MEANS OF DISPERSAL. 1 25 



Amazons, twelve hundred miles from the volcanoes of 

 the Andes from which they must have come ; he once 

 received a large piece which had been found in the 

 middle of the stream about 900 miles further down, and 

 pieces having reached this distance, he remarks, would 

 be pretty sure of being carried out to sea, and thence 

 probably with the north-westerly Atlantic current to 

 shores many thousands of miles away. From the 

 rounded and water-worn appearance of the fragments, 

 it was concluded that they must have been rolled about 

 for a long time in the shallow streams near the sources 

 of the rivers at the feet of the volcanoes before they leapt 

 the waterfalls and embarked on the currents leading 

 direct for the Amazons, and " they may have been 

 originally cast on the land and afterwards carried to 

 the rivers by freshets, in which case the eggs and seeds 

 of land-insects and -plants might be accidentally intro- 

 duced, and safely enclosed with particles of earth in 

 their cavities." It seems very likely also that the eggs of 

 molluscs and small snails in a state of hibernation may 

 sometimes be thus enclosed, and as the speed of the cur- 

 rent, in the Amazons, for instance, during the rainy season, 

 is said to be from three to five miles an hour, they might 

 thus travel, unharmed, on fresh-water to great distances, 

 and might possibly survive for some days on the surface 

 of the ocean. I am not aware, however, that snails 

 or their eggs have ever been found in the interstices 

 of pumice. It was not until after his return to 

 England that Mr. Bates came to regard the fragments 

 he had seen as probable agents for dispersal, and he 



