[[p. xxxiv]] "Lobo Raman, 100 Miles E. of Bencoolen, December 10th, 1861.

My dear Bates,

     I should have written to you before to thank you for your paper on the 'Papilios,' but I somehow never can post up my correspondence till I get into some savage wilderness like that in which I am at present. I have read your paper with great attention, and also with great pleasure, and I trust it is but the first of a long series which will establish your own fame, and at the same time demonstrate the simplicity and beauty of the Darwinian philosophy.

     "Your paper is in every respect an admirable one, and incontestably proves the necessity of minute and exact observation over a wide extent of country to enable a man to grapple with the more difficult groups, unravel the synomy, and mark out the limits of the several species and varieties. All this you have done, and have besides established a very interesting fact in zoological geography,--that of the southern bank of the river having received its fauna from Guyana, and not from Brazil. There is, however, another fact I think of equal interest and importance, which you have barely [[p. xxxv]] touched upon, and yet I think your own materials in this very paper establish it, viz.: that the river, in a great many cases, limits the range of the species, or of well-marked varieties. . . . In mammals this fact was not so much to be wondered at, but few persons would credit that it would extend to birds and winged insects. . . . It would seem that Guyana forms having once crossed the river, have a great tendency to become modified, and then never recross. Why the Brazilian species should not first have taken possession of their own side of the river is a mystery. I should be inclined to think that the river bed is comparatively new, and that the south plains were once continuous with Guyana; in fact that Guyana is older than North Brazil, and after it had pushed out its alluvial plains into what is now North Brazil, an elevation on the Brazilian side made the river cut a new channel to the northward, leaving the Guyana species isolated, exposed to competition with a new set of species, and thus led to their becoming modified as we now find them. The phenomenon of a tract of country having been peopled from one now separated from it, and not from that of which it forms a part, is too extraordinary not to require some special and extraordinary cause, and the one I have mentioned seems capable of producing the effects, and by no mean improbable (however unexpected) in itself. . . .

     "I suppose you will now turn to the Coleoptera, and give us the Cicindelidæ on the same plan . . .

"Yours very sincerely,
Alfred R. Wallace."