354 THE ORIGIN OF SPECIES 



plants were of many kinds, and were altogether 537 in number; 

 and yet the viscid mud was all contained in a breakfast cup I 

 Considering these facts, I think it would be an inexplicable cir- 

 cumstance if water birds did not transport 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 

 fiiat seeds retain their power of germination, when rejected many 

 hours afterward 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 the 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. 



