Our classification of the causes of disease may be set forth in simple form, thus:
1. Pre-natal causation—viz. hereditary transmission of a proclivity to certain disorders, and also the influence of circumstances acting on either parent at the time of conception or on the mother during gestation.
2. Conditional causation—i.e. that belonging to variations of temperature, humidity, etc., affecting individuals.
3. Functional causation—that which is connected with excessive, deficient, or abnormal exercise of any of the functions of the economy.
4. Ingestive causation—e.g. bad diet, intemperance, poisoning.
5. Enthetic causation—viz. that of all contagious, endemic, and epidemic diseases. Closely allied to this is epithelic morbid influence—namely, that of the parasites producing certain affections of the skin, as itch, favus, etc.
6. Mechanical causation. The effects of this belong chiefly, though not exclusively, to the domain of surgery.
Pre-natal causation is of immense consequence, and its study takes in the whole scope of the influences of species, race, family, and individual parentage. Darwin's observations and speculations, and those of other evolutionists, have not ignored the field of human life in considering the struggle for existence and the survival of the fittest. If we are obliged to admit that such a struggle and survival do exist for men as well as for animals and for plants, it is nevertheless obvious that either man's reason and will introduce exceptions to the ordinary laws of development and selection in nature, or else a very peculiar standard of fitness must be recognized in the survivals of humanity. Many feeble, inert, deformed, and diseased forms survive and perpetuate offspring through a long series of generations, while strong and admirable ones perish, often even destroying each other.
Leaving this theme, upon which biological science has not yet pronounced its last word, we may inquire, What diseases are reasonably ascribed to hereditary transmission? First, it must be remarked that seldom is a disease actually received directly from a parent. Putting aside a few asserted instances of variola and allied or analogous affections in utero, congenital constitutional syphilis and (more rarely) scrofulosis seem to afford almost the only examples of this. Nearly always it is a predisposition merely that is inherited. This, however, may be very strongly marked. Its seat is evidently in that (as yet) occult law or process of individual organic development to whose manifestation we give the name of the constitution. In some families all the men grow bald before forty; in others, scarcely so at eighty. Some may expect deafness in middle life, others blindness in old age, and others, again, have a probability of death from disease of the heart at about fifty or apoplexy at about sixty years of age. Such considerations enter into every examination for life insurance, and they are no less important in our prognostications of the results of diseases in practice.
Speaking more definitely, gout is undoubtedly often hereditary. That is, a healthy childhood may be followed by liability to gout in adult or middle age, even in the absence of direct provocatives to that disorder, but much more frequently when they are present. Gout affords an example of the general fact that inherited proclivity to special diseases shows itself at nearly the same time of life in each generation—scrofula in childhood, phthisis in adolescence or early maturity, gout from thirty to forty, apoplexy after sixty, etc. But exceptions to such rules are not at all rare. Gout also exemplifies another important fact—viz. the occasional modification of the transmitted morbid tendency or "diathesis." Parents who have regular gout—i.e. painful attacks of acute inflammation of the smaller joints, followed by deposits of urates, carbonates, etc.—not unfrequently have children who are subject to neuralgia or dyspepsia or modified rheumatic attacks (not sufficiently recognized in practical treatises), to which the name "gouty rheumatism" is most applicable. Again, in one generation there may be a marked tendency to insanity; in the next, to paralysis; in a third, to tubercular meningitis during infancy.1 Or some of these successions may occur in a reverse order.