Elwood's Continuum

Yes. I checked. See Sequel. Continuum, 1-4, edited by Roger Elwood, was a series of anthologies of new additions to already existing series written by their respective authors. See here. Poul Anderson's four additions to his already existing Rustum History were later collected in New America and the new instalments by the other authors were likewise collected so that the original Continuums have become redundant.

This explains some odd features of the Anderson stories. Written to order, they are uncharacteristically didactic. In these four successive instalments, Danny/Dan/Daniel Coffin systematically ages, solves problems of qualitatively different kinds - technical, scientific, personal, economic and political - and steadily grows in social stature, out of the shadow of his imposing father into the most important single individual in the Rustumite lowlands.

Elwood has been accused of editing mediocre anthologies but we are glad that he facilitated the writing of two excellent short stories that fitted into the chronology of Anderson's Technic History.