My dear Lyell, I never thought of showing your letter to any one. I mentioned in a letter to Hooker that I had been much interested by a letter of yours with original objections, founded chiefly on Natural Selection not having done so much as might have been expected...In your letter just received, you have improved your case versus Natural Selection; and it would tell with the public (do not be tempted by its novelty to make it too strong); yet is seems to me, not REALLY very killing, though I cannot answer your case, especially, why Rodents have not become highly developed in Australia. You must assume that they have inhabited Australia for a very long period, and this may or may not be the case. But I feel that our ignorance is so profound, why one form is preserved with nearly the same structure, or advances in organisation or even retrogrades, or becomes extinct, that Icannot put very great weight on the difficulty. Then, as you say often in your letter, we know not how many geological ages it may have taken to make any great advance in organisation. Remember monkeys in the Eocene formations: but I admit that you have made out an excellent objection and difficulty, and I can give only unsatisfactory and quite vague answers, such as you have yourself put; however, you hardly put weight enough on the absolute necessity of variations first arising in the right direction, videlicet, of seals beginning to feed on the shore.
I entirely agree with what you say about only one species of many becoming modified. I remember this struck me much when tabulating the varieties of plants, and I have a discussion somewhere on this point. It is absolutely implied in my ideas of classification and divergence that only one or two species, of even large genera, give birth to new species; and many whole genera become WHOLLY extinct...Please see page 341 of the 'Origin.' But Icannot remember that I have stated in the 'Origin' the fact of only very few species in each genus varying. You have put the view much better in your letter. Instead of saying, as I often have, that very few species vary at the same time, I ought to have said, that very few species of a genus EVER vary so as to become modified; for this is the fundamental explanation of classification, and is shown in my engraved diagram...
I quite agree with you on the strange and inexplicable fact of Ornithorhynchus having been preserved, and Australian Trigonia, or the Silurian Lingula. I always repeat to myself that we hardly know why any one single species is rare or common in the best-known countries. I have got a set of notes somewhere on the inhabitants of fresh water; and it is singular how many of these are ancient, or intermediate forms; which Ithink is explained by the competition having been less severe, and the rate of change of organic forms having been slower in small confined areas, such as all the fresh waters make compared with sea or land.
I see that you do allude in the last page, as a difficulty, to Marsupials not having become Placentals in Australia; but this I think you have no right at all to expect; for we ought to look at Marsupials and Placentals as having descended from some intermediate and lower form. The argument of Rodents not having become highly developed in Australia (supposing that they have long existed there) is much stronger. I grieve to see you hint at the creation "of distinct successive types, as well as of a certain number of distinct aboriginal types." Remember, if you admit this, you give up the embryological argument (THE WEIGHTIEST OF ALL TO ME), and the morphological or homological argument. You cut my throat, and your own throat; and I believe will live to be sorry for it. So much for species.
The striking extract which E. copied was your own writing!! in a note to me, many long years ago--which she copied and sent to Mme. Sismondi; and lately my aunt, in sorting her letters, found E.'s and returned them to her...I have been of late shamefully idle, i.e. observing (Drosera) instead of writing, and how much better fun observing is than writing.
Yours affectionately, C. DARWIN.
CHARLES DARWIN TO C. LYELL.
15 Marine Parade, Eastbourne, Sunday [September 23rd, 1860].
My dear Lyell, I got your letter of the 18th just before starting here. You speak of saving me trouble in answering. Never think of this, for I look at every letter of yours as an honour and pleasure, which is a pretty deal more than I can say of some of the letters which I receive. I have now one of 13CLOSELY WRITTEN FOLIO PAGES to answer on species!...
I have a very decided opinion that all mammals must have descended from a SINGLE parent. Reflect on the multitude of details, very many of them of extremely little importance to their habits (as the number of bones of the head, etc., covering of hair, identical embryological development, etc. etc.). Now this large amount of similarity I must look at as certainly due to inheritance from a common stock. I am aware that some cases occur in which a similar or nearly similar organ has been acquired by independent acts of natural selection. But in most of such cases of these apparently so closely similar organs, some important homological difference may be detected. Please read page 193, beginning, "The electric organs," and trust me that the sentence, "In all these cases of two very distinct species," etc. etc., was not put in rashly, for I went carefully into every case. Apply this argument to the whole frame, internal and external, of mammifers, and you will see why I think so strongly that all have descended from one progenitor. I have just re-read your letter, and I am not perfectly sure that I understand your point.