176 FRESH- WATEE PRODUCTIONS. [Chap. XIII. 



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 after- 

 wards in pellets or in the excrement. When I saw 

 the great size of the seeds of that fine water-lily. 

 the Xelumbium, and remembered Alph. de Candolle's 

 remarks on the distribution of this plant, I thought that 

 the means of its dispersal mnst remain inexplicable; 

 but Audubon states that he found the seeds of the great 

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

 the Xelumbium luteum) in a heron's stomach. Xow 

 this bird must often haye 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 instance, 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 ; consequently an in- 

 truder from the waters of a foreign country woidd 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,* 



