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
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, and then
having become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the same
identical form, or in some degree modified, apparently depends in main part
on the wide dispersal of their seeds and eggs by animals, more especially
by fresh-water birds, which have great powers of flight, and naturally
travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same
species have migrated from some one area, but that allied species, although
now inhabiting the most distant points, have proceeded from a single area,
the birthplace of their early progenitors.
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