324 FRESH- WATER PRODUCTIONS. [CHAP. XIII. 



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 

 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 Colym- 

 betes, 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 dis- 

 persal 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 botantists 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 6| 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 contained in a break- 

 fast cup ! Considering these facts, I think it would be an inex- 

 plicable circumstance if water-birds did not transport the seeds 

 of fresh-water plants to unstocked ponds and streams, situated at 

 very distant points. The same agency may have come into play 

 with the eggs of some of the smaller fresh- water animals. 



