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 13 CLOSELY 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.