« VorigeDoorgaan »
we find that the transmission of any particular trait precisely as it existed in the parent, though not very uncommon, is, nevertheless, far from the invariable rule. It is a vulgar view of heredity which supposes that every or any trait must descend from parent to child in all its original vigor and proportions. But while temporary disappearance is compatible with the physiological law, it is equally true that traits may be transmitted in a latent form, and may reappear at some distant interval. Instances of this law, or a closely correlative one, will be better considered when we come to speak of the transmission of disease. At present we need only say that such instances furnish no support to the idea that a trait not derived from the immediate progenitor, though existing in uncles, aunts, or cousins, is not attributable to heredity. The simple fact is, that in some of the family it is fully developed, while in some it is latent but more or less ready to make its appearance in the next generation. This we are obliged to believe, unless we maintain that the recurrence of a trait, after it has once disappeared, is purely accidental, - a recurrence which is too common to be explained in this manner.
Not only are the bodily traits and peculiarities of structure transmitted, but the same is true of the moral and intellectual qualities, – aptitudes, appetites, passions, feelings, habits of thought, — and with the same apparent irregularity. Considering that these are connected with the cerebral system, and dependent on it for their manifestation, this inheritance might be expected, and the fact is proved as satisfactorily as any other in the whole category of hereditary transmission. The highest manifestation of intellect what passes under the name of genius — is, however, seldom transmitted, and the fact has been inconsiderately regarded as disproving the whole doctrine of the hereditary character of the moral or intellectual nature. Genius, however, is not a simple, definite power, but a highly complex manifestation resulting from the mingled activity of many parts of the brain endowed with the finest qualities by structure. The transmission of genius in its highest forms, therefore, implies the simultaneous descent of a great number and variety of organic peculiarities. The thing is not impossible, but, under the ordinary operation of the law of heredity,
is not likely often to happen. Single talents, simple and circumscribed in their nature, are more frequently transmitted. And the same is true of the moral powers. A benevolent, or devout, or proud, or timid, or bold man often sees his characteristic quality displayed in his child; but that complete, rounded, moral development which occasionally distinguishes some favored mortal is seldom repeated in the next succeeding generation.
As a general rule, peculiarities of structure which are the result of accident subsequent to birth go no further, and the parent who has thus lost a limb or an eye has no apprehension of seeing his offspring born deficient in this organ. But it is well known that aptitudes which are produced by long, special instruction may be transmitted to the offspring. This fact is strikingly manifested in dogs, whose peculiar qualities, such as pointing, setting, and retrieving, strongly hereditary as they are, were originally obtained by a process of training. At first the transmission of such qualities is uncertain and irregular, and not until they have passed through several generations do they become fixed in the constitution of the breed. And there can be no question that the general improvement of the physical and mental character of man, produced by assiduous culture and other elevating influences, is felt by succeeding generations. This is the potent agency effecting the advancement of the race. Without it, one generation would be little if any better than its predecessors, because utterly without the benefit of any cumulative improvement. Instruction raises the individual, but it is heredity which raises the race.
Bearing the above facts in mind, we shall the more readily understand the true origin and propagation of disease, because they are governed by the same general laws as those of the normal and healthy traits. There is this difference in the two cases, that disease originates in conditions and influences adverse to the healthy action of the nervous system. None the less, however, does it become an organic trait, subject to the same laws of propagation as any feature of the face or quality of the mind. The initial step in the morbid process, as we have already seen, is a superexcitation of the vital activities inherent in the cerebral organism, and the final one is obvious lesion of the structure. But the whole process, we must remember, seldom passes through all its stages within the lifetime of a single individual. Within that period, it is usually confined to the first stage, but it becomes a fixed fact in the cerebral economy, and is transmitted with as much persistency as might be expected of a trait so recently engrafted on the common stock. The circumstances of this transmission are pre-eminently matter for curious and important inquiry, and, in view of the painful frequency of mental diseases, deserve to be better understood than they generally are.
The affections of the brain, like those of other organs, are seldom transmitted to all the offspring; and, not unfrequently, all are spared the sad heritage. They disappear before the stronger influences of better blood concerned in the work of reproduction, or are overborne by the more settled traits that belong to the normal condition. It is to be considered, however, that in a large proportion of cases where the offspring seem to have escaped, the abnormal affection may exist in a latent form, to appear, perhaps, fully developed in a subsequent generation.
The observations of contemporary inquirers have made us acquainted with another fact scarcely suspected before. It used to be thought that the idea of hereditary insanity implied the existence of mental disease, in precisely the same form, in the immediate parent. Observation and analogy both lead us to consider every case as hereditary where abnormal action, even the least intense or prominent, existed in the parent. The essential thing is a fixed, persistent deviation from the line of healthy action. Precisely what shape it may take, either in the individual in whom it originates, or in the succeeding generations to which it is transmitted, depends on causes which we have scarcely begun to understand. Of the fact, however, there can be no doubt. Unequivocal, demonstrative mania or lifelong epilepsy may be represented in the offspring by a dormant germ never quickened into action, or by mental peculiarities that pass in the world for originality or affectation, or by some inflammatory or congestive disorder of the brain, or by a repetition of the same form of mania or epilepsy.
Until our own day, little account was made of those minor affections of the brain which are manifested by headaches and other slight forms of neurosis, but which, nevertheless, become the foundation of the severest forms of mental disease. Neither was there supposed to be any connection between the mental or convulsive disease of the offspring, and those habits of the parent - drinking, onanism, etc. - which, without inflicting —
upon their subject any manifest derangement, nevertheless occasion a kind of cerebral deterioration, which may be transmitted to succeeding generations, to make its appearance in every possible form of mental or nervous disorder.
In the current notions on this subject, some confusion of thought has always prevailed respecting the precise thing which is really transmitted. A little attention to the history of other diseases would have prevented any error on the subject. Nobody supposes that phthisis, cancer, or gout, all regarded as hereditary, are conveyed from parent to child with all their distinctive characters. Years must pass before the evil is revealed at all, simply because what is really transmitted is only that abnormal condition designated as tendency to disease, or that initial stage of it which is held in check by the antagonistic forces of early life, and which, perhaps, may never attain a hurtful degree of activity. The multitudes who are understood to have a tendency to consumption may live long and comfortably under a judicious hygiene, and die at last of some other disease. And this is precisely the case with mental disease. It is only the primordial germ — the taint- that is transmitted, in one degree or another of intensity. In answer to the question whether the insanity of any particular person is hereditary, it is not enough to show that the disease has occurred in none of his progenitors. The hereditary element is fairly established if it appears that some near progenitor suffered from any affection of the head. The change in the type of the disease, as it passes from one generation to another, which is so characteristic of insanity, is not less frequently from a higher to a lower grade of intensity than the reverse.
Hence it is that we often see among the children of the insane one maniacal, another imbecile, another epileptic, another hysterical, another eccentric, another passionate, another alternating between exaltation and depression. To recognize the hereditary element in one of these cases and not in another indicates no very broad observation of the course of disease, nor a very nice perception of its affinities and relations.
It would be interesting to know in what proportion of cases well-developed mental disease is transmitted in any form to the offspring. The fact that it may exist in a latent state, giving little or no indication of its presence, must necessarily prevent any very satisfactory statistics on the subject. Common observation shows us that all the children, where there are several, seldom escape, while some remain entirely free from any trace of disorder. Thus is illustrated the operation of the physiological laws that are concerned in the process of transmission. While in some cases the force of heredity carries down the morbid tendency, in others Nature asserts her right to transmit the characters of the race free from foreign admixtures and abnormal ingredients. Besides, the healthier blood of the sound parent may play a controlling part in the formation of the new being. We are not warranted in believing that of these two forces, the normal and the abnormal, the former is less likely to prevail than the latter; so that, in the long run, half the offspring at least may escape. There are conditions in the case, as yet very imperfectly understood, that forbid any more definite conclusion than this. No more can be said, with any approach to certainty, than that the longer the cerebral trouble has existed in the family, and the more decided its action has been on the nervous system, the more likely it is to be transmitted.
The general doctrine is that the morbid movement which ends in insanity is a progressive one, and usually requires more than one generation in order to reach its full development. Of late years the proportion of cases having an hereditary origin (including in this category those where the disease existed in collateral branches) has been estimated by few practical observers at less than one half. Measured by our views of hereditary influence, and with exact information of the parental antecedents, this proportion would be greatly increased. We would not say that insanity never exists ex