346 ON THE ORIGIN OF SPECIES 



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

 scended from a common parent and must have pro- 

 ceeded 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 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 

 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 

 Dyticus 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 flown with a favouring gale no one can 

 tell. 



