Heredity and what it Means.--As I look over the faces of the boys in my
class I notice that each boy seems to be more or less like each other boy
in the class; he has a head, body, arms, and legs, and even in minor ways
he resembles each of the other boys in the room. Moreover, if I should ask
him I have no doubt but that he would tell me that he resembled in many
respects his mother or father. Likewise if I should ask his _parents_ whom
he resembled, they would say, "I can see his grandmother or his grandfather
in him."
This wonderful force which causes the likeness of the child to its parents
and to _their_ parents we call _heredity_. Heredity causes the plants as
well as animals to be like their parents. If we trace the workings of
heredity in our own individual case, we will probably find that we are
molded like our ancestors not only in physical characteristics but in
mental qualities as well. The ability to play the piano or to paint is
probably as much a case of inheritance as the color of our eyes or the
shape of our nose. We are a complex of physical and mental characters,
received in part from all our ancestors.
[Illustration: Variations in the Catalpa caterpillar. (Photographed,
natural size, by Davison.)]
Variation.--But I notice another thing; no boy in the class before me is
_exactly_ like any other boy, even twins having minute differences. In this
wonderful mold of nature each one of us tends to be slightly different from
his or her parents. Each plant, each animal, varies to a greater or lesser
degree from its immediate ancestors and may vary to a very great degree.
This factor in the lives of plants and animals is called _variation_.
Heredity and variation are the cornerstones on which all the work in the
improvement of plants and animals, including man himself, are built.
The Bearers of Heredity.--We have seen that somewhere in every living cell
is a structure known as a nucleus. In this nucleus, which is a part of the
living matter of the cell, are certain very minute structures always
present, known as _chromosomes_. These chromosomes (so called because they
take up color when stained) are believed to be the structures which contain
the _determiners_ of the qualities which may be passed from parent plant to
offspring or from animal to animal; in other words, the qualities that are
inheritable (see page 252).
The Germ Cells.--But it has been found that certain cells of the body, the
egg and the sperm cells, before uniting contain only half as many
chromosomes as do the body cells. In preparing for the process of
fertilization, half of these elements have been eliminated, so that when
the egg and sperm cell are united they will have the full number of
chromosomes that the other cells have.
If the chromosomes carry the determiners of the characters which are
inheritable, then it is easy to see that a fertilized egg must contain an
equal number of chromosomes from the bodies of each parent. Consequently
characteristics from each parent are handed down to the new individual.
This seems to be the way in which nature succeeds in obtaining variation,
by providing cell material from two different individuals.
Offspring are Part of their Ancestors.--We can see that if you or I receive
characteristics from our parents and they received characteristics from
their parents, then we too must have some of the characteristics of the
grandparents, and it is a matter of common knowledge that each of us does
have some trait or lineament which can be traced back to our grandfather or
grandmother. Indeed, as far back as we are able to go, ancestors have added
something.
[Illustration: COMPARISON OF SEXUAL AND ASEXUAL CELL REPRODUCTION]
Charles Darwin and Natural Selection.--The great Englishman Charles Darwin
was one of the first scientists to realize how this great force of heredity
applied to the development or evolution of plants and animals. He knew that
although animals and plants were like their ancestors, they also tended to
vary. In nature, the variations which best fitted a plant or animal for
life in its own environment were the ones which were handed down because
those having variations which were not fitted for life in that particular
environment would die. Thus nature seized upon favorable variations and
after a time, as the descendants of each of these individuals also tended
to vary, a new species of plant or animal, fitted for the place it had to
live in, would be gradually evolved.
Mutations.--Recently a new method of variation has been discovered by a
Dutch naturalist, named Hugo de Vries. He found that new species of plants
and animals arise suddenly by "mutations" or steps. This means that new
species instead of arising from very slight variations, continuing during
long periods of years (as Darwin believed), might arise very suddenly as a
very great variation which would at once breed true. It is easily seen that
such a condition would be of immense value to breeders, as new plants or
animals quite unlike their parents might thus be formed and perpetuated. It
will be one of the future problems of plant and animal breeders to isolate
and breed "mutants," as such organisms are called.
[Illustration: Improvement in corn by selection. To the left, the corn
improved by selection from the original type at the right.]
Artificial Selection.--Darwin reasoned that if nature seized upon favorable
variants, then man, by selecting the variations he wanted, could form new
varieties of plants or animals much more quickly than nature. And so to-day
plant or animal breeders _select_ the forms having the characters they wish
to perpetuate and breed them together. This method used by plant and animal
breeders is known as _selection_.
Selective Planting.--_By selective planting we mean choosing the best
plants and planting the seed from these plants with a view of improving the
yield._ In doing this we must not necessarily select the most perfect
fruits or grains, but must select seeds from the _best plants_. A wheat
plant should be selected not from its yield alone, but from its ability to
stand disease and other unfavorable conditions. In 1862 a Mr. Fultz, of
Pennsylvania, found three heads of beardless or bald wheat while passing
through a large field of bearded wheat. These were probably _mutants_ which
had lost the chaff surrounding the kernel. Mr. Fultz picked them out, sowed
them by themselves, and produced a quantity of wheat now known favorably
all over the world as the Fultz wheat. In selecting wheat, for example, we
might breed for a number of different characters, such as more starch, or
more protein in the grain, a larger yield per acre, ability to stand cold
or drought or to resist plant disease. Each of these characters would have
to be sought for separately and could only be obtained after long and
careful breeding. The work of Mendel (see page 257) when applied to plant
breeding will greatly shorten the time required to produce better plants of
a given kind. By careful seed selection, some Western farmers have
increased their wheat production by 25 per cent. This, if kept up all over
the United States, would mean over $100,000,000 a year in the pockets of
the farmers.
Hybridizing.--We have already seen that pollen from one flower may be
carried to another of the same species, thus producing seeds. If pollen
from one plant be placed on the pistil of another of an _allied_ species or
variety, fertilization _may_ take place and new plants be eventually
produced from the seeds. This process is known as _hybridizing_, and the
plants produced by this process known as _hybrids_.
[Illustration: In hybridizing, all of the flower is removed at the line
(_W_) except the pistil (_P_). Then pollen from another flower of a nearly
related kind is placed on the pistil and the pollinated flower covered up
with a paper bag. Can you explain why?]
Hybrids are extremely variable, rarely breed from seeds, and often are
apparently quite unlike either parent plant. They must be grown for several
years, and all plants that do not resemble the desired variety must be
killed off, if we expect to produce a hybrid that will breed more plants
like itself. Luther Burbank, the great hybridizer of California, destroys
tens of thousands of plants in order to get one or two with the characters
which he wishes to preserve. Thus he is yearly adding to the wealth of this
country by producing new plants or fruits of commercial value. A number of
years ago he succeeded in growing a new variety of potato, which has
already enriched the farmers of this country about $20,000,000. One of his
varieties of black walnut trees, a very valuable hard wood, grows ten to
twelve times as rapidly as ordinary black walnuts. With lumber yearly
increasing in price, a quick growing tree becomes a very valuable
commercial product. Among his famous hybrids are the plumcot, a cross
between an apricot and a plum, his numerous varieties of berries and his
splendid "Climax" plum, the result of a cross between a bitter Chinese plum
and an edible Japanese plum. But none of Burbank's products grow from
seeds; they are all produced _asexually_, from hybrids by some of the
processes described in the next paragraph.
The Department of Agriculture and its Methods.--The Department of
Agriculture is also doing splendid work in producing new varieties of
oranges and lemons, of grain and various garden vegetables. The greatest
possibilities have been shown by department workers to be open to the
farmer or fruit grower through hybridizing, and by budding, grafting, or
slipping.
[Illustration: Steps in budding. _a_, twig having suitable buds to use;
_b_, method of cutting out bud; _c_, how the bark is cut; _d_, how the bark
is opened; _e_, inserting the bud; _f_, the bud in place; _g_, the bud
properly bound in place.]
_Budding._--If a given tree, for example, produces a kind of fruit which is
of excellent quality, it is possible sometimes to attach parts of the tree
to another strong tree of the same species that may not bear good fruit.
This is done by _budding_. A T-shaped incision is cut in the bark; a bud
from the tree bearing the desired fruit is placed in the cut and bound in
place. When a shoot from the embedded bud grows out the following spring,
it is found to have all the characters of the tree from which it was taken.
[Illustration: Steps in tongue grafting. _a_, the two branches to be
formed; _b_, a tongue cut in each; _c_, fitted together; _d_, method of
wrapping.]
Grafting.--Of much the same nature is grafting. Here, however, a small
portion of the stem of the closely allied tree is fastened into the trunk
of the growing tree in such a manner that the two cut layers just under the
bark will coincide. This will allow of the passage of food into the grafted
part and insure the ultimate growth of the twig. Grafting and budding are
of considerable economic value to the fruit grower, as it enables him to
produce at will, trees bearing choice varieties of fruit.[34]
Footnote 34: For full directions for budding and grafting,
see Goff and Mayne, _First Principles of Agriculture_, Chap.
XIX, Mayne and Hatch, _High School Agriculture_, pp.
159-165, or Hodge, _Nature Study and Life_, pages 169-179.
Other Methods.--Other methods of plant propagation are by means of runners,
as when strawberry plants strike root from long stems that run along the
ground; layering, where roots may develop on covered up branches of
blackberry or raspberry plants; slips, roots developing from stems which
are cut off and placed in moist sand; from tubers, as in planting potatoes;
and by means of bulbs, as the tulip or hyacinth. All of the above means of
propagation are asexual and are of importance in our problem of plant
breeding.
[Illustration: Plant breeding plots. (Minnesota Experiment Station.)]
The Work of Gregor Mendel.--Fifty years ago, an Austrian monk, Gregor
Mendel, found in breeding garden peas that these plants passed on certain
_fixed characters_, as the shape of the seed, the color of the pod when
ripe, and others, and that when two pea plants of different characters were
crossed, one of these characters would be likely to appear in the offspring
of the second generation in the ratio of three to one. Such characters as
would appear to the exclusion of others in the first crossing of the plants
were called _dominant_, the ones not appearing, _recessive_
characteristics. When these seeds were again sown the ones bearing a
recessive characteristic would produce only peas with this recessive
characteristic, but the ones with a dominant characteristic might give rise
to a pure dominant or to offspring having partly a dominant and partly a
recessive character; pure dominants being to the mixed offspring in the
ratio of 1 to 2. The pure dominants if bred with others like themselves
would produce only pure dominants, but the cross breeds would again produce
mixed offspring of three kinds in the ratio of one dominant to two cross
breeds and one recessive. The feature of this work that interests us is
that _unit_ characters are passed along by heredity in the germ cells
_pure_, that is, unchanged, from one generation to another, and
independently of each other.
[Illustration: Illustration of Mendel's Law.]
Determiners of Character.--A child then resembles his parents in some
definite particulars because certain _determiners_ of characters have been
present in the germ cells of one of the parents. If the determiner of a
certain character is _absent_ from the germ cells of both parents, it will
be _absent_ in _all_ of their offspring.
These discoveries of Mendel are of the greatest importance in plant and
animal breeding because they enable the breeder to isolate certain
characters and by proper selection to breed varieties which have these
desired characters, instead of waiting for a _chance_ union of the desired
characters by nature.
Animal Breeding.--It has been pointed out that the domestication of wild
animals, the horse, cattle, sheep, goats, and the dog, marked a great
advance in civilization in the history of the earth's peoples. As the young
of these animals came to be bred in captivity the peoples owning them would
undoubtedly pick out the strongest and best of the offspring, killing off
the others for food. Thus they came unconsciously to select and aid nature
in producing a stronger and better stock. Later man began to recognize
certain characters that he wished to have in horses, dogs, or cattle, and
so by slow processes of breeding and "crossing" or hybridizing one nearly
allied form with another the numerous groups of domesticated animals began
to appear.
[Illustration: What has resulted from artificial selection among dogs.
(After Romanes.)]
In Darwin's time animal breeding was so far advanced that he got his ideas
of selection by nature in evolution from the artificial selection practiced
by animal breeders. A glance at the pictures will give some idea of the
changes that have taken place in the form of some animals since man began
to breed them a few thousand years ago.
[Illustration: The four-toed ancestor of the present horse, restored from a
study of its fossil skeleton. (After Knight in American Museum of Natural
History.)]
Some Domesticated Animals.--Our domesticated dogs are descended from a
number of wolflike forms in various parts of the world. All the present
races of cats, on the other hand, seem to be traced back to Egypt. Modern
horses are first noted in Europe and Asia, but far older forms flourished
on the earth in former geologic periods. It is interesting to note that
America was the original home of the horse, although at the time of the
earliest explorers the horse was unknown here, the wild horse of the
Western plains having arisen from horses introduced by the Spaniards. Long
ages ago, the first ancestors of the horse were probably little animals
about the size of a fox. The earliest horse we have knowledge of had four
toes on the fore and three toes on the hind foot. Thousands of years later
we find a larger horse, the size of a sheep, with a three-toed foot. By
gradual changes, caused by the tendency of the animals to vary and by the
action of the surroundings upon the animal in preserving these variations,
there was eventually produced our present horse, an animal with legs
adapted for rapid locomotion, with feet particularly fitted for the life in
open fields, and with teeth which serve well to seize and grind herbage.
Knowledge of this sort was also used by Darwin to show that constant
changes in the form of animals have been taking place since life began on
the earth.
The horse, which for some reason disappeared in this country, continued to
exist in Europe, and man, emerging from his early savage condition, began
to make use of the animal. We know the horse was domesticated in early
Biblical times, and that he soon became one of man's most valued servants.
In more recent times, man has begun to change the horse by breeding for
certain desired characteristics. In this manner have been established and
improved the various types of horses familiar to us as draft horses, coach
horses, hackneys, and the trotters.
It is needless to say that all the various domesticated animals have been
tremendously changed in a similar manner since civilized man has come to
live on the earth. When we realize the very great amount of money invested
in domesticated animals; that there are over 60,000,000 each of sheep,
cattle, and swine and over 20,000,000 horses owned in this country, then we
may see how very important a part the domestic animals play in our lives.
Improvement of Man.--If the stock of domesticated animals can be improved,
it is not unfair to ask if the health and vigor of the future generations
of men and women on the earth might not be improved by applying to them the
laws of selection. This improvement of the future race has a number of
factors in which we as individuals may play a part. These are personal
hygiene, selection of healthy mates, and the betterment of the environment.
Personal Hygiene.--In the first place, good health is the one greatest
asset in life. We may be born with a poor bodily machine, but if we learn
to recognize its defects and care for it properly, we may make it do its
required work effectively. If certain muscles are poorly developed, then by
proper exercise we may make them stronger. If our eyes have some defect, we
can have it remedied by wearing glasses. If certain drugs or alcohol lower
the efficiency of the machine, we can avoid their use. With proper _care_ a
poorly developed body may be improved and do effective work.
Eugenics.--When people marry there are certain things that the individual
as well as the race should demand. The most important of these is freedom
from germ diseases which might be handed down to the offspring.
Tuberculosis, syphilis, that dread disease which cripples and kills
hundreds of thousands of innocent children, epilepsy, and feeble-mindedness
are handicaps which it is not only unfair but criminal to hand down to
posterity. The science of being well born is called _eugenics_.
[Illustration: In this and the following diagrams the circle represents a
female, the square a male. N means normal; F means feeble-minded; A,
alcoholic; T, tubercular; _Sx_, sexually immoral; _Sy_, having syphilis.
This chart shows the record of a certain family for three generations. A
normal woman married an alcoholic and tubercular man. He must have been
feeble-minded also as two of his children were born feeble-minded. One of
these children married another feeble-minded woman, and of their five
children two died in infancy and three were feeble-minded. (After
Davenport.)]
[Illustration: This chart shows that feeble-mindedness is a characteristic
sure to be handed down in a family where it exists. The feeble-minded woman
at the top left of the chart married twice. The first children from a
normal father are all normal, but the other children from an alcoholic
father are all feeble-minded. The right-hand side of the chart shows a
terrible record of feeble-mindedness. Should feeble-minded people be
allowed to marry? (After Davenport.)]
The Jukes.--Studies have been made on a number of different families in
this country, in which mental and moral defects were present in one or both
of the original parents. The "Jukes" family is a notorious example. The
first mother is known as "Margaret, the mother of criminals." In
seventy-five years the progeny of the original generation has cost the
state of New York over a million and a quarter of dollars, besides giving
over to the care of prisons and asylums considerably over a hundred
feeble-minded, alcoholic, immoral, or criminal persons. Another case
recently studied is the "Kallikak" family.[35] This family has been traced
back to the War of the Revolution, when a young soldier named Martin
Kallikak seduced a feeble-minded girl. She had a feeble-minded son from
whom there have been to the present time 480 descendants. Of these 33 were
sexually immoral, 24 confirmed drunkards, 3 epileptics, and 143
_feeble-minded_. The man who started this terrible line of immorality and
feeble-mindedness later married a normal Quaker girl. From this couple a
line of 496 descendants have come, with _no_ cases of feeble-mindedness.
The evidence and the moral speak for themselves!
Footnote 35: The name Kallikak is fictitious.
Parasitism and its Cost to Society.--Hundreds of families
such as those described above exist to-day, spreading disease,
immorality, and crime to all parts of this country. The cost to
society of such families is very severe. Just as certain animals
or plants become parasitic on other plants or animals, these families
have become parasitic on society. They not only do harm to others
by corrupting, stealing, or spreading disease, but they are actually
protected and cared for by the state out of public money. Largely
for them the poorhouse and the asylum exist. They take from
society, but they give nothing in return. They are true parasites.
The Remedy.--If such people were lower animals, we would
probably kill them off to prevent them from spreading. Humanity
will not allow this, but we do have the remedy of separating the
sexes in asylums or other places and in various ways preventing
intermarriage and the possibilities of perpetuating such a low and
degenerate race. Remedies of this sort have been tried successfully
in Europe and are now meeting with success in this country.
Blood Tells.--Eugenics show us, on the other hand, in a study
of the families in which are brilliant men and women, the fact that
the descendants have received the _good_ inheritance from their
ancestors. The following, taken from Davenport's _Heredity in
Relation to Eugenics_, illustrates how one family has been famous
in American History.
In 1667 Elizabeth Tuttle, "of strong will, and of extreme intellectual
vigor, married Richard Edwards of Hartford, Conn., a man of high repute and
great erudition. From their one son descended another son, Jonathan
Edwards, a noted divine, and president of Princeton College. Of the
descendants of Jonathan Edwards much has been written; a brief catalogue
must suffice: Jonathan Edwards, Jr., president of Union College; Timothy
Dwight, president of Yale; Sereno Edwards Dwight, president of Hamilton
College; Theodore Dwight Woolsey, for twenty-five years president of Yale
College; Sarah, wife of Tapping Reeve, founder of Litchfield Law School,
herself no mean lawyer; Daniel Tyler, a general in the Civil War and
founder of the iron industries of North Alabama; Timothy Dwight, second,
president of Yale University from 1886 to 1898; Theodore William Dwight,
founder and for thirty-three years warden of Columbia Law School; Henrietta
Frances, wife of Eli Whitney, inventor of the cotton gin, who, burning the
midnight oil by the side of her ingenious husband, helped him to his
enduring fame; Merrill Edwards Gates, president of Amherst College;
Catherine Maria Sedgwick of graceful pen; Charles Sedgwick Minot, authority
on biology and embryology in the Harvard Medical School; Edith Kermit
Carow, wife of Theodore Roosevelt; and Winston Churchill, the author of
_Coniston_ and other well-known novels."
[Illustration: This record shows the inheritance of artistic ability (black
circles and squares). (After Davenport.)]
Of the daughters of Elizabeth Tuttle distinguished descendants also came.
Robert Treat Paine, signer of the Declaration of Independence; Chief
Justice of the United States Morrison R. Waite; Ulysses S. Grant and Grover
Cleveland, presidents of the United States. These and many other prominent
men and women can trace the characters which enabled them to occupy the
positions of culture and learning they held back to Elizabeth Tuttle.
Euthenics.--Euthenics, the betterment of the environment, is another
important factor in the production of a stronger race. The strongest
physical characteristics may be ruined if the surroundings are unwholesome
and unsanitary. The slums of a city are "at once symptom, effect, and cause
of evil." A city which allows foul tenements, narrow streets, and crowded
slums to exist will spend too much for police protection, for charity, and
for hospitals.
Every improvement in surroundings means improvement of the chances of
survival of the race. In the spring of 1913 the health department and
street-cleaning department of the city of New York cooperated to bring
about a "clean up" of all filth, dirt, and rubbish from the houses,
streets, and vacant lots in that city. During the summer of 1913 the health
department reported a smaller percentage of deaths of babies than ever
before. We must draw our own conclusions. Clean streets and houses, clean
milk and pure water, sanitary housing, and careful medical inspection all
do their part in maintaining a low rate of illness and death, thus reacting
upon the health of the citizens of the future. It will be the purpose of
the following pages to show how we may best care for our own bodies and how
we may better the environment in which we are placed.