It is necessary to say a word here about the political position of the Emperor's kindred. The imperial house embraced the male and female descendants in male (agnatic) line from the founder of the dynasty; the wife of the Emperor; and the wives of the male descendants. Thus Livia and Julia belonged to the house of Augustus, but Octavia did not belong to it, nor Julia's children, until Augustus adopted them. The distinctive privilege possessed by members of the imperial house was that they were inviolable and sacrosanct like the tribunes. This right dated from the triumviral period, and thus is explained how it was that Octavia, though not one of the imperial house, possessed tribunician sacrosanctity. She had acquired it not as the sister of Caesar, but as the wife of Antonius. Soon it became the custom for the soldiers to take an oath of fidelity to the whole house of the Caesars; but this custom hardly existed under Augustus himself. Under the first Princeps the members of his house enjoyed few honours and privileges, compared with those which were acquired by them in later reigns.
It has been already seen that constitutionally the Emperor has no voice in appointing a successor to the Principate; for neither designation nor heredity was recognized. Augustus had to find a practical way for escaping this constitutional principle, and securing that the system which he founded should not come to an end on his own death and that he should have a capable successor. The plan which he adopted was an institution which had no official name, but which was equivalent to a co-regency. He appointed a "consort" in the imperial power. There was no constitutional difficulty in this. The institution of collegial power was familiar to Roman law and Roman practice; and the two elements of the imperial authority—the imperium and the tribunician power—could be held by more than one. But, at the same time, the consort was not the peer of the Emperor; he could only be subsidiary. There could be only one Princeps, only one Augustus. In fact, the consort held, in relation to the Augustus, somewhat the same position as the praetor held to the consul.
Thus from the necessity for making practical provision for the succession arose certain extraordinary magistracies,—proconsular and tribunician offices, which held a middle place between the Princeps on the one hand, and the ordinary magistrates on the other. On the death of the Princeps, the consort would have a practical, though not a legal claim, to be elected Princeps, and nothing short of revolution would, as a rule, hinder him from obtaining the highest position in the state.
The proconsular command was first conferred on the consort, the tribunician power subsequently. Under Augustus both powers were conferred for a limited number of years, but always for more than one year, which was the defined period for the ordinary magistracies. The consort had not command over the troops, like the Emperor, but it was common to assign him some special command. He did not bear the title of Imperator, and he did not wear the laurel wreath. Nor was he included in the yearly vows which were offered up for the Emperor. But he had the right to set up his statues, and his image appeared on coins.
Anyone might be selected as consort. But it was only natural that the Emperor should select his son for that position, and thus it became ultimately the recognized custom that the Emperor's son should become his consort. By this means the danger of elevating a subject so near the imperial throne was avoided, and the natural leaning of a sovran towards the foundation of a dynasty was satisfied. When the Emperor had no children, he used to adopt into his family whomsoever he chose as his successor, and the danger of such a course was mitigated by the paternal power which he possessed over his adopted son.
It was some time, however, before this usage became a stereotyped part of the imperial system. The first consort of Augustus was Agrippa, who married his niece Marcella. The proconsular imperium was conferred on Agrippa, sometime before 22 B.C., but Augustus had certainly no intention that Agrippa should be his successor. He was compelled to assign a distinguished position to his invaluable and ambitious coadjutor,—to take him into a sort of partnership,—in order to secure his cheerful service. But circumstances brought it about that he came to be regarded, if not as the probable successor, yet as something very like it.
As Livia proved unfruitful, Augustus had to look elsewhere for a successor. Within his own family three choices were open to him. Though he had no sons, he might at least have a grandson by the marriage of his daughter Julia. Or he might select his sister's son as his heir and successor. Or he might adopt his Claudian step-children.
His first plan, the marriage of the young Marcellus with Julia, combined two of these courses. The Empire might thus descend through a nephew to grand-children. High hopes were formed of Marcellus, who was attractive and popular and a great favorite of his uncle. The marriage was celebrated in 25 B.C., during the absence of Augustus in Spain, where he suffered from a severe illness, and Agrippa, the brother-in-law of the bridegroom, was called upon to act as the father of the bride. In the following year, Marcellus was elected curule aedile, and a decree of the senate allowed him to stand as candidate for the consulship ten years before the legal age. At the same time Augustus allowed his stepson Tiberius to be elected quaestor, though he was even younger than Marcellus; and this perhaps was a concession to Livia, who may have felt jealous of the son of Octavia and the daughter of Scribonia.
But there was another who certainly felt jealous of the favor shown to Marcellus, and regarded him as an unwelcome rival. This was Agrippa. He had entered, as we have seen, into affinity with the imperial family by his marriage with Marcella; he had been consul, as the Emperor's colleague for two successive years. If Augustus was the Princeps, men were inclined to look upon Agrippa as the second citizen; and in the East, where political facts were often misinterpreted, he was actually thought to be an equal co-regent with the Emperor. He was not popular, like his young brother-in-law, but he was universally respected; his services were recognized, and his abilities were esteemed; and he had every reason to cherish ambitious aspirations. Augustus had left Rome in 27 B.C. in order to devote his attention to the administration of Gaul and Spain. During his absence, which lasted until 24 B.C., there were no disturbances in Rome, although he left no formal representative to take his place. This tranquillity must have been partly due to the personal influence of Agrippa, who lived at Rome during these years, though not filling an official post.
In 23 B.C., the year of his eleventh consulate, Augustus was stricken down by another illness, and he seems to have entertained some idea of abdicating the imperial power. He summoned his colleague, the consul Piso, to his bedside, and gave him a document containing a list of the military forces, and an account of the finances, of the Empire. This act of Augustus displays the constitutional principle, that when the Emperor died, the imperial power passed into the keeping of the senate and the chief magistrates. But Augustus, although he could not appoint, could at least recommend, a successor; and it is to his honor that he did not attempt to forward the interests of his family at the expense of the interests of the state. Marcellus was still very young, and his powers were unproved. Augustus gave his signet-ring to Agrippa, thus making it clear whom he regarded as the one man in the Empire capable of carrying on the work which he had begun. But Augustus was not to die yet. He was healed by the skill of the famous physician Antonius Musa. On his recovery, he learned that his illness had been the occasion of unfriendly collisions between Agrippa and Marcellus. While Marcellus naturally built hopes on his marriage with Julia, Agrippa was elated by the conspicuous mark of confidence which the Emperor had shown in him at such a critical moment. Augustus, therefore, thought it wise to separate them, and he assigned to Agrippa an honorable mission to the eastern provinces of the Empire, for the purpose of regulating important affairs in connection with Armenia. The proconsular imperium was probably conferred on him at this time. Agrippa went as far as Lesbos, but no further, and issued his orders from that island. His friends said that this course was due to his moderation; others suspected that he was sulky, and it is clear that he understood the true meaning of his mission.
But an unexpected and untoward event suddenly frustrated the plan which Augustus had made for the succession, and removed the cause of the jealousy of Agrippa. Towards the end of the same year, Marcellus was attacked by malaria at Baiae, and the skill which cured his father-in-law did not avail for him. He was buried in the great mausoleum which Augustus had erected some years before in the Campus Martius, as a resting-place for his family. The