174 FRESH- WATER PRODUCTIONS. [Chap. XIIL 



weed from one aquarium to another, that I have unin- 

 tentionally stocked the one with fresh-water shells from 

 the other. But another agency is perhaps more effec- 

 tual: I suspended the feet of a duck in an aquarium, 

 where many ova of fresh-water shells were hatching; 

 and I found that numbers of the extremely minute and 

 just-hatched shells crawled on the feet, and clung to 

 them so firmly that when taken out of the water they 

 could not be jarred off, though at a somewhat more ad- 

 vanced age they would voluntarily drop off. These 

 just-hatched molluscs, though aquatic in their nature, 

 survived on the duck's feet, in damp air, from twelve to 

 twenty hours; and in this length of time a duck or 

 heron might fly at least six or seven hundred miles, and 

 if blown across the sea to an oceanic island, or to any 

 other distant point, would be sure to alight on a pool 

 or rivulet. Sir Charles Lyell informs me that a Dytis- 

 eus has been caught with an Ancylus (a fresh-water 

 shell like a limpet) firmly adhering to it; and a water- 

 beetle of the same family, a Colymbetes, once flew on 

 board the ' Beagle,' when forty-five miles distant from 

 the nearest land: how much farther it might have been 

 blown by a favouring gale no one can tell. 



With respect to plants, it has long been known what 

 enormous ranges many fresh-water, and even marsh 

 species, have, both over continents and to the most re- 

 mote oceanic islands. This is strikingly illustrated, ac- 

 cording to Alph. de Candolle, in those large groups of 

 terrestrial plants, which have very few aquatic members; 

 for the latter seem immediately to acquire, as if in con- 

 sequence, a wide range. I think favourable means of 

 dispersal explain this fact. I have before mentioned 

 that earth occasionally adheres in some quantity to the 



