I was delighted to receive your letter, which was forwarded here to me. I am very glad to hear about the new edition of the "Principles," (557/1. The seventh edition of the "Principles of Geology" was published in 1847.), and I most heartily hope you may live to bring out half a dozen more editions. There would not have been such books as d'Orbigny's S. American Geology (557/2. "Voyage dans l'Amerique meridionale execute pendant les Annees 1826-37." 6 volumes, Paris, 1835-43.) published, if there had been seven editions of the "Principles" distributed in France. I am rather sorry about the small type; but the first edition, my old true love, which I never deserted for the later editions, was also in small type. I much fear I shall not be able to give any assistance to Book III. (557/3. This refers to Book III. of the "Principles"--"Changes of the Organic World now in Progress.") I think I formerly gave my few criticisms, but I will read it over again very soon (though I am striving to finish my S. American Geology (557/4. "Geological Observations on South America" was published in 1846.)) and see whether I can give you any references. I have been thinking over the subject, and can remember no one book of consequence, as all my materials (which are in an absolute chaos on separate bits of paper) have been picked out of books not directly treating of the subjects you have discussed, and which I hope some day to attempt; thus Hooker's "Antarctic Flora" I have found eminently useful (557/5. "Botany of the Antarctic Voyage of H.M.S. 'Erebus' and 'Terror' in the Years 1839-43." I., "Flora Antarctica." 2 volumes, London, 1844-47.), and yet I declare I do not know what precise facts I could refer you to. Bronn's "Geschichte" (557/6. "Naturgeschichte der drei Reiche." H.E. Bronn, Stuttgart, 1834-49.) which you once borrowed) is the only systematic book I have met with on such subjects; and there are no general views in such parts as I have read, but an immense accumulation of references, very useful to follow up, but not credible in themselves: thus he gives hybrids from ducks and fowls just as readily as between fowls and pheasants! You can have it again if you like. I have no doubt Forbes' essay, which is, I suppose, now fairly out, will be very good under geographical head. (557/7. "On the Connection between the Distribution of the existing Fauna and Flora of the British Isles, and the Geological Changes which have affected their Area, especially during the Epoch of the Northern Drift," by E. Forbes. "Memoirs of Geological Survey," Volume I., page 336, 1846.) Kolreuter's German book is excellent on hybrids, but it will cost you a good deal of time to work out any conclusion from his numerous details. (557/8. Joseph Gottlieb Kolreuter's "Vorlaufige Nachricht von eininigen das Geschlecht der Pflanzen betreffenden Versuchen und Beobachtungen." Leipzig, 1761.) With respect to variation I have found nothing--but