My dear Lyell

I never thought of showing your letter to anyone.—¹ I mentioned in letter to Hooker that I had been much interested by letter of yours with original objections founded chiefly on natural selection not having done as much as might have been expected.² Even if his mind had not been full of Syria he could never have conjectured your precise line of thought.—³ In your letter just received you have improved your case versus N. Selection;⁴ & it would tell with public (do not be tempted by its novelty to make it too strong) yet it 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, & 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 I cannot 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 Eocene formations.

I see I misunderstand you⁵

But I admit that you have made out an excellent objection & difficulty: & I can give only unsatisfactory & quite vague answers, such as you have yourself put. I think, 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 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. & I have a discussion somewhere on point. It is absolutely implied on my ideas of classification & divergence that only one two species of even large genera give birth to new species; & many whole genera become wholly extinct, ie none of the species. Please see p. 341 of Origin. But I cannot remember that I have stated in 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 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 & is shown in my engraved diagram.⁶

I can see no way of estimating the number of species “extinguished in a given time”.

I do not think passages pp. 168 & 313 are really contradictory; mere variability & variability taken advantage of & selected are widely different considerations.— Look at variability of rudimentary organs,—which from being useless may & do vary greatly, & yet are not selected, & so do not become permanently altered & so make a new specific form.

Am I not right that Ammonites have become wholly extinct in a remarkably sudden manner relatively to most other families; I meant only this, but I see I have not been nearly guarded enough.—⁷ I cannot find that I have, but I thought I had, alluded to much extinction, (as well as of modification) in the great blank intervals between our formations. (I see I have in regard to Ammonites at bottom of p. 321)    But does it not strike you that the extinction of Ammonites has been wonderfully sudden?—   I may add that I suspect, & have always suspected, that in southern Chile, near Concepcion, there are beds, which seem to be Tertiary with Ammonites & Baculites.—

I quite agree with you on the strange & inexplicable fact of Ornithorhynchus having been preserved; & 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 best known countries. I have got a set of notes somewhere on the inhabitants of fresh water; & it is singular how many of these are ancient or intermediate forms, which I think is explained, by the competition having been less severe & 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 last page as a difficulty, 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 & placentals as having descended from some intermediate & 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.—”⁸ Remember if you admit this, you give up the embryological argument (the weightiest of all to me) & the morphological or homological argument.—   You cut my throat, & your own throat; & I believe will live to be sorry for it.—   So much for Species.—

The striking extract which Emma copied was your own writing!! in a note to me many long years ago, which she copied & sent to Mad. Sismondi; & lately my Aunts in sorting her letters, found Emma’s & returned them to her.—⁹

Etty has a wish to try the sea, & we all start there in about a week.—   I have been of late shamefully idle; ie observing instead of writing & how much better fun observing is than writing.—

Yours affect | C. Darwin