FRESH-WATER PRODUCTIONS 429 



and allied species which, on our theory, are descended from 

 a common parent, and must have proceeded from a single 

 source, prevail throughout the world. Their distribution at 

 first perplexed mc much, as their ova are not likely to be 

 transported by birds; and the ova, as well as the adults, are 

 immediately killed by sea-water. I could not even under- 

 stand how some naturalised species have spread rapidly 

 throughout the same country. But two facts, which I have 

 observed — and many others no doubt will be discovered — 

 throw some light on this subject. When ducks suddenly 

 emerge from a pond covered with duck-weed, I have twice 

 seen these little plants adhering to their backs; and it has 

 happened to me, in removing a little 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 

 (luck's feet, in damp air, from twelve to twenty hours ; and 

 ill 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 dew 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 enor- 

 mous 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 immedi- 



