382 FRESH-WATER PRODUCTIONS. 



idly throughout the same country. But two fact*, which 

 I have observed — and many others no doubt will be dis- 

 covered — 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 some- 

 what more advanced age they would voluntarily drop off. 

 These just-hatched mollusks, 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 favoring 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 favorable means of dispersal explain this fact. I 

 have before mentioned that earth occasionally adheres in 

 some quantity to the feet and beaks of birds. Wading 

 birds, which frequent the muddy edges of ponds, if suddenly 

 flushed, would be the most likely to have muddy feet. 

 Birds of this order wander more than those of any other; 

 and they are occasionally found on the most remote and 

 barren islands of the open ocean ; they would not be likely 

 to alight on the surface of the sea, so that any dirt on their 



