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congenial climates, or by seas, mountains, and other 

 physical impediments ; and these barriers they can some- 

 times surmount by abandoning themselves to violent 

 gales, which may in a few hours carry them to very con- 

 siderable distances. On the Andes some sphinxes and 

 flies have been observed by Humboldt, at the height of 

 19,180 feet above the sea, and which appeared to him to 

 have been involuntarily carried into those regions by 

 ascending currents of air*." With respect to the acci- 

 dental conveyance of numerous species across the sea, it 

 is not to the winds alone that we must look for an ex- 

 planation. Large and rapid rivers are liable to inun- 

 date their banks and bring down insects in prodigious 

 masses, which are disgorged into the ocean, and car- 

 ried to a distance from the coast, in proportion to the 

 violence of the ejecting stream. When the body of 

 water is considerable, the sea becomes diluted to an un- 

 usual extent ; and creatures which must have otherwise 

 perished, from the action of the salt, are able to survive 

 for a time, and may be deposited, by means of rapid 

 currents into which they are borne, on neighbouring 

 islands and continents. Even the Hydradephaga are 

 thus occasionally transported ; for Darwin mentions 

 having captured a Colymbetes off Cape S ta Maria (to the 

 north of the Rio de la Plata), when forty-five miles 

 from the shore. And, in his ' Journal of Researches/ 

 he records the following remarkable facts, which bear 

 upon this immediate question. " On another occasion, 

 * Principles of Geology, p. 656. 



