THE NATURALIST IN NICARAGUA by Thomas Belt (University of Chicago: $12.95, paperback; illustrated). Nineteenth-Century Britain (and, to a lesser extent, America) produced an extraordinary class of explorer-writers. Adventurous, curious and intellectual, those men forged the records of a world that was about to disappear forever. So it was with Thomas Belt, an English mining engineer by profession but a naturalist by inclination and his place in history. Having already visited Australia, Belt wrote this account of his travels in Nicaragua while crossing the Caucasus and proceeding to Tibet. That Belt was able to record the detail of these observations a century ago, on muleback, while traveling and living in extraordinarily difficult conditions, is itself fascinating enough for armchair geographers and romantics. But this journal is rather more: Charles Darwin himself relied upon some of the engrossing accounts, and praised it. Whether writing about ants or alligators, swamps or rain forests, Indians, mestizos, or Europeans, Belt's keen eye captured the detail and his keen mind captured the essence of what he was witnessing. Today, while political disturbance precludes much contemporary investigation, exploding populations and a world economy are ravaging the remnants of the Central American world. Belt's book is a superb introduction to the tropics; if we do not act decisively, it will soon be pure remembrance.