MEANS OF DISPERSAL. 49 



from one piece of water to another by birds, etc., I am 

 unable to say, for I have not seen it stated that they 

 are able to withstand exposure to the atmosphere. The 

 same remark applies to the free-roving pelagic larvae of 

 Dreissena. The just-hatched young of certain univalves, 

 we know, are well suited for such transportal, for they 

 cling firmly, and can live out of water for hours. On 

 this head we have Mr. Darwin's celebrated and often 

 quoted experiment with a duck's feet \\-\ an aquarium : 



" 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.^ " 



Unfortunately, however, I cannot find, after inquiring 

 of a number of experienced ornithologists and sports- 

 men, that the creatures have ever been seen adhering to 

 birds shot on the wing ; Mr. Dresser, for instance, 

 though he has shot thousands of aquatic birds in 



* Univalves, as Mr. F. Darwin informs me. 

 2 «' Origin," p. 345. 



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