may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily be
answered; for no terrestrial mammal can be transported across a wide
space of sea, but bats can fly across. Bats have been seen wandering by
day far over the Atlantic Ocean; and two North American species either
regularly or occasionally visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has specially studied
this family, that many of the same species have enormous ranges, and
are found on continents and on far distant islands. Hence we have only
to suppose that such wandering species have been modified
through natural selection in their new homes in relation to their new
position, and we can understand the presence of endemic bats on
islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence
in both of the same mammiferous species or of allied species in a more
or less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean; and this
space separates two widely distinct mammalian faunas. On either side
the islands are situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds. No doubt some
few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man s agency; but we shall
soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr. Wallace. I have not as yet
had time to follow up this subject in all other quarters of the world;
but as far as I have gone, the relation generally holds good. We see
Britain separated by a shallow channel from Europe, and the mammals are
the same on both sides; we meet with analogous facts on many islands
separated by similar channels from Australia. The West Indian Islands
stand on a deeply submerged bank, nearly 1000 fathoms in depth, and
here we find American forms, but the species and even the genera are
distinct. As the amount of modification in all cases depends to
a certain degree on the lapse of time, and as during changes of level
it is obvious that islands separated by shallow channels are more
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