Darwin supposes that terrestrial productions have probably not changed to the same extent as marine organisms. "If the Megatherium, Mylodon...had been brought to Europe from La Plata, without any information in regard to their geological position, no one would have suspected that they had co-existed with sea shells all still living" ("Origin," Edition VI., page 298).); but I should require strong evidence before believing that, in countries at all well-known, so-called Silurian, Devonian, and Carboniferous strata could be contemporaneous. You seem to me on the third point, viz., on non-advancement of organisation, to have made a very strong case. I have not knowledge or presumption enough to criticise what you say. I have said what I could at page 363 of "Origin." It seems to me that the whole case may be looked at from several points of view. I can add only one miserable little special case of advancement in cirripedes. The suspicion crosses me that if you endeavoured your best you would say more on the other side. Do you know well Bronn in his last Entwickelung (or some such word) on this subject? it seemed to me very well done. (567/4. Probably "Untersuchungen uber die Entwickelungsgesetze der organischen Welt wahrend der Bildungszeit unserer Erdoberflache," Stuttgart, 1858. Translated by W.S. Dallas in the "Ann. and Mag. Nat. Hist." Volume IV., page 81.) I hope before you publish again you will read him again, to consider the case as if you were a judge in a court of appeal; it is a very important subject. I can say nothing against your side, but I have an "inner consciousness" (a highly philosophical style of arguing!) that something could be said against you; for I cannot help hoping that you are not quite as right as you seem to be. Finally, I cannot tell why, but when I finished your Address I felt convinced that many would infer that you were dead against change of species, but I clearly saw that you were not. I am not very well, so good-night, and excuse this horrid letter. LETTER 568. TO J.D. HOOKER. Down, June 30th [1866].