Previous diseases constitute an often overlooked class of factors in predisposing to new attacks, and also in determining their course and results. Of some affections one attack prepares the way for another, as is the case with intermittent fever, convulsions, delirium tremens, and insanity. Just the reverse is true of yellow fever and of all the exanthemata, as scarlet fever, measles, small-pox; likewise of the analogous disorders, mumps and whooping cough. The moot question in this regard concerning syphilis may be left for discussion elsewhere. 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.