likely to have been continuously united within a recent period to the
mainland than islands separated by deeper channels, we can understand
the frequent relation between the depth of the sea and the degree of
affinity of the mammalian inhabitants of islands with those of a
neighbouring continent, an inexplicable relation on the view of
independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic
islands, namely, the scarcity of kinds the richness in endemic forms in
particular classes or sections of classes, the absence of whole groups,
as of batrachians, and of terrestrial mammals notwithstanding the
presence of rial bats, the singular proportions of certain orders of
plants, herbaceous forms having been developed into trees, etc., seem
to me to accord better with the view of occasional means of transport
having been largely efficient in the long course of time, than with the
view of all our oceanic islands having been formerly connected by
continuous land with the nearest continent; for on this latter view the
migration would probably have been more complete; and if modification
be admitted, all the forms of life would have been more equally
modified, in accordance with the paramount importance of the relation
of organism to organism.
I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote
islands, whether still retaining the same specific form or modified
since their arrival, could have reached their present homes. But the
probability of many islands having existed as halting-places, of which
not a wreck now remains, must not be overlooked.
I will here give a single instance of one of the cases of difficulty.
Almost all oceanic islands, even the most isolated and smallest, are
inhabited by land-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and
are killed by it. Yet there must be, on my view, some unknown, but
highly efficient means for their transportal. Would the just-hatched
young occasionally crawl on and adhere to the feet of birds roosting on
the ground, and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over
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