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; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds
might be dropped by the bird whilst feeding its young, in the same way
as fish are known sometimes to be dropped.
In considering these several means of distribution,
it should be remembered that when a pond or stream is first formed, for
instance, 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 individuals of the species,
however few, already occupying any pond, yet as the number of kinds is
small, compared with those on the land, the competition will probably
be less severe between aquatic than between terrestrial species;
consequently an intruder from the waters of a foreign country, would
have a better chance of seizing on a place, than in the case of
terrestrial colonists. We should, also, remember that some, perhaps
many, fresh-water productions are low in the scale of nature, and that
we have reason to believe that such low beings change or become
modified less quickly than the high; and this will give longer time
than the average for the migration of the same aquatic species. We
should not forget the probability of many species having formerly
ranged as continuously as fresh-water productions ever can range, over
immense areas, and having subsequently become extinct in intermediate
regions. But the wide distribution of fresh-water plants and of the
lower animals, whether retaining the same identical form or in some
degree modified, I believe mainly depends on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful
gardener, thus takes her seeds from a bed of a particular nature, and
drops them in another equally well fitted for them.
_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, on the
view that all the individuals both of the same and of allied species
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