These later structures become diseased, the inhibiting force is removed, allowing the older structures opportunity to function. again. This they do if suitable stimuli are presented. These stimuli may come from within or from without. The old man may not think continually of the scenes of his boyhood, but just give him the cue and see how the ideas will cause the awakening of slumbering brain-tracts and in turn how the old neuroses reinstate old psychoses. Habits are the last functions to be forgotten. There are two reasons for this: (a) The processes have been so long and frequently continued that the neural structure has grown to that mode. (b) The spinal cord is the oldest formation of the nervous system in racial development-fundamental and hence the last to succumb. When all other functions are deranged and unbalanced in the nervous wreck or in the demented, the early habits still persist intact. In the decline of memory during old age or in nervous debility, proper names are the first to be lost. They are special—accessory— used only occasionally, and largely recently learned, and hence their application is seldom automatic. Other words are so habitually used that the chain of sounds becomes automatically reinstated.

Individual Differences.-There are very great differences of memory among individuals. There are persons who acquire readily, but forget quickly; those that acquire with difficulty, but retain accurately and tenaciously. Again, there are fortunate persons who acquire easily and retain with great persistence and fidelity, as well as some who work hard to acquire only to be chagrined on having what is learned evaporate almost as soon as learned. When one remembers things learned through a given sense better than what is learned through the other senses, we say he has a certain "type" of memory. There are types of memory corresponding to all of the senses. Some persons possess one type, some another. Again, there are persons who have memories that vary within the realm of a given sense. There are also all degrees of variations, from the special power of remembering remarkably, certain words, certain forms, certain sounds,

or certain colors, up to the very exaggerated cases which we find in abnormal persons, or the mathematical, musical, and other "prodigies."

There are also differences in the same individual at different stages of development. Children are usually thought to have better memories than adults. This view is hardly correct, however. Children's memories are different from adults'. Children acquire, even mechanical associations, more slowly than adults. They retain mechanical associations better when once learned, but adults retain thoughtful associations better. Both the power of registering and retaining thoughtfully increase up to about twenty-five years. The powers are relatively stationary then until about fifty, when a gradual decline sets in.1 These various differences suggest a recognition of different methods of teaching children of different ages, and also an adaptation of means and methods for persons of different memory types. Further discussion of the subject will be found in connection with the treatment of individual differences, memory training, and of imagination.

What Experiences Are Remembered. Of how great intensity must sensations or perceptions be in order to be remembered is a question that naturally rises. No stimulus produces much of a sensation until of sufficient intensity to rise above the threshold, i. e., to enter into consciousness. The threshold differs for different senses and in different persons. Vibrations of a sounding body must be as rapid as eight or ten per second, and for most persons as high as twenty-four per second, in order to produce a sensation. Beyond fifty thousand a second they can no longer be detected. However, it is quite probable that many stimuli which do not produce recognizable sensations have some effect upon the nervous organism and upon the mind. And just as every physical influence in the course of evolution has modified things within its scope, so we believe that all stimuli of sufficient intensity to modify nervous matter or

1 See Meumann, Vorlesungen zur Einführung in die Experimentelle Pädagogik, pp. 189-203.

mind in the least have left memories of their action. It is not to be supposed that all impressions of slight intensity, even though they attain the dignity of sensations, perceptions, or even more complex states, are necessarily recalled. They may be recalled in peculiar or abnormal conditions, or in hypnotic states, but even if too slight for recall under such circumstances, they color all our subsequent life and have their influence upon the general course of conscious memories. There are, in fact, exceedingly few things that are recalled exactly, or even need to be. But we may be certain that every influence to which we are subject leaves its "trace" upon our lives, and, of still more far-reaching importance, upon all posterity. As James remarks, "We are spinning our own fates, good or evil, and never to be undone. Every smallest stroke of virtue or of vice leaves its never so little scar. The drunken Rip Van Winkle, in Jefferson's play, excuses himself for every fresh dereliction by saying, 'I won't count this time!' Well! he may not count it and a kind Heaven may not count it; but it is being counted none the less. Down among his nerve-cells and fibres the molecules are counting it, registering and storing it up to be used against him when the next temptation comes. Nothing we ever do is, in strict scientific literalness, wiped out. Of course, this has its good side as well as its bad one. As we become permanent drunkards by so many separate drinks, so we become saints in the moral, and authorities and experts in the practical and scientific spheres, by so many separate acts and hours of work."1 Colgrove' writes on this point: "Perhaps that was not wholly a dream of De Quincey, Swedenborg, and Coleridge that the angels would come in the judgment day and take a complete record of our lives from the traces left in our bodies and nervous systems, and that by these we should be judged. If these are the books which are to be opened, a record trustworthy enough to determine destiny will be found. Each record in itself makes destiny." Prof. Ewald Hering asserts in that pioneer work on the

newer theory of organic memory1 that, "The conscious memory of man dies with his death; but the unconscious memory of nature is faithful and indestructible. Whoever has succeeded in impressing the vestiges of his work upon it, will be remembered forever."

1

Memory as a General Function of Organized Matter, p. 27.

CHAPTER XIV

THE NATURE AND

EDUCATIONAL SIGNIFICANCE OF ASSOCIATION

Illustrations of Mental Associations.-I walk by a certain building and suddenly find myself thinking about a friend whom I have not seen for years. Why should these thoughts dart into my mind so unceremoniously and unbidden? Even as I wrote the above sentence I suddenly found my mind wandering far away to a certain scene near my boyhood home which I have not seen for many years. Why should writing educational books be mixed up with my thoughts of boyhood episodes? Again, I recline in my easy-chair before the hearth and gaze into the fire with no particular thoughts in mind, and with only a comfortable unconcern. I indulge in day-dreams, and suddenly, when aroused to full consciousness, I find that I have wandered to far distant places and to scenes and events long past. Why should the remote be so connected with the present? When I listen to a speaker, or when I read a book, I try to have my thoughts follow the line suggested by the speaker or the writer. When I follow out a particular line of thought of my own I also take a somewhat definite course marked out by the nature of the thinking or by my own former course of thinking, but when I allow my thoughts to wander I find them taking strange and devious paths.

In all these and similar cases, the particular ideas are called into consciousness because of some chain of relations which we have previously forged in our minds, and because of some factor in our present experience which is also common to the chain of relations previously established. To illustrate, on coming to the university, I now recall that the last time I saw my friend we were standing in the doorway of a particular