Sir Charles
Lyell informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering 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 been blown by 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 illustrated, according to
Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally adheres
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 wander more than those of
any other; and are occasionally found on the most remote and barren islands
of the open ocean; they would not be likely to alight on the surface of the
sea, so that any dirt on their feet would not be washed off; and when
gaining the 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
tablespoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; I
kept it covered up in my study for six months, pulling up and counting each
plant as it grew; the plants were of many kinds, and were altogether 537 in
number; and yet the viscid mud was all contained in a breakfast cup!
Considering these facts, I think it would be an inexplicable circumstance
if water-birds did not transport the seeds of fresh-water plants to
unstocked ponds and streams, situated at very distant points.
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