My dear M^r. Thwaites

I thank you very sincerely for your letter & am much pleased that you go a little way with me.² You will think it presumptuous, but I am well convinced from my own mental experience, that if you keep the subject at all before your mind, you will ultimately go further. The present vol. is mere abstract & there are great omissions— one main one, which I have rectified in the foreign Editions is an explanation (which has satisfied Lyell, who made same objection with you) why many forms do not progress or advance.³ (& I quite agree about some retrograding)— I have also M.S. discussion on beauty—but do you really suppose that for instance Diatomaceæ were created beautiful that man after millions of generations sh^d. admire them through the microscope?—⁴

I sh^d. attribute most of such structures to quite unknown laws of growth; & mere repetition of parts is to our eyes one main element of beauty. When any structure is of use (& I can show what curiously minute particulars are often of highest use) I can see with my prejudiced eyes no limit to the perfection of the coadaptations which could be effected by natural selection.—

I rather doubt whether you see how far, as it seems to me, the argument from Homology & Embryology may be carried. I do not look at this as mere analogy. I would as soon believe that fossil shells were mere mockeries of real shells, as that the same bones in foot of dog & wing of bat—or the similar embryo of mammal & bird—had not a direct signification & that signification can be unity of descent or nothing. But I venture to repeat how much pleased I am that you go some little way with me. I find a number of naturalists do the same, & as their halting places are various & I must think arbitrary, I believe they will all go further. As for changing at once ones opinion; I would not value the opinion of a man who could do so: it must be a slow process.—

Thank you for telling me about the Lantana;⁵ & I sh^d. at any time be most grateful for any information which you think would be of use to me.— I hope that you will publish a list of all naturalised plants in Ceylon, as far as known, carefully distinguishing those confined to cultivated soils alone. I feel sure that this most important subject has been greatly undervalued.

With hearty thanks for your letter | Believe me | Yours very sincerely | C. Darwin