386 
GEOGEAPHICAL DISTEIBUTION. 
Chap. XIT. 
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 marshy 
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 wmuld 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 6| ounces ; I 
kept it covered up in my study for six months, pulling 
up and counting each plant as it grew; the plants were 
