FOSSIL INFUSORIA. 61 



DUST SHOWERS. The fossil shells of protophytes, which lie mingled with the soil 

 of the earth, are not unfrequently carried up into the air in the clouds of dust that 

 are raised aloft by the winds, and borne along on the currents of the atmosphere, 

 to a distance almost incredible. Darwin noticed that the atmosphere of St. Jago, 

 one of the Cape de Verde isles, is generally hazy, owing to the fall of an impalpable 

 fine dust, of a brown color. A small quantity of the dust was collected by this gen 

 tleman, who received also, from Mr. Lyell, four packets of the same kind of powder, 

 which fell on a vessel a few hundred miles northward of the Cape Verde Islands. 

 Five parcels were sent to Dr. Ehrenberg for examination, who found it to consist 

 chiefly of the flinty cases of Diatoms, and the siliceous tissue of plants. No less 

 than sixty-seven distinct kinds of rare structures were detected of which sixty-four 

 were fresh-water species, and the remaining two marine. 



The same observer, upon investigation, met with fifteen accounts of dust falling 

 upon vessels when far out on the Atlantic, off the coast of Africa. It has been 

 here known to descend upon the decks of ships, at the distance of several hundred 

 and even a thousand miles from shore, and when land was distant to the north and 

 south, full sixteen hundred miles. The dust is distributed thickly through the air, 

 soiling everything on board, injuring the eyes, and rendering the atmosphere so 

 hazy that vessels have been known to run ashore in consequence of the obscurity 

 thus produced. This dust is believed to come from the African continent, from the 

 fact that it occurs when the wind is from that direction, and at the same time that 

 the liarmattan prevails, which is a periodical wind that blows from the interior of 

 Africa towards the Atlantic. Clouds of the finer particles of sand, from the arid 

 deserts of this continent, are borne aloft by the sweep of the harmattan, and carried 

 far out over the sea upon the higher currents of the atmosphere. At the distance 

 of three hundred miles from land, Darwin discovered, in the fallen dust, particles of 

 stone the thousandth part of an inch square, mixed with matter still finer. 



In reflecting upon the facts just adduced, we see, that in order to become 

 acquainted with the structure of the world we inhabit, it is not sufficient to trust 

 to our unassisted vision. Wonders, and problems the most curious and interesting, 

 will meet, the gaze of the naturalist at every step he takes ; but unless he explores 

 the secrets of nature with the magic glass of the microscope, half of the treasures 

 of truth will be still unrevealed : sealed from his vision by impenetrable dark 

 ness. 



A question naturally arises, What are the ends which these microscopic animal 

 organisms, whose modes of existence, habits, and structures have been discussed in 

 the preceding pages, subserve in the economy of the world ? To them have been 

 attributed malignant influences ; for the various epidemics, which at intervals have 

 swept down our race, have been supposed by some to originate in a &quot; living cloud &quot; 

 of existences, dwelling in the air. But of this we know nothing certain, and a 

 more satisfactory answer cannot be given than that which is contained in the words 

 of Professor Owen, who thus unfolded his views upon this subject, in one of his 

 lectures: -&quot;Consider their incredible numbers, their universal distribution, their 

 insatiable voracity, and that it is the particles of decaying vegetable and animal 

 bodies which they are appointed to devour and assimilate. Surely we must, in 



