Letter to Charles Darwin February 24, 1875
To Mr. Darwin.
CAMBRIDGE, Feb. 24, 1875.
Your letter of last September, after its long wanderings, reached me at length through 1)r. Gray in link to serve as a Valentine, and gave me much pleasure, of which not the least part was the release I had from the discipline of a doubt whether my long letter of last summer was properly mailed or ever reached you.
It seemed to me, and this was my chief motive in writing, that a letter to one interested especially in some of the many points of investigation which lay loose in my mind, would serve to give them a greater degree of coherency, while preserving for me more freedom than was compatible with the more vigorous requisites of an essay. I have found that writing in any other style is apt to crystallize one's meditations into opinions too fixed for clear, open thought. I was quite willing to submit them, however, as comparative crudities to so friendly a critic, and I am much gratified that you found so little to object to in the letter.
I had thought a little upon the point you make that the two motions of the head, that of denial and that of inspection, are widely different, and had conceived of their grading into each other in the expression of the mixed mental states. I have since made a sort of geometrical analysis of them as extremes of a series of movements. Thus, placing and holding fixedly the tip of the forefinger on the top of the head, the head can only move on an axis through this point and the turning-point
( 331) in the neck. This is one extreme, — the gesture of denial, refusal, warning, &c. By placing the finger successively on the forehead, the tip of the nose, and the chin, the axis of rotation is successfully brought forward by stages toward the horizontal direction it has in the most neutral of critical considerations. But already at the forehead there is a decided element of consideration introduced into the gesture, according to my instinct of interpretation. Professor Lowell is unable to recall distinctly the character of the movement, like our negative, which he saw in Southern Italy, and learned to understand as an affirmative one ; but he is so far interested in the question that he has offered to make inquiries of Signor Monti, an Italian gentleman, a native of Sicily, who formerly taught the Italian language in this College. If, as I hope, he gets the true gesture from him, I will preserve and transmit to you as accurate a description of it as I can.
Very lately, while reading for the first time in my life the Memorabilia of Xenophon, in translation, I came to a passage near the beginning of Chapter iv., Book r, where Socrates gives an interesting statement of the argument from the appearance of design for the existence of the gods ; and I was struck with this sentence : " Is it not—," he asks, "like the work of forethought . . . to make the eyelashes grow as a screen, that the winds may not injure it [the eye] ? To make a coping on the parts above the eves with the eyebrows, that the perspiration from the head may not annoy them ?" It was with the latter query that I was most struck ; for it was a new suggestion to me, and seemed truer than the first. I found that the idea of this use was in the minds of several of my friends ; but whence they derived it, they could not tell, whether from literature or direct experience. One gentleman, formerly much devoted to athletic exercises, told me that, in rowing, the perspiration was often annoying from running into the outer corners of his eyes. His eyebrows are rather thin and short. Dr. W.
(333) James, Instructor in Physiology in the College, who went with Professor Agassiz on his first expedition to South America, says that he spent several hours a day in a part of the expedition fishing in the Amazon, under a scorching sun ; and that the sweat, running from his forehead and drying into a brine, irritated his eyes excessively, so that he was obliged to bathe them frequently in the river. Fishing under a broiling sun in a tropical stream seems not far removed from the conditions of existence of primeval man. I thought that if you had referred to this use of the eyebrows, I should have remembered it ; but I made a cursory, though fruitless search for it.
I have lately read, by the way, the principal additions and corrections in your edition of the " Descent of Man ; " and your less qualified adoption of Mr. Wallace's views on the use of the lay of the hair on the gorilla's forearms gave me another hint toward the little speculation on uses, which I venture to propound at the risk of making another long letter. The survival of the panniculus carnosus in the human forehead and scalp (the latter partially rudimentary), the development of the corrugator muscles, the survival, or perhaps even the development of the eyebrows, and the length of the hair on the head, — all seem to me related to the denuding of the forehead, which doubtless was by sexual selection, or for ornament. The arrangement of the hair on the foreheads of most hairy animals, and in the eyebrows as well as in the eyelashes (which do not serve, as Socrates thought, for screens against the wind), seems to be adapted to keep the rain and perspiration out of their eyes, or to serve for shedding water. Now, the loss of this use in the hair on the forehead would have been a considerable expense for beauty, if the correlative adaptations made for it below and above, in the retention or increase, perhaps, of the hair on the brows, and the increase of length in the hair on the head (to serve as a parting thatch for shedding rain, in place of the old shingles), had not taken
( 334) its place, and laid the foundations for later developments of beauty. The prototypes of the long hairs, or vibrissae, in the eyebrows of some families, perhaps served the same use ; I have met with an instance of this occurring in three successive generations, at least. But the eyebrows are sometimes curly, and may serve (as a friend suggests, who has curly ones, and is one of the three who have had vibrissae) to catch the perspiration and rain, which strokes of the hand would remove from time to time. It occurred to me that, in the same way, a negro's woolly mat might serve to catch a tropical shower, and hold it till he has an opportunity to shake it out. Perhaps the pannicules of the scalp served for this latter purpose. The reversal of direction in the hair bordering the forehead in some monkeys may be for a similar service ; the above suggested use of the panniculus could be experimentally determined in this case. The cowlicks on the foreheads of many children may be relics of, or reversions to, a similar normal arrangement in the straight-haired varieties or races of primeval men. The vibrissae of the brows, especially in curly ones, would have served in former times as gargoyles; as in the nose they apparently serve for joining drops, and extending the conducting and evaporating surfaces of the nasal passages, thus promoting the circulation of the lachrymal ducts.
Other features serving the same important end in vision, of shedding water, I have hinted at above ; namely, the muscles which produce the transverse and vertical furrows of the forehead. Their non-appearance or slight development in childhood indicates the lateness of their acquisition by the race. That these furrows have been serviceable as drains or water courses, taking the place of arrangement in the hair formerly on the forehead, is not inconsistent with the uses of the grief muscles which you seem to me to have fully made out. To compress the eyeball in the more energetic action of the corrugators, and to shade the eyes from excessive light by their
( 335) lesser action, seem to be unquestionable uses. That they should also serve this other use, and that their development has largely depended on this use, are, to me, none the less credible and even probable views.
The inquiry as to which of several real uses is the one through which natural selection has acted for the development of any faculty or organ, or stands and has stood in the first rank of essential importance to an animal's welfare in the struggle for life, has for several years seemed to me a somewhat less important question than it seemed formerly and still appears to most thinkers on the subject. The reasons you give why sexual selection should have had much to do with several of the features, of which f have spoken, I still believe are perfectly valid. The uses of the rattling of the rattlesnake, as a protection, by warning its enemies, and as a sexual call, are not rival uses ; neither are the high-reaching and the fore-seeing uses of the giraffe's neck rivals, but are in the most intimate conspiracy to the same effects. Furthermore, it seems to me presumable that in a long course of development, even in cases of highly specialized faculties, existing uses have risen in succession or alternately to the place of first importance, as in the various uses of the hand. This principle of a plurality of existing uses involves a very important influence in secondary` uses, whether these are incidental or correlative acquisitions, or are the more or less surpassed and superseded ones. They seem to connect in some cases the action of natural selection with the inherited effects of habit and exercise. An animal may, for a comfort or convenience, which bears but little reference to its essential welfare, be indirectly furthering, through exercise, certain faculties which, though rarely called into exercise in functions of prime importance, may nevertheless have, or may come to have, such functions. Thus, the constant or frequent use of the corrugators for forming vertical furrows and draining the forehead into the lachry-
( 336) -mal ducts, or down the nose, or drawing the brows together for shading the eyes, may have been a preparation of them for their rarer but more important surgical service of quickly correcting the circulation of the eyes, and thus keeping the vision keen in conditions of exposure to danger.
There is nothing in this principle which is really new or different from what you have set forth in your works, except the emphasis or prominence I am inclined to give it. The value of a plurality of coexisting uses in making the principles of natural selection and that of the inherited effect of habit co-operate in a larger number of cases and to a greater degree than could otherwise happen, ought to raise the principle from the rank of a scholium to that of a main theorem in the development doctrine. At least, my present interest in one of its possible illustrations makes the matter seem so to me. It is, no doubt, a very interesting inquiry how any given organ or faculty is specially related to essential conditions of an animal's existence ; but it is not so important to the theory of natural selection as it would be if the efficacy of this process depended solely or generally on a single or permanent relation of this sort. The aid, too, which sexual selection gets (and gives) from such an association with habits and natural selection, or through a plurality of uses, is worthy of consideration.
I do not conceive the question whether, in a given case, the coloring of an animal is protective or sexually attractive, is a question of alternatives, of which only one can be true. Sex ual selection may in one case take up what natural selection has laid down, as in lengthening the hair beyond its value as a thatch for keeping the rain from the forehead and eyes. Or this agency having perhaps elaborated in another case the woolly mat of the negro, the hair may then have curled still closer than the task demanded, from its value in holding water; and then, later, sexual selection would return to the artificial cultivation of the African savage's coiffure.
( 337) Among the multitude of topics in my head last summer, one, for which I had no space from the length of my letter, related to a class of gestures used in reflection, meditation, and, I may add, continuous thought or speech under distracting circumstances. To some of these gestures you refer when you say, " Why the hand should be raised to the mouth or ',ce in deep thought is far from clear." I came to this question from the speculations of which I wrote; and I hope —since it would make this letter too long to do so now —to discuss it with you some other time. But I may state here one general conclusion which I had reached. The service on which many gestures seem to be founded appears to be to prevent the attention from wandering, by turning it to something upon which it can readily be kept, and from which it can as readily be recovered. This prevents its wandering too far into the swamp of vague, uncontrollable feelings, such as those of self-attention, visceral sensations, and the reflexes from involuntary movements. The great sensibility of the face, especially about the mouth, seems to me to explain the gesture to which you especially refer ; and even the pressure of the hand on the forehead appears to relate rather to vague sensations in it, thus controlled by the hand, than to any direct effects of the pressure on the action of the brain. But the full justification of these conclusions is a long argument, into which I will not here enter.
I send, in the same mail with this letter, a number of the "Nation," which contains a couple of "Notes" by me about books on evolution. They begin at the foot of page 113.
In this letter, Chauncey expresses the purpose of writing again to Mr. Darwin ; but in the six months of life that remained to him he did not do it. Mr. Darwin's latest note to him was written in reply to this, on March 12. He says, " I write to-day, so that there shall be no delay this time in
( 338) thanking you for your interesting and long letter received this morning. I am sure that you will excuse brevity, when I tell you that I am half killing myself in trying to get a book ready for the press. I quite agree with what you say about advantages of various degrees of importance being co-selected and aided by the effects of use, &c. The subject seems to me well worth further development. I do not think I have anywhere noticed the use of the eyebrows, but have long known that they protected the eyes from sweat. During the Voyage of the `Beagle,' one of the men ascended a lofty hill during a very hot day: he had small eyebrows, and his eyes became fearfully inflamed from the sweat running into them. The Portuguese inhabitants were familiar with this evil, as I well remember from a ridiculous incident : they immediately brought a woman who was suckling a baby to squirt milk from her breast into his eyes ; but he ran away in dismay!
I think you allude to the transverse furrows of the head as a protection against sweat; but remember that these incessantly appear on the forehead of baboons.. . . I have been greatly pleased by the notices in the `Nation.’”