To H. W. Bates   15 April [1868]¹

My dear Bates

Will you look at p. 2 of enclosed note (which please return for it has been & is precious to me), at passage with two blue lines, & tell me the name of the one Papilio, in which there is no difference in colour in the two sexes.²

Also should you call this one species a beautiful or plain one, either relatively to most other Butterflies or to the genus Papilio?—

As I am writing I will ask another question: in your communication to W. H. Edwards in Proc. Ent. Soc. of Philadelphia, you speak of the females of Argynnis Diana, Sagena, Paphia, Papilio Turnus, as departing from the type of their family.—³ Now are these more gaudy than their males? The black female of P. turnus cannot be considered so. With Argynnis are the above females mimetic? I suppose you do not know whether in the case of these species the males or females appear most numerous. I hear from Walsh & Edwards that with P. turnus the males are as 4 to 1 to the females.⁴

H. Doubleday has put me up to good way of estimating number of sexes, viz by priced German list, & the results are striking;⁵ thus with Butterflies out of 114 sp. & vars, in which the sexes differ in price (of course there is no difference in very common species) the males in every case but one are cheapest; so that according to this standard, on an average, there ought to be for every 100 females 143 males.—

To return to the female Butterflies, Wallace thinks in the Pieridæ that in all cases where the female is most beautiful she is mimetic.—⁶

My M.S. on Lepidoptera has run to greater length than I like, but has interested me much; & I owe $\frac{4}{5}$ of my information to your great & unvarying kindness—

Yours very sincerely | Ch. Darwin