Anderson has been producing Time Patrol yarns--all eight are collected here, plus an original--since 1954. The idea is that, since time is mutable, anyone possessing a time machine can change history, by accident or design; so the Danellian supermen who will evolve a million years hence have set up the Time Patrol in order to ensure that the past that leads to their development will actually occur. Unattached Agent Manse Everard is free to roam through time, tackling whatever problems arise and straightening kinks in the historical record: in A.D. 464, a do-gooder from the third millennium changes history in order to improve on what actually happened; a friend of Everard's accidentally becomes Cyrus the Great of Persia; Mongols invade America; Carthage defeats Rome; a time researcher changes the destiny of his 4th-century Goth subjects; 31st-century Exaltationist Merau Varagan tries to destroy 9th-century B.C. Phoenicia; Germans war with Romans in A.D. 70; and Varagan, again, meddles in the New World of the 16th century. Absorbing, accurate historical backdrops add depth and conviction to yarns that otherwise are neither startlingly original nor particularly rigorously worked out.