to one of permanent disablement, and would place the individual so attacked in the ranks of the unfit, and therefore it is evident that this power of acquiring immunity, of varying in a fit direction in response to appropriate stimulation,. like all other powers of acquiring fit traits, whereby the individual is brought into harmony with a more complex environment, has resulted from Natural Selection. This power is, in fact, a short cut, by means of which is brought about in every individual that takes the disease, or at least in all such individuals as have not reverted to an ancestral condition, in which the power was not developed, or was less developed, a condition of complete or nearly complete immunity; a condition of immunity more complete than could possibly have been attained by the survival of the inherently immune alone, if only for the reason already stated, that in the latter case, whenever the immunity approached perfection, the consequent relaxation of the stringency of Disease Selection would cause a racial lapsing back to an ancestral condition of less immunity. It should be noted here, that in all microbic diseases, which run a definite course of limited duration in the individual, immunity, for a greater or lesser length of time after a first attack, must necessarily follow the cessation of that attack, for, since the cessation of the disease is due to the death or departure of the organisms which cause it, that which causes the death or departure of the organisms will for a greater or lesser length of time operate against and cause the death or departure of any fresh organisms of the same kind which may happen to invade the body. Hence it is that whoever has had an attack of small-pox or diphtheria, is for a greater or lesser length of time immune to those diseases.
The facts that many zymotic diseases run a definite course, which is limited in point of time, and that one
