CBAP. XIII. J FRESH-WATER PRODUCTIONS. 325 



Other and unknown agencies probably have also played a part. 

 I have stated that fresh-water fish eat some kinds of seeds, though 

 they reject many other kinds after having swallowed them ; even 

 small fish swallow seeds of moderate size, as of the yellow water- 

 lily and Potamogeton, Herons and other birds, century after 

 century, have gone on daily devouring fish; they then take flight 

 and go to other waters, or are blown across the sea ; and we have 

 seen that seeds retain their power of germination, when rejected 

 many hours afterwards in pellets or in the excrement. When I 

 saw the great size of the seeds of that fine water-lily, the Nelum- 

 bium, and remembered Alph. de Candolle's remarks on the distri- 

 bution of this plant, I thought that the means of its dispersal must 

 remain inexplicable ; but Audubon states that he found the seeds 

 of the great southern water-lily (probably, according to Dr. Hooker, 

 the Nelumbium luteum) in a heron's stomach. Now this bird 

 must often have flown with its stomach thus well stocked to distant 

 ponds, and then getting a hearty meal of fish, analogy makes me 

 believe that it would have rejected the seeds in a pellet in a fit 

 state for germination. 



In considering these several means of distribution, it should be 

 remembered that when a pond or stream is first formed, for in- 

 stance, on a rising islet, it will be unoccupied ; and a single seed 

 or egg will have a good chance of succeeding. Although there will 

 always be a struggle for life between the inhabitants of the same 

 pond, however few in kind, yet as the number even in a well- 

 stocked pond is small in comparison with the number of species 

 inhabiting an equal area of land, the competition between them 

 will probably be less severe than between terrestrial species ; con- 

 sequently an intruder from the waters of a foreign country would 

 have a better chance of seizing on a new place, than in the case of 

 terrestrial colonists. We should also remember that many fresh- 

 water productions are low in the scale of nature, and we have 

 reason to believe that such beings become modified more slowly 

 than the high ; and this will give time for the migration of aquatic 

 species. We should not forget the probability of many fresh- water 

 forms having formerly ranged continuously over immense areas, 

 and then having become extinct at intermediate points. But the 

 wide distribution of fresh-water plants and of the lower animals, 

 whether retaining the same identical form or in some degree 

 modified, apparently depends in main part on the wide dispersal 

 of their seeds and eggs by animals, more especially by fresh- water 

 birds, which have great powers of flight, and naturally travel from 

 one piece of water to another. 



