My dear Sir

Your last letter of Jan. 1. is more valuable to me even than some of your previous ones.¹ The fact about the own-pollen being poisonous is quite extraordinary; I will quote your remarks & explanation after giving your former facts.² Can the cause of the decay be due to parasitic cryptogams?³ I sh^d be very much obliged to you if you w^d. inform me soon whether Oncidium flexuosum is a native of your district.⁴ These observations of yours will be a most valuable addition to my discussion on self-impotent plants.⁵ There never was a more curious case than that of the rudimentary condition of the organs in Catasetum. It explains the fact, which I have been assured of, that Catasetum in some countries not rarely produces seed-capsules.⁶ Your facts also about the sucking in of the pollen-masses & of the dispersal of the seeds in Gesneria are all quite new to me.⁷ I hope you keep a record of all your miscellaneous observations, for you might thus hereafter publish a wonderful book.

Although you have aided me to so great an extent in many ways, I am going to beg for any information on two other subjects. I am preparing a discussion on “sexual selection”, & I want much to know how low down in the animal scale sexual selection of a particular kind extends.⁸ Do you know of any lowly organized animals, in which the sexes are separated and in which the male differs from the female in arms of offence, like the horns & tusks of male mammals, or in gaudy plumage & ornaments as with birds & butterflies? I do not refer to secondary sexual characters by which the male is able to discover the female, like the plumed antennæ of Moths, or by which the male is enabled to seize the female, like the curious pincers described by you in some of the lower crustaceans.⁹ But what I want to know is how low in the scale sexual differences occur which require some degree of self-consciousness in the males, as weapons by which they fight for the female, or ornaments which attract the opposite sex. Any differences between males & females which follow different habits of life w^d have to be excluded. I think you will easily see what I wish to learn. A priori it w^d never have been anticipated that insects w^d have been attracted by the beautiful colouring of the opposite sex, or by the sounds emitted by the various musical instruments of the male Orthoptera.¹⁰ I know no one so likely to answer this question as yourself. & sh^d be grateful for any information however small.

My second subject refers to expression of countenance, to which I have long attended, & in which I feel a keen interest; but to which unfortunately I did not attend when I had the opportunity of observing various races of Man.¹¹ It has occurred to me that you might without much trouble make a few observations for me in the course of some months on Negros, or possibly on native S. Americans; though I care most about Negros. Accordingly I enclose some questions as a guide & if you c^d answer me even one or two I sh^d feel truly obliged. I am thinking of writing a little essay on the origin of Mankind, as I have been taunted with concealing my opinions; & I sh^d do this immediately after the completion of my present book. In this case I sh^d add a chapter on the cause or meaning of Expression.¹²

With gratitude for all your great kindness & sincere admiration of all your powers of observation I remain | my dear Sir yours very | sincerely C. Darwin

PS. | You must not give yourself any great trouble about these questions, but possibly you might in the course of a few months be able to observe for me one or two points.

I have sent copies to other quarters of the world

an answer within 6 or 8 months w^d be in time.—¹³

If you kept the subject occasionally before your mind, an opportunity of observing some few cases, such for instance as (4) or (5) or (13) &c would almost certainly occur.—

But you must not plague yourself on a subject which will appear trifling to you, but has, I am sure, some considerable interest.