According to Larry Niven, time travel is logically impossible--sheer fantasy. So when time-agent Svetz heads back from polluted future Earth in search of extinct animals, he tends to sideslip into fantastic, fictional worlds. In short stories collected in The Flight of the Horse (1973), his quests for a horse, Gila monster and whale unearthed a unicorn, a dragon and Moby Dick...Less comic but equally daft, Rainbow Mars combines both space and time travel to explore Mars in the deep past, before it was a dead world. Naturally it's populated by a menagerie of warring fictional Martians from Edgar Rice Burroughs (multi-armed sword-wielders), H.G. Wells (tentacles and heat rays) and less familiar authors. Svetz and companions are soon in big trouble. Complications include a gigantic alien tree extending into Mars' orbit--an organic version of the space elevator from Arthur C. Clarke's The Fountains of Paradise. One of these useful "beanstalks" on Earth seems a highly desirable facility, but there are hidden drawbacks, and most of the multiplying time-lines lead to disaster...Fun for experienced sf readers who can follow the in-jokes and the switchback ride through tangled alternative histories. The earlier, funnier Svetz stories are included as a bonus.