Popular Science Monthly/Volume 32/March 1888/Curious Facts of Inheritance
| CURIOUS FACTS OF INHERITANCE. |
THE strength of the law which determines the transmission of character—physical or otherwise—from parents to children is still far from receiving due attention and recognition. A striking instance of inheritance is often hailed as wonderful and inexplicable; yet such cases are merely exaggerated examples of a phenomenon of which every family, nay, every individual, affords proof. We all inherit, in a more or less variable degree, the physical constitution and the mental aptitudes of our parents; but this law of inheritance is liable to so much modification, that frequently its operation becomes entirely lost to view. When two forces act upon a body, the resultant is a mean between the two components. This mean is not merely in all cases different from either component, but it is a variable mean, the variation depending upon the relative strength of the two component forces. Inheritance affords an exact parallel to this elementary law of mechanics. No child is entirely like either parent; and the inheritance of two sets of tendencies which may be allied, opposed, or indifferent to each other, may result in characters possessed by neither parent. This result is no breach of the law of inheritance, but is in strict harmony with its most precise conditions; yet it is not surprising that a law subject to such indefinite variation should gain scanty recognition except from those who have made it a special study, and can, therefore, readily distinguish an explicable exception to a law from an actual breach of it.
That the law of inheritance should be constant in its operation, however variable in its effects, is not a matter for surprise. That like produces like is the law written upon the universal face of Nature. Sir Henry Holland truly observes that the real subject for surprise is not that any peculiarity should be inherited, but that any should fail to be inherited; and Darwin remarks that the most correct way of viewing the whole subject would be to look at the inheritance of every character as the rule, and non-inheritance as the anomaly.
It is obvious that instances of inheritance are most likely to be noticed and recorded when the inherited peculiarity is striking and abnormal. Countless instances of inheritance come under our notice almost every day; but the vast majority of them are too slight and insignificant to attract attention. A slight peculiarity of feature, complexion, or voice will readily pass unnoticed; but if a striking deformity be inherited, or some disease pursue a family through several generations, it can hardly escape the most careless observation. Cases are on record of families whose members were characterized by the possession of a supernumerary digit on the hands and feet, and this remarkable peculiarity has been transmitted through five generations, showing how strong is the force of inheritance even in such a minor detail of structure. A still more singular instance is that of Lambert, the well-known "porcupine-man," whose skin was thickly covered with warty projections, which were periodically molted. He had six children, who were similarly affected; and two of his grandsons inherited the strange peculiarity. The writer is acquainted with a gentleman who has a marked drooping of the left eyelid. His son inherits this peculiarity, but in a less remarkable degree. One of the most singular instances of inheritance is that recorded by Decandolle. There was a family in France of which the leading representative could, when a youth, pitch several books from his head by the movement of the scalp alone, and he used to win wagers by performing this feat. His father, uncle, grandfather, and his three children possessed the same power to the same unusual degree. This family became divided eight generations ago into two branches, so that the head of the above-mentioned branch is cousin in the seventh degree to the head of the other branch. This distant cousin resided in another part of France, and on being asked whether he possessed the same faculty, immediately exhibited his power.
Haller, the celebrated physiologist, records that the family of the Bentivoglio all possessed a tumor which used to swell when a damp wind blew, and this strange peculiarity was transmitted from father to son. The frequency among the Romans of surnames indicating some physical peculiarity—Naso, Labeo, Bucco, Capito—would seem to show that the fact of certain types of feature being transmitted through several generations had already been remarked. This fact lies almost unnoticed under many current forms of expression. We speak of a certain type of face being aristocratic or the reverse, by which we mean that physical features characterizing certain classes are transmitted so surely as to become the recognized appanage of those classes. The aristocracy of Western Europe pride themselves upon possessing and transmitting small hands, the outward and visible sign of long exemption from manual labor. The aristocracy of China pride themselves on the smallness of their feet. The implication is in each case the same. We often speak of "blue blood" without any clear idea of the meaning of the expression. The phrase probably arose from the recognition of the fact, that the aristocratic and luxurious classes, who are exempt from actual labor, possess a fine white skin, through which the veins show themselves clearly, and that this peculiarity is transmitted from generation to generation. It is a fact of history that Frederick William I of Prussia succeeded in producing a stock of gigantic grenadiers by matching his tallest soldiers with women of similar proportions.
No point of structure is too minute to afford instances of the law of inheritance. A little spot on the iris has been transmitted from parent to child. The possession of a few abnormally long hairs in the eyebrows has been known to characterize the various members of certain families; and the characteristic of a patch of prematurely gray hair has been transmitted through several generations. Many curious records exist of families which possessed and gloried in their scars, moles, and other family marks, faithfully transmitted from parent to child—a sort of secret hall-mark stamped by Nature to attest the genuineness of the line. Peculiarities in the structure, arrangement, and even in the chemical composition of the teeth, frequently run in families. The writer, among whose professional duties the frequent inspection of tongues holds a humble but not unimportant place, has remarked a notable peculiarity in the shape of that organ transmitted from mother to daughter.
Peculiarities in the expression of the face are frequently inherited. Many cases may be remarked where an inherited resemblance is quite latent when the features are in repose, but comes out with startling vividness when they are agitated by emotion. Among the acquaintances of the writer is a gentleman who, when smiling, exhibits a most peculiar and unusual arrangement of lines at the outer angle of the eyes, and this characteristic has been faithfully transmitted to his children.
When we turn to the lower animals, the instances of striking peculiarities being inherited are still more numerous, and have been recorded with greater care and accuracy. Every breeder and trainer is aware of the vast importance of the law of inheritance, and no instance is allowed to escape notice; but it is only in recent years that philosophers have become alive to the fact that in his physical nature man obeys the ordinary biological laws which prevail among the higher animals, and that among these laws the law of inheritance holds the first place. A breed of cattle once existed which possessed only one horn, and this was transmitted. A one-antlered stag has been known to propagate this peculiarity in his offspring. A rabbit produced a litter in which one of the young was one-eared, and this was transmitted. Many of the most famous breeds of sheep and cattle have arisen through the accidental appearance of some striking peculiarity of structure, which has been preserved by careful selection and breeding. Thus the well-known Ancon or otter breed of sheep, now extinct, arose in the last century in Massachusetts by the accidental birth of a ram characterized by crooked legs and a long back like a turnspit. These peculiarities rendered him unable to leap fences, and as this was a point of great importance to the early settlers, this ram was selected for breeding, and his abnormalities of structure were faithfully transmitted. The breeds of Mauchamp sheep and Niata cattle had a somewhat similar origin. Darwin relates how in a litter of pointer pups one was observed to be of a blue color. This remarkable circumstance led to inquiry, and it was found that, four generations earlier, there had been in the same breed a pointer bitch named Sappho, celebrated for her blue color. We have here an instance of one of the secondary laws of inheritance known as the law of Atavism (from atavus, an ancestor). According to this law, any peculiarity, instead of passing directly from parent to child, may skip one or more generations, and reappear lower down in the line of descent. Of this curious law innumerable instances occur. It is not uncommon for a child to resemble his grandparents much more closely than his father or mother. This is frequently noted in the case of animals, where we have the opportunity of observing several generations, and analogy would lead us to expect a similar principle in the case of man. The law of Atavism can only be explained by assuming that the qualities which were patent in grandfather and grandchild were latent in the intervening generation. There is nothing difficult or arbitrary in this hypothesis, as multitudes of facts are on record to prove that physical and intellectual peculiarities may remain dormant for long periods in an individual, and suddenly develop into prominence under some unwonted pressure. Thus, privation or confinement in an unwholesome atmosphere may develop a latent tendency to consumption. A severe illness has been known to determine the onset of insanity, to which the individual had a hereditary predisposition; or to take more hopeful instances, a severe shock, such as bereavement or the sudden loss of fortune, has been frequently known to bring out unexpected traits of character, and to develop a resolution and a magnanimity, of which the individual had previously exhibited no evidence. Our characters, in addition to those prominent traits which attract general attention, have a multitude of secret marks traced as it were in invisible ink, and ready to spring into prominence on condition of the necessary stimulus being applied.
When we leave the domain of structural peculiarities and turn to that of mind, habit, and instinct, we find an inexhaustible store of curious facts of inheritance. Contrary to popular belief, there seems no reason to doubt that genius is hereditary, though, from the obvious conditions of the case, it is rarely transmitted in like quality and degree from parent to child. The subject is too large to be advantageously considered here; but those interested in it will find a vast mass of striking information and ingenious reasoning in Mr. Francis Galton's admirable work on "Hereditary Genius."
A case is on record of a man who possessed the habit of sleeping on his back with the right leg crossed over the left. His daughter, while still an infant in the cradle, exhibited the same peculiarity. The possibility of imitation, conscious or unconscious, is here obviously excluded. A case has been reported to the writer of a man who had the habit of alternately flexing and extending his great toe while lying in bed. His grandson developed the same habit, though quite ignorant of his grandfather's peculiarity. Ribot records a curious instance of a domestic servant who exhibited an incurable vice of loquacity. She talked incessantly to any one who would listen, to animals, to inanimate objects, and even to herself. When upbraided with her folly, she said it was not her fault, as her father had possessed just the same habit, and had almost driven her mother distracted by it!
Instinct is strongly hereditary in animals, even under the most unfavorable conditions. Ducklings hatched by a hen take to water immediately on breaking their shell; and every one is familiar with the spectacle of the distracted mother wildly running to and fro on the margin of the duck-pond, while her youthful family, heedless of her terror, disport themselves delightedly upon its surface. If the eggs of the wild duck be placed under one of the domesticated species, the young, when their feathers are complete, immediately take to the wing. Birds hatched in confinement construct in their cages the same kind of nest as their more fortunate brethren of the same species build in the virgin forest. Many curious and apparently mysterious facts are explicable on the hypothesis of the permanence under changed conditions of traces of aboriginal instincts. Thus, the domesticated dog, even when thoroughly well cared for, is very fond of burying a bone in some secret spot—a lingering trace, probably, of the time when he ran wild in the woods, and the secreting of surplus food for a future occasion was a matter of practical importance to him. When the squirrel is reared in confinement, it stores away in a corner of its cage a portion of the nuts supplied to it, an instinctive preparation for the coming winter, unnecessary, indeed, for this individual squirrel, but highly important for its ancestors and congeners living in the wild state. Every one must have observed how difficult it is to make the common ass leap over a stream, however small. This unwillingness is not the result of an inherent incapacity for jumping, as the ass leaps over other obstacles with ease, while it hesitates obstinately at the tiniest streamlet. We have here, in all probability, a remnant of an instinct dating far back to the time when the ancestors of the ass were exclusively desert animals, and so unaccustomed to the sight of running water as to be confused and terrified by it. If any one observes a field of lambs at play, he will notice with what delight they frisk upon any hillock within their reach. Here we have probably a trace of the time when the progenitors of our sheep were Alpine animals, and possessed the habits of the chamois.
In the realm of disease, the facts of inheritance are most numerous, and are daily accumulating. Here they are no longer, alas, curious and amusing, but terrible, fateful, overwhelming. No fact of Nature is more pregnant with awful meaning than the fact of the inheritance of disease. It meets the physician on his daily rounds, paralyzing his art, and filling him with sadness. The legend of the ancient Greeks pictured the malignant Furies pursuing families from generation to generation, and rendering them desolate. The Furies still ply their work of terror and death; but we have stripped them of the garb which superstition threw around them, and they now appear to our eyes in the more intelligible but not less awful form of hereditary disease. Modern science, which has cast illumination into so many dark corners of Nature, has shed a new and still more lurid light on the words of the Hebrew Scripture: "The sins of the fathers shall be visited upon the children unto the third and fourth generation." Instances of hereditary disease abound on every hand. Fully fifty per cent of cases of gout are inherited. The proportion is not much less in that fell destroyer of families, our national scourge, consumption. Cancer and scrofula run strongly in families. Insanity is hereditary to a marked degree; but fortunately, like many other hereditary diseases, tends to wear itself out, the stock becoming extinct. Nearly all defects of sight are occasionally inherited. Sir Henry Holland says truly that "no organ or texture of the body is exempt from the chance of being the subject of hereditary disease." Probably most chronic diseases which permanently modify the structure and functions of the body are more or less liable to be inherited.
The important and far-reaching practical deductions from such facts—affecting so powerfully the happiness of individuals and families and the collective welfare of the nation—will be obvious to reflective minds, but can not be dwelt upon In the present article.—Chambers's Journal.