Don't be put off by the gimmicky theme or the editors' blathery introduction. This is a provocative and well-planned anthology. The best of the 15 stories are the older ones, notably Simak's ""Eternity Lost"" (the politics of selective immortality) and Damon Knight's ""Dio"" (a deathless society challenged by an unexpected case of mortality). Other works of the same vintage are Tom Godwin's celebrated ""The Cold Equations"" and Avram Davidson's ""Now Let Us Sleep"" (further triumphs of the race which brought you the demise of the passenger pigeon). Among the more recent contributions, J. G. Ballard's poker-faced variant on the time-reversal theme, one of Michael Bishop's future-Atlanta stories, and Gregory Benford's ""Knowing Her""--a glance at the eventual cost of eternal youth--are especially fine. That rare achievement: a ""theme"" anthology that works.