Many people may insist that this immunity or comparative immunity to tuberculosis and increased resistive vitality against the disease is transmitted and illustrates the principle of heredity. The reaction of the system to the disease increases in each generation and this increase is an acquired character which passes down with the family strain. This immunity should be viewed from another standpoint, however. Certain families possess a resistive vitality to the disease; others lack it. The resistant families do not succumb to it, and propagate themselves. The others gradually die out. What caused the resistant families originally to possess this quality we do not know. We have no trace of its being acquired. Like so many other characters by which men differ from one another, we do not know the beginning of it. Once it comes in as a family trait it is transmitted. In successive generations we have no evidence that it is stronger, only the danger is recognized from experience and better precautions are taken; the consequence is that the original resistive vitality has a better chance to make itself felt and so the family is preserved. This is as true with regard to the conquest of the tendency to excess in the taking of toxic substances, as alcohol and opium, as with regard to disease. It is not the transmission of an acquired character, but the descent of a family trait the origin of which we do not know.
Hereditary Syphilis.—Many physicians will protest that, at least, we have ample evidence for the transmission of syphilis by heredity. We have for many years talked of hereditary syphilis as if it were absolutely sure that its transmission by inheritance took place. There is no doubt, of course, that the disease is conveyed from mother to child. If a mother is actively syphilitic, [{630}] then her child will surely have syphilis when it is born. This, however, is no argument for the hereditary transmission of syphilis. We know now that if a mother is tuberculous, in an active stage of that disease, her child will almost surely have the disease, but this is a question of contagion not of inheritance. If a mother with active tuberculosis nurses her child she is likely to give it tuberculosis. Usually the idea is that the milk is not infective unless there are tuberculous lesions in the breasts, and in cattle it is well known that such lesions in the milk apparatus inevitably bring tubercle bacilli into the milk. The demonstration of tubercle bacilli in the blood of patients in the active stage of the disease is now much more frequent than used to be the case and there seems no doubt that the bacillus can pass through glandular structures into the secretions.
In the same way syphilitic nurses are likely to infect nurslings, though, of course, in this case there are usually syphilitic sores on the nipples which directly communicate the disease. It is almost impossible for a syphilitic woman to nurse a child, if she is in an active stage of the disease, without the production of such infective sores on her nipples. When children are born with syphilis it means only that in the process of feeding the child through the placental tissues, a mother has infected her child quite as she might infect it by nursing afterwards, in case she acquired syphilis after the birth. Lesions corresponding to those on the nipple occur in placental tissues and can be demonstrated without much difficulty. Congenital syphilis, however, can always be traced to contagion and the being born with the disease or having the manifestations of it occur shortly after birth is no argument for heredity at all. It merely emphasizes the danger of contagion.
Mothers of Syphilitic Children.—But there are some cases in which the child who shows symptoms of syphilis after birth is born from a mother who never had any manifestations of syphilis and therefore it has been supposed that the infection must have come from the sperm, and that in these cases, at least, there is a true heredity. It is perfectly possible, however, that syphilitic infective material may accompany the spermatozoon and so bring about the occurrence of syphilis in the offspring. Even this would be infection, however, and not heredity. Much more frequently it would seem that the disease in the infant was contracted from the mother while suffering from a latent form of syphilis, rather than from the paternal contributory particle to its existence. The mother gives no sign of the disease, but Colles' Law is that the mother of a syphilitic child may, without danger to herself, be allowed to nurse her own infant even though she herself has never had any symptoms. This can only mean that she is thoroughly protected against the disease. We would not think for a moment of allowing an ordinarily healthy women to nurse a syphilitic child. Such immunity in the mother of the syphilitic child can only come, so far as the present state of our knowledge goes, from her having had the disease. It has been said that as the result of the intimate communication with her child in utero she has acquired an immunity by the passage across the placental membrane which separates maternal and fetal blood of protective substances of various kinds due to the reaction against the disease already beginning in the child. As a matter of fact, however, there is no evidence of any such reactive substances in the blood of the child which after birth proceeds to have a series of acute lesions that are, as a rule, indicative [{631}] of almost complete lack of resistive vitality. Maternal immunity is evidently due to the occurrence of the disease in some form within the maternal tissues which produces the usual protection against the disease in a briefer time than usual. This certainly seems to be a more satisfactory explanation than that of a transmission of an immunity from the child to the mother which the child itself does not possess. It is easier to understand the transmission of an infection that does not manifest itself externally than of an immunity which there has been no time to acquire. Both explanations leave a mystery, but the mystery in the second case can be explained more in consonance with what we know about syphilitic transmission and immunity than in the other case. It does away with the transmission directly from the father almost completely, of course, leaves practically no ground for the heredity of syphilis, but it accords much better than older explanations with biological principles.
Late Lesions and Heredity.—Many physicians will be likely to insist that the late developments of syphilis in children, in which not only three or five years afterwards, but even fifteen or twenty years after birth, there are syphilitic manifestations, are beyond all doubt examples of heredity. In the last twenty-five years, however, our ideas with regard to the after-effects of syphilis have been entirely modified by what we have learned of such diseases as locomotor ataxia, paresis and the like. These are undoubtedly parasyphilitic diseases in most cases, yet they not infrequently develop from ten to twenty years after any manifestation of syphilis and they seem to occur, by preference almost, in cases where the preliminary symptoms have been very mild. In not a few cases, indeed, the symptoms of syphilis have been so transient in these patients that the true significance of them was missed until the later developments showed their real character. Krafft-Ebing, at the International Medical Congress at Moscow in 1897, detailed some experiments that he had made on paretics in Vienna. They were patients in whom no history of syphilis could be found, yet they were suffering from typical paresis. As they were in the ultimate stage of the disease it did not seem unjustifiable to inoculate them with syphilis, and in most cases it was found that they would not take the disease, showing that they were probably protected by a previous attack, though there was no history of it.
The development of the late symptoms of syphilis in the second generation can then be much more satisfactorily explained on the basis of a mild infection with very few primary symptoms, almost lacking in secondary symptoms, yet followed by subsequent symptoms of great severity consequent upon the deterioration of vitality produced by the disease. As for the manifestations in the third generation, they are not directly syphilitic, but are, whenever they occur, due to conditions consequent upon the degeneration that had been effected in the preceding generation and which directly weakened the offspring—on the same principle that weak parents give birth to weak children, and starving parents cannot have strong, healthy children—but not because of any direct influence of the disease. It is worth while to discuss this subject from this standpoint, since it disposes of the only supposed evidence left for the hereditary transmission of disease that we now have, though only a few years ago most diseases were supposed to be hereditary.
Heredity in Cancer.—With regard to other diseases, the evidence for any inheritance has been founded entirely on coincidence. All the human race [{632}] dies and must die some way, and so in families a certain number will die of the same disease. The argument for heredity in cancer is extremely weak. When all the relatives of a person afflicted with cancer as far out as the third generation are taken into account, only about one in five of them are found to have suffered from cancer. When we remember, however, that more than one in thirty of all those who die, die of cancer and that the death-rate of this disease is greater than that of typhoid fever, smallpox, scarlet fever, measles, and all other infectious diseases put together, it is easy to understand how large a role coincidence plays in any such set of statistics, and how little the significance of the occurrence of cancer in different members of the same family means, unless possibly there is an occasional element of contagiousness which must not be left out of the reckoning.
Heredity in Other Affections.—Other ailments present much less possibility or probability of any element of heredity. For instance, over-indulgence in meat or drink may readily bring about various ailments of the gastro-intestinal tract. These are, of course, definitely acquired conditions, some of them temporary and some persistent, that will continue to give trouble so long as the patient continues to produce irritation of them. They may, of course, lead to permanent pathological conditions. To say that any of these are likely to be inherited would be quite as absurd as to say that a corn could be inherited, or the permanent deformities produced in toes by wearing badly-fitting shoes could be transmitted to the next generation. We do not think for a moment that because a man has lost a finger his children are likely to be born without a finger, and still less if by some accident or abuse he has been deprived of the use of an arm or leg, that that is likely to be transmitted to the next generation. Yet people calmly talk of the heredity of similarly produced conditions within the body, and even physicians are not entirely free from the superstition, for such it is, of the influence of heredity in producing pathological conditions.
Habits of various kinds, physical and mental, are calmly accepted by many people as influenced by or having their origin in heredity. Under Alcoholism and Drug Addictions we have discussed this phase of the subject, but a word or two more may make it clearer. A tendency to form the same habits may be a family trait and descend from one generation to another. That a specific habit should be the subject of heredity or transmission is as much out of the question as that a facility for doing anything should be transmitted. The son of an acrobat must practice quite as faithfully as did his father in order to secure his father's skill. He may inherit from his father that particular constitution of body, that specific combination of muscle and sinew and bone that enables him to become an acrobat by practice, though with a different kind of body it would be impossible, but his father's acquired facility influences in no way the son's ability. We often hear of a man being the descendant of a series of generations each of whom has gone to the university, as if that somehow assured him a readier and better facility for education, but we know very well that this is not true and that the boys straight from the soil are often the best students and far ahead of the scions of long-time academic families.
Inheritance of Defects.—Acquired characters are not transmitted, though family traits are the subject of inheritance. Disease is not hereditary, but [{633}] defect is. Crossed eyes occur very commonly in families and are evidently a subject of transmission. Family noses are often very peculiar and may be traced for many generations. The Hapsburg lip has been noted in sixty per cent. of the Hapsburgs since the family came into prominence in the thirteenth century. Features of all kinds are inherited, as anyone who has ever spent some time in a family portrait gallery where the ancestors were genuine and the paintings reasonably true to life, knows very well. Certain features of European families can be traced for many generations. The tendency to have six toes or to have an extra finger runs in certain families. So small a thing as a patch of white hair in a particular part of the head may be the subject of hereditary transmission. Moles on a particular part of the body are inherited. All these, however, are characters with regard to the acquisition of which we know nothing, but that have somehow found their way into the family strain and have become subjects of transmission from generation to generation. They provide no evidence, however, as to the transmission of acquired characters.