What he wrote I know not: I had previously several years ago seen (by whom I forget) some speculation on warmer period in the U. States subsequent to Glacial period; and I had consulted Lyell, who seemed much to doubt, and Lyell's judgment is really admirably cautious. The arguments advanced in your paper and in your letter seem to me hardly sufficient; not that I should be at all sorry to admit this subsequent and intercalated warmer period--the more changes the merrier, I think. On the other hand, I do not believe that introduction of the Old World forms into New World subsequent to the Glacial period will do for the modified or representative forms in the two Worlds. There has been too much change in comparison with the little change of isolated alpine forms; but you will see this in my book. (347/2. "Origin of Species" (1859), Chapter XI., pages 365 et seq.) I may just make a few remarks why at first sight I do not attach much weight to the argument in your letter about the warmer climate. Firstly, about the level of the land having been lower subsequently to Glacial period, as evidenced by the whole, etc., I doubt whether meteorological knowledge is sufficient for this deduction: turning to the S. hemisphere, it might be argued that a greater extent of water made the temperature lower; and when much of the northern land was lower, it would have been covered by the sea and intermigration between Old and New Worlds would have been checked. Secondly, I doubt whether any inference on nature of climate can be deduced from extinct species of mammals. If the musk-ox and deer of great size of your Barren-Grounds had been known only by fossil bones, who would have ventured to surmise the excessively cold climate they lived under? With respect to food of large animals, if you care about the subject will you turn to my discussion on this subject partly in respect to the Elephas primigenius in my "Journal of Researches" (Murray's Home and Colonial Library), Chapter V., page 85. (347/3. "The firm conviction of the necessity of a vegetation possessing a character of tropical luxuriance to support such large animals, and the impossibility of reconciling this with the proximity of perpetual congelation, was one chief cause of the several theories of sudden revolutions of climate...I am far from supposing that the climate has not changed since the period when these animals lived, which now lie buried in the ice. At present I only wish to show that as far as quantity of food alone is concerned, the ancient rhinoceroses might have roamed over the steppes of Central Siberia even in their present condition, as well as the living rhinoceroses and elephants over the karoos of Southern Africa" ("Journal of Researches," page 89, 1888).) In this country we infer from remains of Elephas primigenius that the climate at the period of its embedment was very severe, as seems countenanced by its woolly covering, by the nature of the deposits with angular fragments, the nature of the co- embedded shells, and co-existence of the musk-ox. I had formerly gathered from Lyell that the relative position of the Megatherium and Mylodon with respect to the Glacial deposits, had not been well made out; but perhaps it has been so recently. Such are my reasons for not as yet admitting the warmer period subsequent to Glacial epoch; but I daresay I may be quite wrong, and shall not be at all sorry to be proved so.
I shall assuredly read your essay with care, for I have seen as yet only a fragment, and very likely some parts, which I could not formerly clearly understand, will be clear enough.
LETTER 348. TO J.D. HOOKER. Down, [December] 26th, [1859].
I have just read with intense interest as far as page xxvi (348/1. For Darwin's impression of the "Introductory Essay to the Tasmanian Flora" as a whole, see "Life and Letters," II., page 257.), i.e. to where you treat of the Australian Flora itself; and the latter part I remember thinking most of in the proof-sheets. Either you have altered a good deal, or I did not see all or was purblind, for I have been much more interested with all the first part than I was before,--not that I did not like it at first.