GEOGRAPHICAL DISTRIBUTION 353 



me, in removing a little duck-weed from one aquarium to an- 

 other, 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 

 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 strik- 

 ingly illustrated, according to Alph. de CandoUe, 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 feet would not be washed off; and when gaining the 

 land, they would be sure to fly to their natural fresh-water haunts. 

 I do not believe that botanists are aware how charged the mud 

 of ponds is with seeds; I have tried several little experiments, but 

 will here give only the most striking case : I took in February three 

 tablespoonfuls of mud from three different points, beneath water, 

 on the edge of a little pond; this mud when dried weighed only 

 six and three-fourths ounces; I kept it covered up in my study 

 for six months, pulling up and counting each plant as it grew; the 



