P.S.--I suppose you have read the last number of H. Spencer; I have been struck with astonishment at the prodigality of original thought in it. But how unfortunate it is that it seems scarcely ever possible to discriminate between the direct effect of external influences and the "survival of the fittest." * * * * * _9 St. Mark's Crescent, Regent's Park, N.W. Nov. 19, 1866._ Dear Darwin,--Many thanks for the fourth edition of the "Origin," which I am glad to see grows so vigorously at each moult, although it undergoes no metamorphosis. How curious it is that Dr. Wells should so clearly have seen the principle of Natural Selection fifty years ago, and that it should have struck no one that it was a great principle of universal application in nature! We are going to have a discussion on "Mimicry, as producing Abnormal Sexual Characters," at the Entomological to-night. I have a butterfly (Diadema) of which the female is metallic blue, the male dusky brown, contrary to the rule in all other species of the genus, and in almost all insects; but the explanation is easy--it mimics a metallic _Euploea_, and so gets a protection perhaps more efficient than its allies derive from their sombre colours, and which females require much more than males. I read a paper on this at the British Association. Have you the report published at Nottingham in a volume by Dr. Robertson? If so, you can tell me if my paper is printed in full. 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 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.