I HAVE OFTEN been tempted to try to write a play or a novel based on the life of Robert FitzRoy. FitzRoy was the British aristocrat and naval officer who commanded the Beagle during the five years it took the 74-man crew -- which included Charles Darwin -- to circumnavigate the globe.

When the Beagle left England Dec. 27, 1831, FitzRoy was 26 and Darwin 22, soon to turn 23; his birthday was Feb. 12, 1809. While the two men had in common that they were upperclass Englishmen -- FitzRoy was the son of a lord and Darwin of a prosperous country doctor and a grandnephew of the master potter Josiah Wedgwood -- they differed in most other ways, among them politically and theologically. FitzRoy was a committed Tory, a deep believer in slavery, and a rigid fundamentalist while Darwin leaned toward Whiggery, hated slavery, and had liberal Christian theological views, insofar as he had any.

Most historians agree that Darwin did not become an evolutionist until after the voyage. But by a year after the Beagle returned, Darwin had committed himself to evolution. FitzRoy appears to have blamed himself for being a party to loosing this heresy on the world. He had shown elements of mental instability, even on the voyage, and by 1860 he appeared at a British scientific meeting, where Darwin's theory was being debated, holding a Bible over his head and ranting "The Book, The Book." Five years later he shot himself.

Be that as it may, during the voyage the two men got on well. This is evidenced by many of the letters in this marvelous collection. Everything about the way this book has been put together is both pleasing and intelligent, from the careful and helpful notes, to the illustrations. The period the letters cover is from 1821, when young Master Darwin was 12 and was addressed by his family as either "Bobby" (his middle name was Robert) or "Charley" -- to 1836, just after the Beagle returned, when his three affectionate sisters who had written, in rotation, one letter each month, for five years, were still addressing him as "Charley." What makes the collection so special is that it includes not only letters Darwin wrote, but letters written to Darwin and even letters written about Darwin. Reading them is like watching a novel unfold with all sorts of plots and subplots as various characters, marry, die or simply disappear out of Darwin's life.

When Darwin left England on the Beagle, he was a young man with no special calling and no fixed prospects excepting the possibility of becoming a country parson somewhere where he could continue his passion for shooting. In an early letter he reports to a friend about a hunt, "In the first week I killed 75 head of game." But by the middle of the voyage, by which time Darwin's superbly insightful observations of geology, zoology, ecology and nearly everything else, had been filtering back to England. Adam Sedgwick, one of the foremost geologists of the time, wrote to Samuel Butler, the headmaster of Darwin's public school (and the grandfather of the author of Erehwon) who, in turn, sent the remarks to Darwin's father, who showed Darwin's sister, who quoted them to Darwin: "It was the best thing in the world for him" Sedwick had written, "that he went out on the Voyage of Discovery -- There was some risk of his turning out an idle man: but his character will now be fixed, & if God spare his life, he will have a great name among the Naturalists of Europe."

Indeed, as the tiny fragile Beagle makes its painfully slow way -- FitzRoy kept extending its mission from the original two years and might have continued to do so indefinitely if the crew had not all but rebelled -- around the world, running all sorts of risks, one's thought is how will its most valuable cargo -- Darwin himself -- ever survive? That he did is even more remarkable in view of the fact that Darwin spent months at a time away from the Beagle traveling in difficult and hostile country, often alone. We know, even as we read his letters, that he will survive, but this does not lessen our anxiety for him, and for the future of science.

Among the characters met through the letters there are the delightful Owen sisters Fanny and Sarah, neighbors of the Darwins in Shropshire. He must have saved all of their letters and they none of his, since only theirs to him are reprinted. In reading the letters one finds oneself, from the remove of a century and a half, falling in love with them, especially Fanny who called the young Darwin "Postillion" and was in turn called by him "Housemaid." What an imp was Housemaid. She writes to Darwin in Cambridge in 1828, "We have tried in vain to get your sisters here -- for a little larking & scandal . . ." Eligible young men are referred to as "shootables," the sisters' version of "suitables." Their letters are full of bubble and squeak, and so one is drawn up short when just, after the Beagle has sailed, Darwin gets a letter from his sister Catherine announcing that Fanny has become engaged to one Robert Myddelton Biddulph of Chirk Castle. Darwin's reaction is not recorded. He makes various kindly references to the now Mrs. Biddulph during the voyage. After it is over, when he brings flowers to her, they contain a note which begins "my Dear Mrs. Biddulph." Fanny remarks to Darwin's sister Catherine that she will answer with a note beginning "My dear Mr. Charles." So much for Housemaid and Postillion. Darwin's future wife, his cousin Emma Wedgwood, whom he married in 1839 is scarcely mentioned in most letters except as a possible interest of Darwin's older brother Erasmus who was an "idle man."

Once the voyage is begun, the personality of FitzRoy continually weaves in an out of the letters. FitzRoy, when he wrote to Darwin, always addressed him as "Philos" -- an affectionate abbreviation of Natural Philosopher. (The term "scientist" had not yet come into use.) Darwin, early in the voyage, wrote to his sister Caroline of the captain, "I never before came across a man whom I could fancy being a Napoleon or a Nelson. . . . His ascendancy over every-body is quite curious: the extent to which every officer & man feels the slightest rebuke or praise would have been, before seeing him incomprehensible." For five years Darwin lived in the uneasy company of this man. That their relationship lasted, and on good terms, for a full five years is due, I think, partly to the fact that Darwin spent so much time away from the Beagle and partly due to his wonderful character. As Mr. Owen, the father of Fanny and Sarah, remarked, "Yes, but who could quarrel with Charles?" The last letter in the book is from FitzRoy to Darwin and ends "I wish you a happy Christmas my good Philos." One eagerly awaits the sequel to this superb collection, if only to read what FitzRoy wrote after Darwin had become famous.