174 FRESH-WATER PRODUCTIONS. [Chap. Xin. 



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

 unintentionally stocked the one with fresh-water shells 

 from the other. But another agency is perhaps more 

 effectual: 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 advanced 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 

 Dytiscus 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 

 remote oceanic islands. This is strikingly illustrated, 

 according 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 

 consequence, 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 



