Chap. X1U.'] FRESH-WATER PRODUCTIONS. 345 



have unintentionally stocked the one with fresh-water shells from 

 the other. But another agency is perhaps more effectual : I sus- 

 pended the feet of a duck in an aquarium, where many ova of fresh- 

 water shells were hatching; aud 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 he 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 

 <lamp air, from twelve to twenty hours ; and in this length of time 

 n 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 feet and beaks of birds. "Wading birds, which fre- 

 quent 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 of; 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 table- 

 spoonfuls of mud from three different points, beneath water, on the 

 edge of a little pond : this mud when dried weighed only 61 ounces ; 

 I kept it covered up in my study for six months, pulling up and 

 counting each plant as it grew; the plants were of many kinds, 

 and were altogether 537 in number ; and yet the viscid mud was all 



