Dear Sir.

I send you by to-day’s post a small work of mine entitled “On Protoplasm &c”. The title page sufficiently indicates its object. There is nothing that would afford me greater pleasure than to know that what I have said in defence of your theory has your approval.¹

While reading your “Expression of the Emotions” another explanation of the firm closure of the lips in the expression of determination or decision has occurred to me.² Firm contraction of muscles protects the bones underneath from injury, so much so that a strong muscular man can break a bar of iron accross his arm without any injury being done to the bone. And if we suppose that aboriginal man fought with blunt instruments or with his fists firm closure of the lips would be of great advantage to him as a protection to his jaws and teeth. Imagine the difference it would make to a man to get a blow on the lower jaw when all the muscles are relaxed instead of when they are so firmly contracted as to produce clenching of the teeth. In the former case it is very probable that a moderate blow would produce dislocation of the jaw, while in the latter case it would have very little effect, because the jaw is made to form almost one solid piece with the rest of the head. Nor is this all. If you try the experiment of closing the lips firmly and then giving them a sharp blow with your knuckles, and afterwards giving a similar blow when the lips are relaxed you will find that there is an immense difference in the sensation experienced. When the lips are firmly closed the blow gives little or no pain, and the teeth do not suffer any shock; but when the lips are relaxed the teeth receive a severe shock, and the lips are so sharply compressed between them and the knuckles that one can readily believe that a somewhat stronger blow would cut the lips through. In the expression of determination the muscles about the mouth are not simply contracted, but the lips are pressed strongly against each other, and their borders are drawn inwards so that each lip forms an arch with its convexity outwards. When the lips are in this position a direct blow will fall upon the crowns of two arches, and its force will consequently be resolved so that a large part of it will be diffused along the flanks of the arches, and such a resolution of the force will tend to protect the teeth still further. It is probable that the most determined efforts of aboriginal man would require to be put forth in his personal encounters with his fellow man, and experience would soon teach him the value of firm compression of the lips as a means of defence, and as these experiences became multiplied and transmitted through a long succession of generations the compression of the lips upon any strong determination being formed would become organised as an instinct. That this is the way in which the instinct has become developed is made more manifest when we notice that when a man expresses strong determination that compression of the lips is frequently accompanied by clenching of one fist generally of the right hand, which is held in a threatening attitude or brought down with a thump on the table or some other object near.³

Sir C Bell’s hypothesis, of which Gratiolet’s is a mere modification, would explain why the lips are closed; and Piderit’s hypothesis would explain why they are firmly closed when a man is determined.⁴ But during determination the lips are not only firmly closed; but they are closed in a special way, and the muscles of the jaw are contracted in a special way, and the speciality of these acts shows that they must have been organised into the race by the muscles concerned having to perform a special function during acts requiring strong determination, and I can think of no special function which they could perform except protecting the jaws and teeth during fighting.

It would be interesting to know whether compression of the lips during determination is equally characteristic of all races, and if not, whether this mode of expression is more characteristic of those races amongst whom the pugilistic art has become highly developed. Certain it is that pugilists are remarkable for their powerful lips. My limited knowledge of the subject does not enable me to answer these and many other similar questions which present themselves for solution. I submit the theory, however, to you and wish you to take it for what it is worth.

I may mention that a few very provoking errors have crept into the text of my book. I revised the first proofs carelessly thinking that I should have second proofs; but on applying to the publisher I found to my disappointment that the work was printed off.

With kind regards | Believe me D⁠⟨⁠e⁠⟩⁠ar Sir | yours ve⁠⟨⁠ry⁠⟩⁠ sincerely | Jam⁠⟨⁠es⁠⟩⁠. Ross

Charles Darwin Esq^r F.R.S.