THE DEVELOPMENT OF THE SENSES. 35 



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 primi- 

 tive sense is to become qualitatively different in parts, and this quali- 

 tative 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 ex- 

 ternal 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 object- 

 ive 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 Berke- 

 ley 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. Ches- 

 elden, and he suddenly received his s'.ght. At first he could perceive 

 no such thing as distance or form. Only by repeatedly touching ob- 

 jects 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- 



