No More Learning

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. e. , what one expects if it is a matter
of heredity. This is the coefficient for most physical and mental
characters: it is the coefficient for such pathological traits as
deafness and insanity, which are obviously due in most cases to
inheritance rather than infection.

But, one objects, this high correlation between parent and child does
not prove inheritance,--it obviously proves infection. The family
relations are so intimate that it is folly to overlook this factor in
the spread of the disease.

Very well, Professor Pearson replied, if the relations between parent
and child are so intimate that they lead to infection, they are
certainly not less intimate between husband and wife, and there ought to
be just as much infection in this relationship as in the former. The
correlation was measured in thousands of cases and was found to lie
around . 25, being lowest in the poorer classes and highest in the
well-to-do classes.

At first glance this seems partly to confirm the objection--it looks as
if there must be a considerable amount of tubercular infection between
husband and wife. But when it is found that the resemblance between
husband and wife in the matter of insanity is also . 25, the objection
becomes less formidable. Certainly it will hardly be argued that one of
the partners infects the other with this disability.

As a fact, a correlation of . 25 between husband and wife, for
tuberculosis, is only partly due to infection. What it does mean is that
like tends to mate with like--called assortative mating. This
coefficient of resemblance between husband and wife in regard to
phthisis is about the same as the correlation of resemblance between
husband and wife for eye color, stature, longevity, general health,
truthfulness, tone of voice, and many other characters. No one will
suppose that life partners "infect" each other in these respects.
Certainly no one will claim that a man deliberately selects a wife on
the basis of resemblance to himself in these points; but he most
certainly does so to some extent unconsciously, as will be described at
greater length in Chapter XI. Assortative mating is a well-established
fact, and there is every reason to believe that much of the resemblance
between husband and wife as regards tuberculosis is due to this fact,
and not to infection. [59]

Again, it is objected that the infection of children is not a family
matter, but due to tuberculous cows' milk: how then does it appear
equally among the Japanese, where cows are not tuberculous and cow's
milk rarely used as an infant food: or among such people as the
Esquimaux and Polynesians, who have never seen a cow?

But, it is argued, at any rate bad housing and unsanitary conditions of
life will make infection easier and lower the resistance of the
individual. Perhaps such conditions may make infection easier, but that
is of little importance considering how easy it is for all city
dwellers--for the population as a whole. The question remains, will not
bad housing cause a greater liability to fatal phthisis? Will not
destitution and its attendant conditions increase the probability that a
given individual will succumb to the white plague?

Most physicians think this to be the case, but they have not taken the
pains to measure the respective roles, by the exact methods of modern
science. S. Adolphus Knopf of New York, an authority on tuberculosis,
recognizes the importance of the heredity factor, but says that after
this, the most important predisposing conditions are of the nature of
unsanitary schools, unsanitary tenements, unsanitary factories and
workshops. This may be very true; these conditions may follow after
heredity in importance--but how near do they follow? That is a matter
capable of fairly accurate measurement, and should be discussed with
figures, not generalities.

Taking the case of destitution, which includes, necessarily, most of the
other evils specified, Professor Pearson measured the correlation with
liability to phthisis and found it to be . 02. The correlation for direct
heredity--that is, the resemblance between parent and offspring--it will
be remembered, is . 50. As compared with this, the environmental factor
of . 02 is utterly insignificant. It seems evident that whether or not
one dies from tuberculosis, under present-day urban conditions, depends
mainly on the kind of constitution one has inherited.

There is no escape, then, from the conclusion that in any individual,
death from tuberculosis is largely a matter of natural selection. But
by taking a longer view, one can actually see the change to which
natural selection is one of the contributors. The following table shows
the deaths from consumption in Massachusetts, per 10,000 population:

1851-60 39. 9
1861-70 34. 9
1871-80 32. 7
1881-90 29. 2
1891-1900 21. 4
1901 17. 5
1902 15. 9

F. L. Hoffman further points out[60] that in Massachusetts, Rhode Island,
and Connecticut, 1872-1911, the decline in the death-rate from
tuberculosis has been about 50%. "The evidence is absolutely conclusive
that actually as well as relatively, the mortality from tuberculosis in
what is the most intensely industrial area of America has progressively
diminished during the last 40 years. "

It will be noted that the great increase in death from consumption in
this area began in the decade following 1840, when the large Irish
immigration began. The Irish are commonly believed to be particularly
susceptible to phthisis. Crowded together in industrial conditions, they
rapidly underwent infection, and their weak racial resistance led to a
high death-rate. The weak lines of heredity were rapidly cut off; in
other words, the intensity of natural selection was great, for a while.
The result was to leave the population of these New England states much
more resistant, on the average, than it was before; and as the Irish
immigration soon slowed down, and no new stocks with great weakness
arrived, tuberculosis naturally tended to "burn itself out. " This seems
to be a partial explanation of the decline in the death-rate from
phthisis in New England during the last half century, although it is not
suggested that it represents the complete explanation: improved methods
of treatment and sanitation doubtless played their part. But that they
are the sole cause of the decline is made highly improbable by the low
correlation between phthisis and environmental factors, which was
mentioned above, and by all the other biometric study of tuberculosis,
which has proved that the results ascribed to hygiene, including
sanitorium treatment, are to some degree illusory.

That tuberculosis is particularly fatal to the Negro race is well known.
Even to-day, after several centuries of natural selection in the United
States, the annual death-rate from consumption among Negroes in the
registration area is 431. 9 per 100,000 population (census of 1900) as
compared with 170. 5 for the whites; in the cities alone it is 471. 0.
That overcrowding and climate can not be the sole factors is indicated
by the fact that the Negro race has been decimated, wherever it has met
tuberculosis. "In the years 1803 and 1810 the British government
imported three or four thousand Negroes from Mozambique into Ceylon to
form into regiments, and of these in December, 1820, there were left
just 440, including the male descendants. All the rest had perished
mainly from tuberculosis, and in a country where the disease is not
nearly so prevalent as in England. "[61] Archdall Reid has pointed
out[62] that the American, Polynesian and Australian aborigines, to whom
tuberculosis was unknown before the advent of Europeans, and who had
therefore never been selected against it, could not survive its advent:
they were killed by much smaller infections than would have injured a
European, whose stock has been purged by centuries of natural selection.

These racial histories are the most important evidence available to the
student of natural selection in man. The conclusion to be drawn from
them seems plain. Natural selection, which has in the past never had an
opportunity to act upon the Negro race through tuberculosis, is now
engaged in hastening, at a relatively rapid rate, the evolution of this
race toward immunity from death by tuberculosis. The evolution of the
white race on this line is, as the figures show, going on
simultaneously, but having begun centuries earlier, it is not now so
rapid. The weakest white stocks were cut off hundreds of years ago, in
Great Britain or Europe; those of the black race are only now going.
Despite all the efforts of medicine and sanitation, it is likely that
the Negro death-rate from phthisis will continue high for some years,
until what is left of the race will possess a degree of resistance, or
immunity, not much inferior to that of the whites among whom they live.
The blacks in North America now must be already more resistant than
their ancestors; the mulattoes descended of normal healthy unions should
be more resistant than the pure Negroes, although no statistics are
available on the point; but were a new immigration to take place from
Africa to-day, and the immigrants to be put into villages with their
Americanized brethren, the high death-rate would result.

While the Negroes were thus undergoing the radical surgery of natural
selection, what was happening to the aborigines of America? The answer
of history is unmistakable; they were meeting the same fate, in an even
more violent form. Not tuberculosis alone, but small-pox, measles,
alcohol and a dozen other importations of the conquerors, found in the
aborigines of the New World a stock which had never been selected
against these diseases.

It is the custom of sentimentalists sometimes to talk as if the North
American Indian had been killed off by the white man. So he was,--but
not directly: he was killed off by natural selection, acting through the
white man's diseases and narcotics. In 1841 Catlin wrote, "Thirty
millions of white men are now scuffling for the goods and luxuries of
life over the bones of twelve millions of red men, six millions of whom
have fallen victims to small-pox. " Small-pox is an old story to the
white race, and the death of the least resistant strains in each
generation has left a population that is fairly resistant. It was new to
the natives of America, and history shows the result. Alcohol, too,
counted its victims by the thousand, for the same reason. The process of
natural selection among the North American Indians has not yet stopped;
if there are a century from now any Indians left, they will of
necessity belong to stocks which are relatively resistant to alcohol and
tuberculosis and the other widespread and fatal diseases which were
unknown upon this continent before Columbus.

The decrease of natives following the Spanish conquest of tropical
America has long been one of the most striking events of history.
Popular historians sometimes speak as if most of the native population
had been killed off by the cruelty of the conquistadores. Surely such
talk could not proceed from those who are familiar with the action of
natural selection. It is obvious that when the Spaniard brought the
natives together, making them work in mines and assemble in churches, he
brought them under conditions especially favorable for infection by the
new diseases which he had brought. The aborigines of the New World, up
to the time the Spaniards came, had undergone no evolution whatever
against these diseases; consequently the evolution began at so rapid a
rate that in a few centuries only those who lived in out-of-the-way
places remain unscathed.

The same story is repeated, in a survey of the history of the Pacific
Islands. Even such a disease as whooping-cough carried off adults by the
hundred. Robert Louis Stevenson has left a graphic picture[63] of
natural selection at work:

"The tribe of Hapaa," he writes, "is said to have numbered some four
hundred when the small-pox came and reduced them by one-fourth. Six
months later a woman developed tubercular consumption; the disease
spread like fire about the valley, and in less than a year two
survivors, a man and a woman, fled from the newly-created solitude. . . .
Early in the year of my visit, for example, or late the year before, the
first case of phthisis appeared in a household of 17 persons, and by the
end of August, when the tale was told me, one soul survived, a boy who
had been absent at his schooling. "

In Tasmania is another good illustration of the evolution of a race
proceeding so rapidly as to be fatal to the race. When the first
English settled on the island, in 1803, the native population consisted
of several thousand. Tuberculosis and many other new diseases, and, most
of all, alcohol, began to operate on the aborigines, who were attracted
to the settlements of the whites. In a quarter of a century there were
only a few hundred left. Many, of course, had met violent deaths, but an
enlightened perusal of any history of the period,[64] will leave no
doubt that natural selection by disease was responsible for most of the
mortality. By 1847 the number of native Tasmanians was reduced to 44,
who were already unmistakably doomed by alcohol and bacteria. When the
last full-blood Tasmanian died in 1876, a new chapter was written in the
story of the modern evolution of the human race.

No such stories are told about the white settlements on this continent,
even before the days of quarantine and scientific medicine. There is no
other adequate explanation of the difference, than that the two races
have evolved to a different degree in their resistance to these
diseases. It is easily seen, then, that man's evolution is going on, at
varying rates of speed, in probably all parts of the human race at the
present time.

We do not mean, of course, to suggest that all the natives who have died
in the New World since the landing of Columbus, have died because the
evolution of their race had not proceeded so far in certain directions
as had that of their conquerors. But the proportion of them who were
eliminated for that reason is certainly very large. In the more remote
parts of South America the process is still going on. Recent press
dispatches have carried the account of the University of Pennsylvania's
Amazon Expedition, under the direction of William C. Farrabee. In a
letter dated March 16, 1916, the leader told of the discovery of the
remains of the tribe of Pikipitanges, a once populous tribe of which a
chief, six women and two boys alone are left. The tribe had been almost
wiped out, Dr. Farabee reported, by an epidemic of _influenza_!

If the aborigines of the New World succumb to the diseases of the
European, it is not less true that the European succumbs to diseases
against which his race has not been selected. The deadliness of yellow
fever to Americans in the tropics, and the relative immunity of Negroes,
is familiar; so too is the frequently fatal result of the African
tropical fevers on the white man, while the natives suffer from them
much less, having been made more resistant by centuries of natural
selection.

This long discussion may now be summarized. We dealt with lethal
selection, that form of natural selection which operates by prematurely
killing off the less fit and leaving the more fit to survive and
reproduce their kind. It is of course understood that the word "fit" in
this connection does not necessarily mean morally or mentally superior,
but merely fit for the particular environment. In a community of
rascals, the greatest rascal might be the fittest to survive. In the
slums of a modern city the Jewish type, stringently selected through
centuries of ghetto life, is particularly fit to survive, although it
may not be the physical ideal of an anthropologist.

Two forms of lethal selection were distinguished, one depending on
starvation and the other on causes not connected with the food supply.
Direct starvation is not a factor of importance in the survival of most
races during most of the time at the present day so far as the civilized
portion of the world is concerned. But disease and the other lethal
factors not connected with the food-supply, through which natural
selection acts, are still of great importance. From a half to two-thirds
of all deaths are of a selective character, even under favorable
conditions.

It is also to be noted, however, that with the progress of medicine, and
the diminution of unfit material, this kind of natural selection will
tend to become less and less widespread. For a long time, natural
selection in man has probably done little to cause marked change in his
physical or mental characteristics. Man's interference has prevented. In
recent centuries natural selection has probably done no more on the
whole than keep the race where it was: it is to be feared that it has
not even done that. It is doubtful if there is any race to-day which
attains the physical and mental average of the Athenians of 2,500 years
ago.

Lethal natural selection, then, has been and still is a factor of great
importance in the evolution of the race, but at present it is doing
little or nothing that promises to further the ideal of eugenics--race
betterment.

But lethal natural selection is only half the story. It is obvious that
if the constitution of a race can be altered by excess of deaths in a
certain class, it can equally be altered by excess of births in a
certain class. This is reproductive selection, which may appear in
either one of two forms. If the individual leaves few or no progeny
because of his failure to mate at the proper time, it is called sexual
selection; if, however, he mates, yet leaves few or no progeny (as
compared with other individuals), it is called fecundal selection.

Even in man, the importance of the role of reproductive selection is
insufficiently understood; in the lower animals scientists have tended
still more to undervalue it. As a fact, no species ordinarily multiplies
in such numbers as to exhaust all the food available, despite the
teaching of Malthus and Darwin to the contrary. The rate of reproduction
is the crux of natural selection; each species normally has such a
reproduction rate as will suffice to withstand the premature deaths and
sterility of some individuals, and yet not so large as to press unduly
upon the food supply. The problem of natural selection is a problem of
the adjustment between reproductive rate and death-rate, and the
struggle for subsistence is only one of several factors.

While the reproductive rate must be looked upon as a characteristic
which has its adaptations like other characteristics, it has one
peculiarity--its increase is always opposed by lethal selection.