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BEING WELL-BORN
BEING WELL-BORN
AN INTRODUCTION TO EUGENICS
_By_
MICHAEL F. GUYER, PH. D.
Professor of Zoology, The University of Wisconsin
Childhood and Youth Series
_Edited by_ M. V. O'SHEA
Professor of Education, The University of Wisconsin
INDIANAPOLIS
THE BOBBS-MERRILL COMPANY
PUBLISHERS
COPYRIGHT 1916
THE BOBBS-MERRILL COMPANY
PRESS OF
BRAUNWORTH & CO.
BOOKBINDERS AND PRINTERS
BROOKLYN, N. Y.
TO MY WIFE
HELEN M. GUYER
EDITOR'S INTRODUCTION
The writer recalls that when he was a young boy, he heard the grown-up
people in the community earnestly and incessantly debating the question:
Does heredity play a greater part in shaping one's mind and body than does
his environment? From that day to this he has listened to men and women in
every walk of life discussing the relation of heredity to environment in
determining human traits. Teachers and parents are constantly asking: "Are
such and such characteristics in my children due to their inheritance or
to the way they have been trained?" Students of juvenile delinquency and
of mental defect and deficiency are searching everywhere for light on this
matter. It is not to be wondered at that practically all people are
peculiarly interested in this problem, since it concerns intimately one's
personal traits, and it constantly confronts any one who is responsible
for the care and culture of the young.
It is suggestive to note how people differ in their views regarding the
extent to which a child's physical and mental qualities and capacities are
fixed definitely by his inheritance. The writer has often heard students
in university classes discuss the subject; and their handling of the
problem has shown how superficially and even superstitiously most persons
regard the mechanism and functions of heredity. It is significant also to
observe what extreme views many people hold regarding the possibility of
affecting a child's traits and abilities by subjecting him to specific
influences during his prenatal life. In any group of one hundred persons
chosen at random, probably seventy-five will believe in specific prenatal
influence. Many of them will believe in birthmarks due to peculiar
experiences of the mother. A popular book recently published asserts among
other things that if a mother will look upon beautiful pictures and listen
to good music during the prenatal period of her child, the latter will
possess esthetic traits and interests in high degree. On the other hand,
people generally do not seem to think that degenerate parents beget only
degenerate children. Alcoholics, feeble-minded persons and the like are
permitted to bring children into the world.
Very few people have any precise knowledge of the mechanism of heredity.
The whole thing is inscrutable to them, and is shrouded in mystery.
Superstition flourishes among even intelligent persons in respect to
heredity, and errors due to education, and tragedies resulting from
vicious social organization are all alike ascribed to its uncontrollable
forces. Most people are none the wiser because they do not know to what
extent the physical and mental defects and deviations of individuals are
due to inheritance or to the malign influences of the individual's
environment and training.
Professor Guyer, who has studied the whole problem in a thoroughgoing,
scientific way, has prepared this book with a view to illuminating some of
the mysteries that surround the subject of heredity, and to dispelling
the illusions that persist regarding it. He shows the method which nature
follows in the development of the individual. He presents the laws which
have become established respecting the extent to which and the manner in
which immediate and remote ancestors contribute to the child's physical
and mental organism. He answers many questions which those who are engaged
in social work or in education in the home or the school are asking
to-day. He discusses subjects upon which every serious-minded person
wishes to be informed. He has thus made a book which is both of
theoretical and of practical interest.
He has written in a style which should make his book attractive to the
parent and the teacher as well as to the student of the complicated
mechanism of inheritance. Only a few special terms are used, and these
should not give any reader trouble, because the treatment throughout is so
concrete that the meaning of the terms will be easily grasped. Further,
the book is illustrated, with many attractive and instructive
illustrations which will show at a glance the working of the principles of
inheritance which are developed in the text.
This book may be heartily commended to all who are interested in questions
of human nature, education and social reform. It should enable the parent,
the teacher and the legislator to understand more clearly than most of
them now do in how far children's traits and possibilities are or can be
fixed by inheritance as contrasted with environmental conditions and
nurture in home, school, church and institutional life.
M. V. O'SHEA.
Madison, Wisconsin.
PREFACE
One of the most significant processes at work in society to-day is the
awakening of the civilized world to the rights of the child; and it is
coming to be realized that its right of rights is that of being well-born.
Any series of publications, therefore, dealing primarily with the problems
of child nature may very fittingly be initiated by a discussion of the
factor of well-nigh supreme importance in determining this nature,
heredity.
No principles have more direct bearing on the welfare of man than those of
heredity, and yet on scarcely any subject does as wide-spread ignorance
prevail. This is due in part to the complexity of the subject, but more to
the fact that in the past no clear-cut methods of attacking the manifold
problems involved had been devised. Happily this difficulty has at least
in part been overcome.
It is no exaggeration to say that during the last fifteen years we have
made more progress in measuring the extent of inheritance and in
determining its elemental factors than in all previous time. Instead of
dealing wholly now with vague general impressions and speculations,
certain definite principles of genetic transmission have been disclosed.
And since it is becoming more and more apparent that these hold for man as
well as for plants and animals in general, we can no longer ignore the
social responsibilities which the new facts thrust upon us.
Since what a child becomes is determined so largely by its inborn
capacities it is of the greatest importance that teachers and parents
realize something of the nature of such aptitudes before they begin to
awaken them. For education consists in large measure in applying the
stimuli necessary to set going these potentialities and of affording
opportunity for their expression. Of the good propensities, some will
require merely the start, others will need to be fostered and coaxed into
permanence through the stereotyping effects of proper habits; of the
dangerous or bad, some must be kept dormant by preventing improper
stimulation, others repressed by the cultivation of inhibitive tendencies,
and yet others smothered or excluded by filling their place with desirable
traits before they themselves come into expression.
We must see clearly, furthermore, that even the best of pedagogy and
parental training has obvious limits. Once grasp the truth that a child's
fate in life is frequently decided long before birth, and that no amount
of food or hospital service or culture or tears will ever wholly make good
the deficiencies of bad "blood," or in the language of the biologist, a
faulty germ-plasm, and the conviction must surely be borne home to the
intelligent members of society that one thing of superlative importance in
life is the making of a wise choice of a marriage mate on the one hand,
and the prevention of parenthood to the obviously unfit on the other.
In the present volume it is intended to examine into the natural
endowment of the child. And since full comprehension of it requires some
understanding of the nature of the physical mechanism by which hereditary
traits are handed on from generation to generation, a small amount of
space is given to this phase. Then, that the reader may appreciate to
their fullest extent the facts gathered concerning man, a review of the
more significant principles of genetics as revealed through experiments in
breeding plants and animals has been undertaken. The main applications of
these principles to man is pointed out in a general discussion of human
heredity. Finally, inasmuch as all available data indicate that the fate
of our very civilization hangs on the issue, the work concludes with an
account of the new science of eugenics which is striving for the
betterment of the race by determining and promulgating the laws of human
inheritance so that mankind may intelligently go about conserving good and
repressing bad human stocks.
In order to eliminate as many errors as possible and to avoid oversights I
have submitted various chapters to certain of my colleagues and friends
who are authorities in the special field treated therein. While these
gentlemen are in no way responsible for the material of any chapter they
have added greatly to the value of the whole by their suggestions and
comments. Thus I am indebted to Professor Leon J. Cole for reading the
entire manuscript; to Professors A. S. Pearse and F. C. Sharp for reading
Chapter VII; to Professor C. R. Bardeen for reading special parts; to
Doctor J. S. Evans for reading Chapter VI and part of V; to Doctor W. F.
Lorenz, of the Mendota Hospital, for reading Chapter VIII; to Judge E. Ray
Stevens for reading Chapter IX, and to Helen M. Guyer for several readings
of the entire manuscript.
Grateful acknowledgment is made to all of these readers, to various
publishers and periodicals for the use of certain of the illustrations, to
the authors of the numerous books and papers from which much of the
material in such a work as this must necessarily be selected, and to my
artist, Miss H. J. Wakeman, for her painstaking endeavors to make her work
conform to my ideas of what each diagram should show.
M. F. G.
CONTENTS
CHAPTER PAGE
I HEREDITY 1
Blood heritage--Kind determined by origin--Ancestry a network--
Ancestry in royalty--Offspring derived from one parent only--Dual
ancestry an aid in studying heredity--Reversion--Telegony--
Prenatal influences apart from heredity--Parent body and germ not
identical--A hereditary character defined--Hereditary mingling a
mosaic rather than a blend--Determiners of characters, not
characters themselves, transmitted--Our knowledge of heredity
derived along three lines--The method of experimental breeding--
The statistical method--Galton's law of regression--Correlations
between parents and offspring--The biometrical method,
statistical, not physiological--Mental as well as physical
qualities inheritable.
II THE BEARERS OF THE HERITAGE 20
The cell the unit of structure--Unicellular organisms--Importance
of cell-theory--Heredity in unicellular forms--Reproduction and
heredity in colonial protozoa--Conjugation--Specialization of
sex-cells--The fertilized ovum--Advancement seen in the Volvox
colony--Natural death--Specialization in higher organisms--Sexual
phenomena in higher forms--Cell-division--Chromosomes constant in
number and appearance--Significance of the chromosomes--Cleavage
of the egg--Chief processes operative in building the body--The
origin of the new germ-cells--Significance of the early setting
apart of the germ-cells--Individuality of chromosomes--Pairs of
chromosomes--Reduction of the number of chromosomes by one-half--
Maturation of the sperm-cell--Maturation of the egg-cell--Parallel
between the two processes--Fertilization--Significance of the
behavior of the chromosomes--A single set of chromosomes
sufficient for the production of an organism--The duality of the
body and the singleness of the germ--The cytoplasm in
inheritance--Chromosomes possibly responsible for the
distinctiveness of given characters--Sex and heredity--Many
theories of sex determination--The sex-chromosome--Sex-linked
characters in man--In lower forms.
III MENDELISM 67
New discoveries in the field of heredity--Mendel--Rediscovery of
Mendelian principles--Independence of inheritable characters--
Illustration in the Andalusian fowl--The cause of the ratio--
Verification of the hypothesis--Dominant and recessive--
Segregation in the next generation--Illustrated in guinea-pigs--
Terminology--The theory of presence and absence--Additional
terminology--Dominance not always complete--Modifications of
dominance--Mendel's own work--Dihybrids--Getting new combinations
of characters--Segregations of the determiners--Four kinds of
gametes in each sex--The 9:3:3:1 ratio--Phenotype and genotype--
The question of blended inheritance--Nilsson-Ehle's discoveries--
Such cases easily mistaken for true blends--Skin-color in man--
Questionable if real blends exist--The place of the Mendelian
factors in the germ-cell--Parallel between the behavior of
Mendelian factors and chromosomes--A single chromosome not
restricted to carrying a single determiner.
IV MENDELISM IN MAN 97
Probably applicable to many characters in man--Difficult to get
correct data--A generalized presence-absence formula--Indications
of incomplete dominance--Why after the first generation only half
the children may show the dominant character--Eye-color in man--
Hair-color--Hair-shape--Irregularities--Digital malformations--Eye
defects--Other defects inherited as dominants--Recessive
conditions more difficult to deal with--Albinism--Other recessive
conditions in man--Breeding out defects--Other inheritable
conditions in man.
V ARE MODIFICATIONS ACQUIRED DIRECTLY BY THE BODY INHERITED? 121
Which new characters are inherited?--Examples of somatic
modifications--Use and disuse--The problem stated--Special
conditions in mammals--Three fundamental questions--External
influences may directly affect the germ-cells--Such effects
improbable in warm-blooded animals--Poisons may affect the
germ-plasm--How can somatic modifications be registered in
germ-cells?--Persistence of Mendelian factors argues against such
a mode of inheritance--Experiments on insects--On plants--On
vertebrates--Epilepsy in guinea-pigs--Effects of mutilations not
inherited--Transplantation of gonads--Effects of body on germ,
general not specific--Certain characters inexplicable as inherited
somatic acquirements--Neuter insects--Origin of new characters in
germinal variation--Sexual reproduction in relation to new
characters--Many features of an organism characterized by
utility--Germinal variation a simpler and more inclusive
explanation--Analysis of cases--Effects of training--Instincts--
Disease--Reappearance not necessarily inheritance--Prenatal
infection not inheritance--Inheritance of a predisposition not
inheritance of a disease--Tuberculosis--Two individuals of
tubercular stock should not marry--Special susceptibility less of
a factor in many diseases--Deaf-mutism--Gout--Nervous and mental
diseases--Other disorders which have hereditary aspects--Induced
immunity not inherited--Social, ethical and educational
significance of non-inheritance of somatic modifications--No cause
for discouragement--Improved environment will help conserve
superior strains when they do appear.
VI PRENATAL INFLUENCES 151
All that a child possesses at birth not necessarily hereditary--
The myth of maternal impressions--Injurious prenatal influences--
Lead poisoning--The expectant mother should have rest--Too short
intervals between children--Expectant mothers neglected--
Alcoholism--Unreliability of most data--Alcohol a germinal and
fetal poison--Various views of specialists on the effects of
alcoholism on progeny--The affinity of alcohol for germinal
tissue--Innate degeneracy versus the effects of alcohol--
Experimental alcoholism in lower animals--Further remarks on the
situation in man--Much inebriety in man due to defective nervous
constitution--Factors to be reckoned with in the study of
alcoholism--Venereal diseases--The seriousness of the situation--
Infantile blindness--Syphilis--Some of the effects--A blood test--
Many syphilitics married--Why permit existing conditions to
continue?--Ante-nuptial medical inspection--The perils of venereal
disease must be prevented at any cost--Bad environment can wreck
good germ-plasm.
VII RESPONSIBILITY FOR CONDUCT 195
All mental process accompanied by neural process--Gradations in
nervous response from lower organisms to man--Behavior of many
animals often an automatic adjustment to simple external agents--
Tropisms--Certain apparently complex volitions probably only
tropisms--Complicating factors--Many tropic responses apparently
purposeful--Tropisms grade into reflex actions and instincts--
Adjustability of instincts opens the way for intelligent
behavior--Modification of habits possible in lower animals--Some
lower vertebrates profit by experience--Rational behavior--
Conceptual thought probably an outgrowth of simpler psychic
states--The capacity for alternative action in higher animals--The
elemental units of the nervous system are the same in lower and
higher animals--Neuron theory--Establishment of pathways through
the nervous system--Characteristic arrangements of nerve cells
subject to inheritance--Different parts of the cortex yield
different reactions--Skill acquired in one branch of learning
probably not transferred to another branch--Preponderance of
cortex in highest animals--Special fiber tracts in the spinal cord
of man and higher apes--Great complexity in associations and more
neurons in the brain of man--The nervous system in the main
already staged at the time of birth--Many pathways of conduction
not yet established--The extent of the modifiable zone unknown--
Various possibilities of reaction in the child--Probable origin of
altruistic human conduct--Training in motive necessary--Actual
practise in carrying out projects important--Interest and
difficulty both essential--The realization of certain
possibilities of the germ rather than others is subject to
control--We must afford the opportunity and provide the proper
stimuli for the development of good traits--Moral responsibility.
VIII MENTAL AND NERVOUS DEFECTS 228
Prevalence of insanity--Imperfect adjustments of the brain
mechanism inheritable--Many mental defectives married--
Disproportionate increase in number of mental defectives--Protests
voiced by alienists--Examples of hereditary feeble-mindedness--
Difficult to secure accurate data--Feeble-mindedness and insanity
not the same--Many types of insanity--Not all insanities of the
same eugenical significance--Difficulties of getting genealogies
of specific forms of insanity--Certain forms of insanity seem to
behave as Mendelian recessives--Grades of feeble-mindedness--About
two-thirds of feeble-mindedness inherited--Some results of
non-restraint of the feeble-minded--Not all cases of mental
deficiency inherited--Epileptics--Feeble-mindedness probably a
recessive--Many apparently normal people are carriers of
neuropathic defects--Tests for mental deficiency--The backward
child in school--The exceptionally able child--Cost of caring for
our mentally disordered--Importance of rigid segregation of the
feeble-minded--Importance of early diagnosis of insanity--Opinion
of competent psychiatrists essential--Some insanities not
hereditary--Importance of heredity in insanity not appreciated.
IX CRIME AND DELINQUENCY 263
Heredity and environment in this field--Feeble-mindedness often a
factor--Many delinquent girls mentally deficient--Institutional
figures misleading--Many prisoners mentally subnormal--Inhibitions
necessary to social welfare--The high-grade moron a difficult
problem--Degenerate strains--Intensification of defects by
inbreeding--Vicious surroundings not a sufficient explanation in
degenerate stocks--Not all delinquents defectives--No special
inheritable crime-factor--What is a born criminal?--Epileptic
criminal especially dangerous--The mental disorders most
frequently associated with crime--Bearing of immigration on crime
and delinquency--Sexual vice--School instruction in sex-hygiene--
Mere knowledge not the crux of the sex problem--Early training in
self-restraint an important preventive of crime and delinquency--
Multiplication of delinquent defectives must be prevented.
X RACE BETTERMENT THROUGH HEREDITY 289
Questionable charity--Past protests--An increasing flood of
defectives--Natural elimination of defectives done away with--Why
not prevent our social maladies?--Eugenics defined--Improved
environment alone will not cure racial degeneracy--Heredity and
environment--Inter-racial marriage--Human conservation--Kindness
in the long run--The problem has two phases--Constructive eugenics
must be based on education--Inferior increasing more rapidly than
superior stocks--An unselected population may contain much
valuable material--The lack of criteria for judging fitness--The
college graduate--Native ability, independence and energy
eugenically desirable--Four children to each marriage required to
maintain a stock--Factors contributing to low birth-rate in
desirable strains--The educated public must be made to realize the
situation--Utilization of family pride as a basis for constructive
eugenics--The tendency for like to marry like--Public opinion as
an incentive to action--Choosing a marriage mate means choosing a
parent--The best eugenic marriage also a love match--The
elimination of the grossly unfit urgent--Suggested remedies--
Inefficacy of laws which forbid marriage of mental defectives--
Systems of mating impracticable in the main--Corrective mating
presupposes knowledge of eugenics--Segregation has many
advocates--Sterilization as a eugenic measure--To what conditions
applicable--In insanity--In feeble-mindedness--In cases of
epilepsy--Sterilization laws--Social dangers in vasectomy--Our
present knowledge insufficient--Sterilization laws on trial--An
educated public sentiment the most valuable eugenic agent--The
question of personal liberty--Education of women in eugenics
needed--Much yet to be done--A working program--Which shall it be?
GLOSSARY 343
REFERENCES FOR FURTHER READING AND STUDY 355
INDEX 361
BEING WELL-BORN
CHAPTER I
HEREDITY
It is a commonplace fact that offspring tend to resemble their parents. So
commonplace, indeed, that few stop to wonder at it. No one misunderstands
us when we say that such and such a young man is "a chip off the old
block," for that is simply an emphatic way of stating that he resembles
one or the other of his parents. The same is true of such familiar
expressions as "what's bred in the bone," "blood will tell," and kindred
catch phrases. All are but recognitions of the same common fact that
offspring exhibit various characteristics similar to those of their
progenitors.
=Blood Heritage.--=To this phenomenon of resemblance in successive
generations based on ancestry the term heredity is applied. In man, for
instance, there is a marked tendency toward the reappearance in offspring
of structures, habits, features, and even personal mannerisms, minute
physical defects, and intimate mental peculiarities like those possessed
by their parents or more remote forebears. These personal characteristics
based on descent from a common source are what we may call the blood
heritage of the child to discriminate it from a wholly different kind of
inheritance, namely, the passing on from one generation to the next of
such material things as personal property or real estate.
=Kind Determined by Origin.--=It is inheritance in the sense of community
of origin that determines whether a given living creature shall be man,
beast, bird, fish, or what not. A given individual is human because his
ancestors were human. In addition to this stock supply of human qualities
he has certain well-marked features which we recognize as characteristics
of race. That is, if he is of Anglo-Saxon or Italian or Mongolian
parentage, naturally his various qualities will be Anglo-Saxon, Italian,
or Mongolian. Still further, he has many distinctive features of mind and
body that we recognize as family traits and lastly, his personal
characteristics such as designate him to us as Tom, Harry, or James must
be added. The latter would include such minutiae as size and shape of ears,
nose or hands; complexion; perhaps even certain defects; voice; color of
eyes; and a thousand other particulars. Although we designate these
manifold items as individual, they are in reality largely more or less
duplicates of similar features that occur in one or the other of his
progenitors, features which he would not have in their existing form but
for the hereditary relation between him and them.
"O Damsel Dorothy! Dorothy Q.!
Strange is the gift that I owe to you;
* * * * *
What if a hundred years ago
Those close-shut lips had answered 'No,'
* * * * *
Should I be I, or would it be
One-tenth another, to nine-tenths me?"
"Soft is the breath of a maiden's yes;
Not the light gossamer stirs with less;
But never a cable that holds so fast
Through all the battles of wave and blast,
And never an echo of speech or song
That lives in the babbling air so long!
There were tones in the voice that whispered then
You may hear to-day in a hundred men."
When life steps into the world of matter there comes with it a sort of
physical immortality, so to speak; not of the individual, it is true, but
of the race. But the important thing to note is that the race is made up,
not of a succession of wholly unrelated forms, but a continuation of the
same kind of living organisms, and this sameness is due to the actual
physical descent of each new individual from a predecessor. In other
words, any living organism is the kind of organism it is in virtue of its
hereditary relation to its ancestors.
It is part of the biologist's task to seek a material basis, a continuity
of actual substance, for this continuity of life and form between an
organism and its offspring. Moreover, inasmuch as the offspring is never
precisely similar to its progenitors he must determine also what qualities
are susceptible of transmission and in what measure.
=Ancestry a Network.--=From the fact that each child has all of the
ancestors of its mother as well as of its father, arises the great
complications which are met with in determining the lineage of an
individual. A person has two parents, four grandparents, eight great
grandparents, and thus following out pedigree it is plain to be seen that
through this process of doubling in each generation, in the course of a
few centuries one's ancestry is apparently enormous. By actual
computation, according to Professor D. S. Jordan, if we count thirty
generations back to the Norman invasion of England in 1066, at this ratio
of duplication, the child of to-day would have had at that time an
ancestry of 8,598,094,592 persons. But we know that the total number of
inhabitants in England during the time of William the Conqueror was but a
small fraction of this enormous aggregate. This means that we shall have
to modify our inference that a child has twice as many ancestors as its
parents; a condition which at first sight seems evident, but which is not
literally true. The fact is that the parents of the child, in all
probability, have many ancestors in common--a state of affairs which is
brought about through the intermarriage of relatives, and this is
especially frequent among remoter descendants of common progenitors. Time
after time in genealogy strains of blood have crossed and recrossed until
it is not improbable that a man of to-day who is of English origin has the
blood in his veins from every inhabitant of England who lived during the
time of William the Conqueror and left fruitful descendants. Instead of
conceiving of ancestry as an ever branching and widening tree-like system
as it recedes into the past, it is more accurate, therefore, to regard it
in the light of an elaborate meshwork. The "family tree" in reality
becomes the family net.
=Ancestry in Royalty.--=The pedigrees of royal families have proved to be
of much importance in the study of human inheritance, not that royal
traits are any more heritable than any other, but simply because the
records have been carefully kept so that they are the most comprehensive
and easily followed pedigrees available. The netlike weave of ancestry is
particularly well exemplified in some of these families because of much
close intermarriage. Their heritage typifies on an intensified scale the
heritage of the mass of mankind. For example, if we go six generations
back in the ancestry of Frederick the Great instead of the expected
sixty-four individual ancestors we find only forty; or in a still more
closely woven stock, in the Spanish royal line of Don Carlos we find in
six generations instead of sixty-four individual ancestors, only
twenty-eight. While the present German emperor might have had four
thousand ninety-six ancestors in the twelfth generation back, it is
estimated that owing to intermarriage he probably had only five hundred
thirty-three.
=Offspring Derived from One Parent Only.--=So far in our reckoning of
heredity we have counted elements from both father and mother, and the
complications which arise from such a double ancestry are manifestly very
perplexing ones. If we could do away with the elements of sex and find
offspring that are derived from one parent only, it would seemingly
simplify our problem very much for we should thus have a direct line of
descent, free from intermingling. This, in fact, occurs to a greater or
less extent among lower animals in a number of instances. There may be
only female forms for a number of generations and the eggs which they
produce develop directly into new individuals. Moreover, many of the
simpler organisms have the power of dividing their bodies into two and
thus giving rise to two new forms, each of which resembles the parent.
This shows plainly that we may have inheritance without the appearance of
any male ancestor at all, hence sex is not always a necessary factor in
reproduction or heredity. The development of eggs asexually, that is,
without uniting first with a male cognate, is termed _parthenogenesis_.
The ordinary plant louse or aphid which is frequently found upon geraniums
is a familiar example of an animal which reproduces largely in this way.
During the summer only the females exist and they are so astonishingly
fertile that one such aphid and her progeny, supposing none dies, will
produce one hundred million in the course of five generations. In the last
broods of the fall, males and females appear and fertile eggs are produced
which lie dormant through the winter to start the cycle of the next year.
Again, the eggs of some kinds of animals which normally have to unite with
a male germ before they develop, can be made to develop by merely treating
them with chemical solutions. The difference between an offspring derived
in such a manner, and one which has developed from an egg fertilized by
the male is that it is made up of characteristics from only one source,
the maternal.
=Dual Ancestry an Aid in Studying Heredity.--=Although we have the factors
of heredity in a more simplified form in the case of asexual
transmission, as a matter of fact most of our insight into the problems
of heredity has been attained from a study of sexually reproducing forms,
because the very existence of two sets of more or less parallel features
offers a kind of checking up system by which we can follow a given
characteristic.
=Reversion.--=Occasionally, however, plants and animals do not develop the
complete individuality we might expect, but stop short at or re-attain
some ancestral stage along the line of descent, and thus come to resemble
some progenitor perhaps many generations back of their own time. Thus it
is well known that as regards one or more characteristics a child may
resemble a grandparent or often some remote ancestor much more closely
than it does its immediate parent. The reappearance of such ancestral
traits the student of heredity designates as _Reversion_ or _Atavism_.
Reversion may occur apparently in any class of plants or animals. It is
especially pronounced among domesticated forms, which through man's
selection have been produced under more or less artificial conditions. For
example, among fancy breeds of pigeons, there may be an occasional return
to the old slaty blue color of the ancestral rock-pigeon, with two dark
cross-bars on the wings, from which all modern breeds have been derived.
This is almost sure to happen if the fancy varieties are inter-crossed for
two or three generations. Another example of reversion frequently cited is
the occasional reappearance in domestic poultry of the reddish or brownish
color pattern of the ancestral jungle-fowl to which, among modern forms,
the Indian game seems most nearly related in color. Still another example
is the cross-bars or stripes occasionally to be seen on the forelegs of
colts, particularly mules, reminiscent of the extinct wild progenitors
which were supposedly striped.
Fig. 1, p. 9, is a picture of a hybrid between the common fowl and the
guinea-fowl. The chevron-like markings on certain feathers show a
reversion to a type of color pattern that is prevalent among both the
primitive pheasants (the domestic chicken is a pheasant) and the primitive
guinea-fowls. Although the common spotted guinea-fowl may be crossed with
a black chicken which shows no trace of barring, nevertheless the hybrid
offspring are likely to bear a chevron-like pattern such as that shown in
the picture.
There has been much quibbling over the relative meanings of reversion and
atavism. The general idea, whichever term we use, is that there is a
"throwing back" in a noticeable degree through inheritance to some
ancestral condition beyond the immediate parents. A few recent authors
have taken the term atavism in a restricted sense and use it to signify
specifically those not uncommon cases in which a particular character of
an offspring resembles the corresponding character of a grandparent
instead of a parent. Such, for example, as the blue eye-color of a child
with brown-eyed parents, each of whom in turn has had a blue-eyed parent.
The tendency of other authors is to abandon the term entirely because of
the diversity of meaning that has been attached to it in the past.
[Illustration: FIG. 1
Hybrid between the guinea-fowl and the common fowl, showing in many
feathers reversion to a primitive chevron-like barring.]
Certain classes of so-called reversions, such as the case of the eye-color
just cited, are readily explicable on Mendelian principles as we shall see
in a later chapter, but probably not all kinds of phenomena described as
reversion can be so explained. For example, some seem to be cases of
suppressed development. The word reversion, indeed, must be looked on as
a convenient descriptive term rather than as the name of a single specific
condition.
=Telegony.--=There is yet a wide-spread belief in the supposed influence
of an earlier sire on offspring born by the same mother to a later and
different sire. This alleged phenomenon is termed _telegony_. For example,
many dog-breeders assert that if a thoroughbred bitch has ever had pups by
a mongrel father, her later offspring, although sired by a thoroughbred,
will show taints of the former mongrel mating. In such cases the female is
believed to be ruined for breeding purposes. Other supposed instances of
such influences have been cited among horses, cattle, sheep, pigs, cats,
birds, pets of various kinds and even men. The historic case most
frequently quoted is that of Lord Morton's mare which bore a hybrid colt
when bred to a quagga, a striped zebra-like animal now extinct. In later
years the same mare bore two colts, sired by a black Arabian horse. Both
colts showed stripes on the neck and other parts of the body, particularly
on the legs. It was inferred that this striping was a sort of after effect
of the earlier breeding with the quagga. In recent times, however,
Professor Ewart has repeated the experiment a number of times with
different mares using a Burchell zebra as the test sire. Although his
experiments have been devised so as to conduce in every way possible to
telegony his results have been negative. Moreover, it has been pointed out
that the stripes on the legs of the two foals alleged to show telegony
could not have been derived from the quagga sire for, unlike zebras,
quaggas did not have their legs striped. Furthermore it is known that the
occurrence of dark brown stripes on the neck, withers and legs of ordinary
colts is not uncommon, some cases of which have exhibited more zebra-like
markings than those of the colts from Lord Morton's mare. It seems much
more probable, therefore, that the alleged instances are merely cases of
ordinary reversion to the striped ancestral color pattern which probably
characterized the wild progenitors of the domesticated horse.
Various experiments on guinea-pigs, horses, mice and other forms,
especially devised to test out this alleged after-influence of an earlier
sire, have all proved negative and the general belief of the biologist
to-day is that telegony is a myth.
=Prenatal Influences Apart from Heredity.--=In discussing the problems of
heredity it is necessary to consider also the possibilities of external
influences apart from lineage which may affect offspring through either
parent. Although modifications derived directly by the parent, and
prenatal influences in general, are of extremely doubtful value as of
permanent inheritable significance, nevertheless they must be reckoned
with in any inventory of a child's endowment at birth. Impaired vitality
on the part of the mother, bad nutrition and physical vicissitudes of
various kinds all enter as factors in the birthright of the child, who,
moreover, may bear in its veins slumbering poisons from some progenitor
who has handed on blood taints not properly attributable to heredity. Of
such importance is this kind of influence to the welfare of the immediate
child that it will be necessary to discuss it in considerable detail in a
later chapter.
=Parent Body and Germ Not Identical.--=Inasmuch as each new individual
appears to arise from material derived from its parent, taking the
evidence at its face value one might suppose that any peculiarity of
organization called forth in the living substance of the parent would
naturally be repeated in the offspring, but a closer study of the
developing organism from its first inception to maturity shows this to be
probably a wrong conclusion. The parent-body and the reproductive
substance contained in that body are by no means identical. It becomes an
important question to decide, in fact, how much effect, if any, either
permanent or temporary, the parent-body really has on the germ.
A given fertile germ (Fig. 2, p. 13) gives rise by a succession of
divisions to a body which we call the individual, but such a germ also
gives rise to a series of new germ-cells which reside in that individual,
and it is these germ-cells, not something derived from the body, that pass
on the determiners of distinguishing features or qualities from generation
to generation. It is only by grasping the significance of this fact that
we can understand how in certain cases a totally different set of
characters may appear in an offspring than those manifested in either
parent.
=An Hereditary Character Defined.--=By a _character_, in discussions in
heredity, is meant simply a trait, feature or other characteristic of an
organism. Where we can pick out a single definable characteristic which
acts as a unit in heredity, for greater accuracy we term it a
_unit-character_. Many traits are known to be inherited on a unit basis or
are capable of being analyzed into factors which are so inherited. These
unit-characters are in large measure inherited independently of one
another apparently, although cases of characters inherited as a unit along
with other characters are known.
=Hereditary Mingling a Mosaic Rather Than a Blend.--=The independence of
unit-characters in inheritance leads us to the important conclusion that
the mingling of two lines of ancestry into a new individual is in no sense
bringing them into the "melting pot," as it has been picturesquely
expressed, but it is rather to be regarded as the mingling of two mosaics,
each particle of which retains its own individuality, and which, even if
overshadowed in a given generation, may nevertheless manifest its
qualities undimmed in later generations when conditions favorable to its
expression transpire.
[Illustration: FIG. 2
Diagram illustrating germinal continuity. Through a series of divisions a
germ-cell gives rise to a body or a soma and to new germ-cells. The
latter, not the body, give rise in turn to the next generation.]
=Determiners of Characters, Not Characters Themselves, Transmitted.--=The
fact should be thoroughly understood that the actual thing which is
transmitted by means of the germ in inheritance is not the character
itself, but something which will _determine_ the character in the
offspring. It is important to remember this, for often these
_determiners_, as they are called, may lie unexpressed for one or more
generations and may become manifest only in later descendants. The truth
of the matter is, the child does not inherit its characters from
corresponding characters in the parent-body, but parent and child are
alike because they are both products of the same line of germ-plasm, both
are chips from the same old block.
METHODS OF STUDYING HEREDITY
Before entering into details it will be well to get some idea of the
methods which are commonly employed in arriving at conclusions in the
field of heredity. Some of these are extremely complex and all that we can
do in an elementary presentation is to get a glimpse of the procedures.
=Our Knowledge of Heredity Derived Along Three Lines.--=Our modern
conceptions of heredity have been derived mainly from three distinct lines
of investigation: First, from the study of embryology, in which the
biologist concerns himself with the genesis of the various parts of the
individual, and the mechanism of the germs which convey the actual
materials from which these parts spring; second, through experimental
breeding of plants and animals to compare particular traits or features in
successive generations; and third, through the statistical treatment of
observations or measurements of a large number of parents and their
offspring with reference to a given characteristic in order to determine
the average extent of resemblance between parents and children in that
particular respect.
=The Method of Experimental Breeding.--=A tremendous impetus was given to
the method of experimental breeding when it was realized that we can
itemize many of the parts or traits of an organism into entities which are
inherited independently one of another. Such traits, or as we have already
termed them, unit-characters, may be not only independently heritable but
independently variable as well. The experimental method seeks to isolate
and trace through successive generations the separate factors which
determine the individual unit-characters of the organism. In this attempt
cross-breeding is resorted to. Forms which differ in one or more respects
are mated and the progeny studied. Next these offspring are mated with
others of their own kind or mated back with the respective parent types.
In this way the behavior of a particular character may often be followed
and the germinal constitutions of the individuals concerned can be
formulated with reference to it. Inasmuch as we shall give much
consideration to this method in the chapter on Mendelism we need not
consider it further here.
=The Statistical Method.--=The statistical method seeks to obtain large
bodies of facts and to deal with evidence as it appears through
mathematical analysis of these facts. The attempt of its followers is to
treat quantitatively all biological processes with which it is concerned.
Historically Sir Francis Galton was the first to make any considerable
application of statistical methods to the problems of heredity and
variation. In his attempts to determine the extent of resemblance between
relatives of different degree as regards bodily, mental and temperamental
traits, he devised new methods of statistical analysis which constitute
the basis of modern statistical biology, or _biometry_ as it is termed by
its votaries. Professor Karl Pearson in particular has extended and
perfected the mathematical methods of this field and stands to-day as
perhaps its most representative exponent. The system is in the main based
on the calculus of probability. The methods often are highly specialized,
requiring the use of higher mathematics, and are therefore only at the
command of specially trained workers.
Just as insurance companies can tell us the probable length of human life
in a given social group, since although uncertain in any particular case,
it is reducible in mass to a predictable constant, so the biometrician
with even greater precision because of his improved methods can often,
when a large number of cases are concerned, give us the intensity of
ancestral influence with reference to particular characters.
For example, it is clear that by measuring a large number of adult human
beings one can compute the average height or determine the height which
will fit the greatest number. There will be some individuals below and
some above it, but the greater the divergence from this standard height
the fewer will be the individuals concerned.
Galton compared the heights of 204 normal English parents and their 928
adult offspring. In order to equalize the measurements of men and women he
found he had to multiply each female height by 1.08. Then, to take both
parents into account when comparing height of parents to that of children
he added the height of the father to the proportionately augmented height
of the mother and divided by two, thus securing the height of what he
termed the "mid-parent." He found that the mid-parental heights of his
subjects ranged from 64.5 to 72.5 inches, and that the general _mode_ was
about 68.5 inches. It should be mentioned that the _mode_, in a given
population, represents the group containing the largest number of
individuals of one kind; it may or may not coincide with the average. The
children of all mid-parents having a given height were measured next and
tabulated with reference to these mid-parents. The results of Galton's
measurements may be expressed simply as follows:
-----------------------------------------------------------------
|MODE|
----------------------------------------|----|-------------------
Height of mid-parent| | | | | | | | |
in inches |64.5|65.5|66.5|67.5|68.5|69.5|70.5|71.5|72.5
| | | | | | | | |
Average height of | | | | | | | | |
offspring |65.8|66.7|67.2|67.6|68.3|68.9|69.5|69.9|72.2
-----------------------------------------------------------------
=The Law of Regression.--=It is plain from this table that the offspring
of short mid-parents tend to be under average or modal height though not
so far below as their parents. Likewise children of tall parents tend to
be tall but less tall than their parents. This fact illustrates what is
known as Galton's _law of regression_; namely, that if parents in a given
population diverge a certain amount from the mode of the population as a
whole, their children, while tending to resemble them, will diverge less
from this mode. It is clear that the extent of regression is an inverse
measure of the intensity of inheritance from the immediate parents; if the
deviation of the offspring from the general mode were nearly as great as
that of their parents then the intensity of the inheritance must be high;
if but slight--that is, if the offspring regressed nearly to the
mode--then the intensity of the inheritance must be ranked as low. In the
example in question it must be ranked as relatively high. Computations
show that as regards stature the fraction two-thirds represents
approximately the amount of resemblance between the two generations where
both parents are considered.
=Correlations Between Parents and Offspring.--=In modern researches the
conception of mid-parent and mid-grandparent as utilized by Galton has
been largely abandoned. It has been found more convenient as well as more
accurate to keep the measurements of the two parents separate and to deal
with correlations between fathers and sons, fathers and daughters, mothers
and sons, mothers and daughters, brother and brother, etc. Professor
Pearson and his pupils have found for a number of characters that the
correlation between either parent and children, whether sons or daughters,
is relatively close. The correlation between brother and brother, sister
and sister, and brother and sister, usually ranges a little higher than
the corresponding relation between parents and children.
=The Biometrical Method, Statistical, Not Physiological.--=While biometry
may in certain cases go far toward showing us the average intensity of
the inheritance of certain characters it can not replace the method of
the experimental breeder which deals with particular characters in
individual pedigrees. It must be borne in mind that the biometrical method
is a statistical and not a physiological one and that it is applicable
only when large numbers of individuals are considered in mass. It is most
valuable in cases where we are unable sharply to define single characters,
due probably to the concurrent action of a number of independent causes,
or where experiment is impossible so that we have to depend solely on
numerical data gained by observation.
=Mental Qualities Inheritable.--=Galton showed by this method long ago,
and Pearson and his school have extended and more clearly established the
work, that exceptional mental qualities tend to be inherited. While on the
average the children of exceptional parents tend to be less exceptional
than their parents, still they are far more likely to be exceptional than
are the children of average parents. By this method Professor Pearson has
shown that such mental and temperamental attributes as ability, vivacity,
conscientiousness, temper, popularity, handwriting, etc., are as
essentially determined as are physical features through the hereditary
endowment.