Part 9 (2/2)
How were these cases of feeble-mindedness defined? The definition is purely arbitrary. Ordinarily, any adult who tests much below 12 years by the Binet-Simon scale is held to be feeble-minded; and the results of this test vary a little with the skill of the person applying it and with the edition of the scale used. Furthermore, most of the feeble-minded cases in inst.i.tutions, where the Mendelian studies have usually been made, come from families which are themselves of a low grade of mentality. If the whole lot of those examined were measured, it would be difficult to draw the line between the normals and the affected; there is not nearly so much difference between the two cla.s.ses, as one would suppose who only looks at a Mendelian chart.
[Ill.u.s.tration: A HUMAN FINGER-TIP
FIG. 21.--The palms of the hands and soles of the feet are covered with little ridges or corrugations, which are supposed to be useful in preventing the grasp from slipping; whence the name of friction-skin has been given to these surfaces. The ridges are developed into various patterns; the one above is a loop on the left forefinger.
The ridges are studded with the openings of the sweat glands, the elevated position of which is supposed to prevent them from being clogged up; further, the moisture which they secrete perhaps adds to the friction of the skin. Friction-skin patterns are inherited in some degree. Photograph by John Howard Payne.]
[Ill.u.s.tration: THE LIMITS OF HEREDITARY CONTROL
FIG. 22.--Print of a finger-tip showing a loop-pattern, enlarged about eight times. This is a common type of pattern, and at first glance the reader may think it could be mistaken for one of his own. There are, however, at least sixty-five ”ridge characteristics” on the above print, which an expert would recognize and would use for the purpose of identification. If it were found that the first two or three of them noted corresponded to similar characteristics on another print, the expert would have no doubt that the two prints were made by the same finger. In police bureaus, finger-prints are filed for reference with a cla.s.sification based on the type of pattern, number of ridges between two given points, etc.; and a simple formula results which makes it easy to find all prints which bear a general resemblance to each other. The exact ident.i.ty or lack of it is then determined by a comparison of such _minutiae_ as the sixty-five above enumerated. While the general outline of a pattern is inherited, these small characters do not seem to be, but are apparently rather due to the stretching of the skin as it grows.
Ill.u.s.tration from J. H. Taylor.]
[Ill.u.s.tration: DISTRIBUTION OF I Q'S OF 905 UNSELECTED CHILDREN, 5-14 YEARS OF AGE
THE DISTRIBUTION OF INTELLIGENCE
FIG. 23.--Diagram showing the mentality of 905 unselected children, 5 to 14 years of age, who may probably be taken as representative of the whole population. The median or tallest column, about one-third of the whole number, represents those who were normal or, as a statistician would say, mediocre. Their mental ages and chronological ages were practically identical. To the left of these the diminis.h.i.+ng columns show the number whose mental ages fell short of their chronological ages. They are the mentally r.e.t.a.r.ded, ranging all the way down to the lowest one-third of one per cent who represent a very low grade of feeble-mindedness. On the other side the mentally superior show a similar distribution. A curve drawn over the tops of the columns makes a good normal curve. ”Since the frequency of the various grades of intelligence decreases _gradually_ and at no point abruptly on each side of the median, it is evident that there is no definite dividing line between normality and feeble-mindedness, or between normality and genius. Psychologically, the mentally defective child does not belong to a distinct type, nor does the genius.... The common opinion that extreme deviations below the median are vastly more frequent than extreme deviations above the median seems to have no foundation in fact. Among unselected school children, at least, for every child of any given degree of deficiency there is roughly another child as far above the average as the former is below.” Lewis M. Terman, _The Measurement of Intelligence_, pp. 66-67.]
It would be well to extend our view by measuring a whole population with one of the standard tests. If the intelligence of a thousand children picked at random from the population be measured, it will prove (as outlined in Chapter III) that some of them are feeble-minded, some are precocious or highly intelligent; and that there is every possible degree of intelligence between the two extremes. If a great number of children, all 10 years old, were tested for intelligence, it would reveal a few absolute idiots whose intelligence was no more than that of the ordinary infant, a few more who were as bright as the ordinary kindergarten child, and so up to the great bulk of normal 10-year-olds, and farther to a few prize eugenic specimens who had as much intelligence as the average college freshman. In other words, this trait of general intelligence would be found distributed through the population in accordance with that same curve of chance, which was discussed and ill.u.s.trated when we were talking about the differences between individuals.
Now what has become of the unit character, feeble-mindedness? How can one speak of a unit character, when the ”unit” has an infinite number of values? Is a _continuous quant.i.ty_ a _unit_?
If intelligence is due to the inheritance of a vast, but indeterminate, number of factors of various kinds, each of which is independent, knowledge of heredity would lead one to expect that some children would get more of these factors than others and that, broadly speaking, no two would get the same number. All degrees of intelligence between the idiot and the genius would thus exist; and yet we can not doubt that a few of these factors are more important than the others, and the presence of even one or two of them may markedly affect the level of intelligence.
It may make the matter clearer if we return for a moment to the physical. Height, bodily stature, offers a very good a.n.a.logy for the case we have just been discussing, because it is obvious that it must depend on a large number of different factors, a man's size being due to the sum total of the sizes of a great number of bones, ligaments, tissues, etc. It is obvious that one can be long in the trunk and short in the legs, or vice versa, and so on through a great number of possible combinations. Here is a perfectly measurable character (no one has ever claimed that it is a genetic ”unit character” _in man_ although it behaves as such in some plants) as to the complex basis of which all will agree. And it is known, from common observation as well as from pedigree studies, that it is not inherited as a unit: children are never born in two discontinuous cla.s.ses, ”tall” and ”short,” as they are with color blindness or normal color vision, for example. Is it not a fair a.s.sumption that the difference between the apparent unit character of feeble-mindedness, and the obvious non-unit character of height, is a matter of difference in the number of factors involved, difference in the degree to which they hang together in transmission, variation in the factors, and certainly difference in the method of measurement? Add that the line between normal and feeble-minded individuals is wholly arbitrary, and it seems that there is little reason to talk about feeble-mindedness as a unit character. It may be true that there is some sort of an inhibiting factor inherited as a unit, but it seems more likely that feeble-mindedness may be due to numerous different causes; that its presence in one child is due to one factor or group of factors, and in another child to a different one.[50]
It does not fall wholly into the cla.s.s of blending inheritance, for it does segregate to a considerable extent, yet some of the factors may show blending. Much more psychological a.n.a.lysis must be done before the question of the inheritance of feeble-mindedness can be considered solved. But at present one can say with confidence of this, as of other mental traits, that like tends to produce like; that low grades of mentality usually come from an ancestry of low mentality, and that bright children are usually produced in a stock that is marked by intelligence.
Most mental traits are even more complex in appearance than feeble-mindedness. None has yet been proved to be due to a single germinal difference, and it is possible that none will ever be so demonstrated.
[Ill.u.s.tration: FIG. 24.--The twins whose finger-prints are shown in Fig. 25.]
Intensive genetic research in lower animals and plants has shown that a visible character may be due to
1. Independent multiple factors in the germ-plasm, as in the case of wheat mentioned a few pages back.
2. Multiple allelomorphs, that is, a series of different grades of a single factor.
3. One distinct Mendelian factor (or several such factors), with modifying factors which may cause either (a) intensification, (b) inhibition, or (c) dilution.
4. Variation of a factor.
5. Or several or all of the above explanations may apply to one case.
Moreover, the characters of which the origin has been most completely worked out are mostly color characters, whose physiological development seems to be relatively simple. It is probable that the development of a mental character is much more complicated, and therefore there is more likelihood of additional factors being involved.
To say, then, that any mental trait is a unit character, or that it is due to a single germinal difference, is to go beyond both the evidence and the probabilities.
And if mental traits are, in their germinal foundations, not simple but highly complex, it follows that any advice given as to how human matings should be arranged to produce any precise result in the progeny, should be viewed with distrust. Such advice can be given only in the case of a few pathological characters such as color-blindness, night-blindness, or Huntington's Ch.o.r.ea. It is well that the man or woman interested in one of these abnormalities can get definite information on the subject; and Huntington's Ch.o.r.ea, in particular, is a dysgenic trait which can and should be stamped out. But it can not be pretended that any of man's traits, as to whose inheritance prediction can be made with confidence, is of great importance to national eugenics.
In short, a knowledge of heredity shows that attempts to predict the mode of inheritance of the important human traits (particularly mental traits) are still uncertain in their results. The characters involved are too complex to offer any simple sequences. If two parents have brown eyes, it can not be said that all their children will have brown eyes; still less can it be said that all the children of two musically gifted parents are certain to be endowed with musical talent in any given degree.
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