149 



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 appea-red 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'* 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. 



