Dear Mr. Wallace,--I am citing your two papers in my second volume of
the new edition of the "Principles"--that on the Physical Geography of
the Malay Archipelago, 1863, and the other on Varieties of Man in ditto,
1864. I am somewhat confounded with the marked line which you draw
between the two provinces on each side of the Straits of Lombok. It
seems to me that Darwin and Hooker have scarcely given sufficient weight
to the objection which it affords to some of their arguments. First, in
regard to continental extension, if these straits could form such a
barrier, it would seem as if nothing short of a land communication could
do much towards fusing together two distinct faunas and floras. But here
comes the question--are there any land-quadrupeds in Bali or in Lombok?
I think you told me little was known of the plants, but perhaps you know
something of the insects. It is impossible that birds of long flight
crossing over should not have conveyed the seeds and eggs of some
plants, insects, mollusca, etc. Then the currents would not be idle, and
during such an eruption as that of Tomboro in Sumbawa all sorts of
disturbances, aerial, aquatic and terrestrial, would have scattered
animals and plants.

When I first wrote, thirty-five years ago, I attached great importance
to preoccupancy, and fancied that a body of indigenous plants already
fitted for every available station would prevent an invader, especially
from, a quite foreign province, from having a chance of making good his
settlement in a new country. But Darwin and Hooker contend that
continental species which have been improved by a keen and wide
competition are most frequently victorious over an insular or more
limited flora and fauna. Looking, therefore, upon Bali as an outpost of
the great Old World fauna, it ought to beat Lombok, which only
represents a less rich and extensive fauna, namely the Australian.