Fossils, Finches and Fuegians: Charles Darwin’s Adventures and Discoveries on the Beagle. Richard Keynes
consisted of minute points apparently injected with a coloured fluid. The whole animal presented a most extraordinary mottled appearance, & much surprised every body who saw it. The edges of the sheath were orange, this likewise varied its tint. The animal seemed susceptible to small shocks of galvanism: contracting itself & the parts between the point of contact of wires, became almost black. This in a lesser degree followed from scratching the animal with a needle. The cups were in double rows on the arms & coloured reddish. The eye could be entirely closed by a circular eyelid, the pupil was of a dark blue. The animal was slightly phosphorescent at night.52
Charles was greatly excited by what he thought was a new discovery, and described it enthusiastically in his first letter to Henslow. But Henslow replied that he too had seen the colour changes of an octopus he had caught at Weymouth, and that the phenomenon had also been reported by others. Cuvier had indeed mentioned the ability of an octopus to outdo a chameleon in this respect, but Charles was nevertheless the first to give an accurate description of the properties of its chromatophores, the pigment cells in the skin whose rapid contraction and expansion under nervous control are responsible for the vivid colour changes in octopus and other cephalopods such as cuttlefish and squid. Their function is not only to camouflage the animals when they move to new surroundings, but as has only been appreciated very recently, to provide a means of communication between them.
In his Autobiography, Charles wrote:
The investigation of the geology of all the places visited was far more important than natural history, 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. The very first place which I examined, namely St. Jago in the Cape Verde Islands, showed me clearly the wonderful superiority of Lyell’s manner of treating geology, compared with that of any other author whose works I had with me or ever afterwards read.
His copy of Volume 1 of Lyell’s Principles was inscribed ‘Given me by Capt. F.R. C.Darwin’, and he had been advised by Henslow to read it ‘but on no account to accept the views therein advocated’.
Charles’s first geological project was to examine the structure of Quail Island, which as it happened was painted by the artist Conrad Martens about eighteen months later, on his way out to join the Beagle at Monte Video (see Plate 1). It was a ‘miserable desolate spot less than a mile in circumference’ in the harbour of Porto Praya, but served Charles usefully as a key to the structure of the main island of St Jago. He wrote in his Autobiography:
The geology of St Jago is very striking yet simple: a stream of lava formerly flowed over the bed of the sea, formed of triturated recent shells and corals, which it has baked into a hard white rock. Since then the whole island has been upheaved. But the line of white rock revealed to me a new and important fact, namely that there had been afterwards subsidence round the craters, which had since been in action, and had poured forth lava. It then first dawned on me that I might perhaps write a book on the geology of the various countries visited, and this made me thrill with delight. That was a memorable hour to me, and how distinctly I can call to mind the low cliff of lava beneath which I rested, with the sun glowing hot, a few strange desert plants growing near, and with living corals in the tidal pools at my feet.
In the notes made at the time Charles’s interpretation was that both islands were volcanic, and had at some not too distant time been submerged beneath the sea, where they quietly collected beds of marine material, followed by another layer of molten lava.53 ‘The whole mass was then raised, since which or at the time there has been a partial sinking. I judge of this from the appearance of distortion, & indeed the distant line of coast seen to the East, which is considerably higher, bears me out.’
This was accompanied by a section drawing of Quail Island showing the successive layers. Tests on specimens from the white layer D showed that it ‘Effervesces readily with Mur: Acid, gives precipitate with Oxalate of Ammonia. – Under Blowpipe becomes slowly caustic, & with heat Cobalt remains of a Violet colour. – Carbonate of Magnesia. (?) Carb. of Lime.’ (The white line may be seen in Plate 1.)
In his first independent geological project, Charles’s careful analysis of the sequence of rocks in Quail Island showed with what great effect he had followed the teaching of Sedgwick and Henslow. His notes also reveal how geology allowed him from the start to exercise to the full his latent passion for argument and theorisation. On completing his notes on Quail Island, he immediately reread them and wrote, ‘I have drawn my pen through those parts which appear absurd,’ and a year later he added a long list of further comments and theories. At the same time he immediately fell in wholeheartedly with Lyell’s gradualist and not yet generally accepted approach that geological changes resulted from slow processes operating over a long period of time. Thus his evidence clearly supported the view that both subsidence and elevation of the land must have taken place over an appreciable area in the not too distant past, and he was led to agree with Lyell that the forces involved might act slowly and evenly so as to leave superficial features of the landscape and buildings undisturbed. Another relevant factor in the story was Charles’s identification of the dust that thickly coated the ship throughout their visit to St Jago, ‘to the great injury of fine astronomical instruments’, as volcanic in origin.
Charles reacted to his first day on St Jago with the enthusiasm that never deserted him:
I returned to the shore, treading on Volcanic rock, hearing the notes of unknown birds, & seeing new insects fluttering about still newer flowers. It has been for me a glorious day, like giving to a blind man eyes – he is overwhelmed with what he sees & cannot justly comprehend it. Such are my feelings, & such may they remain.
On the day originally fixed for sailing on across the Atlantic to Brazil, FitzRoy was busy on shore complying with Captain Beaufort’s strict instructions that no port should be quitted before not only the magnetic angle, but also the dip and daily variation had been ascertained. On 8 February the instruments were re-embarked, and after swinging the ship and determining less than twenty minutes’ difference in any position of the bearing of the peak eleven miles away, the Beagle weighed anchor and sailed. On 10 February they came alongside the packet Lyra, on passage from London to Rio de Janeiro, and were pleased to find that she was carrying a box of six sounding-leads for them, modified by their designer to operate satisfactorily at depths well below a hundred fathoms. Charles posted a brief letter to his father, in case it might arrive sooner than a long one due to be dispatched from Bahia, in which he said:
I think, if I can so soon judge, I shall be able to do some original work in Natural History – I find there is so little known about many of the Tropical animals.
At sunset on 15 February the St Paul Rocks were seen on the horizon, these being the summit of a sunken mountain, and further from land than FitzRoy had ever seen a group of such small rocks. At daylight next morning the sea was smooth, and while the Beagle sailed round so that Stokes could take angles and make soundings, two boats were sent out to enable FitzRoy, Charles and a party to land on the rocks and examine them. As FitzRoy described it:
When our party had effected a landing through the surf, and had a moment’s leisure to look about them, they were astonished at the multitudes of birds which covered the rocks, and absolutely darkened the sky. Mr Darwin afterwards said that till then he had never believed the stories of men knocking down birds with sticks; but there they might be kicked before they would move out of the way. The first impulse of our invaders