I thank you most sincerely for sending me your Memoir. (160/1. Etude sur l'Espece a l'occasion d'une revision de la Famille des Cupuliferes. "Biblioth. Univ. (Arch. des Sc. Phys. et Nat.)," Novembre 1862.) I have read it with the liveliest interest, as is natural for me; but you have the art of making subjects, which might be dry, run easily. I have been fairly astonished at the amount of individual variability in the oaks. I never saw before the subject in any department of nature worked out so carefully. What labour it must have cost you! You spoke in one letter of advancing years; but I am very sure that no one would have suspected that you felt this. I have been interested with every part; though I am so unfortunate as to differ from most of my contemporaries in thinking that the vast continental extensions (160/2. See Letters 47, 48.) of Forbes, Heer, and others are not only advanced without sufficient evidence, but are opposed to much weighty evidence. You refer to my work in the kindest and most generous spirit. I am fully satisfied at the length in belief to which you go, and not at all surprised at the prudent reservations which you make. I remember well how many years it cost me to go round from old beliefs. It is encouraging to me to observe that everyone who has gone an inch with me, after a period goes a few more inches or even feet. But the great point, as it seems to me, is to give up the immutability of specific forms; as long as they are thought immutable, there can be no real progress in "Epiontology." (160/3. See De Candolle, loc. cit., page 67: he defines "Epiontologie" as the study of the distribution and succession of organised beings from their origin up to the present time. At present Epiontology is divided into geography and palaeontology, "mais cette division trop inegale et a limites bien vagues disparaitra probablement.") It matters very little to any one except myself, whether I am a little more or less wrong on this or that point; in fact, I am sure to be proved wrong in many points. But the subject will have, I am convinced, a grand future. Considering that birds are the most isolated group in the animal kingdom, what a splendid case is this Solenhofen bird-creature with its long tail and fingers to its wings! I have lately been daily and hourly using and quoting your "Geographical Botany" in my book on "Variation under Domestication." LETTER 161. TO HORACE DOBELL. Down, February 16th [1863].