Chap. XII. 
FKESH-WATER PRODUCTIONS. 
385 
quently 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 
