XVII. HEREDITY, VARIATION, PLANT AND ANIMAL BREEDING

Problems.—To determine what makes the offspring of animals or plants tend to be like their parents.

To determine what makes the offspring of animals and plants differ from their parents.

To learn about some methods of plant and animal breeding.

(a) By selection.

(b) By hybridizing.

(c) By other methods.

To learn about some methods of improving the human race.

(a) By eugenics.

(b) By euthenics.

Suggestions for Laboratory Work

Laboratory exercise.—On variation and heredity among members of a class in the schoolroom.

Laboratory exercise.—On construction of curve of variation in measurements from given plants or animals.

Laboratory demonstration.—Stained egg cells (ascaris) to show chromosomes.

Laboratory demonstrations.—To illustrate the part played in plant or animal breeding by

(a) selection.

(b) hybridizing.

Laboratory demonstration.—From charts to illustrate how human characteristics may be inherited.

heredity and eugenics

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.

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.

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.

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.

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.

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.

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.

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]

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.

Plant breeding plots. (Minnesota Experiment Station.)

Illustration of Mendel's Law.

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.

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.

What has resulted from artificial selection among dogs. (After Romanes.)

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.

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.

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.

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.)

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!

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."

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 coöperated 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.

[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.

[35] The name Kallikak is fictitious.

Reference Books

elementary

Hunter, Laboratory Problems in Civic Biology. American Book Company.

Bailey, Plant Breeding. Macmillan and Company.

Harwood, New Creations in Plant Life. The Macmillan Company.

Jordan, The Heredity of Richard Roe. American Unitarian Association.

Sharpe, Laboratory Manual, pp. 64-72, 345-347. American Book Company.

advanced

Allen, Civics and Health. Ginn and Company.

Coulter, Castle, East, Tower, and Davenport, Heredity and Eugenics. University of Chicago Press.

Davenport, Heredity in Relation to Eugenics. Henry Holt and Company.

De Vries, Plant Breeding. Open Court Publishing Company.

Goddard, The Kallikak Family. The Macmillan Company.

Kellicott, The Social Direction of Human Evolution. Appleton Company.

Punnet, Mendelism. The Macmillan Company.

Richards, Helen M., Euthenics, the Science of Controllable Environment.

Walter, Genetics. The Macmillan Company.