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
_minutiæ_ 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.
3. One distinct Mendelian factor (or several such factors), with
modifying factors which may cause either (a) intensification, (b)
inhibition, or (c) dilution.
4. Variation of a factor.
5. Or several or all of the above explanations may apply to one case.
Moreover, the characters of which the origin has been most completely
worked out are mostly color characters, whose physiological development
seems to be relatively simple. It is probable that the development of a
mental character is much more complicated, and therefore there is more
likelihood of additional factors being involved.
To say, then, that any mental trait is a unit character, or that it is
due to a single germinal difference, is to go beyond both the evidence
and the probabilities.
And if mental traits are, in their germinal foundations, not simple but
highly complex, it follows that any advice given as to how human matings
should be arranged to produce any precise result in the progeny, should
be viewed with distrust. Such advice can be given only in the case of a
few pathological characters such as color-blindness, night-blindness, or
Huntington's Chorea. It is well that the man or woman interested in one
of these abnormalities can get definite information on the subject; and
Huntington's Chorea, in particular, is a dysgenic trait which can and
should be stamped out. But it can not be pretended that any of man's
traits, as to whose inheritance prediction can be made with confidence,
is of great importance to national eugenics.
In short, a knowledge of heredity shows that attempts to predict the
mode of inheritance of the important human traits (particularly mental
traits) are still uncertain in their results. The characters involved
are too complex to offer any simple sequences. If two parents have brown
eyes, it can not be said that all their children will have brown eyes;
still less can it be said that all the children of two musically gifted
parents are certain to be endowed with musical talent in any given
degree.
Prediction is possible only when uniform sequences are found. How are
such sequences to be found in heredity, if they do not appear when a
parent and his offspring are examined? Obviously it is necessary to
examine _a large number_ of parents and their offspring,--to treat the
problem by statistical methods.
But, it may be objected, a uniformity gained by such methods is
spurious. It is merely shutting the eyes to the mass of contradictions
which are concealed by an apparent statistical uniformity.
This objection would be valid, if the statistical results were used for
prediction _in individual cases_. The statistician, however, expressly
warns that his conclusions must not be used for such prediction. They
are intended to predict only general trends, only average results; and
for this purpose they are wholly legitimate. Moreover, evolution itself
is a problem of statistics, and therefore the statistical method of
studying heredity may offer results of great value to eugenics, even
though it can not furnish in individual cases the prediction which would
be desirable.
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-color, 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:
[Illustration: FINGER-PRINTS OF TWINS
FIG. 25.--Above are the finger-prints, supplied by J. H. Taylor
of the Navy Department, of the two young sailors shown in Fig. 24. The
reader might examine them once or twice without seeing any differences.
Systematic comparison reveals that the thumbs of the left hands and the
middle fingers of the right hands particularly are distinguishable.
Finger-prints as a means of identification were popularized by Sir
Francis Galton, the founder of eugenics, and their superiority to all
other methods is now generally admitted. In addition to this practical
usefulness, they also furnish material for study of the geneticist and
zoölogist. The extent to which heredity is responsible for the patterns
is indicated by the resemblance in pattern in spite of the great
variability in this tract.]
"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 reëmphasize
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-color in man and coat-color 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 emphasized 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 crystallizes the vague realization 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.
We do not propose to discuss further the laws of heredity; but it is
likely that the reader who has made no other study of the subject may by
this time find himself somewhat bewildered. "Can we talk only in
generalities?" he may well ask; "Does eugenics know no laws of heredity
that will guide me in the choice of a wife? I thought that was the
purpose of eugenics!"
We reply: (1) The laws of heredity are vastly complicated in man by the
complex nature of most of his characters. The definite way in which some
abnormalities are inherited is known; but it has not been thought
necessary to include an account of such facts in this work. They are set
forth in other books, especially Davenport's _Heredity in Relation to
Eugenics_. The knowledge of how such a trait as color-blindness is
inherited may be of importance to one man out of a thousand in choosing
a wife; but we are taking a broader view of eugenics than this. As far
as the great mass of human characters go, they are, in our opinion, due
to so many separately inheritable factors that it is not safe to
dogmatize about exactly how they will behave in heredity. Such
knowledge, desirable as it may be, is not necessary for race progress.
(2) But it is possible, with present knowledge, to say that human
traits, mental as well as physical, are inherited, in a high degree.
Even before the final details as to the inheritance of all traits are
worked out--a task that is never likely to be accomplished--there is
ample material on which to base action for eugenics. The basal
differences in the mental traits of man (and the physical as well, of
course) are known to be due to heredity, and little modified by
training. It is therefore possible to raise the level of the human
race--the task of eugenics--by getting that half of the race which is,
on the whole, superior in the traits that make for human progress and
happiness, to contribute a larger proportion to the next generation than
does the half which is on the whole inferior in that respect. Eugenics
need know nothing more, and the smoke of controversy over the exact way
in which some trait or other is inherited must not be allowed for an
instant to obscure the known fact that the level can be raised.