Darwin did not receive this work until December 23rd, so that the reference is to proof-sheets.), as usual, with great interest.

The points are awfully intricate, almost at present beyond the confines of knowledge.

The view which I should have looked at as perhaps most probable (though it hardly differs from yours) is that the whole world during the Secondary ages was inhabited by marsupials, araucarias (Mem .-- Fossil wood so common of this nature in South America (345/2.
See Letter 6, Note.)), Banksia, etc.; and that these were supplanted and exterminated in the greater area of the north, but were left alive in the south.

Whence these very ancient forms originally proceeded seems a hopeless enquiry.
Your remarks on the passage of the northern forms southward, and of the southern forms of no kinds passing northward, seem to me grand.
Admirable, also, are your remarks on the struggle of vegetation: I find that I have rather misunderstood you, for I feared I differed from you, which I see is hardly the case at all.

I cannot help suspecting that you put rather too much weight to climate in the case of Australia.