I suppose you have read Agassiz's marvellous theory of the Great Amazonian glacier, 2,000 miles long! I presume that will be a little too much, even for you. I have been writing a little popular paper on "Glacial Theories" for the Quarterly Journal of Science of January next, in which I stick up for glaciers in North America and icebergs in the Amazon!
I was very glad to hear from Lubbock that your health is permanently improved. I hope therefore you will be [pg 177]able to give us a volume per annum of your magnum opus, with all the facts as you now have them, leaving additions to come in new editions.
I am working a little at another family of my butterflies, and find the usual interesting and puzzling cases of variation, but no such phenomena as in the Papilionidæ.—With best wishes, believe me, my dear Darwin, yours very faithfully,
ALFRED R. WALLACE.
6 Queen Anne Street, W. Monday, January, 1867.
My dear Wallace,—I return by this post the Journal.[56] Your résumé of glacier action seems to me very good, and has interested my brother much, and as the subject is new to him he is a better judge. That is quite a new and perplexing point which you specify about the freshwater fishes during the glacial period.
I have also been very glad to see the article on Lyell, which seems to me to be done by some good man.
I forgot to say when with you—but I then indeed did not know so much as I do now—that the sexual, i.e. ornamental, differences in fishes, which differences are sometimes very great, offer a difficulty in the wide extension of the view that the female is not brightly coloured on account of the danger which she would incur in the propagation of the species.
I very much enjoyed my long conversation with you; and to-day we return home, and I to my horrid dull work of correcting proof-sheets.—Believe me, my dear Wallace, yours very sincerely,