Zymotic diseases, both in man and brutes, are due to the presence of living ferments, minute animals or vegetables, very low in the scale of life, which permanently or temporarily take up their abode in the bodies of higher animals, drawing thence their sustenance, and, either by themselves or by their poisonous secretions, causing death or disablement, which latter may be more or less complete and more or less permanent, and which, therefore, may more or less completely and permanently place the individual attacked by them among the ranks of the unfit. In this category are included all infectious and contagious diseases, small-pox, cholera, scarlatina, measles, influenza, syphilis, &c. In some of them the organism, which causes the death or disablement, has been seen and studied under the microscope, as in tuberculosis and cholera; in all the others we are able by analogy to infer with certainty the existence of pathogenic germs, as in small-pox and measles.
Concerning any disease prevalent in any given area—e.g. scarlatina in England—experience teaches that some individuals of the community are much more susceptible to its action than are others. For instance, in England, where scarlatina is prevalent, there are people so constituted that they take the disease in a severe form and perish of it, whereas there are others who are totally immune to its action, or who take it in so mild a form that they do not suffer more than a passing inconvenience. Now it is manifest that, in the presence of such a disease, those individuals who are liable to be attacked by it in its severer forms are at a disadvantage, and tend to be eliminated and to leave no offspring, or if by chance they leave offspring, these, inheriting the peculiarities of their parents, tend in turn to be eliminated; whereas those individuals who are immune against the disease, or are so constituted
