The Voyage of the Beagle - The Original Classic Edition. Darwin Charles

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СКАЧАТЬ fine dust, which appeared to have been filtered from the wind by the gauze of the vane at the masthead. Mr. Lyell has also given me four packets of dust which fell on a vessel a few hundred miles northward of these islands. Professor Ehrenberg finds that this dust consists in great part of infusoria with siliceous shields,

       and of the siliceous tissue of plants. (1/3. I must take this opportunity of acknowledging the great kindness with which this illustrious naturalist has examined many of my specimens. I have sent (June 1845) a full account of the falling of this dust to the Geological Society.) In five little packets which I sent him, he

       has ascertained no less than sixty-seven different organic forms! The infusoria, with the exception of two marine species, are all inhabitants of fresh-water. I have found no less than fifteen different accounts of dust having fallen on vessels when far out in the Atlantic. From the direction of the wind whenever it has

       fallen, and from its having always fallen during those months when the harmattan is known to raise clouds of dust high into the atmosphere, we may feel sure that it all comes from Africa. It is, however, a very singular fact, that, although Professor Ehrenberg knows many species of infusoria peculiar to Africa, he finds none of these in the dust which I sent him. On the other hand, he finds

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       in it two species which hitherto he knows as living only in South

       America. The dust falls in such quantities as to dirty everything

       on board, and to hurt people's eyes; vessels even have run on shore owing to the obscurity of the atmosphere. It has often fallen on ships when several hundred, and even more than a thousand miles from the coast of Africa, and at points sixteen hundred miles distant in a north and south direction. In some dust which was

       collected on a vessel three hundred miles from the land, I was much

       surprised to find particles of stone above the thousandth of an inch square, mixed with finer matter. After this fact one need not be surprised at the diffusion of the far lighter and smaller sporules of cryptogamic plants.

       The geology of this island is the most interesting part of its natural history. On entering the harbour, a perfectly horizontal white band in the face of the sea cliff, may be seen running for some miles along the coast, and at the height of about forty-five

       feet above the water. Upon examination, this white stratum is found to consist of calcareous matter, with numerous shells embedded, most or all of which now exist on the neighbouring coast. It rests

       on ancient volcanic rocks, and has been covered by a stream of basalt, which must have entered the sea when the white shelly bed was lying at the bottom. It is interesting to trace the changes, produced by the heat of the overlying lava, on the friable mass, which in parts has been converted into a crystalline limestone, and in other parts into a compact spotted stone. Where the lime has

       been caught up by the scoriaceous fragments of the lower surface of the stream, it is converted into groups of beautifully radiated

       fibres resembling arragonite. The beds of lava rise in successive

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       gently-sloping plains, towards the interior, whence the deluges of melted stone have originally proceeded. Within historical times no signs of volcanic activity have, I believe, been manifested in any part of St. Jago. Even the form of a crater can but rarely be

       discovered on the summits of the many red cindery hills; yet the

       more recent streams can be distinguished on the coast, forming lines of cliffs of less height, but stretching out in advance of those belonging to an older series: the height of the cliffs thus affording a rude measure of the age of the streams.

       During our stay, I observed the habits of some marine animals. A large Aplysia is very common. This sea-slug is about five inches long; and is of a dirty yellowish colour, veined with purple. On each side of the lower surface, or foot, there is a broad membrane, which appears sometimes to act as a ventilator, in causing a

       current of water to flow over the dorsal branchiae or lungs. It

       feeds on the delicate seaweeds which grow among the stones in muddy

       and shallow water; and I found in its stomach several small pebbles, as in the gizzard of a bird. This slug, when disturbed, emits a very fine purplish-red fluid, which stains the water for

       the space of a foot around. Besides this means of defence, an acrid secretion, which is spread over its body, causes a sharp, stinging sensation, similar to that produced by the Physalia, or Portuguese man-of-war.

       I was much interested, on several occasions, by watching the habits of an Octopus, or cuttlefish. Although common in the pools of water left by the retiring tide, these animals were not easily

       caught. By means of their long arms and suckers, they could drag

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       their bodies into very narrow crevices; and when thus fixed, it

       required great force to remove them. At other times they darted

       tail first, with the rapidity of an arrow, from one side of the

       pool to the other, at the same instant discolouring the water with

       a dark chestnut-brown ink. These animals also escape detection by a very extraordinary, chameleon-like power of changing their colour. They appear to vary their tints according to the nature of the

       ground over which they pass: when in deep water, their general shade was brownish purple, but when placed on the land, or in shallow water, this dark tint changed into one of a yellowish

       green. The colour, examined more carefully, was a French grey, with

       numerous minute spots of bright yellow: the former of these varied in intensity; the latter entirely disappeared and appeared again by turns. These changes were effected in such a manner that clouds, varying in tint between a hyacinth red and a chestnut-brown, were continually 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 СКАЧАТЬ