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 have
descended from a single parent; and therefore have all proceeded from a
common birthplace, notwithstanding that in the course of time they have
come to inhabit distant points of the globe. I have already stated that
I cannot honestly admit Forbes's view on continental extensions, which,
if legitimately followed out, would lead to the belief that within the
recent period all existing islands have been nearly or quite joined to
some continent. This view would remove many difficulties, but it would
not, I think, explain all the facts in regard to insular productions. In
the following remarks I shall not confine myself to the mere question of
dispersal; but shall consider some other facts, which bear on the
truth of the two theories of independent creation and of descent with
modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large
size and varied stations of New Zealand, extending over 780 miles of
latitude, and compare its flowering plants, only 750 in number, with
those on an equal area at the Cape of Good Hope or in Australia,
we must, I think, admit that someth