The percentage having (a) relatives, including brothers, and sisters, is nearly twice as great among the deaf of consanguineous parentage as among the offspring of unrelated parents. This is not inconsistent with the Irish returns which show the average number of deaf children to a family to be so much greater where the parents were cousins, than where they were not.
The statistics of the (c) relatives, or deaf sons and daughters of the deaf, are not very full. Of the 31,334 married deaf who answered the inquiry in regard to deaf relatives, 437 or 1.4 per cent reported deaf children and 30,897 or 98.6 per cent reported no deaf children. Of the totally deaf 2.4 per cent had deaf children, and of the congenitally deaf 5.0 per cent. The percentage of deaf children varied greatly according to the number and class of deaf relatives, as shown by Table XXXIII.
The striking feature of these percentages is the regularity with which they increase in proportion as the number of deaf relatives increases, until among the 242 persons who have (a), (b) and (d) relatives, 23 or 9.5 per cent also have (c) relatives. A consanguineous marriage within a family tainted with deafness would have the same effect as doubling the number of deaf relatives, which as we have seen greatly increases the percentage having deaf children.
It would seem that the number of the married deaf reported as having deaf children is much too small, especially since Dr. Fay[89]produces statistics of 4471 marriages of the deaf of which 300 produced deaf offspring. Counting only the 3,078 marriages of which information in regard to offspring was available these figures show an average of a little less than one such marriage in ten as productive of deaf offspring. The total number of children of these marriages was 6,782, of which 588 were deaf. These 3,078 marriages represented 5,199 deaf married persons as compared with the 31,334 reported in the Twelfth Census, or about one sixth. Increasing the 300 families who had deaf children in the same ratio we have 1800 as compared with the 437 reported by the census. But as it was inevitable that Dr. Fay's cases should be selected somewhat, he has probably collected records of more than one sixth of all the cases where deaf children were born of deaf parents. But we can hardly believe that he found three-fourths of such cases. The true number therefore must be considerably greater than 437, but less than 1800.[90]
Dr. Fay found that 31 out of the 4,471 marriages of the deaf were consanguineous, but he expresses the belief that theactual number and percentage of consanguineous marriages of the deaf are larger. The following table which combines several of Dr. Fay's tables sets forth the main results of his work. In each instance one or both parties to the marriage were deaf. The totals include only those of whom information as to the offspring was available.
Obviously percentages based on these figures are of little value of themselves, especially since Dr. Fay's cases are not entirely typical, but in general this table points us to the same conclusion that we have reached by other means,namely that where a tendency toward deafness exists, a consanguineous marriage is more likely to produce deaf children than a non-consanguineous marriage. If more figures were available the percentage of deaf children would probably increase with the nearness of consanguinity and the number of deaf relatives, but with the present data a further analysis has no significance.[91]
If, then, consanguineous marriages where relatives are deaf have a greater probability of producing deaf offspring, and also a greater probability of producing plural deaf offspring, than ordinary marriages, and two thirds of the congenitally deaf offspring of consanguineous marriages do have deaf relatives, it does not seem necessary to look beyond the law of heredity for an explanation of the high percentage of the congenitally deaf who are of consanguineous parentage.
In those cases of deafness which, in the Census returns, are ascribed to specific causes, the factor of consanguinity is still noticeable, although the percentage of the non-congenitally deaf who are the offspring of cousins never exceeds 5.3 (Table XXVIII). But the influence of heredity is not removed by the elimination of the congenitally deaf. Many instances are known where successive generations in the same family have developed deafness in adult life, often at about the same age and from no apparent cause. The following case well illustrates this point. It is furnished me by a correspondent in whom I have great confidence. The facts are these: A—— aged28 married B—— aged 19, his first cousin who bore the same surname as himself. Both lived to old age and were the parents of eight children, two of whom died in infancy. My informant further states:
Having personally known very well all of the surviving six children of this family, I can truthfully state that all were unusually strong, active and vigorous people and all the parents of healthy children. A—— was troubled with deafness as long as I can remember, and this physical trait he transmitted to all of his children, though some of them did not develop the same till well along in life. C—— (the youngest son), however, began to indicate deafness quite early. No one of his four children is in the least deaf.
Having personally known very well all of the surviving six children of this family, I can truthfully state that all were unusually strong, active and vigorous people and all the parents of healthy children. A—— was troubled with deafness as long as I can remember, and this physical trait he transmitted to all of his children, though some of them did not develop the same till well along in life. C—— (the youngest son), however, began to indicate deafness quite early. No one of his four children is in the least deaf.
It will be noticed here that whereas in the case of the cousin marriage the trait was so strongly inherited, it disappeared entirely in the next generation with a non-consanguineous marriage. The inheritance of tendencies or weaknesses may be more common than the actual inheritance of defects. Dr. Bell's words on this point are suggestive:
Where a tendency toward ear trouble exists in a family, it may lie dormant and unsuspected until some serious illness attacks some member of the family, when the weak spot is revealed and deafness is produced. We are not all built like that wonderful one-horse shay that was so perfectly made in all its parts that when at last it broke down it crumbled into dust. When an accident occurs it is the weak spot that gives way, and it would be incorrect to attribute the damage to the accident alone and ignore the weakness of the part; both undoubtedly are contributing causes.In the case, then, of a deaf person who has deaf relatives, the assigned cause of deafness may not be the only cause involved, or indeed the true cause at all. It may be the causesimply in the same sense that the pulling of a trigger is the cause of the expulsion of a bullet from a rifle, or a spark the cause of the explosion of a gunpowder magazine; hereditary influences may be involved.[92]
Where a tendency toward ear trouble exists in a family, it may lie dormant and unsuspected until some serious illness attacks some member of the family, when the weak spot is revealed and deafness is produced. We are not all built like that wonderful one-horse shay that was so perfectly made in all its parts that when at last it broke down it crumbled into dust. When an accident occurs it is the weak spot that gives way, and it would be incorrect to attribute the damage to the accident alone and ignore the weakness of the part; both undoubtedly are contributing causes.
In the case, then, of a deaf person who has deaf relatives, the assigned cause of deafness may not be the only cause involved, or indeed the true cause at all. It may be the causesimply in the same sense that the pulling of a trigger is the cause of the expulsion of a bullet from a rifle, or a spark the cause of the explosion of a gunpowder magazine; hereditary influences may be involved.[92]
It is thus possible to account for the large proportion of deafness among persons of consanguineous parentage by the simple action of the laws of heredity. Why then should we go out of our way to look for a cause of the defect in consanguinity itself? When two explanations are possible, the simpler explanation is the more probable, other factors being equal; but in the present problem the factors are not equal, for the evidence points strongly toward the simpler hypothesis of intensified heredity, while there is little or no evidence that consanguinity is a causeper se.
As to the probability then of a consanguineous marriage producing deaf offspring, it will readily be seen to be very slight, and in those cases where there is actually no trace of hereditary deafness in the family, perhaps no greater than in non-related marriages. While the census figures in regard to the deaf are not complete they probably include a great majority of the deaf in the United States. The 89,287 deaf would mean an average of 12 deaf persons to every 10,000 inhabitants and the 14,472 congenitally deaf, 2 persons to every 10,000. Assuming then, as before[93]that 1,000,000 persons in continental United States are the offspring of consanguineous marriages within the limits of the term "cousins" as used in the Census report, 41 out of every 10,000 persons of consanguineous parentage would be deaf, and 17 congenitally so. Thus less than one half of one per cent of the offspring of consanguineous marriagesin the United States are deaf, and only one sixth of one per cent are deaf-mutes in the commonly accepted sense of the term.
It is interesting here to quote an opinion given by Dr. Bell in 1891, as to the probable results of the consanguineous marriage of deaf persons.[94]
1. A deaf person, not born deaf, who has no deaf relatives, will probably not increase his liability to have deaf offspring by marrying a blood relative.2. A deaf person, born deaf, who has no deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative.3. A deaf person, whether born deaf or not, who has deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative, especially if that relative should happen to be on the deaf side of the family. For example: If his father has deaf relatives and his mother has none, he will be more likely to have deaf offspring if he marries a relative of his father than if he marries a relative of his mother.The laws of heredity seem to indicate that a consanguineous marriage increases or intensifies in the offspring whatever peculiarities exist in the family. If a family is characterized by the large proportion of persons who enjoy good health and live to old age with unimpaired faculties, then a consanguineous marriage in such a family would probably be beneficial, by increasing and intensifying these desirable characteristics in the offspring. On the other hand, if a large proportion of the members of a family betray weakness of constitution—for example: if many of the children die in infancy, and a large proportion of the others suffer from ill health, only a few living to old age with unimpaired faculties—then a consanguineous marriage in such a family would probably be hurtfulto the offspring. A large proportion of the children would probably die in infancy, and the survivors be subject to some form of constitutional weakness.As there are few families entirely free from constitutional defects of some kind, a prudent person would do well to avoid consanguineous marriage in any case—not necessarily on account of deafness, but on account of the danger of weakening the constitution of the offspring. Remoteness of blood is eminently favorable to the production of vigorous offspring, and those deaf persons who have many deaf relatives would greatly diminish their liability to have deaf offspring by marrying persons very remote in blood from themselves.Children, I think, tend to revert to the type of the common ancestors of their parents. If the nearest common ancestors are very far back in the line of ancestry, the children tend to revert to the common type of the race. Deafness and other defects would be most likely to disappear from a family by marriage with a person of different nationality. English, Irish, Scotch, German, Scandinavian and Russian blood seems to mingle beneficially with the Anglo-Saxon American, apparently producing increased vigor in the offspring.
1. A deaf person, not born deaf, who has no deaf relatives, will probably not increase his liability to have deaf offspring by marrying a blood relative.
2. A deaf person, born deaf, who has no deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative.
3. A deaf person, whether born deaf or not, who has deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative, especially if that relative should happen to be on the deaf side of the family. For example: If his father has deaf relatives and his mother has none, he will be more likely to have deaf offspring if he marries a relative of his father than if he marries a relative of his mother.
The laws of heredity seem to indicate that a consanguineous marriage increases or intensifies in the offspring whatever peculiarities exist in the family. If a family is characterized by the large proportion of persons who enjoy good health and live to old age with unimpaired faculties, then a consanguineous marriage in such a family would probably be beneficial, by increasing and intensifying these desirable characteristics in the offspring. On the other hand, if a large proportion of the members of a family betray weakness of constitution—for example: if many of the children die in infancy, and a large proportion of the others suffer from ill health, only a few living to old age with unimpaired faculties—then a consanguineous marriage in such a family would probably be hurtfulto the offspring. A large proportion of the children would probably die in infancy, and the survivors be subject to some form of constitutional weakness.
As there are few families entirely free from constitutional defects of some kind, a prudent person would do well to avoid consanguineous marriage in any case—not necessarily on account of deafness, but on account of the danger of weakening the constitution of the offspring. Remoteness of blood is eminently favorable to the production of vigorous offspring, and those deaf persons who have many deaf relatives would greatly diminish their liability to have deaf offspring by marrying persons very remote in blood from themselves.
Children, I think, tend to revert to the type of the common ancestors of their parents. If the nearest common ancestors are very far back in the line of ancestry, the children tend to revert to the common type of the race. Deafness and other defects would be most likely to disappear from a family by marriage with a person of different nationality. English, Irish, Scotch, German, Scandinavian and Russian blood seems to mingle beneficially with the Anglo-Saxon American, apparently producing increased vigor in the offspring.
Having thus considered the more important problems which have been connected with the marriage of near kin, we have only to discuss the bearing of the conclusions thus formed upon the social aggregate, and the effect which consanguineous marriages have upon the evolution and improvement of the human species.
It has been shown that the frequency with which consanguineous marriages occur varies greatly with the physical and social environments; that such marriages are more frequent in isolated and in rural communities than in cities; and that with the increasing range of individual activity and acquaintance the relative frequency of consanguineous marriage is decreasing.
Consanguinity in the parents has no perceptible influence upon the number of children or upon their masculinity, and has little, if any, direct effect upon the physical or mental condition of the offspring.
The most important physiological effect of consanguineous marriage is to intensify any or all inheritable family characteristics or peculiarities by double inheritance. The degree of intensification probably varies with the nature of the characteristic; degenerate conditions of the mind, and of the delicate organs of special sense being the most strongly intensified.
It is probable also that in the absence of degenerativetendencies the higher qualities of mind and body are similarly intensified by marriage between highly endowed members of the same family. Dr. Reibmayr believes that inbreeding is necessary to the higher evolution of the race: "A settled abode, natural protection from race mixture and the development of a closely inbred social class are the basic conditions of every culture period." But inbreeding must not be carried too far: "In the course of generations the ruling class begins to degenerate mentally and physically, until not only is the class destroyed, but for lack of capable leadership the people (Volk) itself is subjugated and a crossing of blood again takes place."[95]
In the breeding of animals the closest inbreeding is frequently resorted to in order to improve the stock, and many examples can be given of the closest possible inbreeding for generations without apparent detriment, but it is universally admitted that the animals selected for such inbreeding must be sound constitutionally, and free from disease. After a certain number of generations however, degeneration apparently sets in. The number of generations through which inbreeding may be carried varies with the species, and the purpose for which the animals are bred. Where they are bred primarily for their flesh, as for beef, mutton or pork, it can be pursued farther and closer than where they are bred for achievement in which a special strength is required—for instance in the breeding of race horses. This would indicate that the more delicate brain and nervous system is sooner affected than the lower bodily functions.
In man, however, freedom from hereditary taint cannot so easily be secured. Individuals cannot be selected scientifically for breeding purposes. Furthermore, the human body is more delicately constructed than that of the loweranimals, and the nervous system is more highly developed and specialized, so that it is reasonable to suppose that in man degeneration would set in earlier in the process of inbreeding, and that it would be impossible to breed as closely as with the lower animals. Instances are well known, however, where incestuous unions have been productive of healthy offspring, and successive generations of offspring of incestuous connection are not unknown; but, although statistics are lacking, it seems to be very often true that children of such unions are degenerate. It may be that the reason for this is that with the laws and social sentiments now prevailing in all civilized communities, only degenerates ever contract incestuous alliances. Desirable as it may be from a social point of view that this strong sentiment against incest should continue, it is not yetproventhat even the closest blood relationship between the parents is directly injurious to the offspring. The "instinctive horror of incest" is a myth, for although a horror of incest does very properly exist in civilized, and in some tribal societies, it is purely a matter of custom and education, and not at all a universal law.
Double heredity may account for all the observed ill effects of consanguineous marriage, including the high youthful death-rate, the higher percentage of idiocy, deafness and blindness, and probably also the scrofulous and other degenerate tendencies; nevertheless, there may be in some instances a lowering of vitality which this hypothesis does not fully explain.
The tendency of inbreeding in animals, it is well known, is to fix the type, the tendency of crossing, to variation. Inbreeding then, tends to become simple repetition with no natural variations in any direction, a stagnation which in itself would indicate a comparatively low vitality. Variation and consequent selection is necessary to progress."Sex," according to Ward[96]"is a device for keeping up a difference of potential," and its object is not primarily reproduction, but variation.[97]
It is organic differentiation, higher life, progress, evolution.... But difference of potential is a social as well as a physiological and physical principle, and perhaps we shall find the easiest transition from the physiological to the social in viewing the deteriorating effects of close inbreeding from the standpoint of the environment instead of from that of the organism. A long-continued uniform environment is more deteriorating than similarity of blood. Persons who remain for their whole lives, and their descendants after them, in the same spot, surrounded by precisely the same conditions, and intermarry with others doing the same, and who continue this for a series of generations, deteriorate mentally at least, and probably also physically, although there may not be any mixing of blood. Their whole lives, physical, mental, and moral, become fixed and monotonous, and the partners chosen for continuing the race have nothing new to add to each other's stock. There is no variation of the social monotony, and the result is socially the same as close consanguineal interbreeding. On the other hand, a case in which a man should, without knowing it, marry his own sister, after they had been long separated and living under widely different skies, would probably entail no special deterioration, and their different conditions of life would have produced practically the same effect as if they were not related.[98]
It is organic differentiation, higher life, progress, evolution.... But difference of potential is a social as well as a physiological and physical principle, and perhaps we shall find the easiest transition from the physiological to the social in viewing the deteriorating effects of close inbreeding from the standpoint of the environment instead of from that of the organism. A long-continued uniform environment is more deteriorating than similarity of blood. Persons who remain for their whole lives, and their descendants after them, in the same spot, surrounded by precisely the same conditions, and intermarry with others doing the same, and who continue this for a series of generations, deteriorate mentally at least, and probably also physically, although there may not be any mixing of blood. Their whole lives, physical, mental, and moral, become fixed and monotonous, and the partners chosen for continuing the race have nothing new to add to each other's stock. There is no variation of the social monotony, and the result is socially the same as close consanguineal interbreeding. On the other hand, a case in which a man should, without knowing it, marry his own sister, after they had been long separated and living under widely different skies, would probably entail no special deterioration, and their different conditions of life would have produced practically the same effect as if they were not related.[98]
Professor Ward's idea of "difference of potential," or contrast, as essential to the highest vigor of the race as well as tothat of the individual offspring, offers an alternative explanation of the observed results of consanguineous marriages, and one which does not necessarily conflict with the explanation already given. All the phenomena of intensification are simply due to a resemblance between husband and wife in particular characteristics, such as a common tendency toward deafness or toward mental weakness. This resemblance, which may or may not be the result of a common descent, renders more probable the appearance of the trait in the offspring. If the parents closely resembled each other in many respects they would be more likely to "breed true" and the children would resemble one another in their inherited traits, thus accounting for the high average of deaf-mutes to the family, observed in the Irish statistics.[99]
The theory of contrast and resemblance supplements that of intensified heredity where the resemblance is general, rather than in particular traits or characteristics. In such a case the absence of the stimulating effects of contrast might result in a lowering of vitality, which in turn would react upon the youthful death-rate.
Where then related persons differ greatly in mental and physical traits, and generally speaking, belong to different types, it is very improbable that there would be any ill effects resulting from the mere fact of consanguinity. A case in point is furnished me by a correspondent. A first cousin marriage which turned out exceedingly well was between strongly contrasted individuals; the husband was "short, stocky and dark complexioned" while the wife was "tall, slight of figure, and of exceedingly light complexion." In other cases in which the results were not so good the husband and wife bore a close resemblance to one another, physically and mentally.
This, however, does not agree with the results obtained by Professor Karl Pearson. Basing his conclusions on the correlation of stature between husband and wife, he believes that homogamy is a factor of fertility. Taking 205 marriages from Mr. Francis Galton'sFamily Records, Professor Pearson found the correlation between husband and wife to be .0931 ± .0467, while weighted by their fertility the correlation was .1783 ± .0210, practically doubling the intensity of assortative mating.[100]The value of these correlations, however, is impaired, as he says, by the insufficient number of observations, and by the fact that absolutely taller mothers are the more fertile.
In a subsequent investigation of from 1000 to 1050 pairs of parents of adult children, Professor Pearson found the correlation in stature to be .2804 ± .0189; of span .1989 ± .0204; and of forearm .1977 ± .0205; with cross coefficients varying from .1403 to .2023. If, as he believes, "The parents of adult children are on the average more alike than first cousins, then it follows that any evils which may flow from first cousin marriage depend not on likeness of characters, but on sameness of stock."[101]
But even if it were true, as is very improbable, that parents of adult children are more alike than first cousins, it would still be likely to follow that first cousins who married would be more alike than first cousins in general. A certain degree of resemblance is undoubtedly necessary to complete fertility: husband and wife must be physically compatible, and must both enjoy a certain degree of health and physical strength. These facts are admitted by all, but it does not follow that resemblance beyond a certain point is not in itself detrimental.
Professor Pearson's own experiments in this line, however, do not give consistent results, for in correlating eyecolor with fertility, heterogamy seems to increase fertility. The highest average fertility (4.57) is in those cases where the father is dark-eyed and the mother light-eyed, while the lowest is where both parents have blue-green or gray eyes.[102]
In a recent study an attempt has been made to measure the coefficient of correlation between cousins.[103]In the characteristics of health, success, temper and intelligence the coefficients ranged between .25 and .30. These values differ but little from those found to obtain for the resemblance between avuncular relatives for eye color (.265), or between grandparent and grandchild for the same characteristic (.3164).[104]Positive results were also found, with one doubtful exception, for the occurrence of insanity and tuberculosis in cousins. The writer concludes: "The grandparent, the uncle and aunt, and the cousin are on practically the same footing with regard to relationship or intensity of kinship as measured by degree of likeness of character; and it seems probable that any scientific marriage enactments would equally allow or equally forbid marriage between grandparent and grandchild, uncle and niece, aunt and nephew, and between first cousins."[105]
As we should expect the resemblance between near relatives has been found to be much greater. From a measurement of from 4000 to 4886 pairs, the average correlation of the characteristics of stature, span, forearm length andeyecolor between parent and child was .4695. By similar computations and measuring the same characteristics, the fraternal correlation was found to be .508.[106]From measurements of a greater variety of characteristics in school children the mean fraternal correlation was .539.[107]In athletic power the coefficient was still higher, .72 between brothers, .75 between sisters and .49 between brothers and sisters. Measurements of mental characteristics—vivacity, assertiveness, introspection, popularity, conscientiousness, temper, ability and handwriting proved to be as easily correlated, the mean coefficients being; brothers, .52, sisters. .51, brothers and sisters .52.[108]
The relative amount of degeneracy and disease among the offspring of consanguineous marriages has been enormously exaggerated, and the danger is by no means as great as is popularly supposed. Nevertheless, since it is undoubtedly true that on the average such marriages do not produce quite as healthy offspring as do non-consanguineous unions, and since public sentiment is already opposed to the marriage of cousins, it is perhaps just as well that existing laws on the subject should remain in force. From the standpoint of eugenics however, it is much more important that the marriage of persons affected with hereditary disease should be prevented. Dr. Bell has pointed out the danger of producing a deaf-mute race by the intermarriage of congenitally deaf persons,[109]and this warningshould be made to apply to other congenital defects as well. Some states already prohibit the marriage of the mentally defective, and persons under the influence of intoxicants. Such provisions are wise, and are the most practical means of achieving eugenic ideals—by preventing the propagation of the unfit. The interests of society demand that the mentally and physically defective should not propagate their kind.
From the broader viewpoint of social evolution the problems of inbreeding or crossing of stocks merge into the discussion of the endogamous and exogamous types of society. Whatever may have been the origin of exogamy, the survival of the exogamous type in progressive societies may easily be explained on the ground of superior adaptability, variability and plasticity, which enables such societies to survive a change of environment while the more rigid structure of the endogamous clan brings about its extermination.
Inbreeding leads to caste formation and a rigid and stratified social structure, which is in the end self-destructive, and cannot survive a change of environment. The governing caste may, as Reibmayr says, favor the growth of culture, but it is usually the culture of that caste, and not of the people at large. The ruling caste is usually the result of selection of the strongest and ablest, but after it becomes a caste, the individuals are selected on account of hereditary social position and not primarily on account of ability. Now biological experiments show that although artificial selection may be carried to a point where animals will breed true to a characteristic to within 90 per cent, yet if selection is stopped, and the descendants of the selected individuals are allowed to breed freely among themselves, they will in a very few generations revert to the original type. This is what happens in a social caste, unless, as in the case of the Englisharistocracy, it is continually renewed by selection of the ablest of the other classes.
The superposition and crossing of cultures, the development of secondary civilization, is necessary to social evolution in its broadest sense, and this usually involves crossing of blood as well as crossing of cultures. As a result of the unprecedented migrations of the last half-century we have in the United States the greatest variety of social types ever brought so closely together. An opportunity is offered either for the perpetuation of each racial type by inbreeding, with the prospect of an indefinite stratification of society, or for the amalgamation of all cultural and racial elements into a homogeneous whole, and the development of a race more versatile and adaptable than any the world has yet known. The general tendency will undoubtedly be toward amalgamation, but there are decided tendencies in the other direction, as for instance in the "first families of Virginia," and in that large element of the New England population which prides itself upon its exclusively Puritan ancestry, and which has inherited from its progenitors that intolerance which characterized the early settlers of New England more than the pioneers of the other colonies. The dynamic forces of modern civilization are, however, opposed to caste—the West has long ago obliterated the distinction between the Pennsylvania German and the Puritan, the Scotch-Irish and the Knickerbocker Dutch. These same dynamic forces, which have prevented the formation of caste have at the same time been diminishing the percentage of consanguineous marriage and will undoubtedly continue to operate in the same way for some time to come. And when rational laws prohibit the marriage of the diseased and the degenerate, the problem of consanguineous marriage will cease to be of vital importance.
Barr, M.W.Mental Defectives.Philadelphia, 1904.
Bell, A.G.Marriage—An Address to the Deaf.Second Edition. Washington, 1891.
Memoir upon the Formation of a Deaf Variety of the Human Race.Washington, 1884.
Bemiss, S.M. "Report on the Influence of Marriages of Consanguinity upon Offspring." InTransactions of the American Medical Association, vol. xi, 1858, pp. 319-425.
Boudin, J.C.M.F.J. "Du Croisement des families, des races et de espèces." InMemoires de la Société d'Anthropologie, vol. i. Paris, 1860-1863.
Child, G.W. "On Marriages of Consanguinity."In Medico-Chirurgical Review, Apr. 1862, pp. 461-471. London.
Dally, E. "Rapport sur les mariages consanguins." InBulletins de la Société d'Anthropologie. Paris, 1877.
"Recherches sur les mariages consanguins et sur les races pures." InBulletins de la Société d'Anthropologie.Paris, 1863.
Darwin, G.H. "Marriages of First Cousins in England and Their Effects." InJournal of the Statistical Society of London, vol. 38. London, 1875.
Dean, L.W.The Influence of Consanguinity on the Organs of Special Sense. Chicago, 1903.
De Lapouge, G.V.Les Selections Sociales.Paris, 1896.
Dugdale, R.L.Jukes: a Study in Crime, Pauperism, Disease and Heredity.New York, 1877.
Duesing, C. "Das Geschlechtsverhaeltnis der Geburten in Preussen." InStaatswissenschaftliche Studien, vol. iii, pp. 373-454. Jena, 1890.
Die Regulierung des Geschlechtsverhaeltnisses bei der Vermehrung der Menschen, Tiere und Pflanzen.Jena, 1884.
Fay, E.A.Marriages of the Deaf in America.Washington, Volta Bureau, 1898.
Feer, E.Der Einfluss der Blutsverwandschaft der Eltern auf die Kinder.Berlin, 1907.
Goehlert.Ueber die Vererbung der Haarfarben bei dén Pferden.Quoted by Duesing and Westermarck.
Howard, G.E.History of Matrimonial Institutions.Chicago, 1904.
Huth, A.H. "Consanguineous Marriage and Deaf-mutism." InThe Lancet. London, Feb. 10, 1900.
Marriage of Near Kin.Second Edition. London, 1887.
Ireland, Census of. General Reports for 1861, 1871, 1881, 1891, and 1901.
Joerger. "Die Familie Zero." Reviewed by Gertrude C. Davenport in theAmerican Journal of Sociology.Chicago, Nov. 1907.
Lewis, C.J. and J.N.Natality and Fecundity.London, 1906.
Luckock, H.M.History of Marriage.London, 1894.
McCulloch, O.C.Tribe of Ishmael: a Study in Social Degradation.Indianapolis, 1888.
Maine, H.S.Early Law and Custom.London, 1883.
Mayet, P.Verwandtenehe und Statistik.Berlin, 1903. Quoted by Feer.
Mayo-Smith, R.Statistics and Sociology.New York, 1895.
Montegazza, P.Studj sui Matrimonj Consanguinei.Milan, 1868. Quoted by Darwin.
Mulhall, M.G.Dictionary of Statistics.London, 1892.
New York,Names of Persons for whom Marriage Licenses were Issued by the Secretary of the Province of New York Previous to 1784.Albany, 1860.
Pastor, L.M. "De los Matrimonies entre Parientes." InMemorias de la Real Academia de Ciencias Morales y Políticas, vol. ii, pp. 369-400. Madrid, 1867.
Pearson, K. "Law of Ancestral Heredity." InBiometrika, vol. ii, pp. 211-228. Cambridge (Eng.), 1903.
"Mathematical Contributions to the Theory of Evolution—On the Law of Reversion." InProceedings of the Royal Society, vol. 66, pp. 140-164. London, 1900.
The Grammar of Science.London, 1900.
"On theLaws of Inheritance in Man. Part II." InBiometrika, vol. iii, pp. 131-190. Cambridge, 1904.
Pearson, K., and A. Lee. "Mathematical Contributions to the Theory of Evolution. VIII." InPhilosophical Transactions of the Royal Society, vol. 195 A, pp. 79-150. London, 1901.
"On the Laws of Inheritance in Man." InBiometrika, vol. ii, pp. 357-462. Cambridge, 1903.
Raseri, E. "Sur le nombre des consanguins dans un groupe de population." InArchives italiennes de Biologie, vol. 33, pp. 230-241. 1900.
Reibmayr, A.Insucht und Vermisching beim Menschen.Leipzig, 1897.
Schuster, E. "On Hereditary Deafness." InBiometrika, vol. iv, pp. 465-482. Cambridge, 1906.
Shattuck, L.Memorials of the Descendants of William Shattuck.Boston, 1855.
Thomas, W.I.Sex and Society.Chicago, 1907.
Uchermann.Les Sourds-muets en Norvège.Christiana, 1901. Quoted by Feer.
United States, Twelfth Census.Special Report on the Blind and the Deaf. Washington, 1906.Vital Statistics. Part I.
Ward, L.F.Pure Sociology.Second Edition. New York, 1907.
Webster, N. "Explanation of the Reezons why Marriage iz Prohibited between Natural Relations." InCollection of Essays and Fugitiv Writings on Moral, Historical, Political and Religious Subjects.Boston, 1790.
Westermarck, E.History of Human Marriage.London, 1894.
Woods, F.A.Mental and Moral Heredity in Royalty.New York, 1906.