FRESE-WATER PRODUCTIONS. 407 



Some species of fresh-water shells have very wide ranges, 

 and allied species which, on our theory, are descended from 

 a common parent, and must have proceeded from a single 

 source, prevail throughout the world. Their distribution 

 at first perplexed me much, as their ova are not likely to 

 be transported by birds; and the ova, as well as the adults, 

 are immediately killed by sea- water. I could not even 

 understand how some naturalized species have spread 

 rapidly throughout the same country. But two facts, 

 which I have observed — and many others no doubt will be 

 discovered — 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 

 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 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 



