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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
of the
however
→already occupying any pond,
as the number
→of kinds
is
→compared with those on the
land, the competition
→will
probably be less severe
than between terrestrial species; consequently an intruder from the waters of a foreign
have a better chance of seizing on a
than in the case of terrestrial colonists. We
remember that
fresh-water productions are low in the scale of nature, and
we have reason to believe that
beings
become modified
than the high; and this will give
time
the
→average for the
migration of
aquatic species. We should not forget the probability of many
having formerly ranged
continuously
→as fresh-water productions ever can range,
over immense areas, and
become extinct
intermediate
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
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. |
