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Charles Darwin kept a diary on the voyage of the Beagle in which he recorded his main occupations on an almost daily basis, thus conveying a sense of immediacy and allowing the reader to partake step by step in his experiences as the voyage unfolds. He sent sections regularly back through packet boats e.g. to his sister, Caroline Darwin, for sharing with family and friends. A new edition was edited by Richard Darwin Keynes and first published in 1988. The Beagle Diary traces the voyage from 1831 to 1836 transcribing the originally 751 ink written pages and contains accounts of every step of this often very exacting trip.

Darwin suffered quite badly from seasickness, but refers to it with characteristic understatement. In any event, even when he noted being idle, he actually kept himself busy with extraordinary dedication for studying the geology, the plants, animals and people he encountered during the years of this remarkable voyage. Early on, on the way to Cape Verde, he built a plankton net sketched in the diary and has this entry for 10th January 1832: "... I proved to day the utility of a contrivance which will afford me many hours of amusement & work. - it is a bag four feet deep, made of bunting, & attached to [a] semicircular bow this by lines is kept upright, & dragged behind the vessel. - This evening it brought up a mass of small animals, & tomorrow I look forward to a greater harvest." The editor comments that it "appears to be only the second recorded use of a plankton net, the first being that of J.V. Thompson a few years earlier, of which Charles Darwin may have learnt from Professor Grant in Edinburgh." The following day he notes "I am quite tired having worked all day at the produce of my net. - The number of animal that the net collects is very great & fully explains the manner so many animals of a large size live so far from land. - Many of these creatures so low in the scale of nature are most exquisite in their forms & rich colours. - It creates a feeling of wonder that so much beauty should be apparently created for such little purpose."

The diary is full of commentary about the sea, the conditions of the coast, the gales and bad weather suffered during the time surveying the seas around Tierra del Fuego, the Falkland Islands and Chiloe on the Pacific side. Naturally, for a sail ship as the Beagle this was even more essential than for today's engine-driven vessels. He also undertook many long walks on foot as well as a number of long inland excursions, many on horseback, in Brazil, Uruguay, different parts of Argentina down to Patagonia, Chile and other countries. The descriptions of the different immigrants and native people are remarkably liberal and open-minded for his time. There are constant annotations and critical interpretations of his observations, showing that he did not take either the accounts of earlier travellers or of whalers or local people he encountered at face value. He does refer to other sources than direct observation, but does so always with an open, critical mind, sometimes accepting, sometimes rejecting them, when he can not reconcile them with his own observation and background knowledge.

Among the most remarkable analyses made during the voyage of the Beagle and sketched out in the diary concerned the formation of atolls. Even before seeing one with his own eyes, he had hinted at the underlying phenomenon of subsidence of volcanic craters, when he interpreted his observations of sea shells in the high South American cordillera as indicative of subsequent rise and subsidence of marine rocks over extended periods of time. When visiting the Cocos Islands (then named Keeling Islands) in the Indean Ocean, he rightly identified the atolls "as the summit of a lofty mountain". He realised that the rock-making coral polyps "continue to build upwards, as the foundations of the isld from volcanic agency, after intervals gradually subsides". He correctly identified that the encircling reefs of the Pacific islands, such as Tahiti and Eimeo with their central peaks, which were visited by the Beagle and mentioned in the diary, are precursors of atolls (page 723 of the original diary). In these atolls the volcanic craters and vents do not reach the surface any more. Instead the corals form an often very deep-reaching ridge or calcarous material of organic origin, keeping a balance between subsidence of the crater and build-up of the corals. 

The Beagle Diary is not very explicit on the sealife he observed and collected, however, when looking at his entire work and pursuing the hints further into his prolific other writing, Daniel Pauly elaborated an encyclopedia of "Darwin's Fishes" - see further below.

When he returned after more than five year's absence, Charles Darwin was already a celebrity, yet some of the most revolutionary insights from the study of the rich samples of rocks, animals and plants he had sent back throughout the voyage, were to come later.

The most scientifically oriented materials are contained in his scientific notebooks, which he used as important inputs for his later publications.

He entertained an extensive correspondence with family, friends and colleagues, in which he often voiced more private opinions. He also received from them critical comments and suggestions for future observations, as he himself acknowledged in the second edition of the Beagle Diary. The full text is available for downloading, chapter by chapter, here.

The map of the world showing the principal stations of the Darwin's circumnavigation on the Beagle can be seen">here.

The most private expressions can be found in his personal diary.