Chap. XII. FRESH-WATER PRODUCTIONS. 385 



queutly become modified and adapted to the fresh 

 waters of a distant land. 



Some species of fresh-water shells have a very wide 

 range, and allied species, which, on my theory, are de- 

 scended 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 they 

 are immediately killed by sea water, as are the adults. 

 I could not even understand how some naturalised 

 species have rapidly spread throughout the same 

 country. But two facts, which I have observed — and 

 no doubt many others remain to be observed — throw 

 some light on this subject. When a duck suddenly 

 emerges from a pond covered with duck-weed, I have 

 twice seen these little plants adhering to its back; and 

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

 weed from one aquarium to another, that I have quite 

 unintentionally stocked the one with fresh-water shells 

 from the other. But another agency is perhaps more 

 effectual : I suspended a duck's feet, which might 

 represent those of a bird sleeping in a natural pond, 

 in an aquarium, where many ova of fresh-water shells 

 were hatching; and I found that numbers of the ex- 

 tremely 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 would be sure to alight on a pool 

 or rivulet, if blown across sea to an oceanic island or 

 to any other distant point. Sir Charles Lyell also 



