MEANS OF DISPERSAL. 1 35 



especially If loaded with stones, etc., as Mr. Darwin has 

 remarked, seem likely in many cases to float beneath 

 the surface. In the channels of Tierra del Fuego large 

 quantities of drift-timber are cast upon the beach, yet it 

 is extremely rare to meet a log swimming on the water.^ 

 Even in the case of timber which floats upon the surface 

 the water will gradually penetrate every crevice, and 

 most trees are almost sure to lose their bark before long ; 

 the bark of those seen by Mr. Moseley off the coast of 

 New Guinea was often detached, floating separately, 

 and much wood was held vertically in the water ; but 

 on the other hand, as we have just seen, many of the 

 tree-trunks evidently from the Orinoco, observed by 

 Dr. Nicholls on the coast of Tobago had the bark still 

 attached. The pollard-elm described by Mr. Blomefield 

 is said to have been so weakened by the attacks of 

 Helix lapicida that a slight wind might have overturned 

 it, and one can well imagine that such a tree, after having 

 fallen to the ground, might easily be swept off by the 

 sudden overflow of a river, and the snails contained 

 within it, if ^' in a torpid state " with the mouths of their 

 shells stopped up " with a bung of sawdust and small 

 chips of wood cemented together," as was the case with 

 some of those in the tree in question, would be well 

 fitted to resist salt water, but a tree oi this kind when 

 launched upon the waves would almost immediately 

 become sodden and fall to pieces. Immense numbers of 

 land snails, especially of kinds frequenting damp places 



* Darwin's Journal, ed. 2, 1845, p, 462, 



