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HEREDITY is that biological law by which all beings endowed with life tend to repeat themselves in their descendants : it is for the species what personal identity is for the individual. By it a groundwork remains unchanged amid incessant variation; by it Nature ever copies and imitates herself. Ideally considered, heredity would simply be the reproduction of like by like. But this conception is purely theoretical, for the phenomena of life do not lend themselves to such mathematical precision : the conditions of their occurrence grow more and more complex in proportion as we ascend from the vegetable world to the higher animals, and thence to man.
Man may be regarded either in his organism or in his dynamism : in the functions which constitute his physical life, or in the 'operations which constitute his mental life. Are both of these forms of life subject to the law of heredity ? are they subject to it wholly, or only in part ? and, in the latter case, to what extent are they so subject ?
The physiological side of this question has been diligently studied, but not so its psychological side. We proposé to supply this deficiency in the present work. But the hereditary transmission of mental faculties—considered in its phenomena, its laws, its consequences, and especially in its causes is so closely connected with physiological heredity, that we are compelled to
consider this latter subject at the outset. This we will do very briefly, referring the reader for fuller details to special treatises. It will suffice to show, by means of a few definite and well-ascertained facts, that heredity extends over all the elements and functions of the organism ; to its external and internal structure, its maladies, its special characteristics, and its acquired modifications.
The first thing that attracts the attention, even of the unobservant, is the heredity of the external structure. This is a fact of everyday experience, and nothing is more common than to hear that such and such a child is the image of its father, mother, or grandparents. Hereditary influence may manifest itself in the limbs, the trunk, the head, even in the nails and the hair, but especially in the countenance, expression, or characteristic This is an observation made by the ancients; hence the Romans had their Nasones, Labeones, Buccones, Capitones, and other names, derived from hereditary peculiarities. According to Haller, the Bentivoglios had on their bodies a slightly prominent tumour, transmitted from father to son, which warned them of changes in the weather, and which grew larger whenever a moist wind was coming. The resemblance may be so close as to give rise to doubts concerning personal identity, or at once to betray parentage. Ten years before his death a singer at the opera, named Nourrit, appeared on the stage with one of his sons, who had inherited his physical constitution as well as his pleasing voice; and in a play with a plot like that of the Menachmi, the extraordinary resemblance of the son to the father added a hundred-fold interest to the endless misunderstandings with which the play was filled. These hereditary resemblances have sometimes led to the most unexpected and most romantic adventures, so that it is not surprising that Marryat has turned them to account in his novel, ‘Japhet in Search of a Father.'
It is still more singular that this resemblance between parents and children may undergo such metamorphoses as shall cause the child to resemble at one time the father, and at another the
1 P. Lucas, Traité Physiologique et Philosophique de l' Hérédité Naturelle, vol. i. p. 195.
mother. Girou de Buzareingues, in his work De la Génération, containing some curious facts observed by him, tells us that he knew two brothers who in early life resembled their mother, while their sister resembled the father. These resemblances were such as to strike all who saw them. * But now,' says he, and ever since their youth, the two boys resemble the father, while the daughter has ceased to be like him.' This same author was led, in consequence of numerous observations, to believe that changes of this kind are more frequent and more thorough in the case of boys than in that of girls.
The system of intentional and conscious selection has been applied even to man. Frederick William I., the father of Frederick the Great, who was noted for his love of colossal men, dealt with his regiment of giants as stock-breeders deal with their cattle. He would not allow his guards to marry women of stature inferior to their own. Haller used to boast of his belonging to one of those races whose members, by reason of their imposing stature, seem born to rule other men.'
Heredity may be also traced in all that concerns the complexion of the skin, and the shape and size of the body. Thus, so truly is obesity the result of an organic predisposition, that it has often been known to make its appearance amid privations, and under all the disadvantages of hard labour and poverty.
Heredity influences the internal conformation no less than the external structure. Nothing is more undisputed than the heredity of the form, size, and anomalies of the osseous system ; and universal everyday experience proves the heredity of all the proportions of the cranium, thorax, pelvis, vertebral column, and the smallest bones of the skeleton. Even the heredity of excess or defect in the number of the vertebræ and the teeth has been ascertained. (Lucas.) The circulatory, digestive, and muscular systems obey the same laws which govern the transmission of the other internal systems of the organism. There are some families in which the heart and the size of the principal blood vessels are naturally very large; others in which they are comparatively small ; and others, again, which present identical faults of conform. ation. Lastly—and this is a point that more nearly concerns us-heredity regulates the proportions of the nervous system. It is
evident in the general dimensions of the brain, the principal organ of that system; it is very often apparent in the size, and even in the form, of the cerebral convolutions. This fact was observed by Gall, who thereby accounted for the transmission of mental faculties. We need not here dwell upon this point, for we shall have frequent occasion to revert to it in the course of the present work.
Heredity of the internal elements occurs in the fluids of the organism, as well as in the solid parts : the blood is more abundant in some families than in others, and this superabundance transmits, or may transmit, to the members of such families, a predisposition to apoplexy, hemorrhage, and inflammation. Thus there exists in some families such a liability to hemorrhage that even the prick of a pin may cause in them a flow of blood that cannot be checked. The same may be said with regard to the bile and the lymph.
Nor is it merely, as might be supposed, the structure, whether internal or external, that is thus transmissible ; some quite peculiar characteristics of the mode of existence pass from parent to child. Heredity governs the subordinate no less than the dominant characteristics. Thus fecundity, length of life, and those purely personal characteristics which physicians call idiosyncrasies, are hereditarily transmitted. A few facts will confirm this.
There is no doubt of the influence of heredity on the reproductive power. Some families are noted for their fecundity, and this fecundity descends either through the father or through the mother.
A mother gave birth to twenty-four children, among them five girls, who in turn gave birth to forty-six children in all. The daughter of this woman's son, while still young, was brought to bed with her sixteenth child. (Girou.) The sons, daughters, and grandchildren of a couple who were the parents of nineteen children were nearly all gifted, says Lucas, with the same fecundity.
Several families belonging to the old French nobility possessed extraordinary powers of propagation. Anne de Montmorency (who when over seventy-five years of age was still able, at the battle of St. Denis, to break with his sword the teeth of the Scotch soldier who gave him his death-blow) was the father of
twelve children. Three of his ancestors—Mathieu I., Mathieu II., and Mathieu III.—had altogether eighteen children, of whom fifteen were boys. The son and grandson of the great 'Condé reckoned nineteen children between them; and their great-grandfather, who was slain at Jarnac, had ten. The first four Guises had, in all, forty-three children, thirty of them boys. Achille de Harlay, father of the first President, had nine children; his father, ten; his great-grandfather, eighteen. In some families this fecundity has persisted for five or six generations.
It is now generally understood that longevity depends far less on race, climate, profession, mode of life or food, than on hereditary transmission. If we consult special treatises on this subject, we find centenarians as well among blacks as among whites; in Russia and Scotland as in Italy and Spain ; among those who take the greatest care of their health as among those who have led the hardest lives. A collier in Scotland prolonged his hard and dreary existence over one hundred and thirty-three years, and worked in the mines after he was eighty.
Similar facts are to be met with among prisoners, and even galley-slaves. "The average of life,' says Dr. Lucas, plainly depends on locality, hygiene, and civilization; but individual longevity is entirely exempt from these conditions. Everything tends to show that long life is the result of an internal principle of vitality, which privileged individuals receive at their birth. is so deeply imprinted in their nature as to make itself apparent in every part of their organization. This kind of heredity has long been observed in England, where life-assurance companies require information as to the longevity of the ancestors of those who desire to effect an insurance.
There are, also, on the other hand, many families in which the hair turns grey in early youth, and in which the vigour of the physical and intellectual faculties fails prematurely. In others, early death is of such common occurrence that only a few individuals can escape it by great precaution. In the Turgot family the fifty-ninth year was rarely passed. The man who made that family
i Benoiston de Châteauneuf, Mémoire sur la Durée des Familles Nobles en France.