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Page:Popular Science Monthly Volume 21.djvu/43

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THE DEVELOPMENT OF THE SENSES.

derm, 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 pro-