Roman’s had a strong sense of heredity. They respected their noble ancestors and, in fact, worshipped them. The images of the family’s dead patriarchs were carried at every aristocratic family funeral and displayed the rest of the time in the atrium, the great reception hall of the family home. The faces of the dead ancestors reminded visitors that they were visiting the representative of the genius or spirit of one of the great noble houses – a man whose ancestors held consulships, led great armies and changed history.
The oldest living ancestor of any branch of a family was called its paterfamilias, the father of the family. In prehistory he had the powers of an Old Testament Patriarch, and his sons literally lived in his household. This had all gone; noble Romans maintained their own households, even if the father or grandfather were still living. But the paterfamilias still wielded enormous authority, and still theoretically had power of life and death over his children – a father actually killed a treasonous son in the year of Augustus’ birth.
So in planning his succession a Roman was looking to carry on the great line of his ancestors as much as, or even more than, his own private bloodline. If he could not provide himself with a son, choosing an heir descended from his father or grandfather would honour their memory, maintain the bloodline of the ancestors and help transmit the family genius.
We can see this in operation in the case of Augustus himself. He called himself the son of Julius Caesar – as he was, by adoption. Caesar never had a legitimate son, and his daughter predeceased him. He had a son, Caesarion, by the Egyptian Queen Cleopatra, but he had no intention of making him his heir at Rome. Instead, in his will, he adopted his sister’s grandson. This followed the two principles GIRLS ARE MUMS and DADS ARE BEST; because he could not carry on his father’s line himself, he carried it through his sister.
In 44 BC Julius Caesar was assassinated. The young man he adopted, Gaius Octavius, immediately changed his name to Gaius Iulius Caesar Octavianus – the correct way to show he was now a Julius adopted from the Octavius family. In 27 BC this young man was granted the title Augustus, which he used for the rest of his life.
What about the children of Caesar’s only daughter Julia? He had made a political match for her; she married Pompey the Great who adored her, but she died young in 54 BC leaving no children. After her death, Caesar and Pompey fought each other in Caesar’s Civil War (49-45 BC), which ended with Pompey’s defeat and death. Had Julia lived and had a son, maybe things would have been different.