One of the difficulties of Dickson’s translation is the use of the word ‘burgess’ where we should prefer ‘citizen.’ ‘Burgess’ sounds archaic and suggests a kind of office to English speakers, but Mommsen talked about der Bürger und der Nichtbürger, citizen and non-citizen. These definitions become clearer in chapter six when he discusses the metics (Mommsen borrows the Greek term for foreign workers) and freed slaves who made up the body of the clients of Rome’s citizens and, in time, spawned the social class known as the plebs.
He begins, as usual, with conjectures about the joining of the apparently rival communities centered around the Palatine and Quirinal, and discusses again various sorts of doubling in religion and the military. All of this strikes me as convenient for his earlier arguments but ultimately unimportant.
The non-citizens, in their position as clientes, depend upon the citizens (and despite Mommsen’s derivation from a root meaning ‘listen’, clients rather ‘lean’ upon their patrons). They depend upon them because they have no rights under the law. He’s right that the plebs referred to the multitude, and it’s reasonable to assume, as he does, that a new social class might arise when the ancient dependencies were obscured. Mommsen sees a gradual diminution of the importance of families as against the state, and a natural tendency toward an acknowledgement of the rights of dependents: as the patron-client relationship faded (along with a growing imbalance of numbers), the lack of rights would have been felt more strongly.
The famous Servian reforms, he suggests, were the result, placing upon certain non-citizens some of the responsibilities of citizens (e.g., military service).
He follows this through with numbers and arguments for how and why these various divisions and levies occurred, but it’s enough to say that the narrative makes sense as we move from his reconstruction to the historical Rome. For example, he makes this the impetus for the census, and posits that the new classes existed as cives sine suffragio (‘citizens without the vote’).