Disease.—Perhaps in the realm of disease more than in any other has an interest in the inheritance of somatic acquirements been manifested. The problem arising here is not essentially different from other questions of inheritance but since it is a matter of such practical importance to man, we may well give it special attention. We have to deal simply with the old questions of what is constitutionally in the germ, what is acquired by the body, and lastly, whether the somatically acquired is inherited. While we all know in a general way what is meant by disease, especially if some specific disorder such as scarlet fever, malaria or tuberculosis is mentioned, an attempt to give an accurate definition is much like trying to define a weed, inasmuch as what is functionally all right at one time or place may be all wrong at another, or what is normal in one animal may be abnormal in another. In general we may say that disease is derangement or failure of physiological function.
Reappearance of a Disorder in Successive Generations Not Necessarily Inheritance.—In attempting to study the inheritance of diseases we must recognize clearly at the outset that reappearance of a disease in successive generations by no means necessarily signifies inheritance. Before it can be pronounced such we must make sure that it is not a case of reimpressing similar modifications on the individuals of successive generations. For example, in England there is a well-recognized condition known as collier’s lung which results from constant working in coal mines. And while both father and son may exhibit it, because of their similar occupations, there is nothing hereditary about the malady. Likewise there is what is known as emery grinder’s lung, and practically every large manufacturing city with soot-laden atmosphere leaves its impress on the lungs of the inhabitants. This will occur, of course, generation after generation, as long as such pollutions of the atmosphere continue to exist. It is clear that any unhealthy occupation is likely to cause the reappearance of an associated typical disease generation after generation as long as the children follow the calling of their parents. The common misconception that deformities or postures associated with a trade, such as a shoemaker’s or tailor’s, is genetically stamped on offspring by the end of the third or fourth generation results from failure to discriminate between real inheritance and mere reappearances under similar conditions of environment.
Prenatal Infection Not Inheritance.—Again, we must recognize that prenatal infection is not inheritance. We have already seen that the young mammal undergoes a certain period of intra-maternal development, but influences operating on it during this period of gestation must be reckoned with as environmental, not germinal. For example, it is said that an unborn child may take smallpox from its mother but this and all similar occurrences are cases of contagion. We find the great pathologist, Virchow, who with many others of his time was a believer in the inheritance of acquired characters, saying nevertheless regarding such instances that, “What operates on the germ after the fusion of the sex-nuclei, modifying the embryo, or even inducing an actual deviation in the development, can not be spoken of as inherited. It belongs to the category of early acquired deviations which are therefore frequently congenital.”
Inheritance of a Predisposition Not Inheritance of a Disease.—We must discriminate sharply also between the inheritance of a predisposition and the inheritance of a disease itself.
We often hear the statement made that tuberculosis is inherited and have cited in evidence certain consumptive families or strains. But tuberculosis is a bacterial disease and children of tuberculous parents are never born with the disease except in the rarest of instances.
Tuberculosis.—What is really inherited is a constitutional susceptibility to this particular germ. While almost any individual may contract tuberculosis when in a state of depressed vitality, or under stress of adverse surroundings, there is no doubt that certain families are more easily infected than others and much less resistant to the ravages of the disease when once it gains a foothold. However, a predisposition is a vastly different thing from the inheritance of the actual disease. For just as we are born with a nose well adapted to eye-glasses but not with eye-glasses on our nose, so many of us are born tuberculizable though not tuberculous, and every sanitary advance we make toward lessening the chances of infection is just so much more insurance for the susceptible.
The whole problem of tuberculosis is an extremely complex one. We do not know just the measure of the inheritance of the predisposition. Some writers in the past have maintained that tuberculosis is mainly a question of infection and not of inherent susceptibility, but steadily increasing evidence all points the other way.
Where the predisposition exists the chances of infection are still, even under the conditions of present-day sanitation, very great. The close association between a consumptive and other members of the family through a prolonged period of time, of course, renders the latter likely to infection unless unusual care is exercised. Very often where a parent is consumptive a child contracts the malady shortly after birth and is particularly likely to do so if the mother, who nurses it and cares for it most intimately, is the tubercular member of the family. Where the mother is tubercular, indeed, the probabilities are that the child has already before birth had its vitality lowered through the toxins circulating in her blood or through defective nutrition, and in consequence does not resist well any diseases.
Undoubtedly a large proportion of our infant mortality is of tubercular origin. It is now a well-established fact that much tuberculosis in children is attributable to drinking milk from tuberculous cows, yet we find individuals so uninformed and dairymen so mercenary that they fight all attempts of the commonwealth to test out cattle for tuberculosis so as to condemn the infected individuals and thus save our babies. Recent investigations made in some of our large pork-packing establishments also indicate that hogs, especially such as have been around tubercular cattle, are often shot through and through with tuberculosis and that such flesh when used as food, if not thoroughly cooked, may become a serious menace to our health.
With the wide prevalence of bovine and human tuberculosis it is little wonder that nearly every human being becomes more or less infected at some period of life. Autopsies on large numbers of individuals in some of our great hospitals have shown that as many as ninety-nine per cent. of the subjects show tubercular lesions of some kind. While it is true that the class of people who would come to autopsy in such public hospitals would perhaps be more likely to be tubercular than the average of the community, still it can not be denied that a very large degree of infection exists. Pearson, from statistics gathered in Europe, has shown that about eighty to ninety per cent. of the population have tubercular lesions before the age of eighteen. Hamburger found that in Vienna ninety-five per cent. of the children of the poor, between twelve and thirteen years of age, were infected with tubercular bacilli and he estimates that all would be before maturity. According to Doctor Mott, pathologist to the London County Asylums, the insane between the ages of fifteen and thirty-five are about fifteen times as likely to acquire tuberculosis as the sane are.