Literary Unity

Apparently, Balzac's The Human Comedy is bound together by a large cast of recurring characters weaving in and out of multiple narratives. By contrast, a future history series, essentially bound together by events, is not obliged to feature even one continuing character. Indeed, any series limited to a normal lifespan cannot count as a future history. A futuristic series becomes a future history series as soon as its later stories begin to refer to the events of earlier stories as historical events.

Poul Anderson's Technic History presents the best of three options:

a long, multiple period, future history series;
strong series characters in two of the historical periods;
a few "weaving" characters -

- Adzel first as a student on Earth, then as a continuing character in the trade pioneer crew sub-series;

Desai first on Aeneas, then much later in conversation with Flandry back on Terra;

Tachwyr, reappearing three times after first meeting Flandry on Merseia.

Dornford Yates, who will be our Lurker in the Background for a while longer, found another way to interweave characters. A major villain, "Rose" Noble, is killed so a member of his gang, Punter, works for other bosses in later novels. Thus, the unprepossessing Punter unites the Chandos series with some non-series novels and even eventually appeals to Chandos as "an old friend."

An equivalent would be if Flandry later ran into one of the Scothani working not for Terra but for Merseia but still claiming Flandry's friendship! Or, as Sean Brooks commented, a later meeting with an ennobled Leon Ammon would have delighted if not enlightened.