To Fritz Müller 22 February 
Down Bromley Kent
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 shd be very much obliged to you if you wd. 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 wd have to be excluded. I think you will easily see what I wish to learn. A priori it wd never have been anticipated that insects wd 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. & shd 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 cd answer me even one or two I shd 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 shd do this immediately after the completion of my present book. In this case I shd 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 wd 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.
Queries about Expression
(1) Is astonishment expressed by the eyes & mouth being opened wide & by the eyebrows being elevated?
(2) Does shame excite a blush, when the colour of the skin allows it to be visible?
(3) When a man is indignant or defiant does he frown, hold his body & head erect, square his shoulders & clench his fists?
(4) When considering deeply on any subject or trying to understand any puzzle does he frown, or wrinkle the skin beneath the lower eyelids?
(5) When in low spirits are the corners of the mouth depressed, & the inner corner or angle of the eybrows raised by that muscle which the French call the ‘grief’ muscle?
(6) When in good spirits do the eyes sparkle with the skin round & under them a little wrinkled & with the mouth a little extended?
(7) When a man sneers or snarls at another is the corner of the upper lip over the canine teeth raised on the side facing the man whom he addresses?
(8) Can a dogged or obstinate expression be recognized, which is chiefly shewn by the mouth being firmly closed, a lowering brow & slight frown?
(9) Is contempt expressed by a slight protrusion of the lips & turning up of the nose with a slight expiration?
(10) Is disgust shewn by everted lower lip, slightly raised upper lip with sudden expiration something like incipient vomiting?
(11) Is extreme fear expressed in the same general manner as with Europeans?
(12) Is laughter ever carried to such an extreme as to bring tears into the eyes?
(13) When a man wishes to shew that he cannot prevent something being done, or cannot himself do something does he shrug his shoulders, turn inwards his elbows, extend outwards his hands & open the palms?
(14) Do the children when sulky pout, or greatly protrude their lips?
(15) Can guilty or sly or jealous expressions be recognized? tho’ I know not how these can be defined.
(16) As a sign to keep silent is a gentle hiss uttered?
(17) Is the head nodded vertically in affirmation, & shaken laterally in negation?
Observations on natives who have had little communications with Europeans would be of course the most valuable, tho’ those made on any natives wd be of much interest to me. General remarks on expression are of comparatively little value. A definite description of the countenance under any emotion or frame of mind wd possess much more value, & an answer to any one of the foregoing questions wd be gratefully accepted.
Down Bromley Kent
Observations on orchid self-sterility.
Wants information on characters that may have originated through sexual selection in lower animals.
Encloses queries on expression.