Popular Science Monthly/Volume 21/May 1882/The Development of the Senses
IN the fifth century before Christ, Democritus declared that the senses of sight, hearing, smell, and taste were merely modifications of the sense of touch. Aristotle ridiculed his theory, and so, stamped with his disapproval, it lay untouched for two thousand years, until Telesius, an Italian of the sixteenth century, revived it.
Strange to say, all that modern science has accomplished in embryology and zoölogy tends to confirm this theory of Democritus, that these four senses are only specializations of the universal sense—the sense of touch. In the embryo of all animals the organs of these four senses first appear as infoldings of the outer germinal layer, the ectoderm, from which the outer skin also develops. At an early stage they are all simple pockets in the outer covering. If the history of the embryo is to be taken as the miniature of the history of the race—that is, if the individual in its development follows the same course that the race has followed, and it seems reasonable to suppose that this is the case—it is easy to see the importance of this evidence.
In the animal kingdom the sense of touch is universal; it is even found in those lowest animals, the protozoa, which are only masses of simple protoplasm. But, if this animal with its one sense is to become higher, there must be a division of labor; there is too much work for one sense to do properly, and by a quantitative modification this primitive sense is to become qualitatively different in parts, and this qualitative difference is the difference which we notice between the sense of touch and the other senses of the higher animals; it has come about by an accumulation of the sense of touch.
The waves of air which fall on the body of this protozoan as heat are capable of a higher rendering, they will signify more than heat to the proper organ for perceiving them, they will give the sensations of light and colors. The simplest eyes are merely pigment-spots in the skin, they merely distinguish heat from cold and light from darkness; but later, by the formation of a lens and sensitive membrane, the external world is revealed in all its variety.
The eye is first found in the sea-anemone, where it is merely one of these pigment spots. But all that the most complete eye can give to us is a field of gradated colors. In itself this field of colors conveys no information to us. It must be explained before it can be of any practical use to us, and this necessary explanation can only be furnished by our sense of touch. That is, distance, magnitude, and shape are not directly perceived by the eye, but are suggested by certain objective gradations of color which have been associated with them in our past experience. Thus, sight appears as entirely dependent upon touch for its usefulness. This theory was first advanced by Bishop Berkeley in his famous "Essay toward a New Theory of Vision," and was afterward confirmed in a very wonderful way by some experiments made by Dr. Cheselden, of London. A young man had been blind from his birth on account of cataracts. These were removed by Dr. Cheselden, and he suddenly received his sight. At first he could perceive no such thing as distance or form. Only by repeatedly touching objects could he bring himself to realize that certain experiences of touch were always associated with certain gradations of color. Gradually he connected the sensations of sight with the sensations of touch, and in time became as insensible as we are to their true relation.
The ear first appears, in the jelly-fish, as a pocket in the outer skin. In this simple condition it serves as a general indicator of violent air-motion. But as the animal becomes higher there is a demand for a nicer perception of sound, and this pocket is closed and finally is provided with a complicated acoustic apparatus, in the same way that the eye is provided with a lens, which renders into terms of noise and music those air-waves which to the rest of the skin are imperceptible.
But a sound conveys no more information in itself than does the field of colors presented by the eye; only when we can tell from what it comes, and what consequences have been connected with it in our past experience, does it have any practical meaning to us. And, again, this explanation can only be furnished by our sense of touch, or by our sense of sight, which, as we have seen, is entirely dependent upon our sense of touch.
The senses of smell and taste should properly be resolved into one sense, for they are probably only late modifications of the same property of the mucous membrane lining the mouth and nose. This membrane is only an invading growth of the skin surrounding the mouth, so, morphologically, this sense is the same as the two just examined. The sense of smell is undoubtedly present in some insects, as, for instance, in the burying beetles, and may perhaps be found lower.
In man this double sense is undoubtedly retrogressive, and probably reaches its highest development in some of the lower mammalia. With us it is at best only a source of transitory pleasure, and seems in no way to contribute to our higher mental development.
But the senses of sight and hearing are very different in this respect. If Darwin is right, they have played a most important part in the evolution of the past as the instruments of sexual selection. And, in the future development of our race, it seems as if their perfection would be reached only with the perfection of the human mind. For if the impulse to development is given from without by the environment, these organs must be continually improved so as to perceive the nicer and nicer distinctions in the environment which will be the means of elevating the mind. If the impulse to development is given from within the mind, these sense-organs must be developed more highly in order to provide the enlarging mind with the continually nicer perception which it will require.
Man's mind develops, not his body. With the exception of these two sense-organs, his body has been nearly stationary for thousands of years, but these two organs respond to comparatively little change. The ear of the savage differs from the ear of the civilized man more than the two men differ in any other respect.
Touch, smell, and taste seem as complete as they need be for any conceivable human being, but that the eye is yet incomplete is very strikingly shown by the so-called actinic rays of the solar spectrum. In this spectrum there are rays beyond the violet which have an action on certain chemical substances much like the action of the blue and violet rays. But to the human eye these rays are absolutely invisible. The perception of this unknown color seems but a short step in the development of the eye.
But how different from the others, both in character and history, is the sense of touch! Having with them a common origin, like them it is resident in the outer skin, but it is active alike all over the body; the touch of the finger tips may be more delicate than that of the palms, but it is only a quantitative difference. The sense of touch is the fundamental sense. All the other senses have to render their data into its terms before they can be understood by the mind. Animals can live without sight, hearing, taste, or smell, but the presence of the sense of touch seems a necessary condition of animal existence. The other senses are means of self-preservation; the sense of touch is the manifestation of an animal's existence.
The senses, then, all originate from the outer covering; this covering has from the beginning a special sensation from the resistance to external pressure; this property it retains throughout the animal kingdom. The other sense-organs appear as specialized parts of this universal sense-organ; morphologically they are only parts of the skin, rendered more sensitive than the normal skin.
All the evidence seems to point one way, to the conclusion that the other senses are all modifications of the sense of touch. That such is probably the fact seems to be generally admitted. What I have tried to show is our ground for that conclusion, and that what was with Democritus a random speculation is with us fast assuming the nature of a scientific truth.