Darwin's Journal provides interesting contrasts with On The Origin of Species. Origin is the result of decades of hard thinking about, among other things, the raw data collected during the Voyage. The seminal work of a 50-year old mature scientist, compared to the travel journal of a 22-year-old embryo parson who cared for "nothing but shooting, dogs, and rat-catching".
So in some ways, the Voyage of the Beagle is like Thrilling Adventure Stories Starring Teen Darwin. He rides with the gauchos, climbs mountains, experiences earthquakes, sails the ocean blue, and theorizes about coral polyps. Of course, it's not all excitement, there are plenty of occasions to skim, as Darwin assiduously lists the flora and fauna collected from each island and locale.
There are hints of evolutionary thinking here and there, such as the following passage:
Hence probably it is, that we feel so little surprise at one, of two species closely allied in habits, being rare and the other abundant in one district, and another, filling the same place in the economy of nature, should be abundant in a neighboring district, differing very little in its conditions. If asked ho this is, one immediately replies that it is determined by some slight difference, in climate, food, or the number of enemies: yet how rarely, if ever, we can point out the precise cause and manner of action of the check [upon growth of a species]! We are, therefore, driven to the conclusion, that causes generally quite inappreciable by us, determine whether a given species shall be abundant or scanty in numbers.
Or better yet: "The whole reasoning, of course, is founded on the assumption of the immutability of species; otherwise the difference in the species in the two regions might be considered as superinduced during a length of time."
My geological examination of the country generally created a good deal of surprise amongst the Chilenos: it was long before they could be convinced that I was not hunting for mines. This was sometimes troublesome: I found the most read way of explaining my employment was to ask them how it was that they themselves were not curious concerning earthquakes and volcanoes? -- why some springs were hot and others cold? -- why there were mountains in Chile, and not a hill in La Plata? These bare questions at once satisfied and silenced the greater number; some, however (like a few in England who are a century behindhand), thought that all such inquiries were useless and impious; and that it was sufficient that God had thus made the mountains.
Of course, there are the Galapagos tortoises, with their individual characters on different islands. But one of the identifying characters was a surprise to me... "Captain Porter has described those from Charles and from the nearest island to it, namely Hood Island, as having their shells in front thick and turned up, whilst the tortoises from James Island are rounder, blacker, and have a better taste when cooked."
And from time to time Darwin turns an excellent phrase: "We feel surprise when travellers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals! This is a wonder which does not at first strike the eye of the body, but, after reflection, the eye of reason."
There is also mixed up in the Beagle's story, the strange homecoming of Jemmy Button and Fuegia Basket.