CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued.
2. 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. I have already given my reasons
for disbelieving in continental extensions within the period of existing
species on so enormous a scale that all the many islands of the several
oceans were thus stocked with their present terrestrial inhabitants. This
view removes many difficulties, but it does not accord with all the facts
in regard to the productions of islands. In the following remarks I shall
not confine myself to the mere question of dispersal, but shall consider
some other cases bearing 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. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780 miles
of latitude, together with the outlying islands of Auckland, Campbell and
Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal areas
in Southwestern Australia or at the Cape of Good Hope, we must admit that
some cause, independently of different physical conditions, has given rise
to so great a difference in number. Even the uniform county of Cambridge
has 847 plants, and the little island of Anglesea 764, but a few ferns and
a few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the barren
island of Ascension aboriginally possessed less than half-a-dozen flowering
plants; yet many species have now become naturalised on it, as they have in
New Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and animals
have nearly or quite exterminated many native productions. He who admits
the doctrine of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and animals were not
created for oceanic islands; for man has unintentionally stocked them far
more fully and perfectly than did nature.