The Danish physician, Dr. Mygge, published in 1879 a book on ‛Marriage between Blood-Relations,’ which unfortunately has received much less attention than it deserves.2009 Thanks to the trustworthiness of the method, the number of cases considered, and the author’s impartiality, it is probably the most important statistical contribution hitherto issued on this subject. Dr. Mygge found, from the information he received from various parts of Denmark, that in that country, or at least in the parishes of it which came under his observation, there occur, among the children of related persons, comparatively more idiots, lunatics, epileptics, and deaf-mutes than among others. He considers it probable, too, though not proved, that such children die in a higher ratio and are more liable to certain diseases. But, on the other hand, he did not notice any perceptible difference in fertility between consanguineous and crossed marriages.2010

In these inquiries, Dr. Mygge followed the method applied by the Norwegian physician Ludvig Dahl twenty years earlier. Through careful investigation of 246 marriages, eighty-five of which were between first cousins and four between still nearer relations, this inquirer was led to the conclusions that consanguineous marriages are somewhat less fertile than crossed marriages; that they produce comparatively many more still-born and sickly children; and that insanity, idiocy, deaf-dumbness, and epilepsy occur about eleven times as often among the offspring of relations, as among the offspring of unrelated parents. But he admitted that the numbers compared were too small to make his conclusions decisive.2011

These results are of course to a great extent conjectural. But it is noteworthy that, of all the writers who have discussed the subject, the majority, and certainly not the least able of them, have expressed their belief in marriages between first cousins being more or less unfavourable to the offspring.2012 And no evidence which can stand the test of scientific investigation has hitherto been adduced against this view.

Some writers have, indeed, cited instances of communities where consanguineous marriages have occurred constantly without any evil effects having appeared. Thus the Pitcairn Island, uninhabited till the year 1790, was at that time peopled by nine white men, and six men and twelve women of Tahiti. In 1800 the population consisted of one man, five women, and nineteen children; and the descendants of these persons are stated by later travellers to be strong and healthy without any traces of degeneration. Omitting whatever else may be said against this case as evidence for the harmlessness of consanguineous marriages, I need only call attention to the facts that, since the colonization of this island, a few strangers have joined the little colony; that it was once removed to Norfolk Island, and that, of those who returned, one was a Norfolk Islander who had married a Pitcairn girl; that the island has frequently been visited by ships with their crews;2013 and that, as Beechey expressly states, the same restrictions with regard to intermarriage of relations exist here as in England.2014

There are several isolated communities—in Java, Peru, Great Britain, France, Scandinavia, &c.—which intermarry solely among themselves without any evil effects being discernible. An often-quoted case is the community of Batz (3,300 persons), situated near Croisic on a peninsula. The inhabitants of this community have been in the habit of closely intermarrying among themselves from time immemorial. Nevertheless, they are almost all very well in health without any hereditary affection. But Dr. Voisin observes, “Les conditions climatériques de la commune de Batz, son voisinage de la mer, l’hygiène et les habitudes de ses habitants, semblent s’accorder pour empêcher la dégénérescence de l’espèce et paraissent expliquer l’innocuité des mariages entre consanguins qui s’y pratiquent depuis plusieurs siècles.”2015 In other isolated communities the population is not so numerous, and the sanitary conditions are not perhaps so favourable: but in any case we may say that this local endogamy is generally something quite different from marriage with near relations. Dr. Mitchell found that, in almost all the isolated communities along the coasts of Scotland, which had been given as instances of close interbreeding, such marriages were comparatively rare. According to Dr. Mygge, the like is true of the population of Lyø and Strynø in Denmark.2016 And Dr. Andrew Wood states, of the fisher-folk of Newhaven, that, though they keep themselves much segregated, they are very careful regarding intermarriage, and look upon the union of relatives as an infringement of the laws of morality.2017

Moreover, even if it could be proved that, in particular cases, close intermarrying, though continued for a long time, has been followed by no bad consequences, this would be no evidence that consanguineous marriages are as a rule innocuous. In some parishes of Denmark Dr. Mygge found no evil effects of such marriages, whilst in others they were very conspicuous.2018 And from the investigations of Mr. Darwin it appears that, notwithstanding the injury which most plants suffer from self-fertilization, a few have almost certainly been propagated in a state of nature for thousands of generations without having been once intercrossed. It is impossible to understand, he says, why some individuals even of the same species are sterile, whilst others are quite fertile, with their own pollen.2019

There is evidence that the bad consequences of self-fertilization and close interbreeding may almost fail to appear under favourable conditions of life. In-and-in bred plants, when allowed enough space and good soil, frequently show little or no deterioration: whereas, when placed in competition with another plant, they often perish or are much stunted.2020 Crampe’s experiments with brown rats proved that the breeding in-and-in was much less injurious, if the offspring of the related parents were well fed and taken care of, than if it was otherwise.2021 And this is in striking accordance with Dr. Mitchell’s observations as to consanguineous marriages in Scotland. The results there appear to be least grave, and are frequently almost nil, if the parents and children live in tolerable comfort, without anxiety or much thought for the morrow, and easily earning enough to procure good food and clothing—in short, when they work, but do not struggle for existence. On the other hand, when they are “poor, pinched for food, scrimp of clothing, badly housed, and exposed to misery; when they have to toil and struggle for the bare necessaries of life—never having enough for to-day and being always fearful of to-morrow,”—the evil may become very marked.2022

If this is the case, we must expect to find that consanguineous marriages are much more injurious in savage regions, where the struggle for existence is often very severe, than they have proved to be in civilized society, especially as it is among the well-off classes that such marriages occur most frequently.2023 In England, according to Mr. G. H. Darwin, cousin-marriages among the aristocracy are probably 4½ per cent.; among the middle and upper middle class, or among the landed gentry, 3½ per cent.; but in London, comprising all classes, they are probably only 1½ per cent.2024 He thinks that the slightness of the evils which he found to result from first-cousin marriages perhaps depends upon the fact that a large majority of Englishmen live under what are on the whole very favourable circumstances.2025 We must also, however, remember that there has been a great mixture of races in Europe, and that this necessarily makes marriage of kinsfolk less injurious, so far as the evil results of such unions depend upon too great a likeness between the sexual elements.

The conclusion that closely related marriages produce more destructive effects among savage than civilized peoples, derives perhaps, some additional probability from certain ethnological facts. These facts may, at least, serve to show that such marriages, and the experience of isolated communities, are not everywhere in favour of Mr. Huth’s conclusions. Several statements on the subject have, indeed, scarcely any value as direct evidence for the harmfulness of consanguineous marriages, but to two or three considerable weight must be attached.

According to v. Martius, who is a great authority on Brazilian ethnography, it is a well-established fact, observed everywhere, that the smaller and more isolated of the Indian communities, scarcely any members of which marry members of other communities, are much more liable to every kind of deterioration than the larger groups.2026 “It is probable,” Mr. Bates, another most capable judge, remarks with reference to the savage tribes on the Upper Amazons, “that the strange inflexibility of the Indian organization, both bodily and mental, is owing to the isolation in which each small tribe has lived, and to the narrow round of life and thought, and close intermarriages for countless generations, which are the necessary results. Their fecundity is of a low degree, for it is very rare to find an Indian family having so many as four children, and we have seen how great is their liability to sickness and death on removal from place to place.”2027 Touching the Isánna Indians, Mr. Wallace asserts that they are said not to be nearly so numerous, nor to increase so rapidly, as the Uaupés; which may perhaps be owing to their marrying with relations, while the latter prefer strangers.2028 And v. Tschudi supposes that the low fecundity of the Botocudos is caused by their endogamous habits; for when their women marry out of their own horde, especially with whites or negroes, they are generally very fertile.2029

The Calidonian Indians of the Isthmus of Darien, according to Mr. Gisborne, are bound never to cross the breed with foreigners; hence intermarriage is very constant, and, as he remarks, the race degenerates.2030 The Pueblos in New Mexico, too, are said to deteriorate because of their constant intermarriage in the same village.2031 As regards the Hottentots, Barrow remarks, “The impolitic custom of hording together in families, and of not marrying out of their own kraals, has no doubt tended to enervate this race of men, and reduced them to their present degenerated condition, which is that of a languid, listless phlegmatic people, in whom the prolific powers of nature seem to be almost exhausted.” Few of the women have more than two or three children, and many of them are barren. But this is not the case when a Hottentot woman is connected with a white man. “The fruit of such an alliance,” says Barrow, “is not only in general numerous, but they are beings of a very different nature from the Hottentot.”2032

In too early marriages, the licentious habits of both sexes, and the intermarriage of near relatives, the Rev. J. Sibree finds the causes of the infertility of the women of Madagascar.2033 Among the Garos, the chiefs have, in comparison with the lower classes, degenerated physically, and Colonel Dalton is inclined to think that this degeneration is a result of close interbreeding.2034 The Lundu Sea Dyaks, according to Sir Spenser St. John, have decreased greatly in numbers—from a thousand families to ten. “They complain bitterly,” he says, “that they have no families, that their women are not fertile; indeed, there were but three or four children in the whole place. The men were fine-looking and the women well-favoured and healthy—remarkably clean and free from disease. We could only account for their decreasing numbers by their constant intermarriages.”2035 Mr. Foreman thinks that the low intellect and mental debility perceptible in many families among the domesticated natives of the Philippines are due to consanguineous marriages.2036 Mr. Bachelor connects the rapid decrease of the Ainos with their endogamous habits.2037 And Mr. Meade remarks, with regard to the Maoris, that one of the principal causes of the diminishing population is said to be their intermarriages, which cause barrenness among the women.2038

Of no little interest to us are the Todas of the Neilgherry Hills. Mr. Marshall remarks that, among them, relationship is intimate far beyond that witnessed in any country approaching civilization—“intimate to such a degree, that the whole tribe, where not parents and children, brothers and sisters, are all first cousins, descended from lines of first cousins prolonged for centuries.”2039 As regards the general appearance of the people, a large proportion of both sexes and of all ages are doubtless in excellent health, and their fecundity, according to Dr. Shortt, is by no means of a low degree.2040 Nevertheless, the Todas are dying out. In infancy the mortality is so great that, as a rule, there is in each family only a small number of children.2041 “It is rarely that there are more than two or three children,” says the missionary Metz, “and it is not at all an uncommon thing to find only a single child, while many families have none at all.” The numbers of the Todas have, consequently, for years past been gradually declining, and probably the time is not far distant when they will have passed away.2042 Of course, we do not know whether this depends upon their close intermarriages, but there is, at any rate, some reason to suspect that this is the case. That the intermarrying has not produced more evil effects on the population, may possibly be owing to the wealth for which the Neilgherry Hills are remarkable, and to their climate, which, for mildly invigorating properties and equable seasonal changes throughout the year, is perhaps unrivalled anywhere within the tropics.2043

Another very much in-and-in bred people are the Persians. Among them, husband and wife are generally of the same family, and very often cousins. Yet Dr. Polak who has lived in Persia for nine years, partly as a teacher in the medical school of Teheran, partly as physician to the Shah, and during this residence has had excellent opportunities of acquainting himself with the conditions of the people, has not observed that the diseases which are supposed to result from consanguineous marriages prevail more frequently there than elsewhere. Nor has he found that the Persian women are generally less fertile than others. Yet the families are exceedingly small, as the mortality among children is enormous. Of six, perhaps two as a rule survive, but very often none at all, most of them dying in their second year. Dr. Polak believes, indeed, that, on an average, scarcely more than one living child comes to each woman. A princess in Teheran was looked upon quite as a wonder because she had eight children alive, and the European physician was asked if he ever before, in his own country, had seen a similar case.2044

More important than any of these statements is the following testimony concerning the Karens of Burma, for which I am indebted to the Rev. Dr. Alonzo Bunker, who has been a resident among that people during more than twenty years. He says that, in some of their villages, exogamy prevails, in others endogamy, but marriages between parents and children, brothers and sisters, are prohibited everywhere, and even first cousins very seldom marry, though there is no law against such connections. There is a striking difference with regard to stature, health, strength and fecundity, between the inhabitants of the exogamous and those of the endogamous villages, the latter being much inferior in all these respects. Dr. Bunker has no doubt that this inferiority is owing to the intermarriage of kinsfolk, and he asserts that even the natives themselves ascribe it to this cause, though they obstinately keep up the old custom, regarding marriages out of their own village as highly unbecoming. In cases in which missionaries have been able to persuade young men to choose wives from another village, Dr. Bunker assures me that the good effects of a cross appeared at once.2045

There are some other peoples who ascribe evil results to close intermarriage. Mr. Cousins informs me that the Cis-Natalian Kafirs believe “that their offspring would be of a more sickly nature if such were allowed”; and Mr. Eyles writes that the Zulus, on the border of Pondoland, regard sterility and deformity as consequences of consanguineous unions. The Australian Dieyerie, according to Mr. Gason, have a tradition that, after the creation, fathers, mothers, sisters, brothers, and others of the closest kin intermarried promiscuously, until the bad effects of these marriages became manifest. A council of the chiefs was then assembled to consider in what way the evil might be averted, and the result of their deliberations was a petition to the Muramura, or Good Spirit. In answer to this he ordered that the tribe should be divided into branches, and distinguished one from the other by different names, after objects animate and inanimate, such as dogs, mice, emu, rain, and so forth, and that the members of any such branch should be forbidden to marry other members of the same branch.2046 Again, touching the Kenai, in the north-western part of North America, Richardson states, “It was the custom that the men of one stock should choose their wives from another, and the offspring belonged to the race of the mother. This custom has fallen into disuse, and marriages in the same tribe occur; but the old people say that mortality among the Kenai has arisen from the neglect of the ancient usage.”2047

In a Greenland Eskimo tale, the father of Kakamak, finding that all his grandchildren have died before reaching the age of puberty, suggests to his son-in-law, “Perhaps we are too near akin.”2048 Two Mohammedan travellers of the ninth century tell us that the Hindus never married a relation, because they thought alliances between unrelated persons improved the offspring.2049 In Hadîth, the collection of Mohammedan traditions, it is said, “Marry among strangers; thus you will not have feeble posterity.” “This view,” says Goldziher, “coincides with the opinion of the ancient Arabs that the children of endogamous marriages are weakly and lean. To this class also belongs the proverb of Al-Meydânî, ‘ ... Marry the distant, marry not the near’ (in relationship).” A poet, praising a hero, says, “He is a hero, not borne by the cousin (of his father), he is not weakly; for the seed of relations brings forth feeble fruit.”2050

In opposition to the view that these opinions are the results of experience, it may be urged that any infraction of the customs or laws of ancestors is commonly thought to call down divine vengeance. Father Veniaminof tells us that, among the early Aleuts, incest, which was considered the gravest crime, was believed to be always followed by the birth of monsters with walrus-tusks, beard, and other disfiguration;2051 and among the Kafirs, according to Mr. Fynn, it is a general belief that the offspring of an incestuous union will be a monster—“a punishment inflicted by the ancestral spirit.”2052 But whatever may be said of the other cases referred to, no such explanation can possibly hold good for the Arabs. Among them, marriage with a near relation involved no infringement of their marriage regulations. On the contrary, in spite of the opinions in favour of exogamy, the preference for marriage with a cousin was dominant among them, and a man had even a right to the hand of his “bint ‘amm,” the daughter of a paternal uncle.2053

Taking all these facts into consideration, I cannot but believe that consanguineous marriages, in some way or other, are more or less detrimental to the species. And here, I think, we may find a quite sufficient explanation of the horror of incest; not because man at an early stage recognized the injurious influence of close intermarriage, but because the law of natural selection must inevitably have operated. Among the ancestors of man, as among other animals, there was no doubt a time when blood-relationship was no bar to sexual intercourse. But variations, here as elsewhere, would naturally present themselves; and those of our ancestors who avoided in-and-in breeding would survive, while the others would gradually decay and ultimately perish. Thus an instinct would be developed which would be powerful enough, as a rule, to prevent injurious unions. Of course it would display itself simply as an aversion on the part of individuals to union with others with whom they lived; but these, as a matter of fact, would be blood-relations, so that the result would be the survival of the fittest.

Whether man inherited the feeling from the predecessors from whom he sprang, or whether it was developed after the evolution of distinctly human qualities, we do not know. It must necessarily have arisen at a stage when family ties became comparatively strong, and children remained with their parents until the age of puberty, or even longer. Exogamy, as a natural extension of this instinct, would arise when single families united in small hordes. It could not but grow up if the idea of union between persons intimately associated with one another was an object of innate repugnance. There is no real reason why we should assume, as so many anthropologists have done,2054 that primitive men lived in small endogamous communities, practising incest in every degree. The theory does not accord with what is known of the customs of existing savages; and it accounts for no facts which may not be otherwise far more satisfactorily explained.

The objection will perhaps be made that the aversion to sexual intercourse between persons living very closely together from early youth is too complicated a mental phenomenon to be a true instinct, acquired through spontaneous variations intensified by natural selection. But there are instincts just as complicated as this feeling, which, in fact, only implies that disgust is associated with the idea of sexual intercourse between persons who have lived in a long-continued, intimate relationship from a period of life at which the action of desire is naturally out of the question. This association is no matter of course, and certainly cannot be explained by the mere liking for novelty. It has all the characteristics of a real, powerful instinct, and bears evidently a close resemblance to the aversion to sexual intercourse with individuals belonging to another species.

Besides the horror of incest, there is another feeling to which reference may here be made. “L’amour,” says Bernardin de Saint-Pierre, “ ... ne résulte que des contrastes; et plus ils sont grands, plus il a d’énergie. C’est ce que je pourrois prouver par mille traits d’histoire.... L’influence des contrastes en amour est si certaine, qu’en voyant l’amant on peut faire le portrait de l’objet aimé sans l’avoir vu, pourvu qu’on sache seulement qu’il est affecté d’une forte passion.”2055 Schopenhauer likewise observes that every person requires from the individual of the opposite sex a one-sidedness which is the opposite of his or her own. The most manly man will seek the most womanly woman, and vice versa. Weak or little men have a decided inclination for strong or big women, and strong or big women for weak or little men. Blondes prefer dark persons, or brunettes; snub-nosed persons, hook-nosed; persons with excessively slim, long bodies and limbs, those who are stumpy and short; and so on.2056 A similar view is held by M. Prosper Lucas, Mr. Alexander Walker, Professor Mantegazza, Mr. Grant Allen, and other writers.2057 “In the love of the sexes,” says Professor Bain, “the charm of disparity goes beyond the standing differences of sex; as in contrasts of complexion, and of stature.”2058

Some writers have suggested that love thus excited by differences is favourable to fecundity, those marriages in which it exists being more prolific than others.2059 Thus Mr. Andrew Knight, a most experienced breeder, remarks, “I am disposed to think that the most powerful human minds will be found offspring of parents of different hereditary constitutions. I prefer a male of a different colour from the breed of the female, where that can be obtained, and I think that I have seen fine children produced in more than one instance, where one family has been dark and the other fair. I am sure that I have witnessed the bad effects of marriages between two individuals very similar to each other in character and colour, and springing from ancestry of similar character. Such have appeared to me to be like marriages between brothers and sisters.”2060

These statements, of course, prove nothing, but they may perhaps derive some value from the fact that they are made by so many different observers. The statistical investigation of Professor Alphonse de Candolle, bearing upon the same question, rests on firmer ground. He has found, from facts collected in Switzerland, North Germany, and Belgium, that marriages are most commonly contracted between persons with different colours of the eye, except in the case of brown-eyed women, who are generally considered more attractive than others.2061 He has noted, further, that the number of children is considerably smaller in families where the parents have the same colour of the eye than where the reverse is the case.2062 But Professor Wittrock could not, in Sweden, find any such difference in fecundity between the two categories of marriages;2063 and Mr. Galton observes, “Whatever may be the sexual preferences for similarity or for contrast, I find little indication in the average results obtained from a fairly large number of cases, of any single measurable personal peculiarity, whether it be stature, temper, eye-colour or artistic tastes, influencing marriage selection to a notable degree.”2064

If contrasts instinctively seek each other, this may partly account for the readiness with which love awakens love. Every one knows some unhappy lover who has never been able to win the heart of the person he adores; but in most cases, I should say, love is mutual. And this, perhaps, is owing not only to the contagiousness of the passion, but also to the attractive power of contrasts, which acts equally upon both parties. Thus we might explain, to some extent, the extreme variation of tastes, and the fact that, besides the general standard of beauty common to the whole race, there exists a more detailed ideal special to each individual.