Charles Darwin an Ernst Haeckel, Down, 8. Oktober 
DOWN. | BROMLEY. | KENT. S. E. | Aug.
I thank you sincerely for your letter & the confidence you repose in me. I have been deeply interested in what you say about your poor wife. Her expression ina the photograph is charming. I can to a certain extent understand what your feelings are, for I am fortunate enough to know what a treasure a wife can be & no one thought is so painful to me as the possibility of surviving her.
As you seem interested || about the origin of the “Origin” & I believe do not say so out of mere compliment, I will mention a few points. When I joined the “Beagle” as Naturalist I knew extremely little about Natural History, but I worked hard. In South America three classes of facts were brought strongly before my mind: 1stly the manner in which closely allied species replaceb species in going Southward.
2ndly the close affinity of the species inhabiting the Islands near to S. America to those || proper to the Continent. This struck me profoundly, especially the difference of the species in the adjoining islets in the Galapagos Archipelago. 3rdly the relation of the living Edentata & Rodentia to the extinct species. I shall never forget my astonishment when I dug out a gigantic piece of armour like that of the living Armadillo.
Reflecting on these facts & collecting analogous ones, it seemed to me probable that allied species were descended from a common parent. But for some years I could not conceive how each form became so excellently adapted to its habits of life. I then began systematically to study domestic productions, & || after a time saw clearly that man’s selective power was the most important agent. I was prepared from having studied the habits of animals to appreciate the struggle for existence, & my work in Geology gave me some idea of the lapse of past time. Therefore when I happened to read “Malthus on population” the idea of Natural selection flashed on me. Of all the minor points, the last which I appreciated was the importance & cause of the principle of Divergence. I hope I have not wearied you with this little history of the “Origin” –
I quite agree c with what you say about Kölliker; there is || a capital review of him by Huxley in the Number just published of the “Natural History Review”. This letter was begun several weeks ago, but I have delayed finishing it from having little strength & other things to do. Will you have the kindness to tell this to Prof. Gegenbaur as an apology for not having thanked him for the honour he has done me in sending me his work. By a strange chance I dissected several months ago the hind foot of a toad & was particularly curious to understand what the additional bones were, & this point I see will now be explained to me. As I know from one of the papers which you have || sent me that you have attended to Entomostraca it has occurred to me that you might like to have a copy of my Vol. on the Balanidæ, of which I have a spare copy & would with pleasure send it if you wish for it, & will tell me how to forward it.
With sincere respect
Believe me my dear Sir
yours very faithfully
a eingef.: in; b korr. aus: replaces; c gestr.: agree