GEOGRAPHICAL DISTRIBUTION 183 



it has happened to me, in removing a little duck-weed 

 from one aquarium to another, that I have unintention- 

 ally 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 

 moUusks, 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 fiy 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 ad- 

 hering 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 

 fMiormous 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- 

 ;;orJing to Alph. de Candolle, in those large groups 

 )f terrestrial plants which have very few aquatic mem- 

 bers; for the latter seem immediately to acquire, as if 



in consequence, a wide range. I think favorable means 



—Science — 26 



J 



