Evolution Of Special Senses

SENSATION IS the means of communication between an organism and the outside world, — the material universe.

When a nutrient particle comes in contact with the periphery of an ameba, there would be no contractility, no ingestion, were it not for the fact that the cell possesses sensation, of which the various properties of protoplasm are manifestations. The organism would remain unconscious of its environment, — would starve tho surrounded by an ocean of food. The nervous system is avowedly of the lowest order, but the point to be here noted is that it is sufficient for the needs of its own organization. However meager the knowledge thus obtained may be, it immensely transcends no knowledge at all.

Without attempting to trace the stages from the ameba to man, it may suffice to say that there is evident all along the line an elevation of the function of sensation. This has followed the general law of evolution ” from the simple to the relatively complex,” that is, sensation has become specialized. Besides common sensation, man has the so-called five senses : touch, taste, smell, hearing, and sight.


To most people it has never occurred that this beautiful array of talents leaves anything more to be desired. We think we know the material universe because we can touch, taste, smell, hear, and see some of it. But what reason have we to presume that these are the only phases of matter?

The deaf mute has no conception of music. To him it is a sealed book. A race of deaf mutes would be sure that they knew the material universe, because they could touch, taste, smell, and see it. Imagine their idea of a piano or a barking dog.

Is it not thus apparent that we are probably oblivious to many phases of matter? These five senses are like so many doorways, or windows thru which the ego catches glimpses of the outer world.

Instead of saying that we have in matter something we really know, it may be nearer true to say that we really know more of the attributes of mind, about which we are confessedly ignorant. For example, one may know certain facts about a table, that it is two feet wide and three feet long, that it is harder than his knuckles, but the sum of his knowledge may be so meager, and the special facts so unimportant, when compared with all the facts about it, that he may have an entirely erroneous conception.


John Fiske expresses this in his ” Cosmic Philosophy,” thus : ” The doctrine of relativity affirms the existence of an unknowable reality of which all phenomena whatever are the knowable manifestations.” To this unknowable is given the name of noumenon or the real thing, in distinction from phenomenon, which is the increment man knows of the real thing.

The story is told that when a missionary visited some Indians and explained to them the Christian theology, with considerable emphasis on the final state of the unredeemed, the chief showed considerable skepticism. With an arrow he drew a small circle in the sand, then a larger circle enclosing the first. Pointing to the inner circle, he said : ” This is what Indian know.” Pointing to the outer circle : ” This is what white man know.” Then sweeping the arrow outside the periphery of the larger circle : ” Out here Indian know just as much as white man.”

As a further illustration may be mentioned the old story of the doctor who denied the existence of the soul, because his dissecting knife failed to reveal anything of this nature. He would believe nothing that he could not see. Then the minister asks him if he can see a pain, and if not why he believes in pain. Of course he replies that he feels the pain, whereupon the clergyman replies that he knows of spiritual matters by other senses, which are (perhaps) in the doctor undeveloped.

The mental attitude which is undaunted by the restraints of the evidence of the senses is undoubtedly dangerous to the attainment of truth. To claim that one possesses transcendental royal roads to knowledge is intolerable to the scientific mind, yet a recognition of the limitations of our knowledge of the simplest fact must have a salutary effect upon our egotistic tendencies.

This relativity is especially apparent when we consider the special senses. For example, the membrana tympana is made to vibrate in unison with waves of air emanating from some sonorous body, but those waves must reach a velocity of sixteen per second before they are audible to the human ear. We have abundant evidence that the solar spectrum extends far beyond the visible spectrum. The wonderful development of the sense of smell in the dog simply baffles our comprehension.

” Thus we learn,” as Spencer says in his ” First Principles,” ” the greatness and the littleness of the human intellect — its power in dealing with all that comes within the range of experience; its impotence in dealing with all that transcends experience.”


As the more delicate senses have evolved from simple sensitiveness to contact or the touch sense, it may be interesting to note that a simple refinement of this contact sense is evident thruout. Taste requires that the article brought in contact with the sense organ shall be soluble. In smell, which is closely allied, the contact of finely divided particles of matter is effected by their floating in the air. The matter may be as finely divided as the gaseous state. Hearing requires only the contact of air itself in waves, but still contact is here. The hammer-like form of the organ of Corti is very suggestive of this essential element contact.

The end organs of the optic nerve are the rods and cones of the retina. The contact here is the extreme refinement of touch. The ether wave initiated by the luminous body is condensed by the lenticular system of the eye into an irritant point, the focus, which touches the retina. This principle of contact and the fact that the special senses are merely a refinement of the same are of immense advantage in explaining some of our sight perceptions.

The other general principle which should be recognized is the outward reference of sensation. The common experience of striking the ulnar nerve at the elbow and feeling the sensation at the tip of the little finger illustrates a general principle which applies to all the senses.

This outward reference may be made to extend beyond the finger. Tap the floor with a somewhat flexible cane with the eyes closed and we feel the contact at the end of the cane.

Perhaps we have all had the unpleasant experience of ” seeing stars ” from a blow on the eye. This is because an irritation of the optic nerve gives a sensation of light which is referred outward into space. This is also proof of the specialization of the optic nerve. Moreover, the testimony of those from whom it has been necessary to remove the eye without an anesthetic is that, when the optic nerve is divided, not pain but a flash of light is the resulting sensation.

This outward reference of the sense of hearing is one means of estimating direction. Notice the in-voluntary turning of the head so as to put the auricle at right angles to the sound wave. This is in obedience of the general law that all irritations reaching these special senses are referred back to their source.


The refinement of touch and outward reference reaches its highest development in sight.

Primitive eyes and ears bear a strong resemblance to each other. Later the eye has for its prototype the pigment spot. This suggests that sight developed first as an actinic susceptibility, the pigment spot absorbed the heat of the luminous body, and thus became conscious of that which had heretofore been an unknown world.

It is necessary to touch upon the embryology of the eye only for its psychological bearing, that is, the development of sight. Notice then that the sensitive area becomes depressed, then cupped. This shape protects better and also increases the recognition of direction. For greater protection the cavity becomes closed and the cornea and Iens develop. For what purpose ? Evidently to collect the scattered rays of light and converge them, and focus them in a minute point on the retina. Acute sight demands perfect focusing.

Now let us apply the general law of sense perception and the outward reference of sensation to one of the psychological problems of sight, one which has been very generally misinterpreted.

The Inverted Image. It is well known that the image on the retina is inverted. How does it hap-pen that we see objects erect ? The writer devoted considerable attention to this subject some years ago, ransacking all attainable literature.

Most of the physiologies agree that it is wholly the result of experience, that the child learns by touch to reinvert the retinal picture. Foster’s explanation is as follows :

” As a matter of fact the field of vision, in one important particular, does not correspond to the field of external objects. The image is inverted. The rays of light proceeding from an object which by touch we know to be on what we call our right-hand fall on the left-hand side of the retina. If, therefore, the field of vision corresponded to the retinal image the object would be seen on the left hand. We, how-ever, see it on the right hand, because we invariably associate right hand tactile localization with left hand visual sensation. That is to say, the field of vision, when interpreted by touch, is a reinversion of the retinal image.”

Martin, in his work on ” The Human Body,” says : ” A new-born child, even supposing it could use its muscles perfectly, could not seize a reachable object which it saw. It would not yet have learned that attaining a point exciting that part cf the retina above the fovea (center) meant reaching a position in space below the visual axis ; but very soon it learns that things near its brow, that is, up, excite certain visual sensations, and objects below its eyes, others; and learns to interpret retinal stimuli, so as to localize accurately the directions, with reference of its eyes to outer objects, and never henceforth gets puzzled by retinal inversion.”

These two statements are fairly representative, and altho clear and lucid, are not only inadequate, but erroneous.

First. This reversal by one sense, the tactile, of the testimony of the outer world, as given by Another sense, the visual, is not analogous to the other special senses; and during the learning lapses would occur and pathology would furnish instances of mistakes.

Second. Certain forms of congenital blindness, such as cataracts and complete closure of the pupil, can be remedied by operation. These children learn by touch the correct (erect) position of objects, and their first impression when sight is restored would be an inversion of the object, according to the current theory. As far as the author knows no case of this kind has ever been recorded.

Dr. J. L. Minor, Memphis, Tenn., reported to the writer in November, 1898, two cases of congenital cataracts. The patients were brothers between thirty and forty years of age, and had never seen. After removing the cataracts the doctor kept these men under observation for a month, and assures us ” there was never even a suggestion of inverted images.”

The case of Rev. Mr. Hanna, reported by Sidis, who after falling from his carriage lost all memory of his former life experience, is a unique bit of evidence. He was as a newly born infant opening his eyes for the first time on the world. So totally obliterated from memory were the experiences of his past life that even the simplest mental processes, like the appreciation of distance, form, size, and magnitude, were effaced from his mind, but objects were seen erect.

Mr. Hanna’s subsequent statement is as follows : ” The eyes suddenly opened quite involuntarily, and here indeed was a new world of wonder and study. Objects were all alike as to distance, shape, and thickness, but the variety of color was the feature of interest. The room was a great beautiful picture, absolutely without movement or distance beyond the eye.”

Furthermore, this is a misconception, based on the old theory of special immediate creation of perfected organisms, and finds no place in the scientific thought of to-day. It is inconsistent with the facts of evolution, which means a regular progression from the simple to the relatively complex; and the explanation of the phenomena of sight must cover the primitive eye, as well as the perfected organ.

The function of the primitive eye must have been limited to simple sensitiveness to light, and the implication of the law of natural selection, that every minute change which was continued was of greater advantage to its possessor than a preceding stage, absolutely excludes the tactile reinversion theory. The specialization of a sense organ in such a way that its evidence of the outer world was misleading (inverted) until corrected (reinverted) by some other sense organ (touch), could not have been of more ad-vantage to its possessor than a less highly developed organ which could be trusted ; and natural selection would have carefully avoided propagating any such variation.

The inversion is an accomplished fact as soon as the primitive eye is able to locate an external point in space, for it can never see the point till it can tell its direction.

The subsequent changes are all along this line of so perfecting the mechanism that a luminous point in space shall produce an irritant point on the retina. Thus there is no break in the contemporaneous development of the organ of seeing and the psychical act of seeing. They advance with equal step. There is no catastrophe ; no period when the optical apparatus gives wrong impressions to the sensory.

It is indeed strange that ophthalmologists have so universally neglected to elucidate this puzzling phenomenon, and in what follows the author is borrowing from Le Conte, whose explanation is the only satisfactory one which has come to his notice.

A cone of light emitted by a radiant point falling on a convex refracting surface is again converged to a point behind the refracting surface. These two points are called conjugate foci (literally yoked together), because if the radiant be placed at either focus the light will be brought to a point at the other focus.

In the normal eye, at rest, a luminous point twenty feet or more distant is focused as a point on the retina. (Fig. 2.) If the luminous point be nearer than twenty feet, the refracting or bending effect of the eye must be increased (accommodated) so that the conjugate focus shall still be at the retina. (Fig. 3.) This is accomplished by increasing the thickness of the crystalline lens, shown by the dotted line, Fig. 3. As before stated, when the eye is able to reproduce a luminous point in space as an irritant point on the retina, the optical requirements for perfect vision are secured.

Now “outward projection ” means that the retina is touched at this mathematical point, and, like all other senses, it refers the sensation back to the source; in this case along the central line of the pencil of rays.

The size of the image on the retina of the largest object that can be seen at all clearly at one time without moving the eye or the object is probably not greater than three millimeters.

Conversely, the field of vision — of clear vision — with an immovable eye is extremely limited. At the length of the arm a circle the size of the thumb-nail represents all that can be seen clearly, and it is only by rapid excursions that the eye sees in detail those portions that were only outlined before. The field of vision has been compared to a painting which is hazy and indistinct except a circle one-half inch in diameter, in which the most minute details are worked out. This small area may be any portion of the picture which is desired, by turning the eye toward that spot, but no two places at once. It is hard to believe this, for the eye, by rapid excursions, so quickly covers a large field that the separate sensations are fused into one.

Now, the analogy and bearing of this is important when it is understood that we do not see even this one-half inch object as a whole. Each mathematical point of which the object is composed sends out its bundle of rays, which are again converged to a point upon the retina, and from this irritation conveyed to the brain sensation (sight) results, which refers the irritant right back along the ray-line of each pencil to its source. So point after point irritates the retina, and is referred to its appropriate place in space until the luminous object is reproduced in the external world by the outward projection of an infinite number of luminous points.

To make this clear, a very simple object should be used; let it be a vertical line. (Fig. 4.) Now a luminous point at the top of this line produces an irritant point at the lower side of the retina, which is referred back to its source above and seen there in space.

A luminous point at the bottom of this line produces an irritant point on the upper side of the retina, and is referred back to its position in space, which is below. A point from the center is referred back to its place in the centre for the same reason. And so with a point midway from the center to the top, and a point midway from the center to the bottom; and the process goes on simultaneously for each point of which the line is composed, and a sort of mental composite results, which is the exact counterpart of the object, occupying the identical position in space; somewhat as a spiritual body is conceived to be immanent in the natural body.

The solid lines represent the pencils of rays, the broken lines the axial rays of each pencil, showing the direction of the outward reference of the three irritant points illustrated.

To recapitulate : By the law of conjugate foci, a luminous point in space corresponds to an irritant point on the retina. By the law of outward projection it is referred to its proper place in the object, and, as the object is not seen as a whole, an infinite number of such luminous points of which the object is composed are referred to their respective positions, and furnish a synthetic conception, which must be erect because each of its constituent points is in its place.

Since Newton, scientists have recognized that for one body to act upon another at a distance, some medium must intervene. So with sight, the provisional ether is that intermediate something which reaches from the luminous point and ” touches ” the retina. The unlimited extent of the ether gives to sight unlimited range, and, altho light travels 186,000 miles per second, it takes three and a half years for light to reach us from the nearest star.

Professor Dolbear says : ” The light which reaches us to-day from some of the more distant stars left them before America was discovered ; before Jesus was born; before the pyramids were built, and, for all we should be able to see, they may have ceased to exist long ago, tho their light still shines.”

While this work was in press there appeared in Current Literature, September, 1906, an article entitled ” A New Theory of Vision.” This was a quotation from Cosmos, Paris, of the work of Mr. George Poullaine, who claimed to have discovered ” a loop or twist in the optic nerve.” The twist is in the protuberance of the outer and posterior parts of the optical layer of the brain. ” The peculiar conformation explains,” says Cosmos, ” the reinversion of the retinal image.

” The optic nerves, after emerging from the eye-balls, converge to the optic chiasma. Here they partly cross, or seem to exchange part of their fibers. The two nerve bundles thus modified separate and pass around the peduncles. In this part of their course they are known as the optical bands or Gas-set’s hemopic nerves.

” These bands enter the brain. Their fibers can be traced in the pulvinar, where they describe con-centric curves. They can be traced also in other portions of the optical layer, where they are known as Gratiolet’s optic rays.

” In order to more correctly ascertain the paths of the fibers, Poullaine studied and measured sections of the loop made by a horizontal plane and by two vertical planes, anterior, posterior, and transverse.

The theory,” according to Cosmos, makes it easy to understand the mechanism of the reinversion of the retinal image. The double curve effects a complete reversal of the order of the nerve fibers both from top to bottom and from right to left, the two half-turns being exactly equivalent to a half-twist or rotation thru 180 degrees about the axis of the bundle.”

Individual anatomical variations of all parts of the human body are frequently discovered by surgeons. The location and number of the branches of the arteries, veins, and nerves are not the same in different individuals. Is it reasonable to presume that the optic nerves are an exception to this rule? If not, then there are many people whose optic nerves do not twist the exact 180 degrees required.

If erect vision depends upon this condition, it is evident that a faulty or anomalous development would furnish instances of partial or complete inverted vision. Very many cases of this kind are needed to substantiate this very ingenious theory, and none are given. Moreover, the theory is based on the misconception that we see the image, not on the retina, as the tactile reinversion theory presumes, but at some other portion of the cranium. In a very recent work Doctor Souter says : ” It is apparent that the retinal image is always inverted with respect to the object of vision. The mind, however, takes no cognizance of this inversion, since it possesses the power of external projection so that we see not the image but the object in its true position.”

Then the writer proceeds to repeat the old argument of tactile reinversion as follows : ” This power has doubtless been derived thru association with the sense of touch. We have learned that a stimulus conveyed to the brain from the upper part of the retina proceeds from an object situated below the eye, and vice versa, and that a stimulus on the temporal side of the retina must proceed from an object on the nasal side of the eye, and vice versa.”