WATER SNAILS. 
189 
pearance, even in localities, such as a reservoir 
on the top of a hill, where the ordinary agencies 
will not account for its presence. Setting aside 
the wonderful tales of “ showers of snails,” the 
occurrence of shells in such localities is evidently 
due to transportation by birds, the young snails 
or capsules adhering to their feet. In elucida¬ 
tion, here follows an experiment performed by 
Mr. Darwin, and extracted from his “ Origin of 
Species” :— 
“I suspended a ducVs feet, which might re¬ 
present 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 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 mollusks, 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 informs 
me that a Dytiscus has been caught with an 
Ancylus firmly adhering to it.” 
