126 THE DISPERSAL OF SHELLS. 



then much regretted having neglected, whilst on the 

 spot, the opportunity of ascertaining whether they 

 contained organisms of any kind/ Pumice is occa- 

 sionally washed up on the shores of many islands, and 

 vessels have encountered vast quantities on the ocean, 

 but it may often be derived from volcanoes quite near the 

 sea, or from submarine ones. 



Trees are carried out to sea by rivers in vast num- 

 bers, and being often drifted by ocean-currents to 

 great distances, are occasionally thrown up on foreign 

 shores, even, as Mr. Darwin states, on those of islands 

 in the midst of the widest oceans,- and land-snails of 

 many kinds, concealing themselves under the bark 

 and within the trunks of old and hollow trees or living 

 among the earth and debris at their roots, or depositing 

 their eggs in such situations, must almost certainly be 

 carried with such drift-wood with some frequency. 

 Volumes might probably be filled with accounts of the 

 floating of timber, etc., upon the sea and its stranding 

 on more or less distant coasts ; most naturalists are 

 familiar with such facts ; it may perhaps be useful, how- 

 ever, to refer briefly to one or two observations on the 

 point. At a great distance from the coast of New 

 Guinea, about seventy miles north-east of Point 

 D'Urville, where the great Ambernoh River runs into 

 the sea, the Challenger^ as Mr. Moseley states, found the 

 water blocked with drift-wood, disposed in long curved 



^ H. W. Bates, "Naturalist on the River Amazons," ed. 5, 1884, 

 pp. 247-9 ; see also " Principles," ii. p. 379. 

 - " Origin," p. 326. 



