The point of the voyage of the Beagle was to make an accurate survey of the coastal areas of South America, and to run a chain of chronometric readings around the world. The making of geological maps of all of the regions visited was also on the “to do” list for this expedition. Darwin was well matched for this job, because his main interest and to a large degree his primary training was geology. All this stuff about Evolution and Natural Selection was to come later. Darwin knew he would be making extensive collections of biological materials and prepared for this by seeking advice from his mentors on the methods of preservation and shipping of preserved plants and animals, and he was keenly interested in observing the natural world during his entire trip. I don’t mean to suggest otherwise. But nonetheless it is true that geology was both his focus and his job on the voyage. Of the twenty four notebooks we know about of Darwin’s, from this voyage, most of the written material is about geological observations.
The story of Darwin’s invitation to join Captain Fitzroy on the Beagle is a long, complicated, and fascinating one. You can read about it in Darwin’s Autobiography, and several scholarly works, as well as various fictionalized accounts. For example, there were issues with his father:
ON RETURNING home from my short geological tour in N. Wales, I found a letter from Henslow, informing me that Captain Fitz-Roy was willing to give up part of his own cabin to any young man who would volunteer to go with him without pay as naturalist to the Voyage of the Beagle. … I was instantly eager to accept the offer, but my father strongly objected, adding the words fortunate for me,—”If you can find any man of common sense, who advises you to go, I will give my consent.” So I wrote that evening and refused the offer. On the next morning I went to Maer to be ready for September 1st, and whilst out shooting, my uncle sent for me, offering to drive me over to Shrewsbury and talk with my father. As my uncle (Josiah Wedgwood) thought it would be wise in me to accept the offer, and as my father always maintained that he was one of the most sensible men in the world, he at once consented in the kindest manner. I had been rather extravagant at Cambridge and to console my father said, “that I should be deuced clever to spend more than my allowance whilst on board the Beagle”; but he answered with a smile, “But they all tell me you are very clever.”
Darwin, C. R. 1958. The autobiography of Charles Darwin 1809-1882. With the original omissions restored. Edited and with appendix and notes by his grand-daughter Nora Barlow. London: Collins. (Pages 71-71) [all further quotes below from the same volume]
As I said, the story of how Darwin ended up on the Beagle is very complicated, with many close calls by which he would not have gone, but my favorite is related here by Darwin in his Autobiography. (In fact, I’ll provide an extended exerpt to give a much more detailed picture of Fitzroy):
Afterwards on becoming very intimate with Fitz-Roy, I heard that I had run a very narrow risk of being rejected, on account of the shape of my nose! He was an ardent disciple of Lavater, and was convinced that he could judge a man’s character by the outline of his features; and he doubted whether anyone with my nose could possess sufficient energy and determination for the voyage. But I think he was afterwards well-satisfied that my nose had spoken falsely.
Fitz-Roy’s character was a singular one, with many very noble features: he was devoted to his duty, generous to a fault, bold, determined, indomitably energetic, and an ardent friend to all under his sway. He would undertake any sort of trouble to assist those whom he thought deserved assistance. He was a handsome man, strikingly like a gentleman, with highly courteous manners…
Fitz-Roy’s temper was a most unfortunate one. This was shown not only by passion but by fits of long-continued moroseness against those who had offended him. … He was extremely kind to me, but was a man very difficult to live with on the intimate terms which necessarily followed from our messing by ourselves in the same cabin. We had several quarrels; for when out of temper he was utterly unreasonable. For instance, early in the voyage at Bahia in Brazil he defended and praised slavery, which I abominated, and told me that he had just visited a great slave-owner, who had called up many of his slaves and asked them whether they were happy, and whether they wished to be free, and all answered “No.” I then asked him, perhaps with a sneer, whether he thought that the answers of slaves in the presence of their master was worth anything. This made him excessively angry, and he said that as I doubted his word, we could not live any longer together.
…
I saw Fitz-Roy only occasionally after our return home, for I was always afraid of unintentionally offending him, and did so once, almost beyond mutual reconciliation. He was afterwards very indignant with me for having published so unorthodox a book (for he became very religious) as the Origin of Species. Towards the close of his life he was as I fear, much impoverished, and this was largely due to his generosity. … His end was a melancholy one, namely suicide, exactly like that of his uncle Ld. Castlereagh, whom he resembled closely in manner and appearance.
Of the purpose and outcome of the Voyage, Darwin wrote:
The voyage of the Beagle has been by far the most important event in my life and has determined my whole career; yet it depended on so small a circumstance as my uncle offering to drive me 30 miles to Shrewsbury, which few uncles would have done, and on such a trifle as the shape of my nose. I have always felt that I owe to the voyage the first real training or education of my mind. I was led to attend closely to several branches of natural history, and thus my powers of observation were improved, though they were already fairly developed.
The investigation of the geology of all the places visited was far more important, as reasoning here comes into play. On first examining a new district nothing can appear more hopeless than the chaos of rocks; but by recording the stratification and nature of the rocks and fossils at many points, always reasoning and predicting what will be found elsewhere, light soon begins to dawn on the district, and the structure of the whole becomes more or less intelligible. I had brought with me the first volume of Lyell’s Principles of Geology, which I studied attentively; and this book was of the highest service to me in many ways.
A little about the beagle itself: The beagle was a 10-gun brig built in 1820 in Woolwich Dockyard. This was considered a small ship, not heavily armed, and was one of a very large number of similar ships built at that time. The Beagle was 90 feet in length and weighted 235 tons, and was refitted in 1823 with parque-rigging, and extra mast, and extra guns, all this in connection with her service in the Admiralty survey and scientific work. The Beagle circumnavigated the globe twice . From 1840 through 1870, The Beagle was assigned to the British Coast Guard and used in anti-smuggling operations. She was sold by auction in 1870 but to whom has been lost to history. She was probably broken up for scrap at that point. (see “The Last Days of Darwin’s Beagle”