Two Tetralogies

James Blish's Cities In Flight Tetralogy holds up well as a future history series until somewhere in Volume III. Beyond that point, the antiagathics keep a small group of interstellar travelers alive indefinitely while, we are informed, centuries are elapsing although meanwhile we have lost all sense of chronology.

In Poul Anderson's Harvest Of Stars Tetralogy, either Anson Guthrie or one of his copies, whether inorganic or organic, is always present except in the shorter Volume III, Harvest The Fire, which is set in the Solar System at a time when Guthrie in whatever form is elsewhere. Although this multiplicity of Guthries generates an illusion of personal continuity, the original Guthrie had died before the beginning of Volume I and is seen only in reminiscences and flashbacks. In Volume IV, an organic Guthrie decides to send an inorganic copy of himself to the Solar System but he himself necessarily remains where he is. Each Guthrie has only a finite lifespan.