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.

You may perhaps answer that Lombok is an outpost of an army that may once have been as multitudinous as that of the old continent, but the larger part of the host have been swamped in the Pacific.  But they say that European forms of animals and plants run wild in Australia and New Zealand, whereas few of the latter can do the same in Europe.  In my map there is a small island called Nousabali; this ought to make the means of migration of seeds and animals less difficult.  I cannot find that you say anywhere what is the depth of the sea between the Straits of Lombok, but you mention that it exceeds 100 fathoms.  I am quite willing to infer that there is a connection between these soundings and the line of demarcation between the two zoological provinces, but must we suppose land communication for all birds of short flight?  Must we unite South America with the Galapagos Islands?  Can you refer me to any papers by yourself which might enlighten me and perhaps answer some of these queries?  I should have thought that the intercourse even of savage tribes for tens of thousands of years between neighbouring islands would have helped to convey in canoes many animals and plants from one province to another so as to help to confound them.  Your hypothesis of the gradual advance of two widely separated continents towards each other seems to be the best that can be offered.  You say that a rise of a hundred fathoms would unite the Philippine Islands and Bali to the Indian region.  Is there, then, a depth of 600 feet in that narrow strait of Bali, which seems in my map only two miles or so in breadth?

I have [been] confined to the house for a week by a cold or I should have tried to see you.  I am afraid to go out to-day.—­Believe me ever most truly yours,

CHA.  LYELL.

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SIR C. LYELL TO A.R.  WALLACE

73 Harley Street.  April 4, 1867.