Then it was asserted that by
the proper series of matings, it was possible to squeeze out of the
germ-plasm the particular concrete _something_ of which insanity was the
visible expression, and have left a family stock that was perfectly
sound and sane.
the proper series of matings, it was possible to squeeze out of the
germ-plasm the particular concrete _something_ of which insanity was the
visible expression, and have left a family stock that was perfectly
sound and sane.
Applied Eugenics by Roswell H. Johnson and Paul Popenoe
49; for hair color--it is .
59.
In all of these points,
it will be admitted that no home training, or any other influence except
heredity, can conceivably play an important part. We could go on with a
long list of such measurements, which biometrists have made; and if they
were all summed up it would be found that the fraternal correlation in
these traits as to the heritability of which there can be no dispute, is
about . 52. Here is a good measure, albeit a technical one, of the
influence of heredity from the near ancestry. It is possible, too, to
measure the direct correlation between a trait in parent and the same
trait in offspring; the average of many cases where only heredity can be
thought to have had any effect in producing the result, is . 49. By the
two methods of measurement, therefore, quite comparable results are
obtained.
So much work has been done in this subject that we have no hesitation in
affirming . 5 to represent approximately the average intensity of
heredity for physical characters in man. If any well-marked physical
character be measured, in which training and environment can not be
assumed to have had any part, it will be found, in a large enough number
of subjects, that the resemblance, measured on a scale from 0 to 1, is
just about one-half of unity. Of course, perfect identity with the
parents is not to be expected, because the child must inherit from both
parents, who in turn each inherited from two parents, and so on.
So far, it may be said, we have had plain sailing because we have
carefully chosen traits in which we were not obliged to make any
subtraction whatever for the influence of training. But it is evident
that not all traits fall in that class.
This is the point at which the inheritance of mental traits has been
most often questioned. Probably no one will care to dispute the
inheritance of such physical traits as eye-color. But in considering the
mind, a certain school of popular pseudo-psychological writers question
the reality of mental inheritance, and allege that the proofs which the
geneticist offers are worthless because they do not make account of the
similarity in environment or training. Of course, it is admitted that
some sort of a mental groundwork must be inherited, but extremists
allege that this is little more than a clean slate on which the
environment, particularly during the early years of childhood, writes
its autograph.
We must grant that the analysis of the inheritance of mental traits is
proceeding slowly. This is not the fault of the geneticist, but rather
of the psychologist, who has not yet been able to furnish the geneticist
with the description of definite traits of such a character as to make
possible the exhaustive analysis of their individual inheritance. That
department of psychology is only now being formed.
We might even admit that no inherited "unit character" in the mind has
yet been isolated; but it would be a great mistake to assume from this
admission that proof of the inheritance of mental qualities, in general,
is lacking.
The psychologists and educators who think so appear either to be swayed
by metaphysical views of the mind, or else to believe that resemblance
between parent and offspring is the only evidence of inheritance that
can be offered. The father dislikes cheese, the son dislikes cheese.
"Aha, you think that that is the inheritance of a dislike for cheese,"
cries the critic, "but we will teach you better. " An interesting example
of this sort of teaching is furnished by Boris Sidis, whose feelings are
outraged because geneticists have represented that some forms of
insanity are hereditary. He declaims for several pages[31] in this
fashion:
"The so-called scientific method of the eugenists is radically faulty,
in spite of the rich display of colored plates, stained tables,
glittering biological speculations, brilliant mathematical formulae and
complicated statistical calculations. The eugenists pile Ossa on Pelion
of facts by the simple method of enumeration which Bacon and the
thinkers coming after him have long ago condemned as puerile and futile.
From the savage's belief in sympathetic, imitative magic with its
consequent superstitions, omens, and taboos down to the articles of
faith and dogmas of the eugenists we find the same faulty, primitive
thought, guided by the puerile, imbecile method of simple enumeration,
and controlled by the wisdom of the logical _post hoc, ergo propter
hoc_. "
Now if resemblance between parent and offspring were, as Dr. Sidis
supposes, the only evidence of inheritance of mental traits which the
eugenist can produce, his case would indeed be weak. And it is perfectly
true that "evidence" of this kind has sometimes been advanced as
sufficient by geneticists who should have known better. But this is not
the real evidence which genetics offers. The evidence is of numerous
kinds, and several lines might be destroyed without impairing the
validity of the remainder. It is impossible to review the whole body of
evidence here, but some of the various kinds may be indicated, and
samples given, even though this involves the necessity of repeating some
things we have said in earlier chapters. The reader will then be able to
form his own opinion as to whether the geneticists' proofs or the mere
assurances of those who have not studied the subject are the more
weighty.
1. _The analogy from breeding experiments. _ Tame rats, for instance, are
very docile; their offspring can be handled without a bit of trouble.
The wild rat, on the other hand, is not at all docile.
W. E. Castle, of Harvard University, writes:[32] "We have repeatedly
mated tame female rats with wild males, the mothers being removed to
isolated cages before the birth of the young. These young which had
never seen or been near their father were very wild in disposition in
every case. The observations of Yerkes on such rats raised by us
indicates that their wildness was not quite as extreme as that of the
pure wild rat but closely approached it. "
Who can suggest any plausible explanation of their conduct, save that
they inherited a certain temperament from their sire? Yet the
inheritance of temperament is one of the things which certain
psychologists most "view with alarm. " If it is proved in other animals,
can it be considered wholly impossible in man?
2. _The segregation of mental traits. _ When an insane, or epileptic, or
feeble-minded person mates with a normal individual, in whose family no
taint is found, the offspring (generally speaking) will be mentally
sound, even though one parent is not. On the other hand, if two people
from tainted stocks marry, although neither one may be personally
defective, part of their offspring will be affected.
This production of sound children from an unsound parent, in the first
case, and unsound children from two apparently sound parents in the
second case, is exactly the opposite of what one would expect if the
child gets his unsoundness merely by imitation or "contagion. " The
difference can not reasonably be explained by any difference in
environment or external stimuli. Heredity offers a satisfactory
explanation, for some forms of feeble-mindedness and epilepsy, and some
of the diseases known as insanity, behave as recessives and segregate in
just the way mentioned. There are abundant analogies in the inheritance
of other traits in man, lower animals and plants, that behave in exactly
the same manner.
If mental defects are inherited, then it is worth while investigating
whether mental excellencies may not also be.
3. _The persistence of like qualities regardless of difference in
environment. _ Any parent with open eyes must see this in his own
children--must see that they retained the inherited traits even when
they left home and lived under entirely different surroundings. But the
histories of twins furnish the most graphic evidence. Galton, who
collected detailed histories of thirty-five pairs of twins who were
closely alike at birth, and examined their history in after years,
writes:[33] "In some cases the resemblance of body and mind had
continued unaltered up to old age, notwithstanding very different
conditions of life;" in other cases where some dissimilarity developed,
it could be traced to the influence of an illness. Making due allowance
for the influence of illness, yet "instances do exist of an apparently
thorough similarity of nature, in which such differences of external
circumstances as may be consistent with the ordinary conditions of the
same social rank and country do not create dissimilarity. Positive
evidence, such as this, can not be outweighed by any amount of negative
evidence. "
Frederick Adams Woods has brought forward[34] a piece of more exact
evidence under this head. It is known from many quantitative studies
that in physical heredity, the influence of the paternal grandparents
and the influence of the maternal grandparents is equal; on the average
one pair will contribute no more to the grandchildren than the other. If
mental qualities are due rather to early surroundings than to actual
inheritance, this equality of grandparental influence is incredible in
the royal families where Dr. Woods got his material; for the grandchild
has been brought up at the court of the paternal grandfather, where he
ought to have gotten all his "acquirements," and has perhaps never even
seen his maternal grandparents, who therefore could not be expected to
impress their mental peculiarities on him by "contagion. " When Dr. Woods
actually measured the extent of resemblance to the two sets of
grandparents, for mental and moral qualities, he found it to be the same
in each case; as is inevitable if they are inherited, but as is
incomprehensible if heredity is not largely responsible for one's mental
make-up.
4. _Persistence of unlike qualities regardless of sameness in the_
_environment. _ This is the converse of the preceding proposition, but
even more convincing. In the last paragraph but one, we mentioned
Galton's study (cited at some length in our Chapter I) of "identical"
twins, who are so much alike at birth for the very good reason that they
have identical heredity. This heredity was found to be not modified,
either in the body or the mind, by ordinary differences of training and
environment. Some of Galton's histories[35] of ordinary, non-identical
twins were also given in Chapter I; two more follow:
One parent says: "They have been treated exactly alike; both were
brought up by hand; they have been under the same nurse and governess
from their birth, and they are very fond of each other. Their increasing
dissimilarity must be ascribed to a natural difference of mind and
character, as there has been nothing in their treatment to account for
it. "
Another writes: "This case is, I should think, somewhat remarkable for
dissimilarity in physique as well as for strong contrast in character.
They have been unlike in mind and body throughout their lives. Both were
reared in a country house and both were at the same schools until the
age of 16. "
In the face of such examples, can anyone maintain that differences in
mental make-up are wholly due to different influences during childhood,
and not at all to differences in germinal make-up? It is not necessary
to depend, under this head, on mere descriptions, for accurate
measurements are available to demonstrate the point. If the environment
creates the mental nature, then ordinary brothers, not more than four or
five years apart in age, ought to be about as closely similar to each
other as identical twins are to each other; for the family influences in
each case are practically the same. Professor Thorndike, by careful
mental tests, showed[36] that this is not true. The ordinary brothers
come from different egg-cells, and, as is known from studies on lower
animals, they do not get exactly the same inheritance from their
parents; they show, therefore, considerable differences in their psychic
natures. Real identical twins, being two halves of the same egg-cell,
have the same heredity, and their natures are therefore much more
nearly identical.
Again, if the mind is molded during the "plastic years of childhood,"
children ought to become more alike, the longer they are together. Twins
who were unlike at birth ought to resemble each other more closely at 14
than they did at 9, since they have been for five additional years
subjected to this supposedly potent but very mystical "molding force. "
Here again Professor Thorndike's exact measurements explode the fallacy.
They are actually, measurably, less alike at the older age; their inborn
natures are developing along predestined lines, with little regard to
the identity of their surroundings. Heredity accounts easily for these
facts, but they cannot be squared with the idea that mental differences
are the products solely of early training.
5. _Differential rates of increase in qualities subject to much
training. _ If the mind is formed by training, then brothers ought to be
more alike in qualities which have been subject to little or no
training. Professor Thorndike's measurements on this point show the
reverse to be true. The likeness of various traits is determined by
heredity, and brothers may be more unlike in traits which have been
subjected to a large and equal amount of training. Twins were found to
be less alike in their ability at addition and multiplication, in which
the schools had been training them for some years, than they were in
ability to mark off the A's on a printed sheet, or to write the
opposites to a list of words--feats which they had probably never before
tried to do.
This same proposition may be put on a broader basis. [37] "In so far as
the differences in achievement found amongst a group of men are due to
the differences in the quantity and quality of training which they had
had in the function in question, the provision of equal amounts of the
same sort of training for all individuals in the group should act to
reduce the differences. " "If the addition of equal amounts of practice
does not reduce the differences found amongst men, those differences can
not well be explained to any large extent by supposing them to have been
due to corresponding differences in amount of previous practice. If,
that is, inequalities in achievement are not reduced by equalizing
practice, they can not well have been caused by inequalities in previous
practice. If differences in opportunity cause the differences men
display, making opportunity more nearly equal for all, by adding equal
amounts to it in each case should make the differences less.
"The facts found are rather startling. Equalizing practice seems to
increase differences. The superior man seems to have got his present
superiority by his own nature rather than by superior advantages of the
past, since, during a period of equal advantage for all, he increases
his lead. " This point has been tested by such simple devices as mental
multiplication, addition, marking A's on a printed sheet of capitals and
the like; all the contestants made some gain in efficiency, but those
who were superior at the start were proportionately farther ahead than
ever at the end. This is what the geneticist would expect, but fits very
ill with some popular pseudo-science which denies that any child is
mentally limited by nature.
6. _Direct measurement of the amount of resemblance of mental traits in
brothers and sisters. _ It is manifestly impossible to assume that early
training, or parental behavior, or anything of the sort, can have
influenced very markedly the child's eye color, or the length of his
forearm, or the ratio of the breadth of his head to its length. A
measure of the amount of resemblance between two brothers in such traits
may very confidently be said to represent the influence of heredity; one
can feel no doubt that the child inherits his eye-color and other
physical traits of that kind from his parents. It will be recalled that
the resemblance, measured on a scale from 0 to 1, has been found to be
about 0. 5.
Karl Pearson measured the resemblance between brothers and sisters in
mental traits--for example, temper, conscientiousness, introspection,
vivacity--and found it on the average to have the same intensity--that
is, about 0. 5. Starch gets similar results in studying school grades.
Professor Pearson writes:[38]
"It has been suggested that this resemblance in the psychological
characters is compounded of two factors, inheritance on the one hand and
training and environment on the other. If so, one must admit that
inheritance and environment make up the resemblance in the physical
characters. Now these two sorts of resemblance being of the same
intensity, either the environmental influence is the same in both cases
or it is not. If it is the same, we are forced to the conclusion that it
is insensible, for it can not influence eye-color. If it is not the
same, then it would be a most marvelous thing that with varying degrees
of inheritance, some mysterious force always modifies the extent of home
influence, until the resemblance of brothers and sisters is brought
sensibly up to the same intensity! Occam's razor[39] will enable us at
once to cut off such a theory. We are forced, I think, literally forced,
to the general conclusion that the physical and psychical characters in
man are inherited within broad lines in the same manner, and with
approximate intensity. The average parental influence is in itself
largely a result of the heritage of the stock and not an extraneous and
additional factor causing the resemblance between children from the same
home. "
A paragraph from Edgar Schuster[40] may appropriately be added. "After
considering the published evidence a word must be said of facts which
most people may collect for themselves. They are difficult to record,
but are perhaps more convincing than any quantity of statistics. If one
knows well several members of a family, one is bound to see in them
likenesses with regard to mental traits, both large and small, which
may sometimes be accounted for by example on the one hand or unconscious
imitation on the other, but are often quite inexplicable on any other
theory than heredity. It is difficult to understand how the inheritance
of mental capacity can be denied by those whose eyes are open and whose
minds are open too. "
Strictly speaking, it is of course true that man inherits nothing more
than the capacity of making mental acquirements. But this general
capacity is made up of many separate capacities, all of these capacities
are variable, and the variations are inherited. Such seems to us to be
the unmistakable verdict of the evidence.
Our conclusions as to the inheritance of all sorts of mental capacity
are not based on the mere presence of the same trait in parent and
child. As the psychological analysis of individual traits proceeds, it
will be possible to proceed further with the study of the inheritance of
these traits. Some work has been done on spelling, which is particularly
interesting because most people, without reflection, would take it for
granted that a child's spelling ability depends almost wholly on his
training. Professor Thorndike's exposition[41] of the investigation is
as follows:
"E. L. Earle ('03) measured the spelling abilities of some 800 children
in the St. Xavier school in New York by careful tests. As the children
in this school commonly enter at a very early age, and as the staff and
methods of teaching remain very constant, we have in the case of the 180
pairs of brothers and sisters included in the 600 children closely
similar school training. Mr. Earle measured the ability of any
individual by his deviation from the average for his grade and sex, and
found the coefficient of correlation between children of the same family
to be . 50. That is, any individual is on the average 50% as much above
or below the average for his age and sex as his brother or sister.
"Similarities of home training might account for this, but any one
experienced in teaching will hesitate to attribute much efficacy to
such similarities. Bad spellers remain bad spellers though their
teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of
spelling ability ('97) found little or no relationship between good
spelling and any one of the popular methods, and little or none between
poor spelling and foreign parentage. Cornman's more careful study of
spelling ('07) supports the view that ability to spell is little
influenced by such differences in school or home training as commonly
exist. "
This is a very clear-cut case of a definite intellectual ability,
differences in which might be supposed to be due almost wholly to the
child's training, but which seem, on investigation, to be largely due to
heredity.
The problem may be examined in still greater detail. Does a man merely
inherit manual skill, let us say, or does he inherit the precise kind of
manual skill needed to make a surgeon but not the kind that would be
useful to a watchmaker? Is a man born merely with a generalized
"artistic" ability, or is it one adapted solely for, let us say, music;
or further, is it adapted solely for violin playing, not for the piano?
Galton, in his pioneer studies, sought for data on this question. In
regard to English judges, he wrote: "Do the judges often have sons who
succeed in the same career, where success would have been impossible if
they had not been gifted with the special qualities of their fathers?
Out of the 286 judges, more than _one in every nine_ of them have been
either father, son or brother to another judge, and the other high legal
relationships have been even more numerous. There can not, then, remain
a doubt but that the peculiar type of ability that is necessary to a
judge is often transmitted by descent. "
Unfortunately, we can not feel quite as free from doubt on the point as
Galton did. The judicial mind, if that be the main qualification for a
judge, might be inherited, or it might be the result of training. Such a
case, standing alone, is inconclusive.
Galton similarly showed that the sons of statesmen tended to be
statesmen, and that the same was true in families of great commanders,
literary men, poets and divines. In his list of eminent painters, all
the relatives mentioned are painters save four, two of whom were gifted
in sculpture, one in music and one in embroidery. As to musicians,
Mendelssohn and Meyerbeer are the only ones in his list whose eminent
kinsmen achieved their success in other careers than music.
Havelock Ellis, who likewise studied British men of genius, throws
additional light on the subject. "Painters and sculptors," he found,
"constitute a group which appears to be of very distinct interest from
the point of view of occupational heredity. In social origin, it may be
noted, the group differs strikingly in constitution from the general
body of men of genius in which the upper class is almost or quite
predominant. Of 63 painters and sculptors of definitely known origin,
only two can be placed in the aristocratic division. Of the remainder 7
are the sons of artists, 22 the sons of craftsmen, leaving only 32 for
all other occupations, which are mainly of lower middle class character,
and in many cases trades that are very closely allied to crafts. Even,
however, when we omit the trades as well as the cases in which the
fathers were artists, we find a very notable predominance of craftsmen
in the parentage of painters, to such an extent indeed that while
craftsmen only constitute 9. 2% among the fathers of our eminent persons
generally, they constitute nearly 35% among the fathers of the painters
and sculptors. It is difficult to avoid the conclusion that there is a
real connection between the father's aptitude for craftsmanship and the
son's aptitude for art.
"To suppose that environment adequately accounts for this relationship
is an inadmissible theory. The association between the craft of builder,
carpenter, tanner, jeweller, watchmaker, woodcarver, ropemaker, etc. ,
and the painter's art is small at best, and in most cases is
non-existent. "
Arreat, investigating the heredity of 200 eminent European painters,
reached results similar to those of Ellis, according to the latter's
citation.
Arithmetical ability seems similarly to be subdivided, according to Miss
Cobb. [42] She made measurements of the efficiency with which children
and their parents could do problems in addition, subtraction,
multiplication and division, and could copy a column of figures. "The
measurements made," she writes, "show that if, for instance, a child is
much quicker than the average in subtraction, but not in addition,
multiplication or division, it is to be expected that one at least of
his parents shows a like trait; or if he falls below the average in
subtraction and multiplication, and exceeds it in addition and division,
again the same will hold true of at least one of his parents. " These
various kinds of arithmetic appear to be due to different functions of
the brain, and are therefore probably inherited independently, if they
are inherited at all.
To assume that the resemblance between parent and offspring in
arithmetical ability is due to association, training and imitation is
not plausible. If this were the case, a class of children ought to come
to resemble their teacher, but they do not. Moreover, the child
sometimes resembles more closely the parent with whom he has been less
associated in daily life.
From such data as these, we conclude that mental inheritance is
considerably specialized. This conclusion is in accord with Burris'
finding (cited by Thorndike) that the ability to do well in some one
high school study is nearly or quite as much due to ancestry as is the
ability to do well in the course as a whole.
To sum up, we have reason to believe not only that one's mental
character is due largely to heredity, but that the details of it may be
equally due to heredity, in the sense that for any particular trait or
complex in the child there is likely to be found a similar trait or
complex in the ancestry. Such a conclusion should not be pushed to the
point of assuming inheritance of all sorts of dispositions that might be
due to early training; on the other hand, a survey of the whole field
would probably justify us in concluding that any given trait is _more
likely than not_ to be inherited. The effect of training in the
formation of the child's mental character is certainly much less than is
popularly supposed; and even for the traits that are most due to
training, it must never be forgotten that there are inherited mental
bases.
If the reader has accepted the facts presented in this chapter, and our
inferences from the facts, he will admit that mental differences between
men are at bottom due to heredity, just as physical differences are;
that they are apparently inherited in the same manner and in
approximately the same degree.
CHAPTER V
THE LAWS OF HEREDITY
We have now established the bases for a practicable eugenics program.
Men differ; these differences are inherited; therefore the make-up of
the race can be changed by any method which will alter the relative
proportions of the contributions which different classes of men make to
the following generation.
For applied eugenics, it is sufficient to know that mental and physical
differences are inherited; the exact manner of inheritance it would be
important to know, but even without a knowledge of the details of the
mechanism of heredity, a program of eugenics is yet wholly feasible.
It is no part of the plan of this book to enter into the details of the
mechanism of heredity, a complicated subject for which the reader can
refer to one of the treatises mentioned in the bibliography at the close
of this volume. It may be worth while, however, to outline in a very
summary way the present status of the question.
As to the details of inheritance, research has progressed in the last
few years far beyond the crude conceptions of a decade ago, when a
primitive form of Mendelism was made to explain everything that
occurred. [43] One can hardly repress a smile at the simplicity of those
early ideas,--though it must be said that some students of eugenics have
not yet outgrown them. In those days it was thought that every visible
character in man (or in any other organism) was represented by some
"determiner" in the germ-plasm; that by suitable matings a breeder could
rid a stream of germ-plasm of almost any determiner he wished; and that
the corresponding unit character would thereupon disappear from the
visible make-up of the individual. Was a family reported as showing a
taint, for instance, hereditary insanity?
Then it was asserted that by
the proper series of matings, it was possible to squeeze out of the
germ-plasm the particular concrete _something_ of which insanity was the
visible expression, and have left a family stock that was perfectly
sound and sane.
The minute, meticulous researches of experimental breeders[44] have left
such a view of heredity far behind. Certainly the last word has not been
said; yet the present hypotheses _work_, whenever the conditions are
such as to give a fair chance. The results of these studies have led to
what is called the factorial hypothesis of heredity,[45] according to
which all the visible characters of the adult are produced by (purely
hypothetical) factors in the germ-plasm; it is the factors that are
inherited, and they, under proper conditions for development, produce
the characters. The great difference between this and the earlier view
is that instead of allotting one factor to each character, students now
believe that each individual character of the organism is produced by
the action of an indefinitely large number of factors,[46] and they
have been further forced to adopt the belief that each individual
factor affects an indefinitely large number of characters, owing to the
physiological interrelations and correlations of every part of the body.
[Illustration: HOW DO YOU CLASP YOUR HANDS?
FIG. 16. --If the hands be clasped naturally with fingers
alternating, as shown in the above illustration, most people will put
the same thumb--either that of the right or that of the left
hand--uppermost every time. Frank E. Lutz showed (_American Naturalist_,
xliii) that the position assumed depends largely on heredity. When both
parents put the right thumb uppermost, about three-fourths of the
children were found to do the same. When both parents put the left thumb
uppermost, about three-fifths of the children did the same. No definite
ratios could be found from the various kinds of matings. Apparently the
manner of clasping hands has no connection with one's right-handedness
or left-handedness. It can hardly be due to imitation for the trait is
such a slight one that most people have not noticed it before their
attention is called to it by the geneticist. Furthermore, babies are
found almost always to clasp the hands in the same way every time. The
trait is a good illustration of the almost incredible minuteness with
which heredity enters into a man's make-up. Photograph by John Howard
Paine. ]
The sweet pea offers a good illustration of the widespread effects which
may result from the change of a single factor. In addition to the
ordinary climbing vine, there is a dwarf variety, and the difference
between the two seems to be proved, by exhaustive experimental breeding,
to be due to only one inherited factor. Yet the action of this one
factor not only changes the height of the plant, but also results in
changes in color of foliage, length of internodes, size and arrangement
of flowers, time of opening of flowers, fertility and viability.
Again, a mutant stock in the fruit fly (Drosophila) has as its most
marked characteristic very short wings. "But the factor for rudimentary
wings also produces other effects as well. The females are almost
completely sterile, while the males are fertile. The viability of the
stocks is poor. When flies with rudimentary wings are put into
competition with wild flies relatively few of the rudimentary flies come
through, especially if the culture is crowded. The hind legs are also
shortened. All of these effects are the results of a single
factor-difference. " To be strictly accurate, then, one should not say
that a certain variation affects length of wing, but that its _chief_
effect is to shorten the wing.
"One may venture to guess," T. H. Morgan says,[47] "that some of the
specific and varietal differences that are characteristic of wild types
and which at the same time appear to have no survival value, are only
by-products of factors whose most important effect is on another part
of the organism where their influence is of vital importance. "
"I am inclined to think," Professor Morgan continues, "that an
overstatement to the effect that each factor may affect the entire body,
is less likely to do harm than to state that each factor affects only a
particular character. The reckless use of the phrase 'unit character'
has done much to mislead the uninitiated as to the effects that a single
change in the germ-plasm may produce on the organism. Fortunately the
expression 'unit character' is being less used by those students of
genetics who are more careful in regard to the implications of their
terminology. "
[Illustration: THE EFFECT OF ORTHODACTYLY
FIG. 17. --At the left is a hand with the third, fourth and
fifth fingers affected. The middle joints of these fingers are stiff and
cannot be bent. At the right the same hand is shown, closed. A normal
hand in the middle serves to illustrate by contrast the nature of the
abnormality, which appears in every generation of several large
families. It is also called symphalangism, and is evidently related to
the better-known abnormality of brachydactyly. Photograph from Frederick
N. Duncan. ]
[Illustration: A FAMILY WITH ORTHODACTYLY
FIG. 18. --Squares denote males and circles females, as is usual
in the charts compiled by eugenists; black circles or squares denote
affected individuals. A1 had all fingers affected in the way shown in
Fig. 17; B2 had all but one finger affected; C2 had all but one finger
affected; D2 had all fingers affected; D3 has all but forefingers
affected. The family here shown is a branch, found by F. N. Duncan, of a
very large family first described by Harvey Cushing, in which this
abnormality has run for at least seven generations. It is an excellent
example of an inherited defect due to a single Mendelian factor. ]
One of the best attested single characters in human heredity is
brachydactyly, "short-fingerness," which results in a reduction in the
length of the fingers by the dropping out of one joint. If one lumps
together all the cases where any effect of this sort is found, it is
evident that normals never transmit it to their posterity, that affected
persons always do, and that in a mating between a normal and an affected
person, all the offspring will show the abnormality. It is a good
example of a unit character.
But its effect is by no means confined to the fingers. It tends to
affect the entire skeleton, and in a family where one child is markedly
brachydactylous, that child is generally shorter than the others. The
factor for brachydactyly evidently produces its primary effect on the
bones of the hand, but it also produces a secondary effect on all the
bones of the body.
Moreover, it will be found, if a number of brachydactylous persons are
examined, that no two of them are affected to exactly the same degree.
In some cases only one finger will be abnormal; in other cases there
will be a slight effect in all the fingers; in other cases all the
fingers will be highly affected. Why is there such variation in the
results produced by a unit character? Because, presumably, in each
individual there is a different set of modifying factors or else a
variation in the factor. It has been found that an abnormality quite
like brachydactyly is produced by abnormality in the pituitary gland. It
is then fair to suppose that the factor which produces brachydactyly
does so by affecting the pituitary gland in some way. But there must be
many other factors which also affect the pituitary and in some cases
probably favor its development, rather than hindering it. Then if the
factor for brachydactyly is depressing the pituitary, but if some other
factors are at the same time stimulating that gland, the effect shown in
the subject's fingers will be much less marked than if a group of
modifying factors were present which acted in the same direction as the
brachydactyly factor,--to perturb the action of the pituitary gland.
This illustration is largely hypothetical; but there is no room for
doubt that every factor produces more than a single effect. A white
blaze in the hair, for example, is a well-proved unit factor in man; the
factor not only produces a white streak in the hair, but affects the
pigmentation of the skin as well, usually resulting in one or more white
spots on some part of the body. It is really a factor for "piebaldism. "
For the sake of clear thinking, then, the idea of a unit character due
to some unit determiner or factor in the germ-plasm must be given up,
and it must be recognized that every visible character of an individual
is the result of numerous factors, or differences in the germ-plasm.
Ordinarily one of these produces a more notable contribution to the
end-product than do the others; but there are cases where this statement
does not appear to hold good. This leads to the conception of _multiple
factors_.
In crossing a wheat with brown chaff and one with white chaff, H.
Nilsson-Ehle (1909) expected in the second hybrid generation to secure a
ratio of 3 brown to 1 white. As a fact, he got 1410 brown and 94 white,
a ratio of 15:1. He interpreted this as meaning that the brown color in
this particular variety was due not to one factor, but to two, which
were equivalent to each other, and either one of which would produce the
same result alone as would the two acting together. In further crossing
red wheat with white, he secured ratios which led him to believe that
the red was produced by three independent factors, any one of which
would produce red either alone or with the other two. A. and G. Howard
later corroborated this work,[48] but showed that the three factors were
not identical: they are qualitatively slightly different, although so
closely similar that the three reds look alike at first sight. E. M.
East has obtained evidence from maize and G. H. Shull from
shepherd's-purse, which bears out the multiple factor hypothesis.
[Illustration: WHITE BLAZE IN THE HAIR
FIG. 19. --The white lock of hair here shown is hereditary and
has been traced back definitely through six generations; family
tradition derives it from a son of Harry "Hot-Spur" Percy, born in 1403,
and fallaciously assigns its origin to "prenatal influence" or "maternal
impression. " This young woman inherited the blaze from her father, who
had it from his mother, who had it from her father, who migrated from
England to America nearly a century ago. The trait appears to be a
simple dominant, following Mendel's Law; that is, when a person with one
of these locks who is a child of one normal and one affected parent
marries a normal individual, half of the children show the lock and half
do not. Photograph from Newton Miller. ]
[Illustration: A FAMILY OF SPOTTED NEGROES
FIG. 20. --The piebald factor sometimes shows itself as nothing
more than a blaze in the hair (see preceding figure); but it may take a
much more extreme form, as illustrated by the above photograph from Q.
I. Simpson and W. E. Castle. Mrs. S. A. , a spotted mutant, founded a
family which now comprises, in several generations, 17 spotted and 16
normal offspring. The white spotting factor behaves as a Mendelian
dominant, and the expectation would be equal numbers of normal and
affected children. Similar white factors are known in other animals. It
is worth noting that all the well attested Mendelian characters in man
are abnormalities, no normal character having yet been proved to be
inherited in this manner. ]
Apart from multiple factors as properly defined (that is, factors which
produce the same result, either alone or together), extensive analysis
usually reveals that apparently simple characters are in reality
complex. The purple aleurone color of maize seeds is attributed by R. A.
Emerson to five distinct factors, while E. Baur found four factors
responsible for the red color of snapdragon blossoms. There are, as G.
N. Collins says,[49] "still many gross characters that stand as
simple Mendelian units, but few, if any, of these occur in plants or
animals that have been subjected to extensive investigation. There is
now such a large number of characters which at first behaved as units,
but which have since been broken up by crossing with suitable selected
material, that it seems not unreasonable to believe that the remaining
cases await only the discovery of the right strains with which to
hybridize them to bring about corresponding results. "
In spite of the fact that there is a real segregation between factors as
has been shown, it must not be supposed that factors and their
determiners are absolutely invariable. This has been too frequently
assumed without adequate evidence by many geneticists. It is probable
that just as the multiplicity and interrelation and minuteness of many
factors have been the principal discoveries of genetics in recent years
that the next few years will see a great deal of evidence following the
important lead of Castle and Jennings, as to variation in factors.
Knowing that all the characters of an individual are due to the
interaction of numerous factors, one must be particularly slow in
assuming that such complex characters as man's mental traits are units,
in any proper genetic sense of the word. It will, for instance, require
very strong evidence to establish feeble-mindedness as a unit character.
No one who examines the collected pedigrees of families marked by
feeble-mindedness, can deny that it does appear at first sight to behave
as a unit character, inherited in the typical Mendelian fashion. The
psychologist H. H. Goddard, who started out with a strong bias against
believing that such a complex trait could even _behave_ as a unit
character, thought himself forced by the tabulation of his cases to
adopt the conclusion that it does behave as a unit character. And other
eugenists have not hesitated to affirm, mainly on the strength of Dr.
Goddard's researches, that this unit character is due to a single
determiner in the germ-plasm, which either is or is not present,--no
halfway business about it.
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 institutions, 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
classes, as one would suppose who only looks at a Mendelian chart.
[Illustration: 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. ]
[Illustration: 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
classification 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 identity 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.
Illustration from J. H. Taylor. ]
[Illustration: 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
diminishing columns show the number whose mental ages fell short of
their chronological ages. They are the mentally retarded, 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 illustrated 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 quantity_ 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 analogy 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 classes, "tall" and "short," as they are with
color blindness or normal color vision, for example. Is it not a fair
assumption 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 class of blending inheritance, for it
does segregate to a considerable extent, yet some of the factors may
show blending. Much more psychological analysis 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.
[Illustration: 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.
it will be admitted that no home training, or any other influence except
heredity, can conceivably play an important part. We could go on with a
long list of such measurements, which biometrists have made; and if they
were all summed up it would be found that the fraternal correlation in
these traits as to the heritability of which there can be no dispute, is
about . 52. Here is a good measure, albeit a technical one, of the
influence of heredity from the near ancestry. It is possible, too, to
measure the direct correlation between a trait in parent and the same
trait in offspring; the average of many cases where only heredity can be
thought to have had any effect in producing the result, is . 49. By the
two methods of measurement, therefore, quite comparable results are
obtained.
So much work has been done in this subject that we have no hesitation in
affirming . 5 to represent approximately the average intensity of
heredity for physical characters in man. If any well-marked physical
character be measured, in which training and environment can not be
assumed to have had any part, it will be found, in a large enough number
of subjects, that the resemblance, measured on a scale from 0 to 1, is
just about one-half of unity. Of course, perfect identity with the
parents is not to be expected, because the child must inherit from both
parents, who in turn each inherited from two parents, and so on.
So far, it may be said, we have had plain sailing because we have
carefully chosen traits in which we were not obliged to make any
subtraction whatever for the influence of training. But it is evident
that not all traits fall in that class.
This is the point at which the inheritance of mental traits has been
most often questioned. Probably no one will care to dispute the
inheritance of such physical traits as eye-color. But in considering the
mind, a certain school of popular pseudo-psychological writers question
the reality of mental inheritance, and allege that the proofs which the
geneticist offers are worthless because they do not make account of the
similarity in environment or training. Of course, it is admitted that
some sort of a mental groundwork must be inherited, but extremists
allege that this is little more than a clean slate on which the
environment, particularly during the early years of childhood, writes
its autograph.
We must grant that the analysis of the inheritance of mental traits is
proceeding slowly. This is not the fault of the geneticist, but rather
of the psychologist, who has not yet been able to furnish the geneticist
with the description of definite traits of such a character as to make
possible the exhaustive analysis of their individual inheritance. That
department of psychology is only now being formed.
We might even admit that no inherited "unit character" in the mind has
yet been isolated; but it would be a great mistake to assume from this
admission that proof of the inheritance of mental qualities, in general,
is lacking.
The psychologists and educators who think so appear either to be swayed
by metaphysical views of the mind, or else to believe that resemblance
between parent and offspring is the only evidence of inheritance that
can be offered. The father dislikes cheese, the son dislikes cheese.
"Aha, you think that that is the inheritance of a dislike for cheese,"
cries the critic, "but we will teach you better. " An interesting example
of this sort of teaching is furnished by Boris Sidis, whose feelings are
outraged because geneticists have represented that some forms of
insanity are hereditary. He declaims for several pages[31] in this
fashion:
"The so-called scientific method of the eugenists is radically faulty,
in spite of the rich display of colored plates, stained tables,
glittering biological speculations, brilliant mathematical formulae and
complicated statistical calculations. The eugenists pile Ossa on Pelion
of facts by the simple method of enumeration which Bacon and the
thinkers coming after him have long ago condemned as puerile and futile.
From the savage's belief in sympathetic, imitative magic with its
consequent superstitions, omens, and taboos down to the articles of
faith and dogmas of the eugenists we find the same faulty, primitive
thought, guided by the puerile, imbecile method of simple enumeration,
and controlled by the wisdom of the logical _post hoc, ergo propter
hoc_. "
Now if resemblance between parent and offspring were, as Dr. Sidis
supposes, the only evidence of inheritance of mental traits which the
eugenist can produce, his case would indeed be weak. And it is perfectly
true that "evidence" of this kind has sometimes been advanced as
sufficient by geneticists who should have known better. But this is not
the real evidence which genetics offers. The evidence is of numerous
kinds, and several lines might be destroyed without impairing the
validity of the remainder. It is impossible to review the whole body of
evidence here, but some of the various kinds may be indicated, and
samples given, even though this involves the necessity of repeating some
things we have said in earlier chapters. The reader will then be able to
form his own opinion as to whether the geneticists' proofs or the mere
assurances of those who have not studied the subject are the more
weighty.
1. _The analogy from breeding experiments. _ Tame rats, for instance, are
very docile; their offspring can be handled without a bit of trouble.
The wild rat, on the other hand, is not at all docile.
W. E. Castle, of Harvard University, writes:[32] "We have repeatedly
mated tame female rats with wild males, the mothers being removed to
isolated cages before the birth of the young. These young which had
never seen or been near their father were very wild in disposition in
every case. The observations of Yerkes on such rats raised by us
indicates that their wildness was not quite as extreme as that of the
pure wild rat but closely approached it. "
Who can suggest any plausible explanation of their conduct, save that
they inherited a certain temperament from their sire? Yet the
inheritance of temperament is one of the things which certain
psychologists most "view with alarm. " If it is proved in other animals,
can it be considered wholly impossible in man?
2. _The segregation of mental traits. _ When an insane, or epileptic, or
feeble-minded person mates with a normal individual, in whose family no
taint is found, the offspring (generally speaking) will be mentally
sound, even though one parent is not. On the other hand, if two people
from tainted stocks marry, although neither one may be personally
defective, part of their offspring will be affected.
This production of sound children from an unsound parent, in the first
case, and unsound children from two apparently sound parents in the
second case, is exactly the opposite of what one would expect if the
child gets his unsoundness merely by imitation or "contagion. " The
difference can not reasonably be explained by any difference in
environment or external stimuli. Heredity offers a satisfactory
explanation, for some forms of feeble-mindedness and epilepsy, and some
of the diseases known as insanity, behave as recessives and segregate in
just the way mentioned. There are abundant analogies in the inheritance
of other traits in man, lower animals and plants, that behave in exactly
the same manner.
If mental defects are inherited, then it is worth while investigating
whether mental excellencies may not also be.
3. _The persistence of like qualities regardless of difference in
environment. _ Any parent with open eyes must see this in his own
children--must see that they retained the inherited traits even when
they left home and lived under entirely different surroundings. But the
histories of twins furnish the most graphic evidence. Galton, who
collected detailed histories of thirty-five pairs of twins who were
closely alike at birth, and examined their history in after years,
writes:[33] "In some cases the resemblance of body and mind had
continued unaltered up to old age, notwithstanding very different
conditions of life;" in other cases where some dissimilarity developed,
it could be traced to the influence of an illness. Making due allowance
for the influence of illness, yet "instances do exist of an apparently
thorough similarity of nature, in which such differences of external
circumstances as may be consistent with the ordinary conditions of the
same social rank and country do not create dissimilarity. Positive
evidence, such as this, can not be outweighed by any amount of negative
evidence. "
Frederick Adams Woods has brought forward[34] a piece of more exact
evidence under this head. It is known from many quantitative studies
that in physical heredity, the influence of the paternal grandparents
and the influence of the maternal grandparents is equal; on the average
one pair will contribute no more to the grandchildren than the other. If
mental qualities are due rather to early surroundings than to actual
inheritance, this equality of grandparental influence is incredible in
the royal families where Dr. Woods got his material; for the grandchild
has been brought up at the court of the paternal grandfather, where he
ought to have gotten all his "acquirements," and has perhaps never even
seen his maternal grandparents, who therefore could not be expected to
impress their mental peculiarities on him by "contagion. " When Dr. Woods
actually measured the extent of resemblance to the two sets of
grandparents, for mental and moral qualities, he found it to be the same
in each case; as is inevitable if they are inherited, but as is
incomprehensible if heredity is not largely responsible for one's mental
make-up.
4. _Persistence of unlike qualities regardless of sameness in the_
_environment. _ This is the converse of the preceding proposition, but
even more convincing. In the last paragraph but one, we mentioned
Galton's study (cited at some length in our Chapter I) of "identical"
twins, who are so much alike at birth for the very good reason that they
have identical heredity. This heredity was found to be not modified,
either in the body or the mind, by ordinary differences of training and
environment. Some of Galton's histories[35] of ordinary, non-identical
twins were also given in Chapter I; two more follow:
One parent says: "They have been treated exactly alike; both were
brought up by hand; they have been under the same nurse and governess
from their birth, and they are very fond of each other. Their increasing
dissimilarity must be ascribed to a natural difference of mind and
character, as there has been nothing in their treatment to account for
it. "
Another writes: "This case is, I should think, somewhat remarkable for
dissimilarity in physique as well as for strong contrast in character.
They have been unlike in mind and body throughout their lives. Both were
reared in a country house and both were at the same schools until the
age of 16. "
In the face of such examples, can anyone maintain that differences in
mental make-up are wholly due to different influences during childhood,
and not at all to differences in germinal make-up? It is not necessary
to depend, under this head, on mere descriptions, for accurate
measurements are available to demonstrate the point. If the environment
creates the mental nature, then ordinary brothers, not more than four or
five years apart in age, ought to be about as closely similar to each
other as identical twins are to each other; for the family influences in
each case are practically the same. Professor Thorndike, by careful
mental tests, showed[36] that this is not true. The ordinary brothers
come from different egg-cells, and, as is known from studies on lower
animals, they do not get exactly the same inheritance from their
parents; they show, therefore, considerable differences in their psychic
natures. Real identical twins, being two halves of the same egg-cell,
have the same heredity, and their natures are therefore much more
nearly identical.
Again, if the mind is molded during the "plastic years of childhood,"
children ought to become more alike, the longer they are together. Twins
who were unlike at birth ought to resemble each other more closely at 14
than they did at 9, since they have been for five additional years
subjected to this supposedly potent but very mystical "molding force. "
Here again Professor Thorndike's exact measurements explode the fallacy.
They are actually, measurably, less alike at the older age; their inborn
natures are developing along predestined lines, with little regard to
the identity of their surroundings. Heredity accounts easily for these
facts, but they cannot be squared with the idea that mental differences
are the products solely of early training.
5. _Differential rates of increase in qualities subject to much
training. _ If the mind is formed by training, then brothers ought to be
more alike in qualities which have been subject to little or no
training. Professor Thorndike's measurements on this point show the
reverse to be true. The likeness of various traits is determined by
heredity, and brothers may be more unlike in traits which have been
subjected to a large and equal amount of training. Twins were found to
be less alike in their ability at addition and multiplication, in which
the schools had been training them for some years, than they were in
ability to mark off the A's on a printed sheet, or to write the
opposites to a list of words--feats which they had probably never before
tried to do.
This same proposition may be put on a broader basis. [37] "In so far as
the differences in achievement found amongst a group of men are due to
the differences in the quantity and quality of training which they had
had in the function in question, the provision of equal amounts of the
same sort of training for all individuals in the group should act to
reduce the differences. " "If the addition of equal amounts of practice
does not reduce the differences found amongst men, those differences can
not well be explained to any large extent by supposing them to have been
due to corresponding differences in amount of previous practice. If,
that is, inequalities in achievement are not reduced by equalizing
practice, they can not well have been caused by inequalities in previous
practice. If differences in opportunity cause the differences men
display, making opportunity more nearly equal for all, by adding equal
amounts to it in each case should make the differences less.
"The facts found are rather startling. Equalizing practice seems to
increase differences. The superior man seems to have got his present
superiority by his own nature rather than by superior advantages of the
past, since, during a period of equal advantage for all, he increases
his lead. " This point has been tested by such simple devices as mental
multiplication, addition, marking A's on a printed sheet of capitals and
the like; all the contestants made some gain in efficiency, but those
who were superior at the start were proportionately farther ahead than
ever at the end. This is what the geneticist would expect, but fits very
ill with some popular pseudo-science which denies that any child is
mentally limited by nature.
6. _Direct measurement of the amount of resemblance of mental traits in
brothers and sisters. _ It is manifestly impossible to assume that early
training, or parental behavior, or anything of the sort, can have
influenced very markedly the child's eye color, or the length of his
forearm, or the ratio of the breadth of his head to its length. A
measure of the amount of resemblance between two brothers in such traits
may very confidently be said to represent the influence of heredity; one
can feel no doubt that the child inherits his eye-color and other
physical traits of that kind from his parents. It will be recalled that
the resemblance, measured on a scale from 0 to 1, has been found to be
about 0. 5.
Karl Pearson measured the resemblance between brothers and sisters in
mental traits--for example, temper, conscientiousness, introspection,
vivacity--and found it on the average to have the same intensity--that
is, about 0. 5. Starch gets similar results in studying school grades.
Professor Pearson writes:[38]
"It has been suggested that this resemblance in the psychological
characters is compounded of two factors, inheritance on the one hand and
training and environment on the other. If so, one must admit that
inheritance and environment make up the resemblance in the physical
characters. Now these two sorts of resemblance being of the same
intensity, either the environmental influence is the same in both cases
or it is not. If it is the same, we are forced to the conclusion that it
is insensible, for it can not influence eye-color. If it is not the
same, then it would be a most marvelous thing that with varying degrees
of inheritance, some mysterious force always modifies the extent of home
influence, until the resemblance of brothers and sisters is brought
sensibly up to the same intensity! Occam's razor[39] will enable us at
once to cut off such a theory. We are forced, I think, literally forced,
to the general conclusion that the physical and psychical characters in
man are inherited within broad lines in the same manner, and with
approximate intensity. The average parental influence is in itself
largely a result of the heritage of the stock and not an extraneous and
additional factor causing the resemblance between children from the same
home. "
A paragraph from Edgar Schuster[40] may appropriately be added. "After
considering the published evidence a word must be said of facts which
most people may collect for themselves. They are difficult to record,
but are perhaps more convincing than any quantity of statistics. If one
knows well several members of a family, one is bound to see in them
likenesses with regard to mental traits, both large and small, which
may sometimes be accounted for by example on the one hand or unconscious
imitation on the other, but are often quite inexplicable on any other
theory than heredity. It is difficult to understand how the inheritance
of mental capacity can be denied by those whose eyes are open and whose
minds are open too. "
Strictly speaking, it is of course true that man inherits nothing more
than the capacity of making mental acquirements. But this general
capacity is made up of many separate capacities, all of these capacities
are variable, and the variations are inherited. Such seems to us to be
the unmistakable verdict of the evidence.
Our conclusions as to the inheritance of all sorts of mental capacity
are not based on the mere presence of the same trait in parent and
child. As the psychological analysis of individual traits proceeds, it
will be possible to proceed further with the study of the inheritance of
these traits. Some work has been done on spelling, which is particularly
interesting because most people, without reflection, would take it for
granted that a child's spelling ability depends almost wholly on his
training. Professor Thorndike's exposition[41] of the investigation is
as follows:
"E. L. Earle ('03) measured the spelling abilities of some 800 children
in the St. Xavier school in New York by careful tests. As the children
in this school commonly enter at a very early age, and as the staff and
methods of teaching remain very constant, we have in the case of the 180
pairs of brothers and sisters included in the 600 children closely
similar school training. Mr. Earle measured the ability of any
individual by his deviation from the average for his grade and sex, and
found the coefficient of correlation between children of the same family
to be . 50. That is, any individual is on the average 50% as much above
or below the average for his age and sex as his brother or sister.
"Similarities of home training might account for this, but any one
experienced in teaching will hesitate to attribute much efficacy to
such similarities. Bad spellers remain bad spellers though their
teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of
spelling ability ('97) found little or no relationship between good
spelling and any one of the popular methods, and little or none between
poor spelling and foreign parentage. Cornman's more careful study of
spelling ('07) supports the view that ability to spell is little
influenced by such differences in school or home training as commonly
exist. "
This is a very clear-cut case of a definite intellectual ability,
differences in which might be supposed to be due almost wholly to the
child's training, but which seem, on investigation, to be largely due to
heredity.
The problem may be examined in still greater detail. Does a man merely
inherit manual skill, let us say, or does he inherit the precise kind of
manual skill needed to make a surgeon but not the kind that would be
useful to a watchmaker? Is a man born merely with a generalized
"artistic" ability, or is it one adapted solely for, let us say, music;
or further, is it adapted solely for violin playing, not for the piano?
Galton, in his pioneer studies, sought for data on this question. In
regard to English judges, he wrote: "Do the judges often have sons who
succeed in the same career, where success would have been impossible if
they had not been gifted with the special qualities of their fathers?
Out of the 286 judges, more than _one in every nine_ of them have been
either father, son or brother to another judge, and the other high legal
relationships have been even more numerous. There can not, then, remain
a doubt but that the peculiar type of ability that is necessary to a
judge is often transmitted by descent. "
Unfortunately, we can not feel quite as free from doubt on the point as
Galton did. The judicial mind, if that be the main qualification for a
judge, might be inherited, or it might be the result of training. Such a
case, standing alone, is inconclusive.
Galton similarly showed that the sons of statesmen tended to be
statesmen, and that the same was true in families of great commanders,
literary men, poets and divines. In his list of eminent painters, all
the relatives mentioned are painters save four, two of whom were gifted
in sculpture, one in music and one in embroidery. As to musicians,
Mendelssohn and Meyerbeer are the only ones in his list whose eminent
kinsmen achieved their success in other careers than music.
Havelock Ellis, who likewise studied British men of genius, throws
additional light on the subject. "Painters and sculptors," he found,
"constitute a group which appears to be of very distinct interest from
the point of view of occupational heredity. In social origin, it may be
noted, the group differs strikingly in constitution from the general
body of men of genius in which the upper class is almost or quite
predominant. Of 63 painters and sculptors of definitely known origin,
only two can be placed in the aristocratic division. Of the remainder 7
are the sons of artists, 22 the sons of craftsmen, leaving only 32 for
all other occupations, which are mainly of lower middle class character,
and in many cases trades that are very closely allied to crafts. Even,
however, when we omit the trades as well as the cases in which the
fathers were artists, we find a very notable predominance of craftsmen
in the parentage of painters, to such an extent indeed that while
craftsmen only constitute 9. 2% among the fathers of our eminent persons
generally, they constitute nearly 35% among the fathers of the painters
and sculptors. It is difficult to avoid the conclusion that there is a
real connection between the father's aptitude for craftsmanship and the
son's aptitude for art.
"To suppose that environment adequately accounts for this relationship
is an inadmissible theory. The association between the craft of builder,
carpenter, tanner, jeweller, watchmaker, woodcarver, ropemaker, etc. ,
and the painter's art is small at best, and in most cases is
non-existent. "
Arreat, investigating the heredity of 200 eminent European painters,
reached results similar to those of Ellis, according to the latter's
citation.
Arithmetical ability seems similarly to be subdivided, according to Miss
Cobb. [42] She made measurements of the efficiency with which children
and their parents could do problems in addition, subtraction,
multiplication and division, and could copy a column of figures. "The
measurements made," she writes, "show that if, for instance, a child is
much quicker than the average in subtraction, but not in addition,
multiplication or division, it is to be expected that one at least of
his parents shows a like trait; or if he falls below the average in
subtraction and multiplication, and exceeds it in addition and division,
again the same will hold true of at least one of his parents. " These
various kinds of arithmetic appear to be due to different functions of
the brain, and are therefore probably inherited independently, if they
are inherited at all.
To assume that the resemblance between parent and offspring in
arithmetical ability is due to association, training and imitation is
not plausible. If this were the case, a class of children ought to come
to resemble their teacher, but they do not. Moreover, the child
sometimes resembles more closely the parent with whom he has been less
associated in daily life.
From such data as these, we conclude that mental inheritance is
considerably specialized. This conclusion is in accord with Burris'
finding (cited by Thorndike) that the ability to do well in some one
high school study is nearly or quite as much due to ancestry as is the
ability to do well in the course as a whole.
To sum up, we have reason to believe not only that one's mental
character is due largely to heredity, but that the details of it may be
equally due to heredity, in the sense that for any particular trait or
complex in the child there is likely to be found a similar trait or
complex in the ancestry. Such a conclusion should not be pushed to the
point of assuming inheritance of all sorts of dispositions that might be
due to early training; on the other hand, a survey of the whole field
would probably justify us in concluding that any given trait is _more
likely than not_ to be inherited. The effect of training in the
formation of the child's mental character is certainly much less than is
popularly supposed; and even for the traits that are most due to
training, it must never be forgotten that there are inherited mental
bases.
If the reader has accepted the facts presented in this chapter, and our
inferences from the facts, he will admit that mental differences between
men are at bottom due to heredity, just as physical differences are;
that they are apparently inherited in the same manner and in
approximately the same degree.
CHAPTER V
THE LAWS OF HEREDITY
We have now established the bases for a practicable eugenics program.
Men differ; these differences are inherited; therefore the make-up of
the race can be changed by any method which will alter the relative
proportions of the contributions which different classes of men make to
the following generation.
For applied eugenics, it is sufficient to know that mental and physical
differences are inherited; the exact manner of inheritance it would be
important to know, but even without a knowledge of the details of the
mechanism of heredity, a program of eugenics is yet wholly feasible.
It is no part of the plan of this book to enter into the details of the
mechanism of heredity, a complicated subject for which the reader can
refer to one of the treatises mentioned in the bibliography at the close
of this volume. It may be worth while, however, to outline in a very
summary way the present status of the question.
As to the details of inheritance, research has progressed in the last
few years far beyond the crude conceptions of a decade ago, when a
primitive form of Mendelism was made to explain everything that
occurred. [43] One can hardly repress a smile at the simplicity of those
early ideas,--though it must be said that some students of eugenics have
not yet outgrown them. In those days it was thought that every visible
character in man (or in any other organism) was represented by some
"determiner" in the germ-plasm; that by suitable matings a breeder could
rid a stream of germ-plasm of almost any determiner he wished; and that
the corresponding unit character would thereupon disappear from the
visible make-up of the individual. Was a family reported as showing a
taint, for instance, hereditary insanity?
Then it was asserted that by
the proper series of matings, it was possible to squeeze out of the
germ-plasm the particular concrete _something_ of which insanity was the
visible expression, and have left a family stock that was perfectly
sound and sane.
The minute, meticulous researches of experimental breeders[44] have left
such a view of heredity far behind. Certainly the last word has not been
said; yet the present hypotheses _work_, whenever the conditions are
such as to give a fair chance. The results of these studies have led to
what is called the factorial hypothesis of heredity,[45] according to
which all the visible characters of the adult are produced by (purely
hypothetical) factors in the germ-plasm; it is the factors that are
inherited, and they, under proper conditions for development, produce
the characters. The great difference between this and the earlier view
is that instead of allotting one factor to each character, students now
believe that each individual character of the organism is produced by
the action of an indefinitely large number of factors,[46] and they
have been further forced to adopt the belief that each individual
factor affects an indefinitely large number of characters, owing to the
physiological interrelations and correlations of every part of the body.
[Illustration: HOW DO YOU CLASP YOUR HANDS?
FIG. 16. --If the hands be clasped naturally with fingers
alternating, as shown in the above illustration, most people will put
the same thumb--either that of the right or that of the left
hand--uppermost every time. Frank E. Lutz showed (_American Naturalist_,
xliii) that the position assumed depends largely on heredity. When both
parents put the right thumb uppermost, about three-fourths of the
children were found to do the same. When both parents put the left thumb
uppermost, about three-fifths of the children did the same. No definite
ratios could be found from the various kinds of matings. Apparently the
manner of clasping hands has no connection with one's right-handedness
or left-handedness. It can hardly be due to imitation for the trait is
such a slight one that most people have not noticed it before their
attention is called to it by the geneticist. Furthermore, babies are
found almost always to clasp the hands in the same way every time. The
trait is a good illustration of the almost incredible minuteness with
which heredity enters into a man's make-up. Photograph by John Howard
Paine. ]
The sweet pea offers a good illustration of the widespread effects which
may result from the change of a single factor. In addition to the
ordinary climbing vine, there is a dwarf variety, and the difference
between the two seems to be proved, by exhaustive experimental breeding,
to be due to only one inherited factor. Yet the action of this one
factor not only changes the height of the plant, but also results in
changes in color of foliage, length of internodes, size and arrangement
of flowers, time of opening of flowers, fertility and viability.
Again, a mutant stock in the fruit fly (Drosophila) has as its most
marked characteristic very short wings. "But the factor for rudimentary
wings also produces other effects as well. The females are almost
completely sterile, while the males are fertile. The viability of the
stocks is poor. When flies with rudimentary wings are put into
competition with wild flies relatively few of the rudimentary flies come
through, especially if the culture is crowded. The hind legs are also
shortened. All of these effects are the results of a single
factor-difference. " To be strictly accurate, then, one should not say
that a certain variation affects length of wing, but that its _chief_
effect is to shorten the wing.
"One may venture to guess," T. H. Morgan says,[47] "that some of the
specific and varietal differences that are characteristic of wild types
and which at the same time appear to have no survival value, are only
by-products of factors whose most important effect is on another part
of the organism where their influence is of vital importance. "
"I am inclined to think," Professor Morgan continues, "that an
overstatement to the effect that each factor may affect the entire body,
is less likely to do harm than to state that each factor affects only a
particular character. The reckless use of the phrase 'unit character'
has done much to mislead the uninitiated as to the effects that a single
change in the germ-plasm may produce on the organism. Fortunately the
expression 'unit character' is being less used by those students of
genetics who are more careful in regard to the implications of their
terminology. "
[Illustration: THE EFFECT OF ORTHODACTYLY
FIG. 17. --At the left is a hand with the third, fourth and
fifth fingers affected. The middle joints of these fingers are stiff and
cannot be bent. At the right the same hand is shown, closed. A normal
hand in the middle serves to illustrate by contrast the nature of the
abnormality, which appears in every generation of several large
families. It is also called symphalangism, and is evidently related to
the better-known abnormality of brachydactyly. Photograph from Frederick
N. Duncan. ]
[Illustration: A FAMILY WITH ORTHODACTYLY
FIG. 18. --Squares denote males and circles females, as is usual
in the charts compiled by eugenists; black circles or squares denote
affected individuals. A1 had all fingers affected in the way shown in
Fig. 17; B2 had all but one finger affected; C2 had all but one finger
affected; D2 had all fingers affected; D3 has all but forefingers
affected. The family here shown is a branch, found by F. N. Duncan, of a
very large family first described by Harvey Cushing, in which this
abnormality has run for at least seven generations. It is an excellent
example of an inherited defect due to a single Mendelian factor. ]
One of the best attested single characters in human heredity is
brachydactyly, "short-fingerness," which results in a reduction in the
length of the fingers by the dropping out of one joint. If one lumps
together all the cases where any effect of this sort is found, it is
evident that normals never transmit it to their posterity, that affected
persons always do, and that in a mating between a normal and an affected
person, all the offspring will show the abnormality. It is a good
example of a unit character.
But its effect is by no means confined to the fingers. It tends to
affect the entire skeleton, and in a family where one child is markedly
brachydactylous, that child is generally shorter than the others. The
factor for brachydactyly evidently produces its primary effect on the
bones of the hand, but it also produces a secondary effect on all the
bones of the body.
Moreover, it will be found, if a number of brachydactylous persons are
examined, that no two of them are affected to exactly the same degree.
In some cases only one finger will be abnormal; in other cases there
will be a slight effect in all the fingers; in other cases all the
fingers will be highly affected. Why is there such variation in the
results produced by a unit character? Because, presumably, in each
individual there is a different set of modifying factors or else a
variation in the factor. It has been found that an abnormality quite
like brachydactyly is produced by abnormality in the pituitary gland. It
is then fair to suppose that the factor which produces brachydactyly
does so by affecting the pituitary gland in some way. But there must be
many other factors which also affect the pituitary and in some cases
probably favor its development, rather than hindering it. Then if the
factor for brachydactyly is depressing the pituitary, but if some other
factors are at the same time stimulating that gland, the effect shown in
the subject's fingers will be much less marked than if a group of
modifying factors were present which acted in the same direction as the
brachydactyly factor,--to perturb the action of the pituitary gland.
This illustration is largely hypothetical; but there is no room for
doubt that every factor produces more than a single effect. A white
blaze in the hair, for example, is a well-proved unit factor in man; the
factor not only produces a white streak in the hair, but affects the
pigmentation of the skin as well, usually resulting in one or more white
spots on some part of the body. It is really a factor for "piebaldism. "
For the sake of clear thinking, then, the idea of a unit character due
to some unit determiner or factor in the germ-plasm must be given up,
and it must be recognized that every visible character of an individual
is the result of numerous factors, or differences in the germ-plasm.
Ordinarily one of these produces a more notable contribution to the
end-product than do the others; but there are cases where this statement
does not appear to hold good. This leads to the conception of _multiple
factors_.
In crossing a wheat with brown chaff and one with white chaff, H.
Nilsson-Ehle (1909) expected in the second hybrid generation to secure a
ratio of 3 brown to 1 white. As a fact, he got 1410 brown and 94 white,
a ratio of 15:1. He interpreted this as meaning that the brown color in
this particular variety was due not to one factor, but to two, which
were equivalent to each other, and either one of which would produce the
same result alone as would the two acting together. In further crossing
red wheat with white, he secured ratios which led him to believe that
the red was produced by three independent factors, any one of which
would produce red either alone or with the other two. A. and G. Howard
later corroborated this work,[48] but showed that the three factors were
not identical: they are qualitatively slightly different, although so
closely similar that the three reds look alike at first sight. E. M.
East has obtained evidence from maize and G. H. Shull from
shepherd's-purse, which bears out the multiple factor hypothesis.
[Illustration: WHITE BLAZE IN THE HAIR
FIG. 19. --The white lock of hair here shown is hereditary and
has been traced back definitely through six generations; family
tradition derives it from a son of Harry "Hot-Spur" Percy, born in 1403,
and fallaciously assigns its origin to "prenatal influence" or "maternal
impression. " This young woman inherited the blaze from her father, who
had it from his mother, who had it from her father, who migrated from
England to America nearly a century ago. The trait appears to be a
simple dominant, following Mendel's Law; that is, when a person with one
of these locks who is a child of one normal and one affected parent
marries a normal individual, half of the children show the lock and half
do not. Photograph from Newton Miller. ]
[Illustration: A FAMILY OF SPOTTED NEGROES
FIG. 20. --The piebald factor sometimes shows itself as nothing
more than a blaze in the hair (see preceding figure); but it may take a
much more extreme form, as illustrated by the above photograph from Q.
I. Simpson and W. E. Castle. Mrs. S. A. , a spotted mutant, founded a
family which now comprises, in several generations, 17 spotted and 16
normal offspring. The white spotting factor behaves as a Mendelian
dominant, and the expectation would be equal numbers of normal and
affected children. Similar white factors are known in other animals. It
is worth noting that all the well attested Mendelian characters in man
are abnormalities, no normal character having yet been proved to be
inherited in this manner. ]
Apart from multiple factors as properly defined (that is, factors which
produce the same result, either alone or together), extensive analysis
usually reveals that apparently simple characters are in reality
complex. The purple aleurone color of maize seeds is attributed by R. A.
Emerson to five distinct factors, while E. Baur found four factors
responsible for the red color of snapdragon blossoms. There are, as G.
N. Collins says,[49] "still many gross characters that stand as
simple Mendelian units, but few, if any, of these occur in plants or
animals that have been subjected to extensive investigation. There is
now such a large number of characters which at first behaved as units,
but which have since been broken up by crossing with suitable selected
material, that it seems not unreasonable to believe that the remaining
cases await only the discovery of the right strains with which to
hybridize them to bring about corresponding results. "
In spite of the fact that there is a real segregation between factors as
has been shown, it must not be supposed that factors and their
determiners are absolutely invariable. This has been too frequently
assumed without adequate evidence by many geneticists. It is probable
that just as the multiplicity and interrelation and minuteness of many
factors have been the principal discoveries of genetics in recent years
that the next few years will see a great deal of evidence following the
important lead of Castle and Jennings, as to variation in factors.
Knowing that all the characters of an individual are due to the
interaction of numerous factors, one must be particularly slow in
assuming that such complex characters as man's mental traits are units,
in any proper genetic sense of the word. It will, for instance, require
very strong evidence to establish feeble-mindedness as a unit character.
No one who examines the collected pedigrees of families marked by
feeble-mindedness, can deny that it does appear at first sight to behave
as a unit character, inherited in the typical Mendelian fashion. The
psychologist H. H. Goddard, who started out with a strong bias against
believing that such a complex trait could even _behave_ as a unit
character, thought himself forced by the tabulation of his cases to
adopt the conclusion that it does behave as a unit character. And other
eugenists have not hesitated to affirm, mainly on the strength of Dr.
Goddard's researches, that this unit character is due to a single
determiner in the germ-plasm, which either is or is not present,--no
halfway business about it.
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 institutions, 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
classes, as one would suppose who only looks at a Mendelian chart.
[Illustration: 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. ]
[Illustration: 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
classification 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 identity 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.
Illustration from J. H. Taylor. ]
[Illustration: 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
diminishing columns show the number whose mental ages fell short of
their chronological ages. They are the mentally retarded, 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 illustrated 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 quantity_ 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 analogy 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 classes, "tall" and "short," as they are with
color blindness or normal color vision, for example. Is it not a fair
assumption 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 class of blending inheritance, for it
does segregate to a considerable extent, yet some of the factors may
show blending. Much more psychological analysis 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.
[Illustration: 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.
