I hear from
Mr. Tomes, who has specially studied this family, that many 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 in their new homes in relation to their new position, and we can
understand the presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by Mr.
Wallace's admirable researches, in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean, and this
separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands are
inhabited by the same or by closely allied quadrupeds. I have not as yet
had time to follow up this subject in all quarters of the world; but as far
as I have gone, the relation holds good. For instance, Britain is
separated by a shallow channel from Europe, and the mammals are the same on
both sides; and so it is with all the islands near the shores of Australia.
The West Indian Islands, on the other hand, stand on a deeply submerged
bank, nearly one thousand fathoms in depth, and here we find American
forms, but the species and even the genera are quite distinct.
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