Gradually the rules originating in this way were extended to the intercourse of citizens with citizens, in cases where the rigorous conditions of jus civile were not exactly satisfied, and so precepts of jus gentium were transferred from the edict of praetor peregrinus to the edict of praetor urbanus.
The portion of the edict most fertile in principles of jus gentium would be the clauses in which the praetor announced, as he did in some cases, that he would instruct the judex, whom he appointed to hear and determine a controversy, to govern himself by a consideration of what was aequum et bonum, i. e. by his views of equity and expediency: and if any of the oral formularies of the earliest system of procedure (legis actiones) contained these or equivalent terms, such formularies may be regarded as a source of jus gentium. It may be observed that Gaius does not, like some other Roman jurists and notably Ulpian (cf. Dig. 1, 1, 1, 3; Inst. 1, 2 pr.), make any distinction between jus gentium and jus naturale. There is nothing in his writings, as they have come down to us, to draw attention to the fact that the teaching of nature may not be in accordance with the practice of nations, as the institution of slavery showed.
Another organ of quasi publication, whereby the rules of jus gentium were transformed from ideal law to positive law—from laws of Utopia to laws of Rome—were the writings of the jurists, who, at first with the tacit, afterwards with the express permission of the legislature, engaged, nominally in interpreting, really in extending the law, about the time of Cicero (De Legibus, § 1, 5), transferred to the edict of the praetor the activity which they had formerly displayed in developing the law of the Twelve Tables and the statutes of the Comitia. By these means, supplemented and confirmed by statute law and custom, the jus gentium gradually increased in importance, and gave the Roman empire its universal law.
Jus civile, i. e. jus civium or law peculiar to citizens, was the law of the Twelve Tables, augmented by subsequent legislation, by juristic interpretation, and by consuetudinary law. The institutions of jus civile may be exemplified by such titles to property as Mancipatio and In Jure Cessio, contracts by the form of Nexum and Sponsio, title to intestate succession by Agnatio or civil relationship; while corresponding institutions of jus gentium were the acquisition of property by Tradition, contract by Stipulation without the solemn term Spondeo, title to intestate succession by Cognatio or natural relationship. Other departments of life were not subject to parallel institutes of jus civile and jus gentium, but the mutual relations of citizens with citizens as well as of citizens with aliens were exclusively controlled by jus gentium: e. g. the informal contracts called Consensual, such as buying and selling, letting and hiring, partnership; and the informal contracts called Real, such as the contract of loan for use or loan for consumption.
Titles to ownership (jus in rem), according to jus gentium, which ultimately superseded civil titles, are explained at large in Book II.
In respect of Obligation (jus in personam), jus gentium may be divided into two classes, according to the degree in which it was recognized by Civil law:—
A. A portion of jus gentium was recognized as a ground of Action. To this class belong (1) the simple or Formless contracts to which we have alluded, (2) obligations to indemnify grounded on delict, (3) rights quasi ex contractu to recover property when it has been lost by one side and gained by the other without any right to retain it. Dig. 12, 6, 14 and Dig. 25, 2, 25. Actions founded on this obligation to restore (condictiones), although it was a species of naturalis obligatio, Dig. 12, 6, 15 pr., were as rigorous (stricti juris) as any in the Civil code. In these cases the obligatio, though naturalis as founded in jus gentium, yet, as actionable, was said to be civilis obligatio, not naturalis, Dig. 19, 5, 5, 1.
The two eminently Civil spheres of the law of obligation were (1) specialty or Formal contracts, and (2) penal suits. Yet even into these provinces jus gentium forced a partial entrance. We shall see that aliens could be parties to a Stipulatio or Verbal contract, though not by the Civil formulary, Spondeo 3 § 93; and to Transcriptio, at least of one kind, 3 § 133, which was a form of Literal contract; and could be made plaintiffs or defendants in penal suits by means of the employment of certain Fictions, 4 § 37. This, however, was rather the extension of jus civile to aliens than the intrusion of jus gentium into a Civil province.
B. Other rights and obligations of jus gentium were not admitted as direct grounds for maintaining an action, yet were otherwise noticed by the institutes of civil jurisprudence and indirectly enforced. Thus a merely naturalis obligatio, though not actionable, might (1) furnish a ground of an equitable defence (exceptio): for instance, on payment of a merely natural debt the receiver has a right of retention, and can bar the suit to recover it back as a payment made in error (condictio indebiti soluti) by pleading the naturalis obligatio, Dig. 12, 6, 64; or the defendant can meet a claim by Compensatio, 4 § 61, cross demand or set-off, of a debt that rests on merely naturalis obligatio, Dig. 40, 7, 20, 2: or a merely naturalis obligatio might (2) form the basis of an accessory obligation, such as Suretyship (fidejussio) 3 § 119 a, or Guaranty (constitutum) Dig. 13, 5, 1, 7, or Mortgage (pignus) Dig. 20, 1, 5 pr., or Novation, 3 § 176, Dig. 46, 2, 1, 1, all institutions, which are themselves direct grounds of action. Though these rights and obligations of natural law are imperfect (obligatio tantum naturalis) as not furnishing immediate grounds of action, yet, as being partially and indirectly enforced by Roman tribunals, they clearly compose a portion of Positive law. Cf. 3 §§ 88, 89 comm.
§ 3. Plebiscites as well as the enactments of the Comitia populi were called Leges, and were named after the tribunes by whom they were carried, as the leges proper (rarely called populiscita) were named after the consul, praetor or dictator by whom they were carried. Thus Lex Canuleia, Lex Aquilia, 3 § 210, Lex Atinia, Inst. 2, 6, 2, Lex Furia testamentaria, 2 § 225, were plebiscites named after tribunes, while the Lex Valeria Horatia was named after two consuls, the Lex Publilia and Lex Hortensia were named after dictators, the Lex Aurelia, 70 b. c., after a praetor. (As to the history of plebiscita and leges and of the other sources of Roman law cf. Historical Introduction and see Smith’s Dict. of Greek and Roman Antiquities, 3rd ed. s. v.)
§ 4. The legislative power of the senate was in the time of the republic a matter of controversy. It is certain that it had a power of issuing certain administrative decrees or instructions to magistrates that was hardly distinguishable from legislation. Under the emperors matters were changed. Legislation by the Comitia, though spoken of by Gaius in the present tense, had ceased to be a reality after the time of Tiberius, and the last recorded lex was passed in the reign of Nerva. As early as the time of Augustus the auctoritas of the senate began to be regarded as the essential process in making a law, and the subsequent rogatio of the Comitia as a mere formality, which was finally omitted. Senatusconsults, like laws, were sometimes named after the consuls who proposed them, though this is not in their case an official designation; they are sometimes even called leges: thus the measure which Gaius calls Sc. Claudianum, § 84, is subsequently referred to by him under the name of lex, § 157, 4 §§ 85, 86. Ulpian says, Non ambigitur senatum jus facere posse. Dig. 1, 3, 9. Of course, these senatusconsults were merely a disguised form of imperial constitution. The sovereignty had in fact passed from both patricians and plebeians to the hands of the princeps. A measure was recommended by the emperor in an oratio or epistola to the senate, and then proposed by the consul who convoked the senate, and voted by the senate without opposition. Hence a senatusconsult is sometimes called oratio, e. g. oratio divi Marci, Dig. 2, 12, 1 pr. Even this form was finally disused. No senatusconsult relating