386 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



informs me that a Dyticus has been caught with an 

 Ancylus (a fresh-water shell like a limpet) firmly ad- 

 hering 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. 



With respect to plants, it has long been known what 

 enormous ranges many fresh-water and even marsh- 

 species have, both over continents and to the most 

 remote oceanic islands. This is strikingly shown, as 

 remarked by Alph. de Candolle, in large groups of 

 terrestrial plants, which have only a very few aquatic 

 members ; for these latter seem immediately to acquire, 

 as if in consequence, a very wide range. I think favour- 

 able means of dispersal explain this fact. I have before 

 mentioned that earth occasionally, though rarely, ad- 

 heres in some quantity to the feet and beaks of birds. 

 Wading birds, which frequent the muddy edges of 

 ponds, if suddenly flushed, would be the most likely to 

 have muddy feet. Birds of this order I can show are 

 the greatest wanderers, and are occasionally found on 

 the most remote and barren islands in the open ocean ; 

 they would not be likely to alight on the surface of the 

 sea, so that the dirt would not be washed off their feet ; 

 when making land, they would be sure to fly to their 

 natural fresh-water haunts. I do not believe that 

 botanists are aware how charged the mud of ponds is 

 with seeds : I have tried several little experiments, but 

 will here give only the most striking case : I took in 

 February three table-spoonfuls of mud from three dif- 

 ferent points, beneath water, on the edge of a little 

 pond ; this mud when dry weighed only 6f ounces ; I 

 kept it covered up in my study for six months, pulling 

 up and counting each plant as it grew ; the plants were 



