LUTHER BURBANK-THE BEARING OF HIS WORK ON HUMAN LIFE
ON IMPROVING THE HUMAN PLANT
Students of heredity are becoming more and more agreed that the same laws and principles apply to the organisms of the vegetable and animal worlds. This is quite what might be expected, considering the fundamental identity of protoplasm, which is the physical basis of all life. But quite aside from any theoretical deductions in the matter, a wide range of experiments with many types of animals has brought conclusive evidence that striking analogies are everywhere to be found between the manner of transmission of traits and characteristics in plants and animals. Moreover observations of human genealogy have shown that man himself is subject to precisely the same laws of heredity that apply to the lowliest vegetable or animal organisms. We must of course make allowance for differences incident to the elaborate organism of man, and we must not forget that man differs from the other organisms in that he can take conscious note of the conditions of his heritage and of his environment and can be guided in a measure by what he thus learns. This fundamental fact gives man a place apart in the entire scheme of evolution. But it does not remove mankind from the limitations imposed by the laws of hereditary transmission. He can consciously modify his environment and he can be guided in his selections by his knowledge of heredity; but he cannot free himself from the thralldom of environmental influences or from the inexorable limitations of his ancestral heritage. In some respects, indeed, man is far more hampered when he attempts to apply the laws of heredity to his own race than he is in making application of the same laws to the basis of transient animals under domestication. The necessities of the social organism that he has built up place limitations on his freedom of selection in the mating of individuals and even sharper restrictions on his selections among the progeny for the parents of future generations. Indeed, until very recently it has not been thought fitting that man should give any consideration whatever to the scientific breeding of his own race, notwithstanding the obvious advantages that have resulted from the scientific breeding of races of plants and animals. Of late, however, it has gradually dawned on the intelligent people of the world that the laws of heredity which confessedly apply to man might rationally be given consideration in the breeding of races of men. The new science of eugenics, named and in large part originated by the late Sir Francis Galton, has received an amount of attention in very recent years that it could not possibly have hoped to receive had it been brought to the attention of the public even twenty years ago. And it cannot well be doubted that the demonstrations as to the possibility of improving the races of valued plants by selective breeding made at Santa Rosa and Sebastopol have had their share in calling public attention to the possible benefits that may accrue from the systematic and intelligent application of the principles of heredity. A general appreciation of the unity of life-forces as well as of life substances, due primarily to the spread of the Darwinian doctrine, has prepared the public to look with unbiased eyes for the first time on the human race itself as an evolution product that owes its pre-eminence to the conscious utilization of natural forces and that may obtain still greater heights by the still more intelligent utilization of these forces. So it will be accepted as a mere matter of course that we should attempt, in completing the review of Mr. Burbank's life work with the development of new forms of plant life, to make application of the practical knowledge gained in the experiment garden to what might, without violence to words, be described as the breeding of the human plant. Such an application we shall now attempt, concisely, yet with as much explicitness as is warranted.
THE GREAT PRINCIPLE OF SELECTION
Even the most casual reader of this work will be aware that the great fundamental principle that guides us in all stages of our experiments in plant development is the principle of selection. We select first the kind of plant that is to be utilized in a given series of experiments. We select the best individual or individuals to be found among the entire company of these plants at our disposal. We select other individuals of the same or of different species as mates before cross-pollenizing, and in successive generations we repeat these processes of selection and reselection over and over. Now in the human family precisely analogous processes of selection are being employed, consciously or unconsciously, in every community. Of course the selections are not usually made with the definite and avowed object of producing progeny of an improved type; but the inherent affinities that lead to the selection of marriage partners are themselves determined by principles that might properly be said to be eugenic-providing artificial restrictions do not too greatly interfere with the freedom of choice. Generally speaking, men and women would choose marriage partners having vigor and health and beauty to the exclusion of those showing the opposite traits, were free choice given them. But, of course, under actual social conditions, entire freedom of choice is impossible, and no fact is more distressingly patent than the fact that large numbers of persons who are obviously unfit to assume the duties of parenthood nevertheless enter the marriage state and bring forth abundant progeny. Indeed, under existing conditions, it is the all too general observation that the notoriously unfit members of the community are the ones that produce the largest families. Now it requires no very profound knowledge of the laws of heredity to understand that such a condition of things is not conducive to the betterment of the race. No one could hope to produce an improved variety of plants of any kind if he had not freedom of choice in determining that the more desirable individuals should be mated and their progeny preserved to the exclusion of the progeny of the less desirable. The entire foundation of plant improvement depends, as we have all along seen, on such freedom of choice. And in proportion as the plant developer selects wisely, chooses the individual plants that have the best hereditary tendencies, mates the right individuals, and rigidly selects the best only among their progeny, can he hope to progress in the direction of his ideal plant. It would appear, then, that unless human society can devise a means whereby a preponderant number of the offspring of each successive generation are the progeny of those members of the community who are superior in body and mind and morals, we cannot expect that the human race will improve generation after generation. Any colony of flowers left to breed indiscriminately, good or bad, will inevitably degenerate from the stage of culture to whizh artificial selection has brought it. The reason for this is that the conditions imposed by cultivation are different from the conditions of Nature and the special development of the plant has taken place along the lines of man's tastes and needs without special regard to the needs of the plant itself. But if you remove the artificial conditions, so that the conditions of Nature again prevail, then selection will take place in accordance with the needs of the plant itself, and this will imply a reversion, in the course of a few generations, to something like the original wild state of the plant.
UNNATURAL STANDARDS OF CIVILIZATION
Now the conditions of human civilization are no less artificial. Standards of excellence among civilized men are quite different from the standards of excellence among barbaric races. We do not count a man as the foremost individual in his community because he has the physical ability to wield a heavier club than his neighbor can wield, nor because of the ruthless freedom with which he exercises his superior strength. Among savage tribes mere physical strength, coupled with brute cunning and ferocity, may determine leadership. Such are the natural and necessary standards so long as man is at war with wild beasts and with other savage men that know no law except that of physical supremacy. But under conditions of civilization all that has been changed. The standards of excellence that determine the position of men and women in any given community are mental and moral rather than merely physical. They are in the broad sense of the word unnatural standards, but they are the only standards compatible with the persistence of the unnatural state of society that we term civilized. So it has come about that the condition of men in civilized society is closely comparable to the condition of plants in a hothouse or in a carefully cultivated and weeded garden. The very conditions of civilization make it as essential that the human weed should be removed and the unfit members of the community prevented from propagating their kind as that similar principles should apply in the hothouse or the flower garden. Under the conditions of barbaric life, and even under those of the high civilization of classical antiquity, the principles of eugenic selection thus implied were carried out with a good deal of rigor. Even if the weaklings were not consciously removed-and this was sometimes done-the stress of living was such that the abnormal or weakly infants were claimed by disease, and the adults who lacked strength and intelligence were likely to succumb to the attacks of wild pests, to starvation, or to the onslaught of human enemies. So the principle of selective or eugenic breeding was all along applied, even when no one comprehended its meaning or gave it a name; and the results are seen in the progress of humanity to its present state. In very recent years, however, there has been great progress in the way of ameliorating the environment, in particular the environment of childhood, through improvement in the understanding of hygiene and the prevention of disease, so that there is no longer the weeding out of the unfit in infancy that occurred even a single generation ago; so the generation of tomorrow are confronted with problems of selection in the breeding of the human race more urgent than ever before. The problem is complicated by the fact that the more intelligent members of the community-precisely the ones that should be selected for the propagation of the race-are prone to restrict the number of their offspring, whereas the less desirable parents practice no such restriction. The obvious tendency of this must be comparable to the condition of a flower garden in which the best plants are restricted to the production of one or two seed pods while the poorer varieties are allowed to scatter their seeds by indiscriminate thousands. The plant breeder who permitted such a condition to obtain in his garden would assuredly not produce improved races of plants. And the human system which permits such a condition to obtain cannot hope to better the average condition of the human race. As to the precise methods through which conditions more in accordance with the improvements of the future generations of our race are to be applied, we shall attempt no details of suggestion. It suffices to point out the principle and to suggest that there cannot well be two opinions as to the desirability of restricting the fecundity of the unfit, however wide the diversity of opinion as to the way in which this may be practically accomplished.
THE ARISTOCRATIC AMERICAN RACE
Lest we seem to take a pessimistic view of the situation, however, let me hasten to point out that the average human plant in most communities of America today is somewhat comparable to the average plants in the most highly developed colonies of Mr. Burbank's experiment gardens. The reader will recall the somewhat detailed accounts that have been given of the cherry colonies comprising 400 aristocratic families, and of the various colonies of plums and quinces and chestnuts and lilies and gladioli and Watsonias and countless others that are similarly made up of individuals exclusively of good breeding and of desirable qualities. Now, whoever will properly gauge the condition of the human garden of today, here in America, must realize that in general the races of human beings that make up the population are of correspondingly aristocratic lineage. Here of course we do not use the word "aristocratic" in the conventional sense. We are referring to the qualities that make a good and desirable citizen; and mean to imply that the process of crossing and selection has been carried out so well for the past ten generations or so in America that a race has been developed having a very high average of those traits that determine "fitness" for existence in a civilized community. It is true that there are certain strains of abnormality-of physical degeneracy, mental obliquity, moral perversion-that have made their way, generation after generation, like weeds in the garden, and that must constantly be reckoned with just as the gardener reckons with his weeds. But the main body of citizens that make up the population are at least moderately fit to live in harmony with the normal environment of civilization, and by the same token to reproduce their kind. Unfortunately, however, there has been a very pronounced tendency within recent decades for the individuals who were reared under the healthful conditions of the farm and village to make their way to the cities and to take up the relatively abnormal life that is forced upon a majority of the city population under existing conditions. The offspring of these city dwellers are reared in an environment radically different from the healthful one in which their parents were reared. They are crowded into dark, ill-ventilated tenements, amidst surroundings that not only lack the light and air and joyousness of the country, but are often positively vitiated as to their mental and moral no less than as to their physical atmosphere. It is as if we were to take the plants that have been bred in the rich, well watered, carefully weeded soil of a garden and transplant them into an infertile, dry soil, choked with weeds and away from sunlight. By no chance could we expect the plants under these conditions to attain full growth or to put forth even a fair complement of flowers and fruits. The Burbank giant amaryllis bulbs, which under proper conditions will put forth splendid stalks bearing flowers ten inches across, would be reduced, under such altered conditions, to the throwing up of meager stalks and, at best, the production of a restricted number of dwarf flowers little calculated to add to the reputation of the plant developer.
THE POWER OF ENVIRONMENT
This matter of environment, then, goes hand in hand with heredity and is a final determining factor in deciding the character of the individual product. It is quite useless to have practiced the most rigid selection among plants for any number of generations, and thereby to have produced varieties of the most splendid possibilities-unless the plants of the newest generation are given proper soil and nourishment and sunshine they will come to nothing. And so it is with the human plant. Despite the good heredity of generations of ancestors bred, let us say, from the old Pioneer stock in New England or Virginia or from the transplanted cions of that stock in the Middle or Far West, the coming generations will be dwarfed and perverted representatives of their race if they are denied a normal environment, particularly in childhood. So one of the great problems that confronts the humanitarian of to-day is the problem of providing a proper environment for the human plant. In the decade covered by the most recent census returns (1901-1910) the total population of the United States increased by 21 per cent. But the rural population increased by only 14 percent and the city population by 38 per cent. There are entire states in which the rural population did not increase at all, and these were precisely those middle western farming districts that supply the healthiest of all environments for the production of improved examples of the human plant. It is not meant to imply that the environment of the city is necessarily unwholesome. But it requires no argument to show that the average city dweller is less favorably situated for the development of normal children than is the average dweller on farm or in country village. Children vitally need fresh air and sunlight and the out-of-door life. They need to be allowed to romp in the fields and to come in contact with nature. The city walls and pavements are a pitifully inadequate substitute for the greensward and the trees of the country. And a generation for which this substitution has been made cannot be expected to improve upon the traditions of its parent generation. So the student of the human plant will do well to give full attention to the question of improving the environment of the human colonies with which he is concerned. The story has been told of the way in which the soil of my experiment garden at Santa Rosa was prepared and modified and even metamorphosed until the conditions were attained that were favorable for the growth of my plant charges. Without such attention to the physical environment it would have been quite impossible to produce the improved races that have been developed at Santa Rosa and Sebastopol. And unless a way can be found to make the average environment of successive generations of human beings better and better-instead of allowing it to become worse and worse-we cannot hope that the generations of our grandchildren and great grandchildren will maintain the average standards of our own time, let alone improve upon them.
EDUCATING THE SEEDLING
A word must be said also as to the influence of environment in its bearing on the mental and moral development of the individual in determining the bringing out or the suppression of hereditary potentialities. The mental and moral attributes of man may be likened to the flower or fruit of the cultivated plant, in that they are the qualities most recently developed or transformed through selective breeding. In token of their newness, they are the qualities most easily altered or modified by environing influences or by new racial blendings. There are, for example, the qualities that are prone to "Mendelize" in hereditary transmission, as we shall see illustrated and interpreted in another connection. The direct influence of environment on these highly differentiated and hence unstable characteristics of plant or of man is easily demonstrated in any experiment garden or in any social community. But even the most deep-seated and fundamental qualities may be profoundly modified if the environing influences are applied during the childhood of the seedling plant or the human subject. "As the twig is bent the tree is inclined" is a maxim the literal truth of which is apparent to the least-skilled horticulturist. The application of the maxim to the human sapling is equally familiar matter of fact to even the tyro in human pedagogy. A Shakespeare is not born with a fund of knowledge and a profuse vocabulary stored in his brain; but only with the receptive quality of brain-fibre that will enable him-granted proper surroundings-to acquire knowledge of things and of words. Placed in childhood on a South Sea Island, among savages, Shakespeare could have passed his life without knowing a single word of the English tongue, and without having even the vaguest conception of the existence of a written language of any kind. This extreme example will serve to suggest the extent to which the individual even of the very best heredity is dependent upon environment for the bringing out of his inherent potentialities. As another extreme example might be cited the case of the child who becomes blind and deaf in infancy through some accident or disease. Such a child will commonly remain at a stage of mental culture comparable to that of a congenital idiot. Exceptional cases like those of Laura Bridgman and Miss Helen Keller, in which, through infinite effort, the other senses are made in part to compensate for the loss of sight and hearing, building up the brain through vicarious channels, serve to give further emphasis to the fact under consideration-the all-importance of the environing influences that we commonly speak of as "educational" in completing the work which heredity carries only to the nascent state of development.
THE MIXTURE OF RACES
Yet another respect in which the problems of breeding a better human race in our day run parallel to the problems of the plant developer is with reference to the foreign materials that make up the stock for the propagation of future generations. It will be recalled that some of Mr. Burbank's most important successes were achieved by blending the racial strains of plants brought from different continents. Plants were imported from Japan, from New Zealand, from Siberia, from South Africa, from Canada, from South America. Carefully selecting among them, he inbred the species from widely separated regions, and thus brought together racial strains that had long diverged. And the results were often startling, and sometimes highly gratifying. It is easy to draw the inference from the most casual glimpses into the past history of our race that the development of civilization has been largely conditioned on the mingling of different racial strains. It is scarcely too much to say that each of the great civilizations of the past was built by a mixed race. It was so in Egypt, in Assyria, in Greece, and in Rome in the ancient days. It is true of the important races of central Europe and of Great Britain in modern times. And it is pre-eminently true of the American race of our own day. The point is too obvious for elaboration. No one needs to be told that the colonial stock that came to America in the early part of the seventeenth century was itself made up of mixed ancestral strains. And the most casual inspection of statistics shows to what extent the increase of population of the past hundred years has been due to the coming of immigrants from all parts of Europe, including the representatives of nearly every race of civilized men. That such mixing of racial strains, within certain limitations, is likely to result in the development of exceptional individuals will not be doubted by any student of the subject, least of all by the plant developer who has produced striking results by a corresponding mingling of divergent types. But, on the other hand, it cannot escape attention that there are limits of crossbreeding beyond which the plant developer may not advantageously go. If he attempts to combine species of plants that are too widely divergent, he either gets no result or produces inferior progeny. And if the races that are crossed lie just at the limits of affinity, he may produce a progeny, that, particularly in the second and later generations, become so variable and diversified as to run counter in the main to all of his plans and expectations. We have seen this illustrated in many cases-witness, for instance, the crossing of the tobacco and the petunia, of the European and Chinese quinces, of the oriental and opium poppies, and of the various members of the genus Rubus.
THE NEED OF SELECTION
In some of these cases, to be sure, individual forms were produced that had very exceptional interest and that might even supply material highly prized by the plant developer for the production of new races. But it must be recalled that the plant developer always has full privilege of excluding the undesirables from the hybrid fraternity. He can pick out one or two individual hybrids showing desirable qualities, and can eliminate the thousands that are unfit. As a single illustration, let us recall the Phenomenal berry, a hybrid between the California dewberry and the Cuthbert raspberry. It will be recalled that this individual plant was the only one worth preserving out of a hybrid colony of many thousand individuals. The one valuable plant was carefully preserved and nurtured. The thousands of undesirables were piled in a heap and burned. The blending of different racial strains had produced one highly prized new specific form. Granted the privilege of destroying the undesirables, the experiment was eminently worth making and the results were altogether gratifying. But what if it had been incumbent on the plant developer to preserve the thousands of undesirable hybrids? Not all of them were altogether obnoxious, to be sure. Yet a very large proportion of them combined racial traits of remote ancestors in such a way as to make them very unfit members of a colony of cultivated plants. Lacking the selecting hand of the plant developer, which could ruthlessly rout out these undesirables, the net result of the hybridizing experiment would have been to produce a vast colony of brambles far less desirable on the average than their parent forms.
AMERICA, THE MELTING POT OF NATIONS
Making the application, it becomes at least a very serious question as to whether the recent altogether unprecedented influx of immigrants of many widely divergent races-notably those that have come from the Mediterranean region and Southeastern Europe, from various provinces of Russia, and from the Far East-are not supplying material that, blended with the existing American stock, may produce results as startling and on the whole of as doubtful value as those produced among plants when widely hybridized. A certain admixture of new strains of these varied races might not be without its advantages. It has been urged that there are qualities of temperament associated with a love of music and the arts characterizing the Latin races, for example, that might advantageously be mingled with the somewhat cold and practical temperament of the American race, to give it a new quality, just as new flavors are bred into the racial strains of plums or pears or peaches. There is no gainsaying the possibility that such blending may have its advantages. But there seems danger at the moment that the matter may be overdone. When we read of the coming of as many as a million three hundred thousand aliens in a single year; and when we are told that of those that come from Southeastern Europe more than 35 percent are of such undeveloped or atavistic types that they are unable to read or write-we cannot escape a feeling of solicitude over the introduction of so high a percentage of blood of so doubtful a character into the strains of our developed colony of American races. It must be recalled that when the plant developer brings from Japan or from Europe or from Asia a new race of plants to combine with his native stock, he selects always the very best individuals that are to be found. Very commonly he breeds the newcomers for successive generations and makes repeated selections before he finds an individual suitable for his hybridizing experiment. He knows very well that if he were to choose inferior members of any stock for his experiments he would be working in the wrong direction, and could not hope to produce improved races. But the immigrants that are flooding in on us, in particular those that come from Southeastern Europe, cannot even by the most liberal interpretation be said to represent the best strains of the varied racial stocks from which they have sprung. They are in large proportion confessedly inferior representatives of their races. There is much evidence to show that they even include large numbers of defectives, who, owing to their alien tongues and habits, can with great difficulty be properly adjudged by the immigrant officials and denied admission in accordance with the laws that are intended to prevent the coming of the notoriously unfit.
THE GREATEST MIGRATION IN HISTORY
But even if it were supposed that a large majority of newcomers are really representatives of the best of their alien racial strains, their coming in such numbers would still make them objects of solicitude to the student of heredity. The American race of today has been built up along certain lines not only of physical, but of mental and moral development that have adapted it for a social and political environment that is far different from that from which many of these aliens come. Transplantation to the new environment may have a certain effect on the immigrants, just as transplantation to the soil of California has had its effect on large numbers of plants brought from the Tropics. But in one case, as in the other, such changes are, after all, only matters of minor detail. A plum tree transplanted from Japan may put out a somewhat larger growth of twigs and a somewhat larger and more highly flavored fruit than was its wont in its native habitat. But at best it remains unmistakably a Japanese plum. The modifications wrought by the environment are matters of detail; the fundamentals of heredity, built up by thousands of generations of past environments, are fixed beyond immediate change. Nor can we doubt that the same thing is true of the fundamental physical, mental, and moral traits of the alien races that make up the great army of immigrants that come to our ports in such numbers as to make their migration, in all probability, by far the largest and most rapid migration of human races that ever took place in the history of the world. The total number of immigrants that have come to America since 1880-within the compass, therefore, of a single generation-is more than twenty million. This is a number in excess of the total population of America at any census prior to 1850. Such an influx of new blood must of necessity change in very large measure the aggregate heredity of the population of America. Whatever the American race was in the middle of the nineteenth century, it is something far different today. That at least is axiomatic, regardless of our estimate as to whether the change has been an improvement or otherwise. The aggregate status of the population of the plant colonies at Santa Rosa and Sebastopol today has probably not more greatly changed from the status of the colonies of 1886 than has the average status of the American race changed in the same period. Doubtless it would be impossible for anyone to gauge accurately the precise character of the modifications in one case or the other. But in general terms it may safely be affirmed that the members of the plant colonies have vastly improved in the sense that they have been modified as to leaf or flower or fruit in such wise as to make them better adapted to meet the needs and tastes and desires of men. Whether the crossbred population of America has been similarly improved in its average adjustment to the needs of a highly evolved social environment is a question that we shall not, at the moment, attempt to decide. Here, as before, it suffices to point out the conditions and to suggest analogies with the crossbred plant colonies; but here also we must not overlook the fact that the plant developer's privilege of weeding out the unfit members of his hybrid colony may change the entire complexion of the situation.
As to all this we are taking a wide view and considering the American race as a whole. But in making the final interpretation, it will be well to glance a moment at the needs of the individual and to make application of one or two principles of heredity to individual cases. In so doing we are no longer considering the question of the mingling of different racial strains but more particularly the blending of individual traits as presented in marriages contracted by persons of the same race and even of the same community. Here we are obviously concerned with problems similar to those that confront the plant developer who is making selection among the members of an inbred colony, where his cross-pollenizings do not involve different species or varieties but only members of the same fraternity or of closely related fraternities. In such a case, it is axiomatic to say that the plant developer selects the individual plants that come nearest to his ideal, and combines them. But of course it often happens that the plant developer is looking for the production or the accentuation of some quality that may be said to constitute an abnormality; whereas the human eugenist is concerned above everything else with the accentuation of normal qualities and the elimination of the elements of abnormality. Nevertheless the experiments of the plant developer may afford a demonstration of principles of heredity that are susceptible of useful application. It is fairly demonstrated, for example, that there is no necessary deterioration brought about by the crossing of plants that are related in the degree which in the case of human beings we describe as cousinship. If the strain is normal and healthy, and in particular if the cousins have grown up under different environment, there is no inherent objection to their mating. That is to say, there is no hereditary reason why they may not produce normal and healthy offspring. The great difficulty, however is that very few families are quite free from one taint or another of disease or infirmity, and the mating of cousins brings together the hereditary factors for this defect in such combination as to accentuate them, and enhance the probability that the defect will make itself manifest in the offspring. Take, for example, the case of the wild heuchera with the crinkled leaf from which it proved feasible, by inbreeding, to develop a race of crinkled-leaved heuchera. It would have been impossible to develop and fix this race so rapidly had pains been taken to cross the plant showing the peculiarity with a normal plant, instead of crossing it with a cousin showing the same peculiarity. In quite the same way such a human abnormality as a tendency to deafness or a malady of the eyes or feeblemindedness or susceptibility to tuberculosis may be accentuated through cousin marriages and thus brought out in the progeny, where, had mating occurred with a normal strain, the tendency might have been indefinitely submerged or even eliminated.
MENDELISM AND HUMAN MALADIES
It has been shown that the phenomena of Mendelian inheritance apply to a number of abnormal conditions to which human beings are subject. It seems fairly established, for example, that normality of mind and feeblemindedness constitute a Mendelian pair of unit characters or unit groups in which normality is dominant and feeblemindedness recessive. It follows that the offspring of a normal individual and a feebleminded one may be all normal in mind; and if these individuals mate exclusively with normal individuals, the character for feeblemindedness may be permanently submerged. But, on the other hand, if the individual normal offspring of the first filial generation were to mate with other individuals having the same heritage, the recessive trait of feeblemindedness would reappear in a certain proportion of their offspring. Obviously cousin marriages give opportunity for the bringing together of such recessive traits, and hence may cause the reappearance in the offspring of undesirable or abnormal characters that might otherwise be suppressed. In the celebrated series of experiments made by Professor Biffin at Cambridge, England, it was shown that susceptibility of wheat to the fungous disease known as rust is transmitted as a factor dominant to immunity. Similarly it has been observed that immunity to attacks of the aphis shown by the roots of the Northern Spy apple is a recessive trait and hence that the seedlings from the Northern Spy may be susceptible. These illustrations, among others, show that susceptibility and immunity to disease may constitute a Mendelian pair of factors that are transmitted in a definite way. There is a growing body of evidence to show that the same thing is true with the human subject in case of susceptibility to certain microbic diseases. But it fortunately happens that in some cases, at any rate, immunity to disease appears to be dominant and susceptibility recessive, so that the offspring of an immune and a susceptible individual are all immune. This appears to be the case, for example, with susceptibility to tuberculosis. The children of an immune and a susceptible person appear to be immune, or relatively immune, to the disease. And this is obviously a fact of the very highest practical importance. But we must recall that the children who are thus individually immune contain in their germ plasm the factors for susceptibility. So such individuals should exercise the utmost precaution not to marry into families where there is a corresponding taint of susceptibility to tuberculosis, even though the individuals they select as marriage partners are themselves healthy. Here as in the other case just cited, the union of two individuals who carry the hereditary factors of susceptibility submerged in their germ plasm will result in the reappearance of susceptibility as a tangible trait in about one in four of the offspring. Stated otherwise, in more general terms, it appears that there are a good many human traits that are blended in such wise in the offspring of a given pair of parents who present the trait, as to cause the undesirable trait to be latent and thus seemingly to disappear from the fraternity; but that a segregation and recombination of the hereditary factors may occur in the next generation in such wise that the undesirable trait reappears. We have just seen that the matings of persons who carry an hereditary taint, yet who themselves are normal, will determine whether that taint will reappear in their offspring or whether the offspring will be normal. In selecting a marriage partner, then, you are selecting hereditary potentialities for your future offspring. And in selecting environing influences in the broadest sense-for the offspring from the hour of their birth, you are largely determining whether the best or the worst of those hereditary potentialities shall become realities. In a word, then, we might advantageously apply to the human plant the same general principle which we saw to be the most fundamental one guiding us in our plant experiments, saying that here, no less than in case of the actual plant, selection is the first and last word. What was said in concluding an earlier chapter with reference to plant breeding, may now advantageously be repeated with reference to the breeding of human beings: "The beginning is selection, and the end is selection."
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 12 Chapter 5