CHAPTER IV!
!
!THE INHERITANCE OF MENTAL CAPACITIES!
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We have come to
the climax of the eugenist's preliminary argument; if
the main
differences between human beings are not due to anything in the environment or
training, either of this or previous generations, there
!can be but one
explanation for them.
!They must be
due to the ancestry of the individual--that is, they must be matters ofheredity in the ordinary sense, coupled with the fortuitous variations which
accompany heredity throughout the organic world.
We need not
limit ourselves, however, to the argument by exclusion, for it is not difficult
to present direct evidence that the differences
!between men are
actually inherited by children from parents. The problem, formally stated, is
to measure the amount by which the likeness of individuals of like ancestry
surpasses the likeness of individuals of different ancestry. After subtraction
of the necessary amount for the greater likeness in training, that the
individuals of like ancestry will have, whatever amount is left will
necessarily, represent the actual inheritance of the child from its
ancestors--parents, grandparents, and so on.
!Obviously, the
subtraction for environmental effects is the point at which a mistake is most
probable. We may safely start, therefore, with a problem in which no
subtraction whatever need be made for this cause. Eye colour is a stock
example, and a good one, for it is not conceivable that home environment or
training would cause a change in the colour of brothers' eyes.
The
correlation[30] between brothers, or sisters, or brothers and sisters--briefly,
the fraternal resemblance--for eye-colour was found by
!Karl Pearson,
using the method described in Chapter I, to be .52. We are in no danger of
contradiction if we state with positiveness that this figure represents the
influence of ancestry, or direct inheritance, in respect to this particular
trait.
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-colour. 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 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.
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.
!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?
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.
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.
!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 moulded 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 "moulding 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.
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 equalising 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. Equalising 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.
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
behaviour, or anything of the sort, can have influenced very markedly the
child's eye colour, 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-colour 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-colour. If it is not the
same, then it
would be a most marvellous 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.
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."
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.
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.
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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.
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