My dear Sir

I am much obliged for the reference: I had forgotten what the point was, & I am delighted to recover, & be able to quote it.¹

I will now discuss, as far as it can be done briefly, the point mooted in your letter;² but brevity will make what I say seem dogmatic   I may premise that I have stated that all organs necessary for reproduction must be attributed to nat. selection; & that the term sexual selection is applicable only when an advantage is gained by a struggle between two or more individuals of the same sex.³ There is good evidence (& I have lately collected more) that males are generally more variable than females:⁴ you may consider this as due to masculine “vital force”; but as yet I sh^d. prefer to speak of it as variability, without pretending to say a word against your idea; though some authors hold an opposite view to yours.—⁵

But I do not think this variability or extra vital force will account for well-marked sexual differences of many kinds, even of colour, unless the variations are in some manner accumulated. In my judgment, (1) when the 2 sexes are alike, let them be ever so brilliantly coloured, there is no evidence (sometimes analogy may be a feeble guide) of the action of S. selection; unless indeed the two sexes closely resemble one sex of a closely allied form, in which the sexes differ.

(2.) If the two sexes differ much in colour, especially if the male is the more brilliant, then there seems to me some probability that the brighter colours have been gained by S. selection, in as much the two sexes have generally been exposed to identical conditions. no doubt the two sexes must differ somewhat in constitution, & though exposed to the same conditions, may come to differ somewhat, without any selection; & such cases can be shown to occur.

(3) The evidence of the action of s. selection becomes good only when the one sex, (generally the male) takes elaborate pains to display his colours or other ornaments to the other sex. For can we believe that the male would thus act, of which there is abundant evidence, unless such display was an advantage to him? & if it be an advantage to him we have the groundwork for S. selection.

I presume that you will hardly suppose that the elegantly constructed stridulating organs of insects, which often require two parts of the body to be modified in adaptation to each other, could have been formed by the masculine vital force, without any guiding power; & if these structures have been modified through sexual selection, why not colour?

As spiders (as far as I am aware) do not display themselves, the evidence that s. selection has been here at work is not strong; but Canestrini in recent Italian paper asserts positively that the female selects one out of several males;⁶ & why sh^d not colour guide her, as something must guide her.—

I fear that I shall have wearied you with my dreadfully bad writing & clumsily expressed letter; but I have little strength to spare. With many thanks | Believe me, my dear Sir, Yours very faithfully | Ch Darwin

P.S | I collected spiders largely during the voyage of the Beagle, & deposited them in the B. Museum;⁷ but I daresay they are all decayed now.—