, 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.
two parents], allowing for sexual differences, and still fewer would
differ as widely as the more exceptional of the two parents.
Applied Eugenics by Roswell H. Johnson and Paul Popenoe
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.
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
zoologist. 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 reemphasize
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.
CHAPTER VI
NATURAL SELECTION
Man has risen from the ape chiefly through the action of natural
selection. Any scheme of conscious race betterment, then, should
carefully examine nature's method, to learn to what extent it is still
acting, and to what extent it may better be supplanted or assisted by
methods of man's own invention.
Natural selection operates in two ways: (1) through a selective
death-rate and (2) through a selective birth-rate. The first of these
forms has often been considered the whole of natural selection, but
wrongly. The second steadily gains in importance as an organism rises in
the scale of evolution; until in man it is likely soon to dwarf the
lethal factor into insignificance. For it is evident that the appalling
slaughter of all but a few of the individuals born, which one usually
associates with the idea of natural selection, will take place only when
the number of individuals born is very large. As the reproductive rate
decreases, so does the death-rate, for a larger proportion of those born
are able to find food and to escape enemies.
When considering man, one realizes at once that relatively few babies or
adults starve to death. The selective death-rate therefore must include
only those who are unable to escape their enemies; and while these
enemies of the species, particularly certain microorganisms, still take
a heavy toll from the race, the progress of science is likely to make it
much smaller in the future.
The different aspects of natural selection may be classified as follows:
{ Lethal { Sustentative
{ { Non-sustentative
Natural selection {
{ Reproductive { Sexual
{ { Fecundal
The lethal factor is the one which Darwin himself most emphasized.
Obviously a race will be steadily improved, if the worst stock in it is
cut off before it has a chance to reproduce, and if the best stock
survives to perpetuate its kind. "This preservation of favourable
individual differences and variations, and the destruction of those
which are injurious, I have called natural selection, or the survival of
the fittest," Darwin wrote; and he went on to show that the principal
checks on increase were overcrowding, the difficulty of obtaining food,
destruction by enemies, and the lethal effects of climate. These causes
may be conveniently divided as in the above diagram, into sustentative
and non-sustentative. The sustentative factor has acquired particular
prominence in the human species, since Malthus wrote his essay on
population--that essay which both Darwin and Wallace confess was the
starting point of their discovery of natural selection.
There is a "constant tendency in all animated life to increase beyond
the nourishment prepared for it," Malthus declared. "It is
incontrovertibly true that there is no bound to the prolific plants and
animals, but what is made by their crowding and interfering with each
others' means of subsistence. " His deduction is well known: that as man
tends to increase in geometrical ratio, and can not hope to increase his
food-supply more rapidly than in arithmetical ratio, the human race must
eventually face starvation, unless the birth-rate be reduced.
Darwin was much impressed by this argument and ever since his time it
has usually been the foundation for any discussion of natural selection.
Nevertheless it is partly false for all animals, as one of the authors
showed[52] some years ago, since a species which regularly eats up all
the food in sight is rare indeed; and it is of very little racial
importance in the present-day evolution of man. Scarcity of food may put
sufficient pressure on him to cause emigration, but rarely death. The
importance of Malthus' argument to eugenics is too slight to warrant
further discussion.
When the non-sustentative forms of lethal selection are considered, it
is seen very clearly that man is not exempt from the workings of this
law. A non-sustentative form of natural selection takes place through
the destruction of the individual by some adverse feature of the
environment, such as excessive cold, or bacteria; or by bodily
deficiency; and it is independent of mere food-supply. W. F. R. Weldon
showed by a long series of measurements, for example, that as the harbor
of Plymouth, England, kept getting muddier, the crabs which lived in it
kept getting narrower; those with the greatest frontal breadth filtered
the water entering their gills least effectively, and died.
But, it was objected, man is above all this. He has gained the control
of his own environment. The bloody hand of natural selection may fall on
crabs: but surely you would not have us think that Man, the Lord of
Creation, shares the same fate?
Biologists could hardly think otherwise. Statisticians were able to
supply the needed proof. A selective death-rate in man can not only be
demonstrated but it can be actually measured.
"The measure of the selective death-rate. " says[53] Karl Pearson, to
whom this achievement is due, "is extraordinarily simple. It consists in
the fact that the inheritance of the length of life between parent and
offspring is found statistically to be about one-third of the average
inheritance of physical characters in man. This can only be due to the
fact that the death of parent or of offspring in a certain number of
cases is due to random and not to constitutional causes. " He arrived at
the conclusion[54] that 60% of the deaths were selective, in the Quaker
families which he was then studying. The exact proportion must vary in
accordance with the nature of the material and the environment, but as
A. Ploetz found at least 60% of the deaths to be selective in the
European royal families and nobility, where the environment is
uniformly good, there is no reason to think that Professor Pearson's
conclusion is invalid.
Dr. Ploetz[55] investigated the relation between length of life in
parents, and infant mortality, in about 1,000 families including 5,500
children; half of these were from the nobility and half from the
peasantry. The results were of the same order in each case, indicating
that environment is a much less important factor than many have been
wont to suppose. After discussing Professor Pearson's work, he
continued:
It seems to me that a simpler result can be reached from our
material in the following way. Since the greater child-mortality of
each of our classes of children (divided according to the ages at
death of their parents) indicates a higher mortality throughout the
rest of their lives, the offspring of parents who die young will
therefore be eliminated in a higher degree, that is, removed from
the composition of the race, than will those whose parents died
late. Now the elimination can be non-selective, falling on all
sorts of constitutions with the same frequency and degree. In that
case it will of course have no connection with selection inside the
race. Or it may be of a selective nature, falling on its victims
because they differ from those who are not selected, in a way that
makes them less capable of resisting the pressure of the
environment, and avoiding its dangers. Then we speak of a selective
process, of the elimination of the weaker and the survival of the
stronger. Since in our examination of the various causes of the
difference in infant mortality, in the various age-classes of
parents, we found no sufficient cause in the effects of the
environment, which necessarily contains all the non-selective
perils, but found the cause to be in the different constitutions
inherited by the children, we can not escape the conclusion that
the differences in infant mortality which we observe indicate a
strong process of natural selection.
Our tables also permit us to get an approximate idea of the extent
of selection by death among children in the first five years of
life. The minimum of infant mortality is reached among those
children whose parents have attained 85 years of age. Since these
represent the strongest constitutions, the mortality of their
children would appear to represent an absolute minimum, made up
almost wholly of chance, non-selective, unavoidable deaths. As the
number of children from marriages, both parties to which reached
85 years of age, is so small as to render any safe conclusions
impossible, our only recourse is to take the children of the
85-year-old fathers and the children of the 85-year-old mothers,
add them together, and strike an average. But we must recognize
that the minimum so obtained is nevertheless still too large,
because among the consorts of the long-lived fathers and mothers,
some died early with the result of increasing the infant mortality.
The infant mortality with the 85-year-old fathers and mothers is
found to be 11. 2%-15. 4%, average about 13%. The total
child-mortality reaches 31-32%, of which the 13% make about 40%.
Accordingly at least 60%, and considering the above mentioned
sources of error we may say two-thirds, of the child mortality is
selective in character. That accords reasonably well with the
55-74% which Pearson found for the extent of selective deaths in
his study.
In general, then, one may believe that more than a half of the persons
who die nowadays, die because they were not fit by by nature (i. e. ,
heredity) to survive under the conditions into which they were born.
They are the victims of lethal natural selection, nearly always of the
non-sustentative type. As Karl Pearson says, "Every man who has lived
through a hard winter, every man who has examined a mortality table,
every man who has studied the history of nations has probably seen
natural selection at work. "
There is still another graphic way of seeing natural selection at work,
by an examination of the infant mortality alone. Imagine a thousand
babies coming into the world on a given day. It is known that under
average American conditions more than one-tenth of them will die during
the first year of life. Now if those who die at this time are the
inherently weaker, then the death-rate among survivors ought to be
correspondingly less during succeeding years, for many will have been
cut down at once, who might otherwise have lingered for several years,
although doomed to die before maturity. On the other hand, if only a few
die during the first year, one might expect a proportionately greater
number to die in succeeding years. If it is actually found that a high
death-rate in the first year of life is associated with a low
death-rate in succeeding years, then there will be grounds for believing
that natural selection is really cutting off the weaker and allowing the
stronger to survive.
E. C. Snow[56] analyzed the infant mortality registration of parts of
England and Prussia to determine whether any such conclusion was
justified. His investigation met with many difficulties, and his results
are not as clear-cut as could be desired, but he felt justified in
concluding from them that "the general result can not be questioned.
Natural selection, in the form of a selective death-rate, is strongly
operative in man in the early years of life. We assert with great
confidence that a high mortality in infancy (the first two years of
life) is followed by a correspondingly low mortality in childhood, and
vice-versa. . . . Our work has led us to the conclusion that infant
mortality _does_ effect a 'weeding out' of the unfit. "
"Unfitness" in this connection must not be interpreted too narrowly. A
child may be "unfit" to survive in its environment, merely because its
parents are ignorant and careless. Such unfitness makes more probable an
inheritance of low intelligence.
Evidence of natural selection was gathered by Karl Pearson from another
source and published in 1912. He dealt with material analogous to that
of Dr. Snow and showed "that when allowance was made for change of
environment in the course of 50 years, a very high association existed
between the deaths in the first year of life and the deaths in childhood
(1 to 5 years). This association was such that if the infantile
death-rate _increased_ by 10% the child death rate _decreased_ by 5. 3%
in males, while in females the _fall_ in the child death-rate was almost
1% for every 1% _rise_ in the infantile death-rate. "
To put the matter in the form of a truism, part of the children born in
any district in a given year are doomed by heredity to a premature
death; and if they die in one year they will not be alive to die in some
succeeding year.
Lately a new mathematical method, which is termed the Variate Difference
Correlation method, has been invented and gives more accurate results,
in such an investigation as that of natural selection, than any hitherto
used. With this instrument Professor Pearson and Miss Elderton have
confirmed the previous work. Applying it to the registered births in
England and Wales between 1850 and 1912, and the deaths during the first
five years of life in the same period, they have again found[57] that
"for both sexes a heavy death-rate in one year of life means a markedly
lower death-rate in the same group in the following year of life. " This
lessened death-rate extends in a lessened degree to the year following
that, but is not by the present method easy to trace further.
"It is difficult," as they conclude, "to believe that this important
fact can be due to any other source than natural selection, i. e. , a
heavy mortality leaves behind it a stronger population. "
To avoid misunderstandings, it may be well to add to this review the
closing words of the Elderton-Pearson memoir. "Nature is not concerned
with the moral or the immoral, which are standards of human conduct, and
the duty of the naturalist is to point out what goes on in Nature. There
can now be scarcely a doubt that even in highly organized human
communities the death-rate is selective, and physical fitness is the
criterion for survival. To assert the existence of this selection and
measure its intensity must be distinguished from an advocacy of high
infant mortality as a factor of racial efficiency. This reminder is the
more needful as there are not wanting those who assert that
demonstrating the existence of natural selection in man is identical
with decrying all efforts to reduce the infantile death-rate. " A further
discussion of this point will be found in a later chapter.
The conclusion that, of the infants who die, a large number do so
through inherent weakness--because they are not "fit" to survive--is
also suggested by a study of the causes of death. From a third to a half
of the deaths during the first year of life, and particularly during the
first month, are due to what may be termed uterine causes, such as
debility, atrophy, inanition, or premature birth. Although in many
cases such a death is the result of lack of prenatal care, in still more
it must be ascribed to a defect in the parental stock.
In connection with infant mortality, it may be of interest to point out
that the intensity of natural selection is probably greater among boys
than among girls. There is a steady preponderance of boys over girls at
birth (about 105 to 100, in the United States), while among the
stillborn the proportion is 158 to 100, if the Massachusetts figures for
1891-1900 may be taken as general in application. Evidently a large
number of weak males have been eliminated before birth. This elimination
continues for a number of years to be greater among boys than among
girls, until in the period of adolescence the death-rates of the two
sexes are equal. In adult life the death-rate among men is nearly always
higher than that among women, but this is due largely to the fact that
men pursue occupations where they are more exposed to death. In such
cases, and particularly where deaths are due to accident, the mortality
may not only be non-selective, but is sometimes contra-selective, for
the strongest and most active men will often be those who expose
themselves most to some danger. Such a reversal of the action of natural
selection is seen on a large scale in the case of war, where the
strongest go to the fray and are killed, while the weaklings stay at
home to perpetuate _their_ type of the race.
A curious aspect of the kind of natural selection under
consideration,--that which operates by death without reference to the
food-supply,--is seen in the evolution of a wide pelvis in women. Before
the days of modern obstetrics, the woman born with an unusually narrow
pelvis was likely to die during parturition, and the inheritance of a
narrower type of pelvis was thus stopped. With the introduction and
improvement of instrumental and induced deliveries, many of these women
are enabled to survive, with the necessary consequence that their
daughters will in many cases have a similarly narrow pelvis, and
experience similar difficulty in childbirth. The percentage of
deliveries in which instrumental aid is necessary is thus increasing
from generation to generation, and is likely to continue to increase
for some time. In other words, natural selection, because of man's
interference, can no longer maintain the width of woman's pelvis, as it
formerly did, and a certain amount of reversion in this respect is
probably taking place--a reversion which, if unchecked, would
necessarily lead after a long time to a reduction in the average size of
skull of that part of the human race which frequently uses forceps at
childbirth. The time would be long because the forceps permit the
survival of some large-headed infants who otherwise would die.
But it must not be supposed that lethal, non-sustentative selection
works only through forms of infant mortality. That aspect was first
discussed because it is most obvious, but the relation of natural
selection to microbic disease is equally widespread and far more
striking.
As to the inheritance of disease as such there is little room for
misunderstanding: no biologist now believes a disease is actually handed
down from parent to child in the germ-plasm. But what the doctors call a
diathesis, a predisposition to some given disease, is most certainly
heritable--a fact which Karl Pearson and others have proved by
statistics that can not be given here. [58] And any individual who has
inherited this diathesis, this lack of resistance to a given disease, is
marked as a possible victim of natural selection. The extent to which
and the manner in which it operates may be more readily understood by
the study of a concrete case. Tuberculosis is, as everyone knows, a
disease caused directly by a bacillus; and a disease to which immunity
can not be acquired by any process of vaccination or inoculation yet
known. It is a disease which is not directly inherited as such. Yet
every city-dweller in the United States is almost constantly exposed to
infection by this bacillus, and autopsies show that most persons have
actually been infected at some period of life, but have resisted
further encroachment. Perhaps a fraction of them will eventually die of
consumption; the rest will die of some other disease, and will probably
never even know that they have carried the bacilli of tuberculosis in
their lungs.
Of a group of men picked at random from the population, why will some
eventually die of tuberculosis and the others resist infection? Is it a
matter of environment? --are open-air schools, sanitary tenements, proper
hygiene, the kind of measures that will change this condition? Such is
the doctrine widely preached at the present day. It is alleged that the
white plague may be stamped out, if the open cases of tuberculosis are
isolated and the rest of the population is taught how to live properly.
The problem is almost universally declared to be a problem of infection.
Infection certainly is the immediate problem, but the biologist sees a
greater one a little farther back. It is the problem of natural
selection.
To prove this, it is necessary to prove (1) that some people are born
with less resistance to tuberculosis than others and (2) that it is
these people with weak natural resistance who die of phthisis, while
their neighbors with stronger resistance survive. The proof of these
propositions has been abundantly given by Karl Pearson, G. Archdall Reid
and others. Their main points may be indicated. In the first place it
must be shown that the morbidity from tuberculosis is largely due to
heredity--a point on which most medical men are still uninformed.
Measurement of the direct correlation between phthisis in parent and
child shows it to be about . 5, i.
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.
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
zoologist. 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 reemphasize
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.
CHAPTER VI
NATURAL SELECTION
Man has risen from the ape chiefly through the action of natural
selection. Any scheme of conscious race betterment, then, should
carefully examine nature's method, to learn to what extent it is still
acting, and to what extent it may better be supplanted or assisted by
methods of man's own invention.
Natural selection operates in two ways: (1) through a selective
death-rate and (2) through a selective birth-rate. The first of these
forms has often been considered the whole of natural selection, but
wrongly. The second steadily gains in importance as an organism rises in
the scale of evolution; until in man it is likely soon to dwarf the
lethal factor into insignificance. For it is evident that the appalling
slaughter of all but a few of the individuals born, which one usually
associates with the idea of natural selection, will take place only when
the number of individuals born is very large. As the reproductive rate
decreases, so does the death-rate, for a larger proportion of those born
are able to find food and to escape enemies.
When considering man, one realizes at once that relatively few babies or
adults starve to death. The selective death-rate therefore must include
only those who are unable to escape their enemies; and while these
enemies of the species, particularly certain microorganisms, still take
a heavy toll from the race, the progress of science is likely to make it
much smaller in the future.
The different aspects of natural selection may be classified as follows:
{ Lethal { Sustentative
{ { Non-sustentative
Natural selection {
{ Reproductive { Sexual
{ { Fecundal
The lethal factor is the one which Darwin himself most emphasized.
Obviously a race will be steadily improved, if the worst stock in it is
cut off before it has a chance to reproduce, and if the best stock
survives to perpetuate its kind. "This preservation of favourable
individual differences and variations, and the destruction of those
which are injurious, I have called natural selection, or the survival of
the fittest," Darwin wrote; and he went on to show that the principal
checks on increase were overcrowding, the difficulty of obtaining food,
destruction by enemies, and the lethal effects of climate. These causes
may be conveniently divided as in the above diagram, into sustentative
and non-sustentative. The sustentative factor has acquired particular
prominence in the human species, since Malthus wrote his essay on
population--that essay which both Darwin and Wallace confess was the
starting point of their discovery of natural selection.
There is a "constant tendency in all animated life to increase beyond
the nourishment prepared for it," Malthus declared. "It is
incontrovertibly true that there is no bound to the prolific plants and
animals, but what is made by their crowding and interfering with each
others' means of subsistence. " His deduction is well known: that as man
tends to increase in geometrical ratio, and can not hope to increase his
food-supply more rapidly than in arithmetical ratio, the human race must
eventually face starvation, unless the birth-rate be reduced.
Darwin was much impressed by this argument and ever since his time it
has usually been the foundation for any discussion of natural selection.
Nevertheless it is partly false for all animals, as one of the authors
showed[52] some years ago, since a species which regularly eats up all
the food in sight is rare indeed; and it is of very little racial
importance in the present-day evolution of man. Scarcity of food may put
sufficient pressure on him to cause emigration, but rarely death. The
importance of Malthus' argument to eugenics is too slight to warrant
further discussion.
When the non-sustentative forms of lethal selection are considered, it
is seen very clearly that man is not exempt from the workings of this
law. A non-sustentative form of natural selection takes place through
the destruction of the individual by some adverse feature of the
environment, such as excessive cold, or bacteria; or by bodily
deficiency; and it is independent of mere food-supply. W. F. R. Weldon
showed by a long series of measurements, for example, that as the harbor
of Plymouth, England, kept getting muddier, the crabs which lived in it
kept getting narrower; those with the greatest frontal breadth filtered
the water entering their gills least effectively, and died.
But, it was objected, man is above all this. He has gained the control
of his own environment. The bloody hand of natural selection may fall on
crabs: but surely you would not have us think that Man, the Lord of
Creation, shares the same fate?
Biologists could hardly think otherwise. Statisticians were able to
supply the needed proof. A selective death-rate in man can not only be
demonstrated but it can be actually measured.
"The measure of the selective death-rate. " says[53] Karl Pearson, to
whom this achievement is due, "is extraordinarily simple. It consists in
the fact that the inheritance of the length of life between parent and
offspring is found statistically to be about one-third of the average
inheritance of physical characters in man. This can only be due to the
fact that the death of parent or of offspring in a certain number of
cases is due to random and not to constitutional causes. " He arrived at
the conclusion[54] that 60% of the deaths were selective, in the Quaker
families which he was then studying. The exact proportion must vary in
accordance with the nature of the material and the environment, but as
A. Ploetz found at least 60% of the deaths to be selective in the
European royal families and nobility, where the environment is
uniformly good, there is no reason to think that Professor Pearson's
conclusion is invalid.
Dr. Ploetz[55] investigated the relation between length of life in
parents, and infant mortality, in about 1,000 families including 5,500
children; half of these were from the nobility and half from the
peasantry. The results were of the same order in each case, indicating
that environment is a much less important factor than many have been
wont to suppose. After discussing Professor Pearson's work, he
continued:
It seems to me that a simpler result can be reached from our
material in the following way. Since the greater child-mortality of
each of our classes of children (divided according to the ages at
death of their parents) indicates a higher mortality throughout the
rest of their lives, the offspring of parents who die young will
therefore be eliminated in a higher degree, that is, removed from
the composition of the race, than will those whose parents died
late. Now the elimination can be non-selective, falling on all
sorts of constitutions with the same frequency and degree. In that
case it will of course have no connection with selection inside the
race. Or it may be of a selective nature, falling on its victims
because they differ from those who are not selected, in a way that
makes them less capable of resisting the pressure of the
environment, and avoiding its dangers. Then we speak of a selective
process, of the elimination of the weaker and the survival of the
stronger. Since in our examination of the various causes of the
difference in infant mortality, in the various age-classes of
parents, we found no sufficient cause in the effects of the
environment, which necessarily contains all the non-selective
perils, but found the cause to be in the different constitutions
inherited by the children, we can not escape the conclusion that
the differences in infant mortality which we observe indicate a
strong process of natural selection.
Our tables also permit us to get an approximate idea of the extent
of selection by death among children in the first five years of
life. The minimum of infant mortality is reached among those
children whose parents have attained 85 years of age. Since these
represent the strongest constitutions, the mortality of their
children would appear to represent an absolute minimum, made up
almost wholly of chance, non-selective, unavoidable deaths. As the
number of children from marriages, both parties to which reached
85 years of age, is so small as to render any safe conclusions
impossible, our only recourse is to take the children of the
85-year-old fathers and the children of the 85-year-old mothers,
add them together, and strike an average. But we must recognize
that the minimum so obtained is nevertheless still too large,
because among the consorts of the long-lived fathers and mothers,
some died early with the result of increasing the infant mortality.
The infant mortality with the 85-year-old fathers and mothers is
found to be 11. 2%-15. 4%, average about 13%. The total
child-mortality reaches 31-32%, of which the 13% make about 40%.
Accordingly at least 60%, and considering the above mentioned
sources of error we may say two-thirds, of the child mortality is
selective in character. That accords reasonably well with the
55-74% which Pearson found for the extent of selective deaths in
his study.
In general, then, one may believe that more than a half of the persons
who die nowadays, die because they were not fit by by nature (i. e. ,
heredity) to survive under the conditions into which they were born.
They are the victims of lethal natural selection, nearly always of the
non-sustentative type. As Karl Pearson says, "Every man who has lived
through a hard winter, every man who has examined a mortality table,
every man who has studied the history of nations has probably seen
natural selection at work. "
There is still another graphic way of seeing natural selection at work,
by an examination of the infant mortality alone. Imagine a thousand
babies coming into the world on a given day. It is known that under
average American conditions more than one-tenth of them will die during
the first year of life. Now if those who die at this time are the
inherently weaker, then the death-rate among survivors ought to be
correspondingly less during succeeding years, for many will have been
cut down at once, who might otherwise have lingered for several years,
although doomed to die before maturity. On the other hand, if only a few
die during the first year, one might expect a proportionately greater
number to die in succeeding years. If it is actually found that a high
death-rate in the first year of life is associated with a low
death-rate in succeeding years, then there will be grounds for believing
that natural selection is really cutting off the weaker and allowing the
stronger to survive.
E. C. Snow[56] analyzed the infant mortality registration of parts of
England and Prussia to determine whether any such conclusion was
justified. His investigation met with many difficulties, and his results
are not as clear-cut as could be desired, but he felt justified in
concluding from them that "the general result can not be questioned.
Natural selection, in the form of a selective death-rate, is strongly
operative in man in the early years of life. We assert with great
confidence that a high mortality in infancy (the first two years of
life) is followed by a correspondingly low mortality in childhood, and
vice-versa. . . . Our work has led us to the conclusion that infant
mortality _does_ effect a 'weeding out' of the unfit. "
"Unfitness" in this connection must not be interpreted too narrowly. A
child may be "unfit" to survive in its environment, merely because its
parents are ignorant and careless. Such unfitness makes more probable an
inheritance of low intelligence.
Evidence of natural selection was gathered by Karl Pearson from another
source and published in 1912. He dealt with material analogous to that
of Dr. Snow and showed "that when allowance was made for change of
environment in the course of 50 years, a very high association existed
between the deaths in the first year of life and the deaths in childhood
(1 to 5 years). This association was such that if the infantile
death-rate _increased_ by 10% the child death rate _decreased_ by 5. 3%
in males, while in females the _fall_ in the child death-rate was almost
1% for every 1% _rise_ in the infantile death-rate. "
To put the matter in the form of a truism, part of the children born in
any district in a given year are doomed by heredity to a premature
death; and if they die in one year they will not be alive to die in some
succeeding year.
Lately a new mathematical method, which is termed the Variate Difference
Correlation method, has been invented and gives more accurate results,
in such an investigation as that of natural selection, than any hitherto
used. With this instrument Professor Pearson and Miss Elderton have
confirmed the previous work. Applying it to the registered births in
England and Wales between 1850 and 1912, and the deaths during the first
five years of life in the same period, they have again found[57] that
"for both sexes a heavy death-rate in one year of life means a markedly
lower death-rate in the same group in the following year of life. " This
lessened death-rate extends in a lessened degree to the year following
that, but is not by the present method easy to trace further.
"It is difficult," as they conclude, "to believe that this important
fact can be due to any other source than natural selection, i. e. , a
heavy mortality leaves behind it a stronger population. "
To avoid misunderstandings, it may be well to add to this review the
closing words of the Elderton-Pearson memoir. "Nature is not concerned
with the moral or the immoral, which are standards of human conduct, and
the duty of the naturalist is to point out what goes on in Nature. There
can now be scarcely a doubt that even in highly organized human
communities the death-rate is selective, and physical fitness is the
criterion for survival. To assert the existence of this selection and
measure its intensity must be distinguished from an advocacy of high
infant mortality as a factor of racial efficiency. This reminder is the
more needful as there are not wanting those who assert that
demonstrating the existence of natural selection in man is identical
with decrying all efforts to reduce the infantile death-rate. " A further
discussion of this point will be found in a later chapter.
The conclusion that, of the infants who die, a large number do so
through inherent weakness--because they are not "fit" to survive--is
also suggested by a study of the causes of death. From a third to a half
of the deaths during the first year of life, and particularly during the
first month, are due to what may be termed uterine causes, such as
debility, atrophy, inanition, or premature birth. Although in many
cases such a death is the result of lack of prenatal care, in still more
it must be ascribed to a defect in the parental stock.
In connection with infant mortality, it may be of interest to point out
that the intensity of natural selection is probably greater among boys
than among girls. There is a steady preponderance of boys over girls at
birth (about 105 to 100, in the United States), while among the
stillborn the proportion is 158 to 100, if the Massachusetts figures for
1891-1900 may be taken as general in application. Evidently a large
number of weak males have been eliminated before birth. This elimination
continues for a number of years to be greater among boys than among
girls, until in the period of adolescence the death-rates of the two
sexes are equal. In adult life the death-rate among men is nearly always
higher than that among women, but this is due largely to the fact that
men pursue occupations where they are more exposed to death. In such
cases, and particularly where deaths are due to accident, the mortality
may not only be non-selective, but is sometimes contra-selective, for
the strongest and most active men will often be those who expose
themselves most to some danger. Such a reversal of the action of natural
selection is seen on a large scale in the case of war, where the
strongest go to the fray and are killed, while the weaklings stay at
home to perpetuate _their_ type of the race.
A curious aspect of the kind of natural selection under
consideration,--that which operates by death without reference to the
food-supply,--is seen in the evolution of a wide pelvis in women. Before
the days of modern obstetrics, the woman born with an unusually narrow
pelvis was likely to die during parturition, and the inheritance of a
narrower type of pelvis was thus stopped. With the introduction and
improvement of instrumental and induced deliveries, many of these women
are enabled to survive, with the necessary consequence that their
daughters will in many cases have a similarly narrow pelvis, and
experience similar difficulty in childbirth. The percentage of
deliveries in which instrumental aid is necessary is thus increasing
from generation to generation, and is likely to continue to increase
for some time. In other words, natural selection, because of man's
interference, can no longer maintain the width of woman's pelvis, as it
formerly did, and a certain amount of reversion in this respect is
probably taking place--a reversion which, if unchecked, would
necessarily lead after a long time to a reduction in the average size of
skull of that part of the human race which frequently uses forceps at
childbirth. The time would be long because the forceps permit the
survival of some large-headed infants who otherwise would die.
But it must not be supposed that lethal, non-sustentative selection
works only through forms of infant mortality. That aspect was first
discussed because it is most obvious, but the relation of natural
selection to microbic disease is equally widespread and far more
striking.
As to the inheritance of disease as such there is little room for
misunderstanding: no biologist now believes a disease is actually handed
down from parent to child in the germ-plasm. But what the doctors call a
diathesis, a predisposition to some given disease, is most certainly
heritable--a fact which Karl Pearson and others have proved by
statistics that can not be given here. [58] And any individual who has
inherited this diathesis, this lack of resistance to a given disease, is
marked as a possible victim of natural selection. The extent to which
and the manner in which it operates may be more readily understood by
the study of a concrete case. Tuberculosis is, as everyone knows, a
disease caused directly by a bacillus; and a disease to which immunity
can not be acquired by any process of vaccination or inoculation yet
known. It is a disease which is not directly inherited as such. Yet
every city-dweller in the United States is almost constantly exposed to
infection by this bacillus, and autopsies show that most persons have
actually been infected at some period of life, but have resisted
further encroachment. Perhaps a fraction of them will eventually die of
consumption; the rest will die of some other disease, and will probably
never even know that they have carried the bacilli of tuberculosis in
their lungs.
Of a group of men picked at random from the population, why will some
eventually die of tuberculosis and the others resist infection? Is it a
matter of environment? --are open-air schools, sanitary tenements, proper
hygiene, the kind of measures that will change this condition? Such is
the doctrine widely preached at the present day. It is alleged that the
white plague may be stamped out, if the open cases of tuberculosis are
isolated and the rest of the population is taught how to live properly.
The problem is almost universally declared to be a problem of infection.
Infection certainly is the immediate problem, but the biologist sees a
greater one a little farther back. It is the problem of natural
selection.
To prove this, it is necessary to prove (1) that some people are born
with less resistance to tuberculosis than others and (2) that it is
these people with weak natural resistance who die of phthisis, while
their neighbors with stronger resistance survive. The proof of these
propositions has been abundantly given by Karl Pearson, G. Archdall Reid
and others. Their main points may be indicated. In the first place it
must be shown that the morbidity from tuberculosis is largely due to
heredity--a point on which most medical men are still uninformed.
Measurement of the direct correlation between phthisis in parent and
child shows it to be about . 5, i.
