Two Kinds Of Series

One kind of series goes somewhere. Another does not. Some TV series have episodes that could be shown in any order. When someone cited I, Robot as an example of a series that went nowhere, I argued at length that Asimov's I, Robot went all the way from the mute humanoid nursemaid, Robbie, to the Machines controlling human destiny. The other guy heard me out, agreed, then said that he had meant Otto Binder's I, Robot.

Poul Anderson's Time Patrol series goes a long way. Originally, it was four short stories collected as Guardians Of Time. Then, over a decade later, new installments began to be added.

The original tetralogy gave us:

Manse Everard;
the Time Patrol;
the Danellians;
Sherlock Holmes;
a brief encounter with a non-humanoid Danellian;
the Neldorians;
the Carthaginian timeline;
causality violation caused only by time travelers (this is explicitly stated);
world-lines like tough rubber bands that spring back to their original shape.

Later installments added:

Wanda Tamberly;
more about other Patrol agents;
one passage set in the future;
the Exaltationists;
Guion;
the Altamont case (Holmes using an alias);
the alpha and beta timelines;
the quantum timeline;
random fluctuations in space-time-energy;
a personal causal nexus;
a peaceful humanoid Danellian;
the real meaning of the Patrol.