I’m reading I, Claudius by Robert Graves, and I’m really having trouble wrapping my head around the Julio-Claudian family tree. I kind of know why Tiberius passed Augustus, but I don’t know what happened to Augustus’s three natural sons, and I can’t see why Germanicus wouldn’t have come after Tiberius, and then Nero after him. And that begs the question of how Claudius became emperor in the first place (I know that’s the point of the book, but I’m not done with it, I’m at the part where Augustus just died). A concise explanation or simplified family tree (google images does NOT help) would be greatly appreciated.
that begs the question
I can’t see why Germanicus wouldn’t have come after Tiberius, and then Nero after him
There's really not much to say here. Germanicus was clearly being groomed for the imperial seat, but he could not possibly have succeeded Tiberius simply because he died before Tiberius, in 19 AD. Germanicus was adopted by Tiberius at the suggestion of Augustus, even though Tiberius already had a son in Drusus. If it was not already clear enough that the imperial line was to pass through Germanicus' branch, Germanicus' marriage to Agrippina made it beyond question.
Augustus’s three natural sons
Augustus had no natural sons. After the death of Marcellus, Augustus adopted Agrippa's two sons, C. and L. Caesar. When they died young, he adopted Tiberius and Agrippa's last son Agrippa Postumus. But initially the succession was clearly to skip a generation, past Agrippa (who was by then quite old, and who died in 12). Even had Claudius been a legitimate contender, which he was not, he was from the wrong branch and the precedent for following the same line to the youngest reasonable direct descendants was already established.
A note: I, Claudius is fiction. In many parts it is not even particularly firmly based on the texts. Claudius the God is even more speculative. Graves notes in his introduction in particular that the machinations of Livia regarding Tiberius are basically his imagination.