Back to Methuselah by George Bernard Shaw is a series of plays set in successive future periods. Therefore, this work is a British future history like The Shape of Things to Come by HG Wells, Last and First Men by Olaf Stapledon, A Short History of the Future by RC Churchill and Galaxies like Grains of Sand by Brian Aldiss. Like the novel, Methuselah’s Children, in Robert Heinlein’s (American) Future History, Back to Methuselah addresses the issue of the prolongation of human life. Like Last and First Men, it ends in a far future with an ultimate evolutionary stage of humanity.
Back to Methuselah is not classified as a science fiction (sf) future history partly, of course, because Shaw is not classified as an sf writer but mainly because of its different medium: stage drama, not prose fiction. However, screen drama makes original contributions to sf and Karel Capek’s stage play, R.U.R., contributed the term “robot." Star Trek, whose complete canon comprises both TV and cinema screen drama plus both prose and graphic fiction, approaches future history status and includes the TV episode, “Requiem for Methuselah,” about the Biblical Methuselah surviving into the interstellar period. Thus, there is a curious “Methuselan” conceptual trilogy of Shaw, Heinlein and Star Trek. The titles form a sequence.
Immortality, whether mutational or medical, is a major sf theme covered, for example, in One Million Tomorrows by another Shaw (Bob), This Immortal by Roger Zelazny and “Now That Man Has Gone” by James Blish. For a brief account of immortality in future histories by Heinlein, Blish, Poul Anderson and Larry Niven, see “Immortality”, here.