The Voyage of the Beagle - The Original Classic Edition. Darwin Charles
passing over the body. (1/4. So named according to Patrick Symes's nomenclature.) Any part, being subjected to a
slight shock of galvanism, became almost black: a similar effect, but in a less degree, was produced by scratching the skin with a needle. These clouds, or blushes as they may be called, are said to be produced by the alternate expansion and contraction of minute vesicles containing variously coloured fluids. (1/5. See
"Encyclopedia of Anatomy and Physiology" article "Cephalopoda.")
This cuttlefish displayed its chameleon-like power both during the
act of swimming and whilst remaining stationary at the bottom. I
was much amused by the various arts to escape detection used by one individual, which seemed fully aware that I was watching it.
Remaining for a time motionless, it would then stealthily advance
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an inch or two, like a cat after a mouse; sometimes changing its
colour: it thus proceeded, till having gained a deeper part, it darted away, leaving a dusky train of ink to hide the hole into which it had crawled.
While looking for marine animals, with my head about two feet above the rocky shore, I was more than once saluted by a jet of water, accompanied by a slight grating noise. At first I could not think
what it was, but afterwards I found out that it was this
cuttlefish, which, though concealed in a hole, thus often led me to its discovery. That it possesses the power of ejecting water there is no doubt, and it appeared to me that it could certainly
take good aim by directing the tube or siphon on the under side of its body. From the difficulty which these animals have in carrying their heads, they cannot crawl with ease when placed on the ground. I observed that one which I kept in the cabin was slightly phosphorescent in the dark.
ST. PAUL'S ROCKS.
In crossing the Atlantic we hove-to, during the morning of February
16th, 1832, close to the island of St. Paul's. This cluster of rocks is situated in 0 degrees 58' north latitude, and 29 degrees
15' west longitude. It is 540 miles distant from the coast of
America, and 350 from the island of Fernando Noronha. The highest point is only fifty feet above the level of the sea, and the entire circumference is under three-quarters of a mile. This small point
rises abruptly out of the depths of the ocean. Its mineralogical
constitution is not simple; in some parts the rock is of a cherty,
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in others of a feldspathic nature, including thin veins of serpentine. It is a remarkable fact that all the many small
islands, lying far from any continent, in the Pacific, Indian, and
Atlantic Oceans, with the exception of the Seychelles and this
little point of rock, are, I believe, composed either of coral or
of erupted matter. The volcanic nature of these oceanic islands is evidently an extension of that law, and the effect of those same causes, whether chemical or mechanical, from which it results that a vast majority of the volcanoes now in action stand either near
sea-coasts or as islands in the midst of the sea.
(PLATE 4. INCRUSTATION OF SHELLY SAND.)
The rocks of St. Paul appear from a distance of a brilliantly white colour. This is partly owing to the dung of a vast multitude of seafowl, and partly to a coating of a hard glossy substance with a pearly lustre, which is intimately united to the surface of the
rocks. This, when examined with a lens, is found to consist of
numerous exceedingly thin layers, its total thickness being about the tenth of an inch. It contains much animal matter, and its origin, no doubt, is due to the action of the rain or spray on the
birds' dung. Below some small masses of guano at Ascension, and on the Abrolhos Islets, I found certain stalactitic branching bodies, formed apparently in the same manner as the thin white coating on these rocks. The branching bodies so closely resembled in general appearance certain nulliporae (a family of hard calcareous
sea-plants), that in lately looking hastily over my collection I
did not perceive the difference. The globular extremities of the branches are of a pearly texture, like the enamel of teeth, but so
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hard as just to scratch plate-glass. I may here mention, that on a part of the coast of Ascension, where there is a vast accumulation of shelly sand, an incrustation is deposited on the tidal rocks, by the water of the sea, resembling, as represented in Plate 4,
certain cryptogamic plants (Marchantiae) often seen on damp walls. The surface of the fronds is beautifully glossy; and those parts formed where fully exposed to the light, are of a jet black colour,
but those shaded under ledges are only grey. I have shown specimens of this incrustation to several geologists, and they all thought
that they were of volcanic or igneous origin! In its hardness and
translucency--in its polish, equal to that of the finest
oliva-shell--in the bad smell given out, and loss of colour under the blowpipe--it shows a close similarity with living sea-shells. Moreover in sea-shells, it is known that the parts habitually covered and shaded by the mantle of the animal, are of a paler colour than those fully exposed to the light, just as is the case with this incrustation. When we remember that lime, either as a phosphate or carbonate, enters into the composition of the hard parts, such as bones and shells, of all living animals, it is an interesting physiological fact to find substances harder than the
enamel of teeth, and coloured surfaces as well polished as those of
a fresh shell, reformed through inorganic means from dead organic matter--mocking, also, in shape, some of the lower vegetable productions. (1/6. Mr. Horner and Sir David Brewster have described ("Philosophical Transactions" 1836 page 65) a singular "artificial substance resembling shell." It is deposited in fine, transparent,
highly polished, brown-coloured laminae, possessing peculiar optical properties, on the inside of a vessel, in which cloth, first prepared with glue and then with lime, is made to revolve
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rapidly in water. It is much softer, more transparent, and contains more animal matter, than the natural incrustation at Ascension; but we here again see the strong tendency which carbonate of lime and
animal matter evince to form a solid substance allied to shell.)
We found on St. Paul's only two kinds of birds--the booby and the noddy. The former is a species of gannet, and the latter a tern.
Both are of a tame and stupid disposition, and are so unaccustomed to visitors, that I could have killed any number of them with my
geological hammer. The booby lays her eggs on the bare rock; but
the tern makes a very simple nest with seaweed. By the side of many of these nests a small flying-fish was placed; which I suppose, had been brought by the male bird for its partner. It was amusing to watch how quickly a large and active crab (Graspus), which inhabits the crevices of the rock, stole the fish from the side of the nest,
as soon as we had disturbed the parent birds. Sir W. Symonds, one
of the few persons who have landed here, informs me that he saw the crabs dragging even the young birds out of their nests, and
devouring them. Not a single plant, not even a lichen, grows on this islet; yet it is inhabited by several insects and spiders. The following list completes, I believe, the terrestrial fauna: a fly
(Olfersia) living on the booby, and a tick which must have come here as a parasite on the birds; a small brown moth, belonging to a genus that feeds on feathers; a beetle (Quedius) and a woodlouse from beneath the dung; and lastly, numerous spiders, which I
suppose