The great number of neurones and the complexity of their connections account for the fact that an impulse leaving a particular sense organ may find its way to one group of muscles at one time and to another group of muscles at another time.

The muscles and glands are the effectors, or organs of response. They are so situated and so connected with sense organs by nervous structures that their responses are coördinated and meet suitably most situations. Placing something in a baby's hand causes the hand to grasp the object. The nervous impulse that starts from sense organs in the skin finds its way to the muscles which cause the fingers to close. Without such established pathways of conduction, behavior would be inappropriate.

2. Neural Basis of Learning

[SMITH, Stevenson, and GUTHRIE, Edwin R., General Psychology in Terms of Behavior, pp. 97-99. New York, D. Appleton & Co., 1924.]

Conditioned responses involve the formation of new pathways and the possibility for this is best afforded by the intricate association fibers of the cortex. When a neural arc is acting, impulses received from sense organs not previously connected with this neural arc are likely to be drained into its outgoing motor pathway. This drainage establishes new synapses and thus connects new sense organs with the responding muscle or gland. This is the neural basis of the conditioned response.

Impulses aroused by accompanying conditioning stimuli are drained into the motor system that is active at the time. Thus when the original stimulus and the conditioning stimulus act together, the combined energy from the two is drained into a single motor system. For this reason the conditioning stimulus facilitates the action of the original mechanism and this mechanism may act with a stimulation less intense than was first required.

The changes in the nervous system that account for positive adaptation are presumably an increase in conductivity at synapses. Resistance at a synapse is decreased each time a nervous impulse passes through it, and an improvement in conductivity of the synapse results from use. A lessened resistance in the synapses of a neural arc means a reduced threshold of

response.

The nervous changes underlying negative adaptation are rather more hypothetical. We may suppose, however, that an

impulse that starts to traverse a neural arc, but which does not reach the terminal effector, must of necessity drain into other pathways. Any motor pathway, when active, may drain to itself afferent impulses from other neural arcs. With use, drainage pathways become better established, with the result that later impulses show a lessened tendency to traverse the original neural arc and an increased tendency to traverse the new drainage pathway. Thus negative adaptation of one response always means the substitution of another response. This substitution is brought about when drainage establishes new association pathways. In this way impulses from the stimulus that is apparently disregarded actually reënforce some routine activity. The drained impulses may reënforce respiration, or any system that is active, or they may occasion emotional responses. Thus they establish a habit of doing something other than the act to which negative adaptation has been developed.

Figure 4 is a schematic representation of some nine billion two hundred million of neurones or nerve elements. This drawing would have to be made millions of times more complex in order to duplicate the human nervous system for being stimulated by situations and for connecting these stimuli with each other and with appropriate responses. The connections between the neurones are subject to modification in the course of education. The modifiability of connections between neurones is the physiological basis of education.

By means of the nervous system and its accessories man is able to control his own behavior. This mechanism may be thought of as a threefold system of receptors, effectors, and connectors, for being sensitive to situations, making responses, and connecting responses with situations.

3. The Physiological Age Is the Basic Age

[BALDWIN, Bird T., "The Physical Growth of Children from Birth to Maturity," University of Iowa Studies in Child Welfare, 1921, Vol. 1, No. 1.]

Physiological age is, the writer believes, directly correlated with stages of mental maturation. . . . The physiologically more mature child has different attitudes, different types of emotions, different interests, than the child who is physically younger though of the same chronological age. . . . Physiological age

has a direct bearing on pedagogical age, as many of our schools are beginning to recognize. The larger and physiologically more mature child may be able to do certain types of school work better, although of inferior ability in specific traits which have

G

A

B

FIG. 4. DIAGRAM SHOWING THE OPERATION OF
THE RECEIVING, THE CONNECTING, AND THE
REACTING MECHANISMS.

The arrows indicate the course of the nerve
impulse when E is stimulated. The receiving
mechanism is labeled E, and the reacting
mechanism (in this case a muscle) is labeled
C. B is the spinal cord, and G, the cortex of
the brain. Note how neurones D and H are
connected. (After Cajal.)

been greatly emphasized by school curricula. . . . That there is a direct relationship between social age and physiological maturity needs only to be mentioned to be evident.

4. Chromosomes, Endocrines, and Heredity [DAVENPORT, Charles B., "Chromosomes, Endocrines and Heredity," Scientific Monthly, May, 1925, Vol. 2, pp. 491-498.] (Abridged.)

The word "heredity" has had various connotations at different times. Very early it was recognized that the children are the heirs of certain traits of the father and mother and hence arose the primitive conception of heredity. Studies of genetics during the past quarter of a century have established the plea that what is inherited is not the visible traits of the parents (the phenotype), but the genes of such, which are carried in the germ cells (the genotype). With the recognition of the importance of the genotype in heredity attention has been directed to the genes themselves as they are located in the germ cells; and during the last fifteen years there has developed a new conception of the rôle of the chromosomes as carrier of these genes. On the one hand, through the remarkable studies of T. H. Morgan and his associates, the architecture of the chromosomes is being determined in detail so that maps can be drawn indicating graphically the relative linear position of the genes in the chromosome. . . . Whenever a particular abnormality appears in the eyes, wings or body of this insect (the banana fly, Drosophila virilis), a corresponding change apparently has occurred in the material occupying a particular point in the particular chromosome in which is located the material chiefly responsible for the production of that abnormality. This discovery marked so great an advance in our knowledge of heredity that it is not strange that in his enthusiasm the principal investigator in this field should have exclaimed: "The problem of heredity is solved !"

...

However, it has not been possible to explain all mutations on the ground of the modifications of the genes lying in the chromosomes. On the contrary a series of mutations have been worked out, chiefly by A. F. Blakeslee and John Belling, which depend upon modifications in the number of entire chromosomes. In the Jimson weed, Datura stramonium, which has normally 12 sets of chromosomes, 2 in each set, the 12 pairs of chromosomes obviously carry, like the different chromosomes of the Drosophila, each its special genes. These chromosomes differ thus in structure as indeed they do in size so that we can recognize chromosomes a, b, c and so on. Now if the chromosome "Set a" comprises, as it occasionally does, 3 chromosomes in

stead of 2, there is a corresponding difference in the form of the Jimson weed. If "Set b" has the extra chromosome, still a different form of plant results. If "Set c" is the one that has the additional number still another form and so on. By the addition of one extra chromosome to each of the 12 sets we may have produced, in turn, 12 different kinds of Jimson weeds. In rare cases 2 extra chromosomes may be added to each of 2 of the sets and thus a new set of modifications of form may be brought about, or a chromosome may be subtracted from one of the sets or may be subtracted from one and added to another set. Again all the sets may be represented by one chromosome only instead of 2 each. Or all the sets may include 3 chromosomes or 4 chromosomes. These are possibilities which permit the plants to survive. Many others are thinkable which probably may cause an early death of the plant.

The relation between structure of the chromosomes-of mutations in its constituent genes, on the one hand, and, on the other, the relation of variations in the number of whole chromosomes or fragments of them, to mutations, seems to justify the conclusion to which the man in the street comes in contemplating the findings of the geneticist, namely, "We are what our chromosomes make us."

The findings of the geneticist are further supported by the similar observation of resemblances between different members of a family. This resemblance is frequently striking among brothers and sisters of one fraternity. That it is not more striking-that brothers and sisters so often show points of difference -is due to the fact that the germ cells of the father and of the mother are not at all alike, but may be very diverse, just because of the hybrid nature of their ancestors.

There is one phenomenon, however, of striking. similarity which has long been noticed and this is the phenomenon of identical twins. Here we have pairs of persons who resemble each other so closely in form and features, in behavior, in resistance to disease, that they are frequently almost indistinguishable. Even their own parents may confuse them. . . . Now this great resemblance of identical twins is commonly explained on the ground that they are derived from one and the same egg which has produced by budding upon its surface two embryos in the place of one. This view is supported by the fact, among others, of a certain peculiarity of identical twins that distinguishes them from other twins in that the embryos at birth are enveloped in the same envelope, called chorion, whereas in