 the question of the civil effects of subsequent marriage was ambiguous. La Roque (Traité de la Noblesse, 1678, p. 408) is of the opinion that "comme la légitimation est un droit Royal et procédant de la Souveraineté, il semble que les Bâtards legitimez par le mariage suivant, sont encore obligez de prendre des lettres du Roy deuement verifiées, pour estre reconnus lgtitimes, et pour user des droits qui leur appartiennent: car la seule coûtume de mettre un Bâtard sous le drap, ne suffit pas pour devenir capables de tous hooneurs & successions; il faut au regard du temporel que le Prince accorde la legitimation pour succeder, de la mesme maniere que les Constitutions de l'Eglise favorisent la legitimation au regard du spirituel."
The English common law system was largely unaffected by Roman law. English feudal law considered children born before marriage to be illegitimate, and hence incapable of inheriting a hereditament. When a court action over land involved a dispute over the legitimacy of a party, the king's court referred the issue to the ecclesiastical court; in the case of a child born before marriage, however, the "plea of special bastardy" framed the question as: "Was the child born before marriage?" At the parliament of Merton under Henry III prealtes objected to this, but the barons refused to cede ("Nolumus leges Angliae mutari" was their famous reply). Thus the statute of Merton (20 Henry III) enshrined this rule in law.
The Legitimacy Act of 1926 (16 and 17 Geo. 5 c. 60) for the first time provided for legitimation by subsequent marriage, provided the parents were not married to a third party at the time of birth, but it explicitly excluded peerages and honours from the effects of such legitimation (s.10: "Nothing in this Act shall affect the succession to any dignity or title or honour or render any person capable of succeeding to or transmitting a right to succeed to any such dignity or title"). The Legitimacy Act 1959 (c. 73) removed the former limitation, and also allowed for the legitimacy of the children of a void marriage when both or either of the parties reasonably believed it valid at the time. Legitimation occurs only if the father was domiciled in England and Wales at the time of birth of the child or of his own death if it occurred before. It only applies to a living child. The Legitimacy Act 1976 (c. 31) allowed the legitimacy of the child of a putative marriage to apply to titles, honours and dignities, but confirmed that legitimation by subsequent marriage did not ("Apart from section 1, nothing in this Act shall affect the succession to any dignity or title of honour or render any person cpable of succeeding to or transmitting a right to succeed to any such dignity or title"), nor did it to the Crown ("It is hereby declared that nothing in this Act affects the Succession to the Throne").
Scotland, being under Roman law, did have legitimation by subsequent marriage. This was formally codified by the Legitimation (Scotland) Act 1968 (c. 22). Its provisions are applicable "in relation to any question as to succession to, or devolution of, any title, honour or dignity".
German common law (das gemeine Recht) is a mixture of Roman law (as interpreted by medieval commentators), statute (imperial and local) and local custom. Roman law was considered one of the basic sources of law (on the belief that the law of the Roman empire was applicable to the Holy Roman Empire). Thus, legitimation by subsequent marriage made its way into German law independently of canon law, during the "reception" of Roman law in the 14th-16th century.
German law usually widened its applicability still further, to children of adultery and incest (children called spurii), as long as the subsequent marriage itself is legal. That point was disputed, but Glück states that that was the case for children of incest as long as a proper dispensation was available for the marriage (he says that protestant church law offers many examples where illegitimate children of all kinds were legitimated by subsequent marriage).
What was the effect of legitimation by subsequent marriage? One effect, with respect to public law, was to remove any dishonor attached to the condition of illegitimacy and grant full civil rights, including access to trades, corporations, guilds, but also to offices, charges, and dignities, unless excluded by specific legislation requiring legitimate birth. (This effect was also obtained by "German" legitimation, that is, legitimation by prince's rescript; but this last form of legitimation did not carry its effects in the area of family law, in particular inheritances).
There were, however, exceptions: rights which legitimated children did not enjoy to the detriment of children of the same parents but born in wedlock.
One was the ability to succeed to fiefs, based on a prescription of the Liber Feudorum (II LF 26.11): "naturales filii, licet postea fiant legitimi, ad successionem feudi nec soli, nec cum aliis admittuntur". The Liber Feudorum, which appears in written form in the 11th c. and was complemented with various Imperial edicts, was a source of feudal (originally longobardic) law mainly for Germany and Northern Italy, but was also used as secondary source in other parts of continental Europe. However, Glück adds that commentators had limited the interpretation of this law to those legitimated by prince's rescript, although he doesn't believe that this is a valid interpretation, and that the nature of fiefs requires legitimate birth. He also cites in his favor a number of 18th c. authors (Jo. Go. Bauer: de successione legitimatorum per nuptias exule in feudis, Leipzig 1734; Ge. Henr. Ayrer, de exclusione legitimatorum a successione feudali, Gottingen 1755; and the very aptly titled book by Jo. Ge. Estor: Dissertatio de generato extra nuptias cum equistri vel plebeia, matrimonio licet ante partum vel conjugio post illum secuto, in feudis nec jure germanico nec longobardico succedente, Marburg, 1771; Schorcht: von der Unfähigkeit der Mantelkinder zur Lehnfolge, Iena 1780; etc.).
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On this point, Heinrich Zöpfl (Grundsätze des gemeinen deutschen Staatsrechts, §233-235) is more categorical. He claims that the old German law (e.g., Sachsenspiegel, c. 377) agreed with canonical law, and that the restrictive interpretation of the Liber Feudorum was legitimate. He further claims that, until the 18th c., the literature was unanimous in considering legitimated children apt to succeed. For him, there was no general principle excluding such children in the nobility.
Glück cites other rights denied to legitimated children: the right of first-born, when the father already has sons born in wedlock at the time of legitimation, succession to family fideicommis (estates subject to entails), succession to german Reichslands.
Legitimation by subsequent marriage can be found in Wurtemberg, the
Prussian ALR, the Austrian Allgemeines Bürgerliches Gesetzbuch
(ABGB) of 1811, the Saxon bürgerliches Gesetzbuch of 1863,
the Entwurf eines bürgerlichen Gesetzbuches of Hesse-Darmstadt
of 1842-53, Nürnberg, Franken, Hohenlohe, etc. These codes followed
the broad applicability of legitimation (to adulterous children) of the
earlier common law; but with exceptions. The original ALR excluded
adulterous children when the adultery had been the cause of the break-up
of the original
marriage, but that was because marriage between the adulterers was
forbidden. A law of 1875 made dispensation available in this case.
Also, the Bavarian Landrecht limited legitimation to naturales children,
and an Austrian law of 1791 required that the parents had been free to
marry at the time of birth, but that was not retained in the GB of 1811,
and a court decision of 1890 explicitly accepted that spurii could
be legitimated by subsequent marriage.
The effect of legitimation by subsequent marriage was usually complete (ALR of 1794: II 2 § 596; Austrian GB of 1811 §161, 752, 756; Saxon GB of 1863 §1780, 2018, 2025; Hesse-Darmstadt Entwurf III 60; also Weimar by a law of 6 Apr 1833, Altenburg by a law of 6 Apr 1841, Gotha by a law of 2 Jan 1844, Reuss cadet line by a law of 10 Dec 1853). I have not been able to check all of these regional codes, but here are some cases:
The law of 16 Dec 1997 (BGBl 2942) eliminates all difference in civil law between legitimate and illegitimate children; consequently, legitimation became an empty concept and §1719 of the BGB was removed.
Rehm is of a different opinion: for him, no legitimated child is apt to succeed. A number of house laws make that requirement explicit: the house laws of Oldenburg and Saxe-Coburg-Gotha say: "aus ebenbürtiger Ehe durch rechtmäßige Geburt abstammen." He also interprets less specific requirements in the same sense. That the house laws of Bavaria, Saxony, Württemberg, Waldeck, require "durch/aus rechtmäßige/ebenbürtige Ehen" must be interpreted to mean that the birth itself must be grounded in a marriage. The fact that the constitution of Bavaria makes the legitimate birth requirement is to him proof that this is the implicit meaning of those house laws. This seems to be the majority opinion of publicists (Schulze, Zachariä, Rönne, etc.)
Examples of house laws requiring legitimate birth: