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Ch. XL.] 



MIGRATION OF SPECIES. 



389 



Dr. Franklin tells us, in one of his letters, tliat lie saw, in 

 Maryland, a whirlwind which began by taking up the dust 

 which lay in the road, in the form of a sugar-loaf with the 

 pointed end downwards, and soon after grew to the height 



diameter 



It 



advanced in a direction contrary to the wind ; and although 

 the rotatory motion of the column was surprisingly rapid, 

 its onward progress was sufficiently slow to allow a man to 

 keep pace with it on foot. Franklin followed it on horse- 



accom 



and saw it enter a wood^ where it twisted and turned round 

 large trees with surprising force. These were carried up in 

 a spiral line, and were seen flying in the air, together with 

 boughs and innumerable leaves, which, from their height, ap- 

 peared reduced to the apparent size of flies. As this cause 

 operates at different intervals of time throughout a great 

 portion of the earth's surface, it may be the means of bear- 

 ing not only plants but insects, land testacea and their eggs, 

 with many other species of animals, to points which they 

 could never otherwise have reached, and from which they 

 may then begin to propagate themselves again as from a 



new centre. 

 Agency of 



and currents. — In considering, in the 



instrumentality 



persion, I cannot do better than cite the words of one of 



our ablest botanical writers. 



The mountain stream or 



torrent,' observes Keith, ' washes down to the valley the seeds 

 which may accidentally fall into it, or which it may happen 

 to sweep from its banks when it suddenly overflows them. 

 The broad and majestic river, winding along the extensive 

 plain, and traversing the continents of the world, conveys to 

 the distance of many hundreds of miles the seeds that may 

 have vegetated at its source. Thus the southern shores of 

 the Baltic are visited by seeds which grew in the interior of 

 Germany, and the western shores of the Atlantic by seeds 

 that have been generated in the interior of America.'^ Fruits, 

 moreover, indigenous to America and the West Indies, such 



* System of Physiological Botany, vol. ii, p. 405 



