Pivotal Volumes In Future Histories

Should a future history series have a pivotal volume? If there are earlier and later periods, then something can happen between them. The pivotal volume of Robert Heinlein's Future History would have been the one between The Green Hills Of Earth and Revolt In 2100 but those "stories to be told" never did get told so that, instead, there is an abrupt transition from the interplanetary economic imperialism of "Logic of Empire" to the American theocracy and interregnum of space travel in "If This Goes On -".

In Poul Anderson's Psychotechnic History, there is a gap between the disintegration of the Solar Union in "Brake" and the already established faster than light interstellar travel of "Gypsy." In Larry Niven's Known Space series, maybe A Gift From Earth is pivotal because it is set on an extra-solar colony planet and ends with the approach of an Outsider that will sell the hyperdrive to another human colony.

In Anderson's Technic History, the pivotal volume is The Earth Book Of Stormgate in the original publication order but Rise Of The Terran Empire in the fictional chronological order of Baen Books' The Technic Civilization Saga. At the mid-point of Rise..., the Afterword to the Earth Book is followed by the Introduction to the seminal story about the Founder of the Terran Empire. Pivotal indeed.