Animal Life and Intelligence

by C. Lloyd Morgan

To account for this process of localization, it is supposed that every sensation, apart from its special quality as a touch, a taste, or a smell, has a more or less defined spatial quality, or local sign, dependent upon the part of the body to which the stimulus is applied. These local signs have, doubtless, in the long run, been established by experience—if under this term we may include a more or less unconscious process, the outcome of evolution. But they are so rapidly established in the individual, that we are forced to conclude that we inherit very highly developed aptitudes for localization.

The refinement of localization is very different in the different senses. In smell and taste there seems no more than a general localization in the organ affected—the nose or the mouth. In hearing there is not much more, unless we regard the discrimination of pitch as a mode of localization. In touch (and temperature) the refinement is much higher, but it varies with the part of the body affected.

If the back be touched by two points less than two inches and a third apart, the sensation will be that of a single point; the finger-tips, however, can distinguish two points separated by less than one-tenth of an inch; and the tip of the tongue is still more refined in its power of discrimination, distinguishing as two, points separated by less than the twenty-fifth part of an inch. So that the tongue is about sixty times as refined in its discrimination as the skin of the back. Moreover, the delicacy of localization may be cultivated, so that in some cases the refinement may, by practice, be doubled.

When we come to sight, the refinement of localization reaches its maximum, the local signs in the retina showing the highest stage of differentiation, the distance on the retina between two points distinguishable by local signs being, according to Helmholtz, not much more than 1/6000 of an inch (.0044 millimetre), which nearly corresponds with the space between two cones in the yellow spot.

We must remember that the presentations of sense are in all cases given in a stippled form, that is, by the stimulation of a number of separate and distinct points. In vision the stippling is very fine, owing to the minute size and close setting of the retinal cones. In the case of hearing, the stippling, if we may so extend the use of this term, is also very fine, as is shown by the fact that musicians can, according to Weber, distinguish notes separated in the scale of sounds by only one-sixtieth part of a musical tone. In touch the stippling is comparatively coarse. But in all cases there is a stippling; and yet from these stippled sensations the mind in all cases elaborates a continuum. The visual image is continuous, notwithstanding the retinal stippling and the existence of the blind spot. When we lay our hands on a smooth table we fill in the interstices between the sensational points, and feel the surface as continuous. In all cases out of the stippled sense-stimuli we form a continuum.

The next thing that we have to note is that it is not so much the sensation itself, as that which gives origin to it, that we habitually refer outwards to the recipient end of the afferent fibre. In referring a sensation of touch to a certain part of the skin, it is of something touching us that we seem to be immediately conscious. We refer the stimulus to an object in the external world, which we localize, and which we believe to have given rise to the sensation.

This, however, is more clearly seen in the case of vision. When we look through the window and see an object such as a house before us, we do not habitually localize the sensation in a certain part of the retina, but we refer the object to a particular position more or less distant in the world around us. This projection of the object outwards in a right line from the eyes is really a marvellous process, though the wonder of it is lost in its familiarity. It is the outcome of the experience of hundreds of generations. And the experience is not gained through vision alone, but through this in combination with other senses and activities. We see an object, but we have to go to it before we can touch it. It is not in contact with us, but distant from us. Its outness and distance is a matter of what is termed the geometry of the senses; and this geometry has been elaborated through many generations of organized beings, from data given by sight, touch, and the muscular sense. It is true that I can now estimate the distance of the house without going to it; but my eyes go to it, and I can feel them go. The panes of my window are separated by iron bars. As I look from them to the distant house and back to them again, I can feel my eyes going from one to the other. The lens of the eye is adjusted for near or far distance by the action of a ciliary muscle, through which its anterior surface can be flattened, returning again by its own elasticity to the more convex form when the muscle ceases to act. Each eye, moreover, is moved in its orbit by six eye-muscles, and in normal vision the two eyes act as one organ. For near distances they converge; for far distances there is less convergence. Through the muscular sense, which is here extraordinarily delicate, we can feel the amount of accommodation and convergence; and thus we can feel the eyes going to or coming from a near and a distant object. Of course, we are aided in judging or estimating distances by the apparent size of the object when the real size is known, by the clearness of its outlines in a slightly hazy atmosphere, and so forth. But apart from such judgments, it would probably be impossible to perceive that an object is near or distant in the absence of muscles of accommodation and convergence affording the data of the muscular sense. Not only the distance of two objects from the eye, but their distance apart, can be measured by the aid of the muscular sense as we move the eyes from one to the other. And in us this is so delicate that, according to Weber, a distinct muscular sensation is attached to a displacement of a sensitive point of the yellow spot through less than 1/6000 of an inch.

Now, if it be true that the consciousness aroused by objects around us, through sensation, is an accompaniment of certain physiological changes in the brain, it is clear that the localization of their points of origin in special parts of the skin, and the outward projection of the objects exciting vision, is an act of the mind quite distinct from the mere passive response in consciousness which we call an impression, and more complex than that mental activity which, through discrimination and recognition, converts the bare impression into a sensation. It is, in fact, part of that mental process which is called perception.[FP] Sensation has nothing to do with the objects around us as such; it is by perception that we are aware of their existence. Let us now follow the process of perception a little further, always remembering that it involves certain activities of the mind.

These activities are too often ignored. We often speak of the senses as the avenues of knowledge, and John Bunyan, likening the soul to a citadel, spoke of the five gateways of knowledge, Eye-gate, Ear-gate, Mouth-gate, Smell-gate, and Feel-gate. Hence arises a vague notion that through the eye-gate, for example, a sort of picture of the external object somehow enters the mind. And this idea is no doubt fostered by the fact that an inverted image of the object is formed on the retina, though how the inverted image is turned right way up again in passing into the mind bothers some people not a little.[FQ]

A much closer analogy is this: Something stands without and knocks at the doorway of sense, and from the nature of the knocks we learn somewhat concerning that which knocks. In other words, at the bidding of certain stimuli from without we construct that mental product which we call the object of sense. It is of these mental constructions—"constructs"[FR] I will call them for convenience—that I have now to speak.