Popular Science Monthly/Volume 47/July 1895/Morbid Heredity

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THE study of morbid heredity is full of interest, because the knowledge of its laws may assist us in finding preventive measures against it, and because it may thereby be a means of comforting persons who are under those laws. In seeking a definition of morbid heredity, we first take Sanson's definition of biological heredity as the transmission from ascendants to descendants, by sexual generation, of natural or acquired properties. With acquired properties we may include morbid ones. Heredity of morbid properties seems to obey the same law as heredity of natural properties, for which we may accept Darwin's formulas of—1. The law of direct and immediate heredity, under which parents tend to transmit their physical and moral characteristics to their descendants. 2. The law of predominance of direct heredity, under which the character of one of the two progenitors is predominant in the product. 3. The law of heredity in reversion, racial heredity, which is applicable to the often-observed facts of atavism, or the reappearance in descendants of the characteristics of a more or less remote ancestor; and 4. The law of homochronous heredity, or the reappearance of hereditary characteristics at the same periods of life in ascendants and descendants.

Morbid heredity does not inevitably obey the laws of direct heredity. It is a well-known fact that diseases in morbid families are not usually transmitted with a perfect likeness. The homologous or similar heredity, which is observed chiefly as to mental diseases, is rare as to other diseases. Usually the disease is modified in descent. A diabetic patient produces an ataxic son, or a hysterical daughter, or an epileptic child. John Hunter seems to have anticipated these variations when he maintained the existence not of hereditary diseases proper, but of a hereditary disposition to contract them—a hypothesis which, though somewhat vague, may account for dissimilar heredity and also for the frequent happy absence of heredity. The probability of morbid heredity manifesting itself is increased when both the parents are attainted with the same defect. Consanguineous marriages, which have been charged with being an important factor in the genesis of neuropathy, of deaf-mutism, and of degeneration in general, really are of effect only through the accumulation of heredity. Consanguinity favors the heredity both of good and of bad family qualities. In healthy families it is desirable; in morbid families it should be shunned.

Pathological selection of nervous parties, who seem to be attracted to one another by invincible sympathies, involves the same probabilities of degeneration as morbid consanguinity. It appears with nervous, hysterical, and veneric persons, and with criminals, among whom vice becomes the basis of unions leading to progressive degeneracy.

Some infectious diseases, usually propagated by contagion, may be transmitted to the child by the mother, or even by the father, while the mother remains free from them. The disease being due to a special agent of infection, that is, to a being with an existence of its own, such transmission can not, properly speaking, be regarded as a fact of heredity. The generative agents have served only as vehicles for the morbid agent or its products. What has been transmitted is not a natural characteristic, or even a definitely acquired characteristic, but a strange and accidentally imposed property, susceptible of disappearing or of being destroyed. Transmission of this kind does not correspond with the definition of biological heredity. Direct heredity of certain diseases has attracted the attention of observers of all times, and has been most regularly noticed in mental diseases.

The family defect is very often exhibited gradually. One or more generations manifest slight troubles, which we might call preparatory. Heredity has to be accumulated, capitalized, as it were, before displaying itself as a morbid entity to which we can give a name. We often find individuals among the ancestors of insane persons, individuals subject to overexcitement, enthusiasts, originals, unfortunate inventors, dissipated persons, or men of irregular life or afflicted with mental or moral eccentricities.

Heredity is not manifested in the same degree in all the forms of madness, and is less evident in the acute than in the chronic forms. Mental troubles generally are most likely to transmit themselves by heredity when they are active at the moment of conception. They are less surely transmissible if their activity in the progenitors is suspended at the time, and especially if the first attack does not come on till after the birth of the child. The fact that we occasionally see a person who has not yet been insane transmit the predisposition to become so to his descendants demonstrates that it is not the disease itself, but the aptitude to acquire it that is transmitted. Accumulated heredity often results in the production of individuals distinguished by physical malformations or by abnormal emotionalisms, constituting what are called the physical and the psychical stigmata of degeneracy. Yet we can not say that heredity impresses special characteristics on madness. But persons inheriting morbid tendencies are more sensitive to excitement of every kind, and more frequently suffer acute irritations under the influence of insignificant causes, while most usually these irritations disappear as easily and as abruptly as they came on.

It is now impossible to deny the heredity of mental troubles, as well of those in the case of which we do not know the accompanying anatomical lesions as of those with which we think we are better acquainted, as in general paralysis and senile dementia. Still less doubtful is it that our cases are most frequently not of direct and identical heredity, but usually of what we call collateral and dissimilar heredity. The son may not inherit from his father, and if the nephew inherits, he will generally seem to be afflicted with a different mental affection from that of his uncle. It must, therefore, be understood that what is meant by heredity in mental diseases does not necessarily correspond with the definition of normal biological heredity.

This frequent dissimilarity in the inheritance of madness becomes more clearly denned when we regard the alliances of the psychopathic family. Nervous troubles very different in their manifestations are frequently met with in families of insane. Prichard has given the name of moral insanity to a mental trouble which prompts to abnormal or mischievous acts while consciousness of their moral nature is wanting. This kind of insanity differs from the impulsive insanity, in which the patient is urged to violent, harmful, or criminal acts by a force which, though invincible, leaves him able to appreciate more or less sanely the character of those acts.

Vice and crime are, furthermore, often hereditary, like insanity. More frequently they are met in families combined with the most various mental disorders—insanity, imbecility, idiocy, etc. The combination of insanity and crime is observed not only in the same family, but often, too, in the same person. Physicians of penitentiaries have long insisted on the frequency of mental disorders among the convicts, and have become convinced that the causes of what is called prison-madness are inherent in the prisoners and not in the prison. It has, moreover, been remarked that debauchery and instinctive perversions are often met with in the hereditary antecedents of insane persons.

Not criminality only has family connections with insanity, but the artistic temperament and genius are frequently associated with it. An old writer has said that there never was a great genius who had not some tinge of insanity. Numerous men, illustrious under different titles, have been attacked with various mental troubles, or have belonged to families in which such troubles were common. The frequency of such associations suggested to Moreau de Tours his saying that genius is a nervous disorder. Further, while great men are rarely exempt from a trace of folly, madmen have no less frequently had a share of genius. Thus we sometimes find in the asylums calculators and musicians of remarkable aptitude in their respective lines. M. H. Nordau ( Degeneration, 1893) has endeavored to demonstrate the constancy of these associations in a special category of artists and literary men whose imagination seems to rejoice in its departure from common ideas. He is liable to criticism for not having comprehended that the madness of these supposed degenerates consisted simply in seeking to surprise or scandalize, and that at bottom their thoughts were not much different from those of their contemporaries. While this criticism may be just as to M. Nordau, it can not clear the authors concerned from the suspicion of insanity.

Civilization favors the production of exceptional beings, men of genius as well as those most degraded by vice or by mental perturbations. The most civilized nations are as much distinguished by the number of their insane and criminals as by that of their men of talent. Civilization produces variation or excites the tendency to it, and it is manifested chiefly in the masculine sex. The parallel development of insanity, genius, and crime constitutes one of the most interesting illustrations of the tendency to variation which characterizes the evolution of mankind, and which results in a progressive inequality, against which the restrictive laws of individualism are of no avail.

Psychical disorders are often associated in families and individuals with other diseases of the nervous system, either growing out of lesions or not connected with known lesions—nervous affections. The relative frequency or nervous manifestations, whether isolated or associated with nervous or other diseases which we shall consider, is so predominant that all such family morbid manifestations may be designated under the name of neuropathic family.

Nervous diseases may be hereditary, and pass directly from father to son; examples of such are locomotor ataxy, epilepsy, and hysteria; but indirect and dissimilar heredity, as in psychopathies, is more usually observed. Family connections between diseases from lesions of the nervous system and nervous affections are proved by frequent coincidences among relatives, and also by their manifestations in the same person, either at the same time or in different periods of his life. Not rarely, further, are mental and neuropathic troubles met with in the history of the same person; and, moreover, a number of diseases are marked by both classes of symptoms.

The already somewhat chaotic picture of morbid heredity would still be incomplete if we omitted to add that among the members of a nervous family we often meet individuals affected with disorders of nutrition—gout, chronic rheumatism, diabetes—quite often hereditary diseases which, as much by their course as by their relationship, deserve the name of nutritional nervous affections. It should be observed further that other diseases, parasitic or suspected to be so, like tuberculosis and cancer, appear more frequently in the same families. The last coincidence may be explained by the fact that the system of nerves regulating nutrition may, when its activity is weak, diminish the resistance of the organism and favor the action of morbid agents.

The question of morbid heredity is still more complicated by the established facts that in a large number of tainted families there exist individuals wholly exempt, while the exceptional character of their cases can not be interpreted by the uncertainties of paternity; and that a considerable number of affections usually regarded as hereditary or peculiar to the family may appear in a family independently of all heredity. Many diseases are known that merit the title of family disorders and attack several children of a single generation without its being possible to find anything like them in either the paternal or the maternal line. The persistence of healthy individuals in an unhealthy family may be explained by atavism; but the appearance of a family disease without any resemblance among the ascendants constitutes an exception to the laws of normal heredity.

We are justified in charging certain toxic or infectious agents with being capable of determining, by the influence they exercise upon progenitors, the same morbid predispositions as heredity. Thus, we can attribute to chronic alcoholism, to saturnism, to morphinism, and to other habits of intoxication of parents a considerable number of nervous affections and psychopathies which are developed in the children at different ages, and confer upon them characters quite different from the characters of their parents. Acute transitory intoxications may have the same effect; and drunkenness of parents at the moment of conception or during gestation has been charged with producing imbecility, idiocy, epilepsy, and other diseases in the children.

The effect of intoxication by drugs may likewise be induced by emotional intoxication. The acute or chronic emotions of the mother during gestation may undoubtedly have a noxious influence upon the child and determine troubles of development in it, which may be manifested by anomalies of forms or by functional troubles revealing anomalies of structure. Bad food or defective hygiene, acting directly upon the nutrition of the mother, may have the same effects. All these conditions, finally, may be accumulated under certain circumstances.

In short, the predisposition to disease may be hereditary or congenital. Hereditary transmission is, however, not inevitable, and most frequently it is due to very diverse conditions in the nutrition of the progenitors. Some authors have associated the idea of degeneration with that of heredity, and designate a whole category of disorders under the name of hereditary degenerations; but many persons who exhibit the characteristics of this category have not inherited them. The necessity of this connection between degeneration and heredity ought to disappear along with the notion of inevitable heredity. We may degenerate without a hereditary tendency thereto, and we may escape morbid heredity. Diseases which are developed simply on account of a hereditary or congenital disposition constitute manifestations of a tendency to degeneration. Morel showed long ago that a race of insane, whatever its origin, tends to exhaust itself in the fourth generation. The fact is found to apply to other hereditary diseases. The tendency to become sterile is, like dissimilarity, an indication of the diminishing vitality which constitutes degeneration, and may be found in plant as well as in animal species. Mr. Dixon has shown, as Morel demonstrated for pathological families, that mulatto stocks die out unless they are crossed with negroes or with whites, and the fourth generation usually marks the limit of their continuance. We have, therefore, a right to infer that it is by degeneration that various diseases which rarely arise except in consequence of a morbid predisposition are met with in the same families.

Congenital malformations act also frequently like the diseases with which they are found associated. Teratological heredity includes facts very like those which have been marked in pathological heredity. While we observe such malformations as sexdigitism, syndactyly, and ectrodactyly transmitted directly for several generations, we more usually see different deformities in the same family. This is because the malformation may vary in form and seat according to the age of the embryo in which a disorder of nutrition is produced. It has even been assumed that variation of species may have had a teratological origin; but we are acquainted with very few deformities that have been definitely established. If the tailless cats of Japan and the Isle of Man are of teratological origin, they constitute a unique exception. While we often observe various deformities in the same family, it is not more rare to meet a number of anomalies in the same person—a phenomenon which deserves special attention.

Most of the deformities compatible with life may be coincident with affections of the nervous system; and the patients whose nervous systems are most gravely affected are just those who present multiple deformities; idiots and imbeciles nearly always exhibit congenital anomalies, which likewise frequently occur in deafmutes, epileptics, etc. The anomalies found in the insane are less gross, but appear more frequently in proportion as the morphology of that class of patients is more carefully studied. The study of physical anomalies in neuropaths, though they are not less common, is still more frequently neglected. With epileptics who have been attacked at an advanced age, and who had consequently resisted a large number of provocative agencies, fewer anomalies are found than among those who were attacked in infancy or youth. If the former held out against a larger number of occasional causes, it was because they were less predisposed, as they were less abnormal.

Teratological abnormities resemble neuropathies not only in their origin and the characteristics of their hereditability, but there can be found in their genesis, besides heredity, all the defective conditions of generation and gestation that have been charged, and justly, with the faculty of giving rise to disorders of the nervous system—emotions, shocks, defective food, alcoholism or any other intoxication, infections, etc. The greater frequency has been noticed of deformities among natural children in cases of conception during intoxication, disproportion in the age of progenitors, etc.

As the masculine sex appears to present a more marked tendency to variation in respect to development and in mental disorders, so it seems to do likewise as regards morphology. Mr. Francis Warner, who has recently examined fifty thousand children in English schools, found 8·27 per cent of physical anomalies among the boys and only 6·78 per cent among the girls. Functional anomalies were also more frequent among the boys.

Like monstrosity, morbid predisposition is the result of a disturbed evolution. In the same way as in families anomalies in form may manifest themselves in very diverse localizations, so anomalies in structure may be variously seated. It is thus comprehensible how, under the influence of the different conditions that usually provoke the manifestations of hereditary diseases—puberty, menopause, fatigue, physical or moral shocks, intoxications, or infections—we observe diverse affections appearing in the same family, but most usually bearing upon the same system. It is worthy of remark that most of these provocative conditions act simply by virtue of the exhaustion that results from them. Growth is usually included among the conditions favorable to the development of disease; but in reality periods of growth, when the processes of nutrition are most energetic, can be and are nothing but periods of resistance; and the susceptibility to attack is developed in the times following periods of growth, particularly of active growth.

The disturbances in evolution of the nervous system are most important in the consideration of the origin of diseases because that system is dominant in the phenomena of the life of nutrition, as well as in those of the life of relation. These disturbances may account for the numerous varieties of morbid manifestations in pathological families.

The want of resemblance in descent observed in pathological and teratological families evidences the want of embryogenic energy which is so accentuated in those families as to end in sterility after a few generations. The attenuation of embryogenic force which may be signalized by failures of very different elements may serve to interpret what is called dissimilar morbid heredity and that paradoxical heredity designated as collateral morbid heredity.

It should be remarked that dissimilarity in morbid families is not absolutely fortuitous. The head of a family gives rise to offspring suffering from different and differently seated disorders of evolution, that cause various morbid predispositions, the variety of which is, however, not so great but that we can find analogies in the manifestation capable of giving a family resemblance to them. Degeneration, in fact, does not take effect except under a kind of rule. As Morel has well observed, unlike degenerates of one family resemble unlike degenerates of another family to such an extent that, like monsters, they are susceptible, wheresoever they may come from, of a scientific classification. Degeneration has its laws the same as normal evolution; whatever may be its cause, it is manifested under a relatively restricted number of common forms.

The theory of the teratological origin of manifestations of morbid heredity is really the only one that will allow us to explain how very diverse conditions of generation, such as extreme youth or too advanced age of progenitors, disproportion in their ages, permanent or even transitory disorders in their vitality, drunkenness, intoxications, infections, accidental exhaustion of the nervous system, or acquired neurasthenia, can produce the same effects as morbid heredity. We should not, in fact, be surprised at finding that degenerates by heredity are not different from degenerates in consequence of disorders of nutrition in progenitors, since degeneration in general results from embryogenical troubles which are reduced, as a whole, to troubles of nutrition. The teratological theory of morbid heredity and of degeneration permits us to comprehend not only unlikeness in morbid heredity, but also the absence of heredity in diseases of the group presumed to be hereditary, but which might be more correctly called degenerative.

Greater importance attaches to disorders of development, when we regard their consequences, as they are produced at a period nearer the beginning of the evolution. External forms are fixed long before the structure of the organs has reached perfection. Thus in man birth finds some parts of the nervous system and the most important ones, when the light of relation is regarded in full development. It is therefore easily comprehensible that evolutionary disorders of the nervous system due to morbid heredity or provoked by influences of the medium may exist without external morbid aberrations. Furthermore, many lesions of the centers met with in neuropathic families in which no external deformities have been found have been attributed to evolutionary troubles of the nervous system.

A race is formed by the fixing of the specific characteristics transmissible by sexual generation. The families and the individuals composing the race transmit to their descendants characteristics of the family and individual characteristics combining themselves in infinite variety to constitute personalities which are yet capable of differing only in limited measure. When the specific qualities that characterize the race cease to be transmitted by heredity; when the children in a family cease to resemble their parents and their brothers and sisters without recovering an ancestral type, and there results a defective change in the adaptation to the physical and social medium, we say that the race is degenerating. By degeneration should be understood the loss of the hereditary qualities that have determined and fixed the characteristics of the race. The characteristic of what is called in human races morbid heredity, which is simply a degeneration, is an abnormal tendency to variation in the posterity, which becomes, in consequence of physical, mental, and moral faults, progressively capable of adapting itself. In the artificial races of domestic animals the result of degeneration is often reversion to a primitive type of the species with capacity to recover the old adaptations. The designation race has in this case been really given to variety, the hereditary qualities of which had not the fixity that characterizes a race. No reversions are observed in the natural races. In the human races in particular degeneration is not manifested, whatever some authors may have said about it, by returns to ancestral forms, but rather by evolutionary disorders bringing on somatic deformities and functional perversions incompatible both with the adaptations now necessary and with ancestral adaptations. Harelip, spina bifida, malconformations of the genital organs, so frequent in degenerates, have nothing to do with ancestral types; and sterility, which is the inevitable outcome of degeneration, can have but little relation to atavism. Considering the matter more closely, we find that the vices in the conformation of degeneration, which we call the stigmata of degeneration, are teratological deformities. If the degenerate fails to give origin to beings that resemble him, it is not because he has acquired the special faculty of transmitting characteristics that do not belong to him, but because degeneration is the dissolution of heredity.

The similarity which we find in the human species among degenerates of different origin—a similarity that permits us to make a classification the scale of which is as a whole narrow enough, has been reproduced in experiments having the provocation of artificial monsters as their object. If the incubation of hen's eggs is disturbed by eccentricities of temperature, if they are warmed too much or not enough, if they are deprived of air, if poisonous substances or substances capable of modifying the nutrition of the embryo—ether, chloroform, alcohol, essences, or nicotine—are introduced into the medium in which they respire, if the same substances are caused to penetrate into the albumen, if they are shaken by abrupt shocks or feeble but repeated blows, monstrosities are generally produced; but it does not appear that any of these causes will provoke exclusively the formation of a special monstrosity. Each of these causes will produce a variety of deformities, any of which may resemble other deformities produced by other causes. In short, the general facts already noticed in degenerating descent may be found in hatches experimentally disturbed—unlikeness in the same families and resemblance of unlike types of one family with those of another.

Besides resulting in ultimate sterility, morbid heredity and degeneration contribute to the destruction of families and races by producing mental and moral differences among them that lead to dissensions and conflicts as mischievous as diseases. When multiple crossings of normal individuals have been effected in a single locality or country, they create in the end both physical resemblances—a family air, a national type—and also psychical likenesses, which entrain a community of tastes and consequently of moral ideas susceptible of becoming fixed for a long series of generations and of constituting a family or national character. The dissolution of heredity, which may be realized either by the introduction of strangers of too different races, or under the influence of native causes of degeneration, is marked both by physical unlikenesses and by the psychical and moral ones that necessarily accompany them. The social discords that spring up among a people like those that so often divide the families of degenerates, taken together, constitute a manifestation of the dissolution of heredity; their source is in a biological fact.

The facts that authorize us to regard morbid heredity or degeneration in general as the consequence of disorders of nutrition during the developmental period of evolution permit us to comprehend the exceptions to the laws of heredity, and consequently to conceive the possibility of securing means of favoring these exceptions and of contending against degeneration.

A strong temptation arises to propose a law prohibiting the marriage of certain categories of degenerates, whereby the natural process of their extinction might be initiated by an artificial sterility. Such a measure would be impracticable, because it would be impossible to fix a limit; and it would certainly be inefficacious in proportion as the temperaments of the persons concerned should be averse to their submission to the laws. The contest may be made by less uncertain processes.

Restoration of a degenerated race—return to mediocrity, as it has been called—may be effected through crossing with individuals of healthy races. M. Sanson has shown, by good examples drawn from zoötechny, that heredity of biological characteristics, and even perhaps of the sex, is generally influenced by the condition of nutrition of the progenitors. The stronger of them attracts the resemblance to his side. It may be assumed that in a union including a morbid factor the healthy factor is in the better position to prevail, and all the more so because it has the atavistic tendency of the other side in its favor. But whether because of the rarity in our time of absolutely healthy elements, or for some other reason, we usually find that, in crossing, the good are more likely to lose than the bad to improve.

There are still other means than happy crossing that may help in the return to mediocrity. Less and less deficient children may be observed to be born in a family of degenerates as the biological conditions of the parents improve. There is nothing surprising in the fact that disorders of nutrition have an injurious influence; all improvements of nutrition may, on the other hand, be accompanied by a correlative amelioration of the products. Generation is, as a whole, the resultant of an excess of nutrition; the lower organisms, absorbing in the medium in which they live more elements than they need for the repair of their losses, increase in volume. When this increase exceeds a certain limit, the individual breaks itself up to form new beings. The process is much more complex in the higher animals, but it is fundamentally the same; and Haeckel has felt free to call reproduction the excrescence of the individual. The best conditions for generation are the best conditions of nutrition. To the regularity of their nutrition is due the regularity of the folding of blastodermic leaflets and the regularity of their further evolution. The arrest of development of a single cell in the earlier periods of evolution is susceptible of determining grave deformations.

Facts observed in human families, in which we see degenerates producing offspring less and less deficient as their own conditions of nutrition improve, indicate that under the influence of a superactivity of nutrition defective organisms might furnish a normal epigenesis. Further, the possibility of combating during the embryonary period the degenerative tendency which is manifested by the delay of development and the frequency of morphological anomalies may be established on the basis of experimental facts which are significant although not very numerous. We find that in the artificial incubation of eggs certain conditions capable of accelerating the normal development are also susceptible of resisting the retarding and deforming influence of disturbing agencies that came into play previous to incubation.

Darwin has remarked that the reproductive function is the most delicate of all, and is also considerably influenced by the medium; in spite of a superabundant alimentation, a large number of wild animals become sterile or produce aborted or deformed offspring by reason simply of being kept in captivity; domestic animals, on the other hand, become more fruitful under the influence of a better régime.

If the influences of the medium are reduced simply to modifications of nutrition; if, on the other side, the embryogenic processes are of the same nature as the processes of nutrition in general, we may assume that the influences of the medium which are capable of happily modifying the nutrition of a defective organism are likewise capable of putting it in better conditions to contribute to the development of the embryo.

Finally, observation and experiment indicate that in order to contend with success against morbid heredity and degeneration, which are besides not inevitable, none of the conditions of nutrition, none of the influences of the medium capable of acting on the development should be neglected.—Translated for The Popular Science Monthly from the Revue des Deux Mondes.


One day, relates Mr. Murray, the publisher, in Good Words, Mr. Darwin came to the late Mr. Murray with a manuscript. As he laid it upon the table he said: "Mr. Murray, here is a book that has cost me many years of hard labor; the preparation of it has afforded me the greatest interest, but I can hardly hope that it will prove of any interest to the general public. Will you bring it out for me, as you have my other books?" It was the work on Earthworms, which had a large and rapid sale. The incident illustrates the modesty of the man.

A successful experiment in telegraphing by induction without connecting wires has been performed by Mr. W. H. Preece between Oban and Auchnacraig, Scotland, while the submarine cable was broken. A guttapercha wire a mile and a half long was laid along the ground from Morven, while on the island of Mull use was made of the ordinary overhead wire connecting Craignure with Aros. The distance between the two parallel wires was about three miles and a half. Using a vibrator as transmitter, and a telephone as receiver, the usual messages were successfully dealt with till the cable was repaired.