=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.