Definition
In Roman and civil law, substitutio is the testamentary appointment of one person as heir in place of another, to take effect if the first-named heir does not or cannot accept the inheritance. The device allowed a testator to designate a fallback heir — or a succession of fallback heirs — so that the estate would not pass intestate simply because the primary heir predeceased the testator, refused the inheritance, or was otherwise incapable of taking it.
The classical formula was: si ille hæres non erit, ille hæres esto — "if such a one will not be heir, let such a one be heir." A testator could chain substitutions, appointing one heir after another in sequence, each taking only if all those named before them failed to inherit.
Recognized Forms
/SUBTYPES
Roman law recognized several distinct forms of substitutio, each serving a different purpose:
Substitutio vulgaris (common or ordinary substitution): The standard form — a fallback heir named in the event the primary heir fails to take. This is the core meaning captured by Burrill.
Substitutio pupillaris (pupillary substitution): A substitution made on behalf of a minor child (pupillus) who was incapable of making a will. The paterfamilias, while instituting the child as heir, simultaneously appointed a substitute to take the child's estate if the child died before reaching the age of testamentary capacity. This was effectively a will made for the child.
Substitutio quasi-pupillaris (quasi-pupillary substitution): An extension of pupillary substitution developed under later Roman law and retained in some civil law systems, applying to persons of any age who were permanently incapable of making a will due to mental incapacity.
Why It Matters in Research
Researchers working in Roman law sources, civil law jurisdictions, or early American and colonial legal materials derived from civil law traditions will encounter substitutio as a technical term of art. Several navigational points matter:
First, the term does not translate cleanly into common law vocabulary. Common law achieved similar outcomes through different instruments — contingent remainders, executory devises, and later, alternative beneficiary designations. When historical sources use substitutio or its English calques ("substitution," "substitute heir"), a researcher should not assume the full common law property framework applies. The civil law concept is heir-centered, not estate-centered in the common law sense.
Second, the pupillaris and quasi-pupillaris subtypes appear in Justinian's Institutes and Digest and resurface in treatises on civil law that circulated among American lawyers in the eighteenth and early nineteenth centuries. Sources touching on Louisiana law, Spanish land grants, or any jurisdiction with a civil law inheritance tradition may invoke these subtypes without explanation.
Third, Burrill's entry is truncated — the source text cuts off mid-sentence. Researchers relying on Burrill alone for the full doctrine will need to supplement with the Institutes directly (Inst. 2.15) or with a more complete civil law treatise such as those of Domat or Pothier, which were widely available and cited in American courts on civil law questions.
Fourth, the chain-substitution device — appointing heirs in series — has modern relevance in jurisdictions that retain fideicommissary substitution (a related but distinct form in which the first heir is required to preserve and pass property to the second). Do not conflate simple substitutio with fideicommissum; they operate differently and have different histories.
Historical Dictionary Support
Burrill's entry correctly identifies the essential structure of substitutio: one person placed in the position of another for purposes of inheritance, triggered by the first heir's failure to take. The classical formula Burrill quotes — si ille hæres non erit, ille hæres esto — is drawn directly from Justinian's Institutes (2.15 pr.) and is accurate.
The limitation of Burrill's entry is its incompleteness. The text as reproduced breaks off before addressing the pupillaris and quasi-pupillaris forms, which are the more technically demanding variants and the ones most likely to appear in disputed succession contexts. A researcher finding only Burrill's entry has a solid foundation for vulgaris substitution but an incomplete picture of the doctrine as a whole.
No other historical dictionary entries were available for comparison. Researchers should treat the Institutes and the Digest as the primary sources for this term, with Domat's Civil Law and Pothier's Treatise on Obligations as secondary authority where those texts were cited by the relevant court or jurisdiction.
Jurisdictional Note
Substitutio as a formal term of art remains operative in Louisiana succession law and in other mixed civil law jurisdictions. In purely common law jurisdictions, the concept has no direct procedural equivalent, though analogous outcomes are achieved through will drafting rather than a named legal institution.