6. By studying domestic animals, see what you can learn about heredity.

Enumerate all the points that you find bearing upon heredity.

7. In a similar way, make a study of heredity in your family. Consider such characteristics as height, weight, shape of head, shape of nose, hair and eye color. Can you find any evidence of the inheritance of mental traits?

8. Make a complete outline of Chapter II.

REFERENCES FOR CLa.s.s READING

DAVENPORT: _Heredity in Relation to Eugenics_.

KELLICOTT: _The Social Direction of Human Evolution_.

CHAPTER III

MIND AND BODY

=Gross Dependence.= The relation of mind to body has always been an interesting one to man. This is partly because of the connection of the question with that of life after death. An old idea of this relation, almost universally held till recently, was that the mind or spirit lived in the body but was more or less independent of the body. The body has been looked upon as a hindrance to the mind or spirit. Science knows nothing about the existence of spirits apart from bodies. The belief that after death the mind lives on is a matter of faith and not of science. Whether one believes in an existence of the mind after death of the body, depends on one's religious faith. There is no scientific evidence one way or the other. The only mind that science knows anything about is bound up very closely with body. This is not saying that there is no existence of spirit apart from body, but that at present such existence is beyond the realm of science.

The dependence of mind upon body in a general way is evident to every one, upon the most general observation and thought. We know the effect on the mind of disease, of good health, of hunger, of fatigue, of overwork, of severe bodily injury, of blindness or deafness. We have, perhaps, seen some one struck upon the head by a club, or run over by an automobile, and have noted the tremendous consequences to the person's mind. In such cases it sometimes happens that, as far as we can see, there is no longer any mind in connection with that body. The most casual observation, then, shows that mind and body are in some way most intimately related.

=Finer Dependence.= Let us note this relation more in detail, and, in particular, see just which part of the body it is that is connected with the mind. First of all, we note the dependence of mind upon sense organs. We see only with our eyes. If we close the eyelids, we cannot see. If we are born blind, or if injury or disease destroys the retinas of the eyes or makes the eyes opaque so that light cannot pa.s.s through to the retinas, then we cannot see.

Similarly, we hear only by means of the ears. If we are born deaf, or if injury destroys some important part of the hearing mechanism, then we cannot hear. In like manner, we taste only by means of the taste organs in the mouth, and smell only with the organs of smell in the nose. In a word, our primary knowledge of the world comes only through the sense organs. We shall see presently just how this sensing or perceiving is accomplished.

=Dependence of Mind on Nerves and Brain.= We have seen how in a general way the mind is dependent on the body. We have seen how in a more intimate way it is dependent on the special sense organs. But the part of the body to which the mind is most directly and intimately related is the nervous system. The sense organs themselves are merely modifications of the nerve ends together with certain mechanisms for enabling stimuli to act on the nerve ends. The eye is merely the optic nerve spread out to form the retina and modified in certain ways to make it sensitive to ether vibrations. In addition to this, there is, of course, the focusing mechanism of the eye. So for all the sense organs; they are, each of them, some sort of modification of nerve-endings which makes them sensitive to some particular force or substance.

Let us make the matter clear by an ill.u.s.tration. Suppose I see a picture on the wall. My eyes are directed toward the picture. Light from the picture is refracted within the eyes, forming an image on each retina.

The retina is sensitive to the light. The light produces chemical changes on the retina. These changes set up an excitation in the optic nerves, which is conducted to a certain place in the brain, causing an excitation in the brain. Now the important point is that when this excitation is going on in the brain, _we are conscious, we see the picture_.

As far as science can determine, we do not see, nor hear, nor taste, nor smell, nor have any other sensation unless a sense organ is excited and produces the excitation in the brain. There can be no doubt about our primary, sensory experience. By primary, sensory experience is meant our immediate, direct knowledge of any aspect of the world. In this field of our conscious life, we are entirely dependent upon sense organs and nerves and brain. Injuries to the eyes destroying their power to perform their ordinary work, or injuries to the optic nerve or to the visual center in the brain, make it impossible for us to see.

These facts are so self-evident that it seems useless to state them. One has but to hold his hands before his eyes to convince himself that the mind sees by means of eyes, which are physical sense organs. One has but to hold his hands tight over his ears to find out that he hears by means of ears--again, physical sense organs.

But simple and self-evident as the facts are, their acceptance must have tremendous consequences to our thinking, and to our view of human nature. If the mind is dependent in every feature on the body with its sense organs, this must give to this body and its sense organs an importance in our thought and scheme of things that they did not have before. This close dependence of mind upon body must give to the body a place in our scheme of education that it would not have under any other view of the mind. We wish to emphasize here that this statement of the close relation of the mind and body is not a theory which one may accept or not. It is a simple statement of fact. It is a presupposition of psychology. By "presupposition" is meant a fundamental principle which the psychologist always has in mind. It is axiomatic, and has the same place in psychology that axioms have in mathematics. All explanations of the working of the mind must be stated in terms of nerve and brain action, and stimulation of sense organs.

Since the sense organs are the primary and fundamental organs through which we get experience, and since the sensations are the elementary experiences out of which all mental life is built, it is necessary for us to have a clear idea of the sense organs, their structure and functions, and of the nature of sensations.

=Vision.= _The Visual Sense Organs._ The details of the anatomy of the eye can be looked up in a physiological textbook. The essential principles are very simple. The eye is made on the principle of a photographer's camera. The retina corresponds to the sensitive plate of the camera. The light coming from objects toward which the eyes are directed is focused on the retina, forming there an image of the object. The light thus focused on the retina sets up a chemical change in the delicate nerve tissue; this excitation is transmitted through the optic nerve to the occipital (back) part of the brain, and sets up brain action there. Then we have visual sensation; we see the object.

The different colors that we see are dependent upon the vibration frequency of the ether. The higher frequencies give us the colors blue and green, and the lower frequencies give us the colors yellow and red.

The intermediate frequencies give us the intermediate colors blue-green and orange. By vibration frequencies is meant the rate at which the ether vibrates, the number of vibrations a second. If the reader wishes to know something about these frequencies, such information can be found in a textbook on physics.

It will be found that the vibration rates of the ether are very great.

It is only within a certain range of vibration frequency that sunlight affects the retina. Slower rates of vibration than that producing red do not affect the eye, and faster than that producing violet do not affect the eye. The lightness and darkness of a color are dependent upon the intensity of the vibration. Red, for example, is produced by a certain vibration frequency. The more intense the vibration, the brighter the red; the less intense, the darker the red.

When all the vibration frequencies affect the eyes at the same time, we see no color at all but only brightness. This is due to the fact that certain vibration frequencies neutralize each other in their effect on the retina, so far as producing color is concerned. Red neutralizes green, blue neutralizes yellow, violet neutralizes yellowish green, orange neutralizes bluish green.

All variations in vision as far as color and brightness are concerned are due to variations in the stimulus. Changes in vibration frequency give the different colors. Changes in intensity give the different brightnesses: black, gray, and white. All explanations of the many interesting phenomena of vision are to be sought in the physiological action of the eye.

Besides the facts of color and light and shade, already mentioned, some further interesting visual phenomena may be mentioned here.

_Visual Contrast._ Every color makes objects near it take on the antagonistic or complementary color. Red makes objects near appear green, green makes them appear red. Blue makes near objects appear yellow, while yellow makes them appear blue. Orange induces greenish blue, and greenish blue induces orange. Violet induces yellowish green, and yellowish green induces violet. These color-pairs are known as antagonistic or complementary colors. Each one of a pair enhances the effect of its complementary when the two colors are brought close together. In a similar way, light and dark tints act as complementaries.

Light objects make dark objects near appear darker, and dark objects make light objects near seem lighter.

These universal principles of contrast are of much practical significance. They must be taken account of in all arrangements of colors and tints, for example, in dress, in the arrangement of flowers and shrubs, in painting.

_Color-Mixture._ If, on a rotating motor, disks of different colors--say red and yellow--are placed and rotated, one sees on looking at them not red or yellow but orange. This phenomenon is known as _color-mixture_.

The result is due to the simultaneous stimulation of the retina by two kinds of ether vibration. If the colors used are a certain red and a certain green, they neutralize each other and produce only gray. All the pairs of complementary colors mentioned above act in the same way, producing, if mixed in the right proportion, no color, but gray. If colored disks not complementary are mixed by rotation on a motor, they produce an intermediate color. Red and yellow give orange. Blue and green give bluish green. Yellow and green give yellowish green. Red and blue give violet or purple, depending on the proportion. Mixing pigments gives, in general, the same results as mixing by means of rotating the disks. The ordinary blue and yellow pigments give green when mixed, because each of the two pigments contains green. The blue and yellow neutralize each other, leaving green.

_Visual After-Images._ The stimulation of the retina has interesting after effects. We shall mention here only the one known as _negative after-images_. If one will place on the table a sheet of white paper, and on this white paper lay a small piece of colored paper, and if he will then gaze steadily at the colored paper for a half-minute, it will be found that if the colored paper is removed one sees its complementary color. If the head is not moved, this complementary color has the same size and shape as the original colored piece of paper. The negative after-image can be projected on a background at different distances, its size depending on the distance of the background. The after-image will be found to mix with an objective color in accordance with the principles of color-mixture mentioned above.

After-image phenomena have some practical consequences. If one has been looking at a certain color for some time, a half-minute or more, then looks at some other color, the after-image of the first color mixes with the second color.

_Adaptation._ The fact last mentioned leads us to the subject of adaptation. If the eyes are stimulated by the same kind of light for some time, the eyes become adapted to that light. If the light is yellow, at first objects seem yellow, but after a time they look as if they were illuminated with white light, losing the yellow aspect. But if one then goes out into white light, everything looks bluish. The negative after-image of the yellow being cast upon everything makes the surroundings look blue, for the after-image of yellow is blue. All the other colors act in a similar way, as do also black and white. If one has been for some time in a dark room and then goes out to a lighter place, it seems unusually light. And if one goes from the light to a dark room, it seems unusually dark.

=Hearing or Audition.= Just as the eye is an organ sensitive to certain frequencies of ether vibration, so the ear is an organ sensitive to certain air vibrations. The reader should familiarize himself with the physiology of the ear by reference to physiologies. The drum-skin, the three little bones of the middle ear, and the cochlea of the inner ear are all merely mechanical means of making possible the stimulation of the specialized endings of the auditory nerve by vibrations of air.

As the different colors are due to different vibration frequencies of the ether, so different pitches of sound are due to differences in the rates of the air vibrations. The low ba.s.s notes are produced by the low vibration frequencies. The high notes are produced by the high vibration frequencies. The lowest notes that we can hear are produced by about twenty vibrations a second, and the highest by about forty thousand vibrations a second.

=Other Sense Organs.= We need not give a detailed statement of the facts concerning the other senses. In each case the sense organ is some special adaptation of the nerve-endings with appropriate apparatus in connection to enable it to be affected by some special thing or force in the environment.

In the case of taste, we find in the mouth, chiefly on the back and edges of the tongue, organs sensitive to sweet, sour, salt, and bitter.

In the nose we have an organ that is sensitive to the tiny particles of substances that float in the air which we breathe in through the nose.

In the skin we find several kinds of sense organs that give us the sensations of cold and warmth, of pressure and pain. These are all special and definite sensations produced by different kinds of organs.

The sense of warmth is produced by different organs from those which produce the sense of cold. These organs can be detected and localized on the skin. So, also, pain and touch or pressure have each its particular organ.

Within the body itself we have sense organs also, particularly in the joints and tendons and in the muscles. These give us the sensations which are the basis of our perception of motion, and of the position of the body and its members. In the semicircular ca.n.a.ls of the inner ear are organs that give us the sense of dizziness, and enable us to maintain our equilibrium and to know up from down.

The general nature of the sense organs and of sensation should now be apparent. The nervous system reaches out its myriad fingers to every portion of the surface of the body, and within the body as well. These nerve-endings are specially adapted to receive each its particular form of stimulation. This stimulation of our sense organs is the basis or cause of our sensations. And our sensations are the elementary stuff of all our experience. Whatever thoughts we have, whatever ideas or images we have, they come originally from our sensations. They are built up out of our sensations or from these sensations as they exist in memory.

=Defects of Sense Organs.= The organs of sight and hearing are now by far the most important of our sense organs. They enable us to sense things that are at a distance. We shall therefore discuss defects of these two organs only. Since sensations are the primary stuff out of which mind is made, and since sight and hearing are the most important sense organs, it is evident that our lives are very much dependent on these organs. If they cannot do their work well, then we are handicapped. And this is often the case.

The making of the human eye is one of the most remarkable achievements of nature. But the making of a perfect eye is too big a task for nature.

She never makes a perfect eye. There is always some defect, large or small. To take plastic material and make lenses and shutters and curtains is a great task. The curvature of the front of the eye and of the front and back of the crystalline lens is never quite perfect, but in the majority of cases it is nearly enough perfect to give us good vision. However, in about one third of school children the defect is great enough to need to be corrected by gla.s.ses.

The principle of the correction of sight by means of gla.s.ses is merely this:[1] When the focusing apparatus of the eye is not perfect, it can be made so by putting in front of the eye the proper kind of lens. There is nothing strange or mysterious about it. In some cases, the eye focuses the light before it reaches the retina. Such cases are known as nearsightedness and are corrected by having placed in front of the eyes concave lenses of the proper strength. These lenses diverge the rays and make them focus on the retina. In other cases, the eye is not able to focus the rays by the time they reach the retina. In these cases, the eyes need the help of convex lenses of the proper strength to make the focus fall exactly on the retina.

[1] The teacher should explain these principles and ill.u.s.trate by drawings. Consult a good text in physiology. Noyes' University of Missouri Extension Bulletin on eye and ear defects will be found most useful.

Another defect of the eye, known as astigmatism, is due to the fact that the eye does not always have a perfectly spherical front (cornea). The curvature in one direction is different from that in others. For example, the vertical curvature may be more convex than the horizontal.

Such a condition produces a serious defect of vision. It can be corrected by means of cylindrical lenses of the proper strength so placed before the eye as to correct the defect in curvature.

Still another defect of vision is known as presbyopia or farsightedness due to old age. It has the following explanation: In early life, when we look at near objects, the crystalline lens automatically becomes thicker, more convex. This adjustment brings the rays to a focus on the retina, which is required for good vision. As we get old, the crystalline lens loses its power to change its adjustment for near objects, although the eye may see at a distance as well as ever. The old person, therefore, must wear convex gla.s.ses when looking at near objects, as in reading and sewing.