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It has been indicated here and there throughout the previous chapters that, despite the fact that there are certain laws governing the various mental traits and processes, still there is variation in the working of those laws. It was pointed out that people differ in kind of memory or imagination in which they excel, in their ability to appreciate, in the speed with which they form habits, and so on. In other words, that boys and girls are not exact duplicates of each other, but that they always differ from each other. Now a knowledge of these differences, their amounts, interrelations, and causes are very necessary for the planning of a school system or for the planning of the education of a particular child. What we plan and how we plan educational undertakings must always be influenced by our opinion as to inborn traits, sex differences, specialization of mental traits, speed of development, the respective power of nature and of nurture. The various plans of promotion and grouping of children found in different cities are in operation because of certain beliefs concerning differences in general mental ability. Coeducation is urged or deplored largely on the ground of belief in the differing abilities of the sexes.

Exact knowledge of just what differences do exist between people and the causes of these differences is important for two reasons. First, in order that the most efficient measures may be taken for the education of the individual, and second, in order that the race as a whole may be made better. Education can only become efficient and economical when we know which differences between people and which achievements of a given person are due to training, and which are due more largely to original equipment or maturity. It is a waste of time on the one hand for education to concern itself with trying to make all children good spellers if spelling is a natural gift; and on the other hand, it is lack of efficiency for schools to be largely neglecting the moral development of the children, if morality is dependent primarily on education. Exact knowledge, not opinions, along all these lines is necessary if progress is to be made.

The principal causes for individual differences are sex, remote ancestry, near ancestry, maturity, and training. The question to be answered in the discussion of each of these causes is how important a factor is it in the production of differences and just what differences is it responsible for. That men differ from women has always been an accepted fact, but exact knowledge of how much and how they differ has, until recent years, been lacking. Recently quantitative measurement has been made by a number of investigators. In making these investigations two serious difficulties have to be met. First, that the tests measure only the differences brought about by differences in sex, and not by any other cause, such as family or training. This difficulty has been met by taking people of all ages, from all sorts of families, with all kinds of training, the constant factor being the difference in sex. The second difficulty is that of finding groups in which the selection agencies have been the same and equally operative. It would be obviously unfair to compare college men and women, and expect to get a fair result as to sex differences, because college women are a more highly selected group intellectually than the college men. It is the conventional and social demands that are primarily responsible for sending boys to college, while the intellectual impulse is responsible to a greater extent for sending girls. Examination of children in the elementary schools, then, gives a fairer result than of the older men and women. The general results of all the studies made point to the fact that the differences between the sexes are small. Sex is the cause of only a small fraction of the differences between individuals. The total difference of men from men and women from women is almost as great as the difference between men and women, for the distribution curve of woman’s ability in any trait overlaps the men’s curve to at least half its range. In detail the exact measurements of intellectual abilities show a slight superiority of the women in receptivity and memory, and a slight superiority of the men in control of movement and in thought about concrete mechanical situations. In interests which cannot be so definitely measured, women seem to be more interested in people and men in things. In instinctive equipment women excel in the nursing impulse and men in the fighting impulse. In physical equipment men are stronger and bigger than women. They excel in muscular tests in ability to “spurt,” whereas women do better in endurance tests. The male sex seems on the whole to be slightly more variable than the female, i.e., its curve of distribution is somewhat flatter and extends both lower and higher than does that of the female; or, stated another way, men furnish more than their proportion of idiots and of geniuses.

Slight though these differences are, they are not to be disregarded, for sometimes the resulting habits are important. For instance, girls should be better spellers than boys. Boys should excel in physics and chemistry. Women should have more tact than men, whereas men should be more impartial in their judgments. With the same intellectual equipment as women, men should be found more often in positions of prominence because of the strength of the fighting instinct. The geniuses of the world, the leaders in any field, as well as the idiots, should more often be men than women. That these differences do exist, observation as well as experiment prove, but that they are entirely due to essential innate differences in sex is still open to question. Differences in treatment of the sexes in ideals and in training for generation after generation may account for some of the differences noted.

What these differences mean from the standpoint of practice is still another question. Difference in equipment need not mean difference in treatment, nor need identity of equipment necessarily mean identity of training. The kind of education given will have to be determined not only by the nature of the individual, but also by the ideals held for and the efficiency demanded from each sex.

Another cause of the differences existing between individuals is difference in race inheritance. In causing differences in physical traits this factor is prominent. The American Indians have physical traits in common which differentiate them from other races; the same thing is true of the Negroes and the Mongolians. It has always been taken for granted that the same kind of difference between the races existed in mental traits. To measure the mental differences caused by race is an extremely difficult problem. Training, environment, tradition, are such potent factors in confusing the issue. The difficulty is to measure inborn traits, not achievement. Hence the results from actual measurement are very few and are confined to the sensory and sensorimotor traits. Woodworth, in summing up the results of these tests, says, “On the whole, the keenness of the senses seems to be about on a par in the various races of mankind.... If the results could be taken at their face value, they would indicate differences in intelligence between races, giving such groups as the Pygmy and Negrito a low station as compared with most of mankind. The fairness of the test is not, however, beyond question." The generality of this conclusion concerning the differences in intelligence reveals the lack of data. No tests of the higher intellectual processes, such as the ability to analyze, to associate in terms of elements, to formulate new principles, and the like, have, been given. Some anthropologists are skeptical of the existence of any great differences, while others believe that though there is much overlapping, still differences of considerable magnitude do exist. At present we do not know how much of the differences existing between individuals is due to differences in remote ancestry.

Maturity as a cause of differences between individuals gives quite as unsatisfactory results as remote ancestry. Every thoughtful student of children must realize that inner growth, apart from training, has something to do with the changes which take place in a child; that he differs from year to year because of a difference in maturity. This same cause, then, must account to some extent for the differences between individuals of different ages. But just how great a part it plays, what per cent of the difference it accounts for, and what particular traits it affects much or little, no one knows. We say in general that nine-year-old children are more suggestible than six-year-old, and than fourteen-year-old; that the point of view of the fifteen-year-old is different from that of the eleven-year-old; that the power of sense discrimination gradually increases up to about sixteen, and so on. That these facts are true, no one can question, but how far they are due to mere change in maturity and how far to training or to the increase in power of some particular capacity, such as understanding directions, or power of forced attention, is unknown. The studies which have been undertaken along this line have failed in two particulars: first, to distribute the actual changes found from year to year among the three possible causes, maturity, general powers of comprehension and the like, and training; second, to measure the same individuals from year to year. This last error is very common in studies of human nature. It is taken for granted that to examine ten year olds and then eleven year olds and then twelve year olds will give what ten year olds will become in one and two years’ time respectively. To test a group of grammar grade children and then a group of high school and then a group of college students will not show the changes in maturity from grammar school to college. The method is quite wrong, for it tests only the ten year olds that stay in school long enough to become twelve year olds; it measures only the very small per cent of the grammar school children who get to college. In other words, it is measuring a more highly selected group and accepting the result obtained from them as true of the entire group. Because of these two serious errors in the investigations our knowledge of the influence of maturity as a cause of individual differences is no better than opinion. Two facts, however, such studies do make clear. First, the supposition that “the increases in ability due to a given amount of progress toward maturity are closely alike for all children save the so-called ‘abnormally-precocious’ or ‘retarded’ is false. The same fraction of the total inner development, from zero to adult ability, will produce very unequal results in different children. Inner growth acts differently according to the original nature that is growing. The notion that maturity is the main factor in the differences found amongst school children, so that grading and methods of teaching should be fitted closely to ‘stage of growth,’ is also false. It is by no means very hard to find seven year olds who can do intellectual work in which one in twenty seventeen year olds would fail."

The question as to how far immediate heredity is a cause of differences found between individuals, can only be answered by measuring how much more alike members of the same family are in a given trait than people picked at random, and then making allowance for similarity in their training. The greater the likenesses between members of the same family, and the greater the differences between members of different families, despite similarities in training, the more can individual differences be traced to differences in ancestry as a controlling cause. The answer to this question has been obtained along four different lines: First, likenesses in physical traits; second, likenesses in particular abilities; third, likenesses in achievement along intellectual and moral lines; fourth, greater likenesses between twins, than ordinary siblings. In physical traits, such as eye color, hair color, cephalic index, height, family resemblance is very strong (the coefficient of correlation being about .5), and here training can certainly have had no effect. In particular abilities, such as ability in spelling, the stage reached by an individual is due primarily to his inheritance, the ability being but little influenced by the differences in home or school training that commonly exist. In general achievement, Galton’s results show that eminence runs in families, that one has more than three hundred times the chance of being eminent if one has a brother, father, or son eminent, than the individual picked at random. Wood’s investigation in royal families points to the same influence of ancestry in determining achievement. The studies of the Edwards family on one hand and the so-called Kallikak family on the other, point to the same conclusion. Twins are found to be twice as much alike in the traits tested as other brothers and sisters. Though the difficulty of discounting the effect of training in all these studies has been great, yet in every case the investigators have taken pains to do so. The fact that the investigations along such different lines all bear out the same conclusion, namely, that intellectual differences are largely due to differences in family inheritance, weighs heavily in favor of its being a correct one.

The fifth factor that might account for individual differences is environment. By environment we mean any influence brought to bear on the individual. The same difficulty has been met in attempting to measure the effect of environment that was met in trying to measure the effect of inner nature namely, that of testing one without interference from the other. The attempts to measure accurately the effect of any one element in the environment have not been successful. No adequate way of avoiding the complications involved by different natures has been found. One of the greatest errors in the method of working with this problem has been found just here. It has been customary when the effect of a certain element in the environment is to be ascertained to investigate people who have been subject to that training or who are in the process of training, thus ignoring the selective influence of the factor itself in original nature. For instance, to study the value of high school training we compare those in training with those who have never had any; if the question is the value of manual training or Latin, again the comparison is made between those who have had it and those who haven’t. To find out the influence of squalor and misery, people living in the slums are compared with those from a better district. In each case the fact is ignored that the original natures of the two groups examined are different before the influence of the element in question was brought to bear. Why do some children go to high school and others not? Why do some choose classical courses and some manual training courses? Why are some people found in the slums for generations? The answer in each case is the same the original natures are different. It isn’t the slums make the people nearly so often as it is the people make the slums. It isn’t training in Latin that makes the more capable man, but the more intellectual students, because of tradition and possibly enjoyment of language study, choose the Latin. It is unfair to measure a factor in the environment and give it credit or discredit for results, when those results are also due to original nature as well, which has not been allowed for. It must be recognized by all those working in this field that, after all, man to some extent selects his own environment. In the second place, it must be remembered that the environment will influence folks differently according as their natures are different. There can be no doubt that environment is accountable for some individual differences, but just which ones and to what extent are questions to which at present the answers are unsatisfactory.

The investigations which have been carried on agree that environment is not so influential a cause for individual differences in intellect as is near ancestry. One rather interesting line of evidence can be quoted as an illustration. If individual differences in achievement are due largely to lack of training or to poor training, then to give the same amount and kind of training to all the individuals in a group should reduce the differences. If such practice does not reduce the differences, then it is not reasonable to suppose that the differences were caused in the first place by differences in training. As a matter of fact, equalizing training increases the differences. The superior man becomes more superior, the inferior is left further behind than ever. A common occurrence in school administration bears out this conclusion reached by experimental means. The child who skips a grade is ready at the end of three years to skip again, and the child who fails a grade is likely at the end of three years to fail again. Though environment seems of little influence as compared with near ancestry in determining intellectual ability per se, yet it has considerable influence in determining the line along which this ability is to manifest itself. The fact that between 1840-44, 9.4 per cent of the college men went into teaching as a profession and 37.5 per cent into the ministry, while between 1890-94, 25.4 per cent chose the former and only 14 per cent the latter, can be accounted for only on the basis of environmental influence of some kind.

Another fact concerning the influence of environment is that it is very much more effective in influencing morality than intellect. Morality is the outcome of the proper direction of capacities and tendencies possessed by the individual, and therefore is extremely susceptible to environmental influences. We are all familiar with the differences in moral standards of different social groups. One boy may become a bully and another considerate of the rights of others, one learns to steal and another to be honest, one to lie and another to be truthful, because of the influence of their environments rather than on account of differences in their original natures. We are beginning to recognize the importance of environment in moral training in the provisions made to protect children from immoral influences, in the opportunities afforded for the right sort of recreation, and even in the removal of children from the custody of their parents when the environment is extremely unfavorable.

Though changes in method and ideals cannot reduce the differences between individuals in the intellectual field to any marked extent, such changes can raise the level of achievement of the whole group. For instance, more emphasis on silent reading may make the reading ability of a whole school 20 per cent better, while leaving the distance between the best and worst reader in the school the same. Granting that heredity, original nature, is the primary cause of individual differences in intellect (aside from those sex differences mentioned) there remains for environment, education in all its forms, the tremendous task of: First, providing conditions favorable for nervous health and growth; second, providing conditions which stimulate useful capacities and inhibit futile or harmful capacities; third, providing conditions which continually raise the absolute achievement of the group and of the race; fourth, providing conditions that will meet the varying original equipments; fifth, assuming primary responsibility for development along moral and social lines.

Concerning those individual differences of which heredity is the controlling cause, two facts are worthy of note. First, that human nature is very highly specialized and that inheritance may be in terms of special abilities or capacities. For instance, artistic, musical, or linguistic ability, statesmanship, power in the field of poetry, may be handed down from one generation to the next. This also means that two brothers may be extremely alike along some lines and extremely different along others. Second, that there seems to be positive combinations between certain mental traits, whereby the presence of one insures the presence of the other to a greater degree than chance would explain. For instance, the quick learner is slow in forgetting, imagery in one field implies power to image in others, a high degree of concentration goes with superior breadth, efficiency in artistic lines is more often correlated with superiority in politics or generalship or science than the reverse, ability to deal with abstract data implies unusual power to deal with the concrete situation. In fact, as far as exact measures go, negative correlations between capacities, powers, efficiencies, are extremely rare, and, when they occur, can be traced to the influence of some environmental factor.

Individuals differ from each other to a much greater degree than has been allowed for in our public education. The common school system is constructed on the theory that children are closely similar in their abilities, type of mental make-up, and capacities in any given line. Experimentation shows each one of these presuppositions to be false. So far as general ability goes, children vary from the genius to the feeble-minded with all the grades between, even in the same school class. This gradation is a continuous one there are no breaks in the human race. Children cannot be grouped into the very bright, bright, mediocre, poor, very poor, failures each group being distinct from any other. The shading from one to the other of these classes is gradual, there is no sharp break. Not only is this true, but a child may be considered very bright along one line and mediocre along another. Brilliancy or poverty in intellect does not act as a unit and apply to all lives equally. The high specialization of mental powers makes unevenness in achievement the common occurrence. Within any school grade that has been tested, even when the gradings are as close as those secured by term promotions, it has been found in any subject there are children who do from two to five times as well as others, and from two to five times as much as others. Of course this great variation means an overlapping of grades on each side. In Dr. Bonser’s test of 757 children in reasoning he found that 90 per cent of the 6A pupils were below the best pupils of 4A grade and that 4 per cent of 6A pupils were below the mid-pupils of the 4A, and that the best of the 4A pupils made a score three times as high as the worst pupils of 6A. Not only is this tremendous difference in ability found among children of the same class, but the same difference exists in rate of development. Some children can cover the same ground in one half or one third the time as others and do it better. Witness the children already quoted who, skipping a grade, were ready at the end of three years to skip again. Variability, not uniformity, is what characterizes the abilities and rate of intellectual growth of children in the schools, and these differences, as has already been pointed out, are caused primarily by a difference in original nature.

There is also great difference between the general mental make-up of children a difference in type. There is the child who excels in dealing with abstract ideas. He usually has power also in dealing with the concrete, but his chief interest is in the abstract. He is the one who does splendid work in mathematics, formal grammar, the abstract phases of the sciences. Then there is the child who is a thinker too, but his best work is done when he is dealing with a concrete situation. Unusual or involved applications of principles disturb him. So long as his work is couched in terms of the concrete, he can succeed, but if that is replaced by the x, y, z elements, he is prone to fail. There is another type of child the one who has the executive ability, the child of action. True, he thinks, too, but his forte is in control of people and of things. He is the one who manages the athletic team, runs the school paper, takes charge of the elections, and so on. For principles to be grasped he must be able to put them into practice. The fourth type is the feeling type, the child who excels in appreciative power. As has been urged so many times before, these types have boundaries that are hazy and ill defined; they overlap in many cases. Some children are of a well-defined mixed type, and most children have something of each of the four abilities characteristic of the types. Still it is true that in looking over a class of children these types emerge, not pure, but controlled by the dominant characteristics mentioned.

The same variation is found among any group of children if they are tested along one line, such as memory. Some have desultory, some rote, some logical memories; some have immediate memories, others the permanent type. In imagery, some have principally productive imagination, others the matter-of-fact reproductive; some deal largely with object images that are vivid and clear-cut, others fail almost entirely with this type, but use word images with great facility. In conduct, some are hesitating and uncertain, others just the reverse; some very open to suggestions, others scarcely touched at all by it; some can act in accordance with principle, others only in terms of particular associations with a definite situation. So one might run the whole gamut of human traits, and in each one any group of individuals will vary: in attention, in thinking, in ideals, in habits, in interests, in sense discrimination, in emotions, and so on. This is one of the greatest contributions of experimental psychology of the past ten years, the tremendous differences between people along all lines, physical as well as mental.

It is lack of recognition of such differences that makes possible such a list of histories of misfits as Swift quotes in his chapter on Standards of Human Power in “Mind in the Making.” Individual differences exist, education cannot eliminate them, they are innate, due to original nature. Education that does not recognize them and plan for them is wasteful and, what is worse, is criminal.

The range of ability possessed by children of the same grade in the subjects commonly taught seems not always to be clear in the minds of teachers. It will be discussed at greater length in another chapter, but it is important for the consideration of individual differences to present some data at this time. If we rate the quality of work done in English composition from 10 to 100 per cent, being careful to evaluate as accurately as possible the merit of the composition written, we will find for a seventh and an eighth grade a condition indicated by the following table:

Quality of
7 8
No. of Pupils
Rated at 10 2 1
Rated at 20 6 6
Rated at 30 8 8
Rated at 40 7 8
Rated at 50 2 4
Rated at 60 1 1
Rated at 70 1 1
Rated at 80 1 1
Rated at 90 1 1

The table reads as follows: two pupils in the seventh grade and one in the eighth wrote compositions rated at 10; six seventh-grade and six eighth-grade pupils wrote compositions rated at 20, and so on for the whole table.

A similar condition of affairs is indicated if we ask how many of a given type of addition problems are solved correctly in eight minutes by a fifth- and a sixth-grade class.

Number of
5 6
No. of Pupils
0 2 3
1 6 6
2 6 6
3 6 6
4 4 5
5 4 5
6 3 4
7 1 2
8 1 1
9 1 1

In like manner, if we measure the quality of work done in penmanship for a fifth and sixth grade, with a system of scoring that ranks the penmanship in equal steps from a quality which, is ranked four up to a quality which is ranked eighteen, we find the following results:

Quality of
5 6
No. of Pupils
Rated at 4 5 6
Rated at 5 1 1
Rated at 6 0 0
Rated at 7 2 4
Rated at 8 10 4
Rated at 9 12 1
Rated at 10 3 6
Rated at 11 3 8
Rated at 12 3 3
Rated at 13 1 2
Rated at 14 1 1
Rated at 15 0 1
Rated at 16 1 1
Rated at 17 0 0
Rated at 18 0 0

Results similar to those recorded above will be found if any accurate measurement is made of the knowledge possessed by children in history or in geography, or of the ability to apply or derive principles in physics or in chemistry, or of the knowledge of vocabulary in Latin or in German, and the like.

All such facts indicate clearly the necessity for differentiating our work for the group of children who are classified as belonging to one grade. Under the older and simpler form of school organization, the one-room rural school, it was not uncommon for children to recite in one class in arithmetic, in another in geography or history, and in possibly still another in English. In our more highly organized school systems, with the attempt to have children pass regularly from grade to grade at each promotion period, we have in some measure provided for individual differences through allowing children to skip a grade, or not infrequently by having them repeat the work of a grade. In still other cases an attempt has been made to adapt the work of the class to the needs and capacities of the children by dividing any class group into two or more groups, especially in those subjects in which children seem to have greatest difficulty. Teachers who are alive to the problem presented have striven to adjust their work to different members of the class by varying the assignments, and in some cases by excusing from the exercises in which they are already proficient the abler pupils.

Whatever adjustment the school may be able to make in terms of providing special classes for those who are mentally or physically deficient, or for those who are especially capable, there will always be found in any given group a wide variation in achievement and in capacity. Group teaching and individual instruction will always be required of teachers who would adapt their work to the varying capacities of children. A period devoted to supervised study during which those children who are less able may receive special help, and those who are of exceptional ability be expected to make unusual preparation both in extent and in quality of work done, may contribute much to the efficiency of the school. As paradoxical as the statement may seem, it is true that the most retarded children in our school systems are the brightest. Expressed in another way, it can be proved that the more capable children have already achieved in the subjects in which they are taught more than those who are tow or three grades farther advanced. Possibly the greatest contribution which teachers can make to the development of efficiency upon the part of the children with whom they work is to be found in special attention which is given to capable children with respect to both the quantity and quality of work demanded of them, together with provision for having them segregated in special classes or passed through the school system with greater rapidity than is now common. In an elementary school with which the writer is acquainted, and in which there were four fifth grades, it was discovered during the past year that in one of these fifth grades in which the brighter children had been put they had achieved more in terms of ability to solve problems in arithmetic, in their knowledge of history and geography, in the quality of English composition they wrote, and the like, than did the children in any one of the sixth grades. In this school this particular fifth grade was promoted to the seventh grade for the following year. Many such examples could be found in schools organized with more than one grade at work on the same part of the school course, if care were taken to segregate children in terms of their capacity. And even where there is only one teacher per grade, or where one teacher teaches two or three grades, it should be found possible constantly to accelerate the progress of children of more than ordinary ability.

The movement throughout the United States for the organization of junior high schools (these schools commonly include the seventh, eighth, and ninth school years) is to be looked upon primarily as an attempt to adjust the work of our schools to the individual capacities of boys and girls and to their varying vocational outlook. Such a school, if it is to meet this demand for adjustment to individual differences, must offer a variety of courses. Among the courses offered in a typical junior high school is one which leads directly to the high school. In this course provision is made for the beginning of a foreign language, of algebra, and, in some cases, of some other high school subject during the seventh and eighth years. In another course emphasis is placed upon work in industrial or household arts in the expectation that work in these fields may lead to a higher degree of efficiency in later vocational training, and possibly to the retention of children during this period who might otherwise see little or no meaning in the traditional school course. The best junior high schools are offering in the industrial course a variety of shop work. In some cases machine shop practice, sheet metal working, woodworking, forging, printing, painting, electrical wiring, and the like are offered for boys; and cooking, sewing, including dressmaking and designing, millinery, drawing, with emphasis upon design and interior decoration, music, machine operating, pasting, and the like are provided for girls. Another type of course has provided for training which looks toward commercial work, even though it is recognized that the most adequate commercial training may require a longer period of preparation. In some schools special work in agriculture is offered.

Our schools cannot be considered as satisfactorily organized until we make provision for every boy or girl to work up to the maximum of his capacity. The one thing that a teacher cannot do is to make all of his pupils equal in achievement. Whatever adjustment may have been made in terms of special classes or segregation in terms of ability, the teacher must always face the problem of varying the assignment to meet the capacities of individual children, and she ought, wherever it is possible, especially to encourage the abler children to do work commensurate with their ability, and to provide, as far as is possible, for the rapid advancement of these children through the various stages of the school system.


1. What are the principal causes of differences in abilities or in achievement among school children?

2. What, if any, of the differences noticed among children may be attributed to sex?

3. Are any of the sex differences noticeable in the achievements of the school children with whom you are acquainted?

4. To what extent is maturity a cause of individual differences?

5. What evidence is available to show the fallacy of the common idea that children of the same age are equal in ability?

6. How important is heredity in determining the achievement of men and women?

7. To what extent, if any, would you be interested in the immediate heredity of the children in your class? Why?

8. To what extent is the environment in which children live responsible for their achievements in school studies?

9. What may be expected in the way of achievement from two children of widely different heredity but of equal training?

10. For what factor in education is the environment most responsible? Why?

11. If you grant that original nature is the primary cause of individual differences in intellectual achievements, how would you define the work of the school?

12. Why are you not justified in grouping children as bright, ordinary, and stupid?

13. Will a boy who has unusual ability in music certainly be superior in all other subjects?

14. Why are children who skip a grade apt to be able to skip again at the end of two or three years?

15. Are you able to distinguish differences in type of mind (or general mental make-up) among the children in your classes? Give illustrations.

16. What changes in school organization would you advocate for the sake of adjusting the teaching done to the varying capacities of children?

17. How should a teacher adjust his work to the individual differences in capacity or in achievement represented by the usual class group?