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[Read before the Missouri State Dental Association, at Sweet Springs, Mo.,

July 8, 1885.]

There is a growing conviction amongst the scientific members of the profession that the forces controlling development of the teeth, and especially the causes of defective tissual formation, must be looked for further back than the life history of the individual; that in the pre-natal influences will yet be discovered those forces dictating the variations in dental formation which are so interesting, but too often most disastrous in their effects upon the arrangement and quality of the teeth; in short, that, like other organs and tissues, the physical peculiarities and characteristics of the teeth are under the irresistible control of the omnipresent dictations of hereditary influences, be they immediate or remote. They are descended from the teeth of the ancestors of the individual in regular generations, in obedience to the law that "like produces like." Mere existence is inherited, and to be formed at all is to be formed like the being from whom organisms are descended and after whom they are copied. In view of this fact, the laws of heredity have an absorbing interest to us, and demand our study and investigation.

Briefly, then, heredity in general is that law by which every organized living being, plant, or animal is developed in the likeness— is the counterpart, more or less exact—of the parent forms which produced it. Its external form and features, its internal organs and tissues, are copied after the type of the species to which it belongs; all its distinguishing peculiarities being characteristic of that species. There is a law of type which governs the evolution of every molecule, the placing of that molecule in the building of tissues, the arrangement of those tissues in the growth and position Vol. xxvu.—33.

of organs in the structure of the individual so that it shall be a typal representative organization. That law is heredity. But the force in its entirety is not so simple as this easy definition. Within the limits of typal requirement there is such a range of variation that no two examples of any species are precisely alike. The activity of this variation brings before us such innumerable differences in the quality or quantity of structure, of external form and feature, of internal organ and tissue, that the complexity is bewildering. The labyrinths of the possibilities of variation can never be unraveled by finite man. TMieir classification would demand superhuman grasp, so that, with all the illumination the best intellects of the age have been able to throw upon the subject,—from Cuvier and Laplace to Darwin and Huxley and Haeckel, and the host of minor workers who are less known because occupied in special fields,—there still remains a mass of darkness that seems unfathomable. And of the great mass of learning already accumulated upon the subject, there are few men who can master the details of the knowledge we already have of heredity and variation in all its complexity.

Mr. Charles Darwin is an authority upon the natural history of inheritance, and in his various works describes it at length. Without detailing, we must here acknowledge our indebtedness for the substance of many of the generalizations which we will submit, and for some of the special rules given.

Man is the epitome of the experience of his ancestors. His every organ is made in that form which was most useful to them in the struggle for existence, and these, being adapted to the preservation and perpetuation of the species in his environments, he transmits to posterity; but he does not transmit the form and substance of tissues and organs unimpaired to his successors, but varies them in relation to his own experiences and the demands for readaptation to new surroundings. The individual experience of our species, as of other organized beings, is constantly exerting a pressure in one or another direction upon every organ, as it is used or disused,—beneficial or cumbersome and superfluous,—which tends to its modification. Yariations are readily created and are as readily transmitted, thus exhibiting the flexibility of organisms. Yariations generally arise for the benefit of the species, but even if injurious are as liable to be passed onwards. Yariations occur for readaptation to changing environments, and, as these are ever altering, change is ever present, and an organ is never transmitted in precisely the same form and structure in which it was received. In civilized man the evolution of the species is now under the domination of that artificial life, that retirement from nature, in which he has incased himself. So the variations at present in progress in his organization may be considered unnatural, if not detrimental, and injurious to physical perfection and natural integrity of organization. This is especially noticeable in the tendency to transmit imperfection and disease, as is so frequently noticed, amongst civilized people.

Dr. Lincoln Kay says that "two great powers or laws of nature cooperate in the perpetuation of a species. One of these laws produces difference, diversity, individuality, by which no two beings are precisely alike. The other great law produces similarity, likeness, uniformity, and it has two kingdoms or fields of action,—first, the species, in which it is supreme and only ruler; second, the individual, in whom it shares its sovereignty with the first-named law. The first is the law of diversity, the second the law of uniformity or heritage. The law of hereditary transmission is identical with the great law which preserves the immutability of species. The difference is in the scope, not in the nature, of the law. In its first field of action it transmits inevitably specific traits; in the second it transmits, not inevitably, individual traits." He concludes, "first, that from healthy and non-consanguineous ancestors proceed a posterity of which a very large proportion are born perfect, sound, and with tendencies toward healthy procreation; second, from unhealthy or consanguineous parents proceeds a posterity of which a very much less proportion are born perfect, etc. Or, to make the correlative statement,—healthy and unrelated ancestors produce a posterity of which a very small proportion are imperfect or unsound; while unhealthy or related ancestors produce a posterity of whom a much larger percentage are imperfect or diseased."

Speaking of the persistence of race as one of the wonders of heredity, M. Topinard, in his "Anthropology," says, "From inheritance emanates the law of permanence of types. In the pure race all the individuals resemble each other as regards their main features. The law of inheritance is that the son is the exact reproduction of his father and mother, but there is also a conflict of all the other elements which figure in his genealogy. In every individual, as in every generation, there are two opposite tendencies,—the one to divergence or variability of characters; the other to concentration or perpetuation of characters. The force presiding over the latter is inheritance, the property of living beings to reproduce themselves under the same forms and with the same attributes." But individual experience causes variation, as apposed to racial experience, which causes permanence. "There is a struggle between the characters; some are added, others neutralize each other, others are reciprocal and mutually assistant, others totally destroy and abort apposing characters. The most remote ancestors have their share in forming the individual as well as the immediate parents. In atavism the reappearance of characters is a matter of chance, or rather there are in the germ latent powers which are awakened into activity by favorable influences, The principal forms of inheritance are (a) continuous inheritence, when the son resembles both parents, and these their parents ; "(6) interrupted inheritance, when, without resembling either father or mother, he is like a grand-parent,—this being very noticeable in the transmission of disease, it frequently appearing in alternate generations; (c) collateral inheritance, when a child resembles an uncle or grand-uncle or aunt; (d) atavic inheritance, when the resemblance goes further back to more remote ancestors, its appearance in succeeding generations being erratic. Some children are exactly like the father, others like the mother, others again like other ancestors or relatives. * * * Examples of interrupted, collateral, and atavic inheritance are numerous among mixed races, the characters which mongrels exhibit being notable examples of the law. Thus, a mongrel of the first blood may be exactly intermediate between the two parents, or may have a predominance of the peculiarities of one race in some of the features and of the other in others. As further mixture goes on, some racial characters are apt to be retained to remote generations. * * * Two pure races will have abetter progeny than two impure races. Where one is pure and the other impure, there will be a progeny intermediate in purity."

One feature of the laws of inheritance does not seem to have received the attention which its importance demands, and that is the action of sexual alternation in the transmission of natural or abormai peculiarities,—that is, that the child usually reproduces the physical peculiarities, temperament, size, structure, disease tendencies, etc., of the parent of the opposite sex. Thus, the sons are likely to resemble the mother, and the daughters the father, in physical form and character. Or, if they do not resemble the immediate parent, the resemblance will be traceable through them to that parent's parent of the opposite sex. Thus, if the son do not resemble his mother, he will be found to be like her father, or his mother. It is interesting to trace these resemblances in regard to the teeth, and find with What exactness the law is carried out. The rule is not invariable, of course,—no rules are so,—but there is sufficient stability in its action to elevate it to the dignity of a law and to proceed upon the assumption of alternate sexual inheritance in generalizations. Atavism, which is so imperfectly understood, may be but the persistent effect of the workings of this law down through remote generations, when features appear which have been totally forgotten in a family. Thus, a child will appear in a family with red or other colored hair which is entirely different from all the rest of the members, and no one remembers when an ancestor had such a feature. Or, a peculiarity will crop out which is a distinct characteristic of a race totally foreign to that of the family, the time of such crossing having been so remote as to evade the possibilities of investigation. Thus it is that features are constantly reappearing in an apparently erratic manner, without regard to traceable inheritance; but if the history of the family could be accurately traced, there would always be found an ancestry to explain unusual characters. Mr. Wm. Sedgwick has given us some observations upon the influence of sex in hereditary disease, in which he says, "Sexual limitations, although met with in all forms of hereditary disease, is more constant and more strongly defined in those diseases affecting hereditarily the skin and its appendages than in those affecting the other organs or tissues of the body. And this should not be surprising when we consider that sex modifies the structures in man as in the lower animals,—thus, the absence of the beard in woman, the variation of the plumage in birds, the weapons of combat possessed by the male in many animals, and their absence in the female, etc. In abnormal excess or deficiency of hair abnormal development of the teeth often also appears, some of these cases occurring according to the laws of atavism. These are often limited to one sex, or appear in alternate sexes."

But let us now apply these laws to the study of the development of the teeth. We find that, being dermal organs, they are peculiarly susceptible to the influences of heredity, and also to causes of variation. ISTot only is the type of tooth usually determined by the impress of one or another of the parents, but its particular defects are the result of the parental impressions, of unfortunate temperamental, consanguineous, or diseased combinations. Of course the immediate influences are not all that is to be considered in the structure of the teeth, but, regarding their deficient formation, these are usually paramount to all the influences of remote inheritance. Special defects are usually traceable to recent inheritance.

Carpenter says: "The influence of parents upon offspring is strikingly manifested not merely in the mixture of their characters, normally displayed, but by the tendency to hereditary transmission of perverted modes of nutrition and functional activity which may have been habitual to either. Many diseases are accounted as hereditary, and perhaps others may be added to the list. The predisposition may have been congenital on the part of the parents or acquired by themselves. The intensification which almost any kind of perversion of nutrition derives from being common to both parents is most remarkably evinced by the lamentable results which too often accrue from the marriage of individuals nearly related to each other and partaking of the same 'taint.' Aside from taint, even a strong

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