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.
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
recognised 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 colour 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.
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 colour of maize seeds is attributed by R. A. Emerson to five
distinct factors, while E. Baur found four factors responsible for the red
colour 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."
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.
!THE DISTRIBUTION OF INTELLIGENCE
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 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 colour blindness or
normal colour 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.
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.
!Intensive
genetic research in lower animals and plants has shown that a visible character
may be due to
!Independent
multiple factors in the germ-plasm.
!Multiple
allelomorphs, that is, a series of different grades of a single factor.
One distinct
Mendelian factor (or several such factors), with modifying factors which may
cause either (a) intensification, (b) inhibition, or (c) dilution.
!Variation of a
factor.
!Or several or
all of the above explanations may apply to one case.
!Moreover, the
characters of which the origin has been most completely worked out are mostly
colour characters, whose physiological development seems to be relatively
simple. It is probable that the development of a mental character is much more
complicated, and therefore there is more likelihood of additional factors being
involved.
To say, then,
that any mental trait is a unit character, or that it is
!due to a single
germinal difference, is to go beyond both the evidence and the probabilities.
From this
standpoint, we return to attack the problem of the relation between parent and
offspring. We noted that there is no uniform sequence in a single family, and
illustrated this by the case of brown eyes. But
!if a thousand
parents and their offspring be selected and some trait, such as eye-colour, or
stature, or general intelligence, be measured, a uniformity at once appears in
the fact of regression. Its discoverer, Sir Francis Galton, gives this account
of it:
!
"If the
word 'peculiarity' be used to signify the difference between the
amount of any
faculty possessed by a man, and the average of that possessed by the population
at large, then the law of regression may be described as follows: each
peculiarity in a man is shared by his kinsmen, but on the average in a less
degree. It is reduced to a
!definite
fraction of its amount, quite independently of what its amount might be. The
fraction differs in different orders of kinship, becoming smaller as they are
more remote. When the kinship is so distant that its effects are not worth
taking into account, the peculiarity of the man, however remarkable it may have
been, is reduced to zero in his kinsmen. This apparent paradox is fundamentally
due to the greater frequency of mediocre deviations than of extreme ones,
occurring between limits separated by equal widths."
As to the
application of this law, let Galton himself speak: "The Law of Regression
tells heavily against the full hereditary transmission of any gift. Only a few
out of many children would be likely to differ from mediocrity so widely as
their Mid-Parent [i. e., the average of their two parents], allowing for sexual
differences, and still fewer would differ as widely as the more exceptional of
the two parents. The more bountifully the parent is gifted by nature, the more
rare will be his
good fortune if
he begets a son who is as richly endowed as himself, and still more so if he
has a son who is endowed yet more largely. But the law is evenhanded; it levies
an equal succession-tax on the transmission of badness as of goodness. If it
discourages the extravagant hopes of a gifted parent that his children on the
average will inherit all his
powers, it not
less discountenances extravagant fears that they will inherit all his weakness
and disease.
"It must be
clearly understood that there is nothing in these statements to invalidate the
general doctrine that the children of a gifted pair
!are much more
likely to be gifted than the children of a mediocre pair." To this it
should be added that progeny of very great ability will arise more frequently
in proportion to the quality of their parents.
It must be
reiterated that this is a statistical, not a biological, law; and that even
Galton probably goes a little too far in applying it to individuals. It will
hold good for a whole population, but not
!necessarily for
only one family. Further, we can afford to reëmphasise the fact that it in no way prevents the improvement of a race
by selection and assortative mating.
Stature is the
character which Dr. Galton used to get an exact measurement of the amount of
regression. More recent studies have changed the value he found, without
invalidating his method. When large numbers are taken it is now abundantly
proved that if parents exceed the average stature of their race by a certain
amount their offspring will,
!in general,
exceed the racial average by only one-half as much as their parents did. This
is due, as Galton said, to the "drag" of the more remote ancestry, which when considered
as a whole must represent very nearly mediocrity, statistically speaking.
The general
amount of regression in heredity, then, is one-half. If it be expressed as a
decimal, .5, the reader will at once note its identity with the coefficient of
correlation which we have so often cited in this book as a measure of heredity.
In fact, the coefficient of correlation
!is nothing more
than a measure of the regression, and it is probably simpler to think of it as
correlation than it is to speak of a Law of Regression, as Sir Francis did.
This correlation
or regression can, of course, be measured for other ancestors as well as for
the immediate parents. From studies of
eye-colour in
man and coat-colour in horses, Karl Pearson worked out the necessary
correlations, which are usually referred to as the law of Ancestral
Inheritance. Dr. Galton had pointed out, years before, that
!the
contributions of the several generations of individuals probably formed a
geometrical series, and Professor Pearson calculated this series, for the two
cases mentioned, as:
!Parents Grandparents G-Grandparents G-G-Grandparents
!.6244 .1988 .0630 .0202 ... etc.
In other words,
the two parents, together, will on the average of a great many cases be found
to have contributed a little more than three-fifths of the hereditary
peculiarities of any given individual; the four grandparents will be found
responsible for a little less than one-fifth, and the eight great-grandparents
for about six hundredths,
and so on, the
contribution of each generation becoming smaller with ascent, but each one
having, in the average of many cases, a certain definite though small
influence, until infinity.
!
It can not be
too strongly emphasised that this is a statistical law,
not a biological
law. It must not be applied to predict the character of the offspring of any
one particular mating, for it might be highly misleading. It would be wholly
unjustified, for example, to suppose that a certain man got three-tenths of his
nature from his father, because the Law of Ancestral Heredity required it: in
point of fact, he might
!get one-tenth
or nine-tenths, none or all of a given trait. But, when dealing with a large
population, the errors on one side balance the errors on the other, and the law
is found, in the cases to which it has been applied, to express the facts.[51]
While,
therefore, this Galton-Pearson law gives no advice in regard to individual
marriages, it is yet of great value to applied eugenics. In the first place, it
crystallises the vague realisation that remote ancestry is of much less
importance than immediate ancestry, to an
!individual,
while showing that every generation has a part in making a man what he is. In
the second place, it is found, by mathematical reasoning which need not here be
repeated, that the type of a population may be quickly changed by the mating of
like with like; and that this newly established type may be maintained when not
capable of further progress. Regression is not inevitable, for it may be
overcome by selection.
To put the
matter in a more concrete form, there is reason to think that if for a few
generations superior people would marry only people on the average superior in
like degree (superior in ancestry as well as individuality), a point would be
reached where all the offspring would tend to be superior, mediocrities of the
former type being eliminated; and this superiority could be maintained as long
as care was taken to
avoid mating
with inferior. In other words, the Galton-Pearson Law gives statistical support
for a belief that eugenic marriages will create an improved breed of men. And
this, it seems to us, is the most important implication of that law for
eugenics, although it is an implication that
!is generally
ignored.
No comments:
Post a Comment