My dear Wallace

Your kind & sympathetic letter pleased me greatly & did me good, but as you are so busy, I did not answer it.³ I write now because I have just received a very remarkable letter from Fritz Müller (with butterflies’ wings gummed on paper as illustrations) on mimicry &c.⁴ I think it is well worth your reading, but I will not send it, unless I receive a $\frac{1}{2}$^d card to this effect.⁵ He puts the difficulty of first start in imitation excellently, & gives wonderful proof of closeness of the imitation.⁶ He hints a curious addition to the theory, in relation to sexual selection which you will think madly hypothetical: it occurred to me in a very different class of cases, but I was afraid to publish it.—⁷ It w^d. aid the theory of imitative protection, when the colours are bright.— He seems much pleased with your caterpillar theory.⁸ I wish the letter c^d be published, but without coloured illustrations w^d, I fear, be unintelligible.

I have not yet made up my mind about Wright’s Review: I shall stop till I hear from him: your suggestion w^d. make the Origin, already too large, still more bulky.—⁹ By the way did M^r Youmans of the U. States apply to you to write a popular sketch of Natural Selection? I told him you w^d. do it immeasureably better than anyone in the world.—¹⁰ My head keeps very rocky & wretched, but I am better.

Ever yours | Most truly | C. Darwin