Hereditament

“ So many factors are at work and there are possible so many combinations of factors that no one can tell; for it is during the period of adolescence that hereditary characteristics show themselves. Up to this time the child is a child of the race; during this period it becomes the offspring of its parents. And the factors of heredity—father, mother, ancestry—are mingling and clashing and combining with the factors of environment, and what the outcome is going to be, nobody knows, in specific cases, in advance. ”

“ When there is inheritance from but one parent, the inheritance is as it were complete; the child as a rule resembles its parent in all hereditary characteristics; this is the result of the so-called "pure line" work. But when we have biparental inheritance, a great number of different combinations of the characteristics of the two parents are produced, so that the process of fertilization is one that in this respect completely alters the face of organic nature, producing infinite variety in place of relative uniformity. ”

“ What, let me ask, are the principal phenomena of heredity? There must be physical continuity between parent, or parents, and offspring, so that the offspring is, as Erasmus Darwin well said, a kind of elongation of the life of the parent. ”

“ The exposition of the doctrine of germinal continuity, (b) More precise investigation of the material basis of inheritance, (c) Suspicions regarding the inheritance of acquired characteristics, (d) Application of statistical methods which have led to the formulation of the law of ancestral heredity." We shall take these up in order.
Exposition of the Doctrine of Germinal Continuity.—From parent to offspring there passes some hereditary substance; although small in amount, it is the only living thread that connects one generation with another. ”

“ Formation of Instincts.—The structure produced by repetition of voluntary acts, and which, as we have seen, determines habits, by the law of inheritance is transmitted in a slight degree to the next generation. I say in a slight degree only, because inheritance is from the whole line of ancestry and not from the immediate parents alone. The inheritance from the immediate parents is greater, it is true, than from any one of the series of previous generations, but infinitely less than the sum of inheritances from all previous generations. ”

“ The inheritance of any character means the transmission in the germinal material of matter which, brought under the necessary external conditions, develops into the character of the parent. There is necessarily an acquired or epigenetic side to every character; but there is nothing in our knowledge of the actual processes to make necessary or even probable the supposition that the result of that factor in one generation appears in the germ-plasm of the subsequent generations, in those cases where an embryological development separates parent and offspring. ”

“ In various other species of animals, and particularly in the human family, a certain nobility of race is observed; numerous qualities, in fact, both of mind and body, are derived by hereditary descent.
I have frequently wondered how it should happen that the offspring, mixed in so many particulars of its structure or constitution, with the stamp of both parents so obviously upon it, in so many parts, should still escape all mixture in the organs of generation; that it should so uniformly prove either male or female, so very rarely an hermaphrodite. ”

“ While it is true that inheritance is most clearly recognized in those characters in which offspring resemble their parents, even characters in which they differ from their parents may be inherited, as is plainly seen when, in any character, a child resembles a grandparent or a more distant ancestor more than either parent. ”

“ To sum up, we have reason to believe not only that one's mental character is due largely to heredity, but that the details of it may be equally due to heredity, in the sense that for any particular trait or complex in the child there is likely to be found a similar trait or complex in the ancestry. Such a conclusion should not be pushed to the point of assuming inheritance of all sorts of dispositions that might be due to early training; on the other hand, a survey of the whole field would probably justify us in concluding that any given trait is more likely than not to be inherited. ”

“ When one parent alone displays some newly-acquired and generally inheritable character, and the offspring do not inherit it, the cause may lie in the other parent having the power of prepotent transmission. But when both parents are similarly characterised, and the child does not, whatever the cause may be, inherit the character in question, but resembles its grandparents, we have one of the simplest cases of reversion. ”

“ Offspring Derived from One Parent Only.—So far in our reckoning of heredity we have counted elements from both father and mother, and the complications which arise from such a double ancestry are manifestly very perplexing ones. If we could do away with the elements of sex and find offspring that are derived from one parent only, it would seemingly simplify our problem very much for we should thus have a direct line of descent, free from intermingling. ”

“ In common parlance, heredity is the transmission of any trait or peculiarity from the parent to the offspring, as the color of the hair, the form of the nose, the tones of the voice; or any disease, or any special character that may exist in either parent.
If a horse has a star on its forehead like one of its ancestors, we say it is due to heredity. If an ox has color marks on its body like its parent, it is a case of heredity. If a human being has a disease which his ancestors had, very often he declares he inherited it from them, even if it be only a common catarrh. ”

“ Possibly we have here an explanation of some hereditary phenomena in human beings. If there is an unequal division, and more of the male than of the female nucleus, the child might, as a result, inherit more of the father's than of the mother's characteristics, or the reverse. ”

“ We know that many traits, good and bad, are transmitted by heredity. And naturally when traits are possessed by both father and mother they stand a much greater chance of being transmitted to the offspring than if possessed by one of the parents alone. Now then, if a certain bad trait, such as epilepsy or insanity, is present in a family that trait is present in both cousins, and the likelihood of children from such a marriage inheriting that trait is much greater than when the parents are strangers, the taint being present in the family of only one of the parents. ”

“ HEREDITY AND GERMINAL CONTINUITY—MENDEL, GALTON, WEISMANN
It is a matter of common observation that in the living world like tends to produce like. The offspring of plants, as well as of animals, resembles the parent, and among all organisms endowed with mind, the mental as well as the physical qualities are inherited. This is a simple statement of the fact of heredity, but the scientific study of inheritance involves deep-seated biological questions that emerged late in the nineteenth century, and the subject is still in its infancy. ”

“ The hereditary action produces tendencies in a particular direction in the offspring, which renders its development easier in that direction. It is only gradually, through education and the influence of heredity in a different direction, that the original tendency can be removed. But if all the circumstances of life favour its development, the individual, the family, and the nation will certainly display the result of these tendencies in full force. ”

“ There had been a wondrous involution from centuries of great history, according to subtle, silent laws of hereditary inheritance, in very blood and life, of tone, and quality, and temper, and now there is evolved, evoked, just that of power which tells of kinship with those who have gone before. ”

“ But now the question arises,—How can we explain the presence of such characters consistently with a belief in the continuity of the germ-plasm, a theory which implies the rejection of the supposition that acquired characters can become hereditary? How can the individuals of any species come to possess various characters which are undoubtedly hereditary, if all changes which are due to the influence of external conditions are transient and disappear with the individual in which they arose? ”

“ Every one must have heard of cases of albinism, prickly skin, hairy bodies, &c., appearing in several members of the same family. If strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. ”

“ But is it less certain that the egg of an eagle developes into an eagle, or that the peculiarities of the father and mother are transmitted to the child, than that a stone falls to the ground when its support is taken away? Again, is it not possible to draw a perfectly distinct and certain conclusion as to the relative quantity of the material basis of heredity, present in the germ-cells of either parent, from the fact that the father and mother possess an equal or nearly equal share in heredity? ”

“ The belief that acquired characters are also inherited, therefore, involves belief that in some way the trait acquired by the parent is incorporated in the germ-plasm of the parent, to be handed on to the child and reappear in the course of the child's development. ”

“ In deciding upon marriage, hereditary influences deserve careful consideration in respect alike of the family of the prospective husband and that of the prospective wife. For it is well established that the law of inheritance relates not only to the peculiarities of external configuration, to the features, the stature, the tint of the skin, but also that children inherit from their parents their mode of bodily development, the functional activity of their organs, the duration 265of their life, their predisposition to disease, and even their intellectual and moral qualities. ”

“ All are but recognitions of the same common fact that offspring exhibit various characteristics similar to those of their progenitors.
Blood Heritage.—To this phenomenon of resemblance in successive generations based on ancestry the term heredity is applied. In man, for instance, there is a marked tendency toward the reappearance in offspring of structures, habits, features, and even personal mannerisms, minute physical defects, and intimate mental peculiarities like those possessed by their parents or more remote forebears. ”

“ When the hereditary energy is equal on both sides there necessarily ensues a kind of process of which the consequence is the fusion of the maternal and paternal characters in an intermediate character. ”

“ If we now try to explain the preponderating influence of one parent by the supposition of a greater strength in hereditary power, without first inquiring after some deeper cause, I think the only conclusion warranted by the facts before us is that this power is rarely or never equal in both of the conjugating germ-cells, but that even within the same species, sometimes the male and sometimes the female is the stronger, and that the strength may even vary in the different offspring of the same individuals, as we so frequently see in human families. ”

“ There remains the terminological difficulty that, as I have tried to show, the individual may display characters which were potential in the germ, inherent and necessarily inherited, though they did not appear in the parent nor yet in any ancestor. We have to face the paradox, then, that in natural inheritance a parent can transmit what he has not got, though this does not apply to the unnatural inheritance of property in human society. ”

“ There is no really great scientist in biology at the present moment who teaches the hereditary transmission of acquired characteristics. Modifications of the organism that become matter for heredity have existed for many generations and we cannot tell just how they began. There is no doubt that there is some hereditary influence, for insanity in the same family is likely to keep recurring in successive generations. More than this, affections of certain less important organs are evidently a common trait in certain family strains. ”

“ Let us now turn to another phase of our subject and consider the biological methods of the description and measurement of heredity, as a preliminary to our next chapter in which we shall discuss the bearings of the facts of human heredity upon the possibility of the formation of a permanently improved human breed.
The fact of heredity is one of the most familiar and patent things about organisms. "Do men gather grapes of thorns or figs of thistles?" For we may define heredity as the fact of general resemblance between parent and offspring. ”

“ The solution is easy to suggest, that the offspring of very simple forms are sometimes exactly like their parents, and sometimes not exactly like. From what has been said above of Inheritance and Variation, this is in the highest degree probable, and, this being admitted, it will follow that according to circumstances the progeny that are like their parents, or those that are unlike, will have the best of it. Why this follows will now have to be explained.
All over the surface of our globe there is a struggle for life going on. ”