246 Fresh-water Productions. Chap, xiil 



contained in a breakfast cup ! Considering these facts, I think it 

 would be an inexplicable circumstance if water-birds did not trans- 

 port the seeds of fresh-water plants to unstocked ponds and streams, 

 situated at very distant points. The same agency may have come 

 into play with the eggs of some of the smaller fresh-water animals. 



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 cen- 

 tury, 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 Nelumbium, and 

 remembered Alph. de Candolle's remarks on the distribution 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 he 

 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 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 pro- 

 ductions 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 dis- 

 tribution of fresh- water plants and of the lower animals, whether 



